1
MECHANICAL PROPERTIES OF DENTAL MATERIALS
Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONTENTS
bull Introductionbull Forcebull Stress
bull Tensilebull Compressivebull Shearbull Flexural
bull Strainbull Elastic amp Plastic Deformationbull Stress ndash Strain Curve
3
bull Mechanical Properties based onElastic Deformationbull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength
shear strength compressivestrength amp flexural strength
4
bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops
bull Stress Concentration Effects
bull Methods To Minimize Stress Concentration
bull Conclusion
bull References
5
Mechanical Properties of Dental Materials - Dr Nithin Mathew
INTRODUCTION
bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges
bull These challenges can cause deformation of materials
bull The mechanical properties of a material define how materials respond tomechanical challenges
bull Mechanical properties are defined by the laws of mechanics
ie It is the physical science that deals with energy forces and their effects on thebodies
6
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
MECHANICAL PROPERTIES OF DENTAL MATERIALS
Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONTENTS
bull Introductionbull Forcebull Stress
bull Tensilebull Compressivebull Shearbull Flexural
bull Strainbull Elastic amp Plastic Deformationbull Stress ndash Strain Curve
3
bull Mechanical Properties based onElastic Deformationbull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength
shear strength compressivestrength amp flexural strength
4
bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops
bull Stress Concentration Effects
bull Methods To Minimize Stress Concentration
bull Conclusion
bull References
5
Mechanical Properties of Dental Materials - Dr Nithin Mathew
INTRODUCTION
bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges
bull These challenges can cause deformation of materials
bull The mechanical properties of a material define how materials respond tomechanical challenges
bull Mechanical properties are defined by the laws of mechanics
ie It is the physical science that deals with energy forces and their effects on thebodies
6
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONTENTS
bull Introductionbull Forcebull Stress
bull Tensilebull Compressivebull Shearbull Flexural
bull Strainbull Elastic amp Plastic Deformationbull Stress ndash Strain Curve
3
bull Mechanical Properties based onElastic Deformationbull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength
shear strength compressivestrength amp flexural strength
4
bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops
bull Stress Concentration Effects
bull Methods To Minimize Stress Concentration
bull Conclusion
bull References
5
Mechanical Properties of Dental Materials - Dr Nithin Mathew
INTRODUCTION
bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges
bull These challenges can cause deformation of materials
bull The mechanical properties of a material define how materials respond tomechanical challenges
bull Mechanical properties are defined by the laws of mechanics
ie It is the physical science that deals with energy forces and their effects on thebodies
6
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength Propertiesbull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength
shear strength compressivestrength amp flexural strength
4
bull Mechanical Properties based onPlastic Deformationbull Flexural Strengthbull Impact Strengthbull Toughnessbull Fracture toughnessbull Brittlenessbull Ductilitybull Malleabilitybull Hardness
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops
bull Stress Concentration Effects
bull Methods To Minimize Stress Concentration
bull Conclusion
bull References
5
Mechanical Properties of Dental Materials - Dr Nithin Mathew
INTRODUCTION
bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges
bull These challenges can cause deformation of materials
bull The mechanical properties of a material define how materials respond tomechanical challenges
bull Mechanical properties are defined by the laws of mechanics
ie It is the physical science that deals with energy forces and their effects on thebodies
6
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Hardness testsbull Brinellbull Rockwellbull Vickersbull Knoops
bull Stress Concentration Effects
bull Methods To Minimize Stress Concentration
bull Conclusion
bull References
5
Mechanical Properties of Dental Materials - Dr Nithin Mathew
INTRODUCTION
bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges
bull These challenges can cause deformation of materials
bull The mechanical properties of a material define how materials respond tomechanical challenges
bull Mechanical properties are defined by the laws of mechanics
ie It is the physical science that deals with energy forces and their effects on thebodies
6
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
INTRODUCTION
bull In the oral environment restorative materials and dental appliances are exposed tochemical thermal and mechanical challenges
bull These challenges can cause deformation of materials
bull The mechanical properties of a material define how materials respond tomechanical challenges
bull Mechanical properties are defined by the laws of mechanics
ie It is the physical science that deals with energy forces and their effects on thebodies
6
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull So it is necessary to understand the principles involved in a variety of mechanicalproperties to optimise the clinical service of a material
bull Mechanical properties are measured responses both elastic (reversible on forceremoval) and plastic (irreversible on force removal) of materials under an appliedforce or distribution of forces
bull They are expressed most often in units of stress and strain
7
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull They can represent measurements of
bull Elastic deformation (reversible)bull Proportional limitbull Resiliencebull Modulus of elasticity
bull Plastic deformation (irreversible)bull Percentage of elongation
bull Combination of bothbull Toughnessbull Yield strength
8
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FORCE
bull In physics a force is any influence that causes an object to undergo a certainchange either concerning its movement direction or geometrical construction
bull A force is defined by 3 characteristicsbull Point of applicationbull Magnitudebull Direction of application
bull The SI unit of force is Newton (N)
9
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Occlusal Forces
bull Max occlusal forces 200 ndash 3500N
bull Occlusal forces between adult teeth are highest in the posterior region closest to themandibular hinge axis and decrease from the molar to the incisors
bull Forces on first and second molars vary from 400 to 800N
bull Average on bicuspids cuspids and incisors is about 300 200 and 150N
bull Increase in force from 235 ndash 494N in growing children with an average yearlyincrease of about 22N
10
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS amp STRAIN
Stressbull The force per unit area acting on millions of atoms or molecules in a given
plane of a materialbull Force acting per unit area
bull Unit of measurement is Megapascal (Mpa)
bull Stress is the internal resistance of a material to an external load applied onthat material
Stress (σ)= 119865119865 (119873119873)119860119860 (1198981198982)
11
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Type Of Stress Produced By Examples
Residual Stress
Stress caused within the material during the manufacturing process
During welding
StructuralStress
Stresses produced in the structure during function Weights they support provide the loadings
In abutments of fixed partial denture
Pressure Stress
Induced in vessels containing pressurized materials In dentures during processing under pressure and heat
Flow Stress Force of liquid striking against the wall acts as the load
Molten metal alloy striking the walls of the mould during casting
Thermal Stress
Material is subjected to internal stress due to different temperatures causing varying expansions in the material
Materials that undergo thermal stress such as inlay wax soldering and welding alloys
Fatigue Stress Stress caused due to cyclic rotation of a material Rotary instruments undergo rotational or cyclic fatigue
12
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull By means of the direction of force stresses can be classified as
bull Tensile stressbull Compressive stressbull Shear stressbull Flexural stress
13
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Tensile Stress
bull Tensile stress occurs when 2 sets of forces are directed away from each other in thesame straight line
bull Also when one end is constrained and the other end is subjected to a force awayfrom the constraint
14
bull It is caused by a load that tends to stretch or elongate a body
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 15
bull In fixed prosthodontics a sticky candy (Jujube) can be used toremove crowns by means of a tensile force when the patienttries to open the mouth after the candy has mechanicallybonded to opposing teeth or gums
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Compressive Stress
bull Compressive stress occurs when 2 sets of forces are directed towards each other inthe same straight line
bull Also when one end is constrained and the other end is subjected to a force towardsthe constraint
bull It is caused by a load that tends to compress or shorten a body
16
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Shear Stress
bull Shear stress occurs when 2 sets of forces are directed parallel to each other but notalong the same straight line
bull A shear stress tends to resist the sliding of one portion of a body over another
bull Shear stress can also be produced by a twisting ortorsional action on a material
17
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 18
bull Eg If a force is applied along the surface of tooth enamel by a sharp-edgedinstrument parallel to the interface between the enamel and the orthodonticbracket the bracket may debond by shear stress failure of the resin luting agent
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 19
In the oral cavity shear failure is unlikely to occur due to
1 Many of the brittle materials in restored tooth surfaces generally have roughcurved surfaces
2 The presence of chamfers bevels or changes in curvature of a bonded toothsurface
3 To produce shear failure the applied force must be located immediately adjacentto the interface The farther away from the interface the load is applied more likelythat it is a tensile failure
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 20
5 Since the tensile strength of brittle materials is usually well below their shearstrength values tensile failure is more likely to occur
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Flexural Stress (bending)
bull Force per unit area of a material that is subjected to flexural loading (bending)
bull A shear stress tends to resist the sliding of one portion of a body over another
bull A flexural force can produce all the three types of stresses in a structure but in mostcases fracture occurs due to the tensile component
21
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 22
bull Flexural stresses produced in a three-unit fixeddental prosthesis
bull Flexural stresses produced in a two-unit cantileverbridge
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull StrengthStrength of a material is defined as the average level of stress
at which a material exhibits a certain amount of plastic deformation or atwhich fracture occurs in several test specimens of the same shape andsize
23
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 24
bull Clinical strength of brittle material (ceramics amalgams composites) may appearto be low when large flaws are present or if stress concentration areas existbecause of improper design of a prosthetic component
bull So under these conditions such appliances may fractureat a much lower applied force because the localizedstress exceeds the strength of the material at the criticallocation of the flaw or stress concentration
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic stress in ductile material such as gold alloys donot cause any permanent damage
bull Plastic stresses does cause permanent deformationand sometimes it may be high enough to produce afracture
25
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the stress in structure varies directly with the force ad inversely with area thearea over which the force acts is an important consideration
bull This is true when considering dental restorative materials where the area overwhich the occlusal forces acts are extremely small such as the cuspal areas ofcontact
26
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRAIN
bull Defined as the change in length per unit original length
bull Strain of a material is reported as percentage()
bull Strain may be either elastic plastic or a combination of both elastic and plastic
bull Elastic strain is reversible ie it disappears when force is removed
bull Plastic strain represents permanent deformation of the material which never recoverswhen the force is removed
Strain (ε)= 119862119862119862119862119862119862119862119862119862119862119862119862119862 119894119894119862119862 119862119862119862119862119862119862119862119862119897119897119862 (120549120549119862119862)119880119880119862119862119894119894119897119897 119900119900119900119900119894119894119862119862119894119894119862119862119862119862119862119862 119862119862119862119862119862119862119862119862119897119897119862 (1198621198620)
27
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Eg
bull A tensile force of 200N is applied to an orthodontic wire of cross-sectional area of0000002 m2
bull If the wire is 01m long and if it stretches 0001m under the load
bull ie the wire will fracture at a tensile stress of 100 Mpa and at a tensile strain of001
Strain (ε)= (120549120549119862119862)(1198621198620)
= 0001 11989811989801119898119898
= 00001 = 001
Stress (σ)= 200 119873119873
0000002 1198981198982 = 2002
x 106 = 100 MPa
28
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC AND PLASTIC DEFORMATION
29
Elastic Shear Deformation Plastic Shear Deformation
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
PL = 1020
YS = 1536
UTS = 1625
0
200
400
600
800
1000
1200
1400
1600
1800
0 05 1 15
STRESS ndash STRAIN CURVESt
ress
(Mpa
)
Strain ()
YS (02) = 1536 Mpa
UTS = 1625 Mpa
E = 10200053 = 192 GPa
bull PL Proportional Limit
bull YS Yield Strength
bull UTS Ultimate Tensile Strength
bull E Elastic Modulus
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON ELASTIC DEFORMATION
bull Mechanical properties that are measures of the elastic strain of dental materialsincludes
bull Youngrsquos Modulus Modulus of Elasticitybull Dynamic Youngrsquos Modulusbull Shear Modulusbull Flexibilitybull Resiliencebull Poissonrsquos Ratio
31
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YOUNGrsquoS MODULUS
bull Elastic modulus describes the relative stiffness of a material which is measured bythe slope of the elastic region of the stress strain graph
bull It is the stiffness of a material that is calculated asthe ratio of the elastic stress to elastic strain
bull ie a stiff material will have a high modulus ofelasticity while a flexible material will have a lowmodulus of elasticity
32
E = 10200053 = 192 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Elastic modulus of a tensile test specimen can be calculated as
bull By definition
Stress (σ)=119865119865119860119860
Strain (ε)=1205491205491198621198621198621198620
33
F Applied ForceA Crossectional Area120549120549120549120549 Change In Length1205491205490 Original LengthE Elastic Modulus
E = 119878119878119897119897119900119900119862119862119878119878119878119878119878119878119897119897119900119900119862119862119894119894119862119862
= σε =frasl119865119865 119860119860frasl120549120549119862119862 1198621198620
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
34
E = 10200053 = 192 GPa
Steep Line Higher modulus and more rigidity
Flat Line Lower modulus and less rigidity
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Principle of Elastic Recovery
bull Burnishing of an open metal margin where a dental abrasivestone is rotated against the metal margin to close themarginal gap as a result of elastic and plastic strain
bull After the force is removed the metal springs back to anamount equal to the total elastic strain
bull A final clearance of 25microm is available for the cement
35
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Eg Impression Material
bull The impression materials should have a low modulusof elasticity to enable it to be removed from theundercut areas in the mouth
bull Modulus of elasticity should not be very low that thematerial cannot withstand tearing
36
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Hookersquos Law
bull According to this law within the limits of elasticity the strain produced by a stress(of any one kind) is proportional to the stress
bull The stress at which a material ceases to obey Hookes Law is known as the limit ofproportionality
bull Hookes law can be expressed by the formula
bull The value of the constant depends on the material and the type of stress
37
119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930119930
= 119914119914119914119914119930119930119930119930119930119930119930119930119930119930119930119930
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
STRESS ndash STRAIN CURVE Enamel amp Dentin
PL = 235CS = 262
PL = 176
CS = 234
0
50
100
150
200
250
300
Com
pres
sive
Sre
ss( M
pa)
Strain
Enamel Dentin
bull Dentin is capable ofsustaining significant plasticdeformation under acompressive load before itfractures
bull Enamel - more stiffer andbrittle than dentin
bull But dentin more flexible andtougher
EEnamel = 117 GPaEDentin = 336 GPa
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
POISSONrsquoS RATIO
bull During axial loading in tension or compression there is a simultaneous strain in theaxial and transverse or lateral directions
bull Under tensile loading as a material elongates in thedirection of the load there is a reduction in cross-section
bull Under compressive loading there is an increase in thecross-section
39
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Within the elastic range the ratio of the lateral to the axial strain is called thePoissonrsquos Ratio
bull Poissonrsquos ratio is a unit-less value since it is a ratio of 2 strains
bull Most rigid materials such as enamel dentin amalgam composite etc exhibit apoissonrsquos ratio of about 03
bull More ductile materials such as soft gold alloys show a higher degree of reduction incross-sectional area and higher poissonrsquos ratio
40
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DYNAMIC YOUNGrsquoS MODULUS
bull Elastic modulus can be measured by a dynamic method as well as a static method
bull The velocity at which the sound travels through a solid can be readily measured byultrasonic transducers and receivers
41
bull The velocity of the sound wave and the density of thematerial can be used to calculate the elastic modulusand poissonrsquos ratio
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull If a shear stress was induced instead of a uniaxial tensile or compressive stress theresulting shear strain could be used to define a shear modulus for the material
bull The Shear Modulus (G) can be calculated from the Elastic Modulus (E) and PoissonrsquosRatio (v)
bull The value of 025 to 030 for Poissonrsquos ratio is typical
bull Therefore the shear modulus is usually 38 of the elastic modulus value
42
G = 1198641198642(1+119907119907)
= 1198641198642(1+03)
= 038E
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXIBILITY
bull Defined as the flexural strain that occurs when the materialis stressed to its proportional limit
bull Materials used to fabricate dental appliances andrestorations a high value for the elastic limit is a necessaryrequirement
43
bull This is because the structure is expected to return to its original shape after it hasbeen stressed and the force removed
bull Maximum flexibility ndash flexural strain that occurs when the material is stressed to itsproportional limit
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull There are instances where a large strain or deformationmay be needed with a moderate or slight stress such as inan orthodontic appliance
bull Here a spring is often bent a considerable distance underthe influence of a small stress
bull In this case the structure is said to possess the propertyof flexibility
44
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
RESILIENCE
bull It is the amount of energy per unit volume that is sustained on loading and releasedupon unloading of a test specimen
45
bull Term resilience is associated with springiness of a material butit means precisely the amount of energy absorbed within a unitvolume of a structure when it is stressed to its proportionallimit
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
bull Resilience of 2 or more materials can be compared by observing the areas under theelastic region of their stress-strain graph
bull ie The material with the longer elastic area has the higher resilience
46
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
bull When a dental restoration is deformed during mastication it absorbs energy
bull If induced stress is not greater than proportional limit the restoration is notpermanently deformed
bull ie only elastic energy is stored in it
bull So restorative material should exhibit a moderately high elastic modulus andrelatively low resilience ndash limiting the elastic strain produced
47
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
bull Chewing force on dental restoration causes Deformation(determined by the magnitude of the induced stress)
bull Large deformation do not occur due to the proprioceptivereceptors in the periodontal ligament
bull The pain stimulus causes the force to be decreased andinduced stress to be reduced
bull This prevent damage to teeth or restoration
48
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRENGTH PROPERTIES
bull Strength can be defined as the
bull Maximum stress that a structure can withstand withoutsustaining a specific amount of plastic strain (yieldstrength)
ORbull Stress at the point of fracture (ultimate strength)
49
bull When we describe the strength of a material we are most often referring to themaximum stress that is required to cause fracture of the material
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Strength of a material can be described by one or more of the following properties
bull Proportional Limitbull Elastic Limitbull Yield Strengthbull Ultimate tensile strength shear strength compressive strength amp flexural
strength
50
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
PROPORTIONAL LIMIT
bull Defined as the magnitude of elastic stress above which plastic deformation occurs
51
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull As stress is increased the strain is alsoincreased
bull Initial 0 ndash A portion shows that stress islinearly proportional to strain
bull As strain is doubled stress is also doubled
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
bull After the point A stress is no longer linearly proportional to strain
bull Here the value of stress at A is known as the proportional limit
52
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull So it can also be defined as the higheststress at which the stress-strain curve is astraight line
bull ie stress is linearly proportional to strain
bull Below the proportional limit there is nopermanent deformation in a structure iethe object will return to its originaldimension when force is removed
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
bull The material is elastic in nature below the proportional limit
53
A
B C D
0
10
20
30
40
50
60
70
80
90
100
0 2 4 6 8 10 12 14
Stre
ss (M
pa)
StrainElastic Plastic
bull The region of the stress ndash strain curvebefore the proportional limit is called theelastic region and the region beyond iscalled the plastic region
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
bull When a material is said to have high value of proportional limit it indicates that thematerial is more likely to withstand applied stress without permanent deformation
54
bull The connectors of partial dentures should have highproportional limit
bull Materials like CobaltChromium (alloy) which has highproportional limit is widely used for the fabrication ofconnectors because they can withstand high stresseswithout permanent deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ELASTIC LIMIT
bull Defined as the maximum stress that a material will withstand without permanentdeformation
bull For linearly elastic materials the proportional limit and the elastic limit representsthe same stress within the structure
55
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
YIELD STRENGTH
bull Defined as the stress at which a test specimen exhibits a specific amount of plasticstrain
bull It is a property that represents the stress value at which a small amount of (01 -02) plastic strain has occurred
bull It is a property often used to describe the stress at which the material begins tofunction in a plastic manner
bull The amount of permanent strain is referred to as the percent offset
56
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A value of either 01 or 02 of the plastic strain is often selected and is referredto as the percent offset
57
02
02 offset
bull The point at which at the parallel lineintersect the stress-strain curve is the yieldstrength
bull Elastic limit proportional limit amp yieldstrength are defined differently but theirvalues are fairly close to each other inmany cases
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull These values are important in the evaluation of dental materials because theyrepresent the stress at which permanent deformation begins
bull If these values are exceeded by the masticatory stresses the restoration orappliance may no longer function as originally designed
58
bull Also A fixed partial denture becomes permanentlydeformed through the application of excessiveocclusal forces since a stress equal to or greater thanyield strength is developed
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull In the process of shaping an orthodontic appliance or adjusting the clasp of on aremovable partial denture it is necessary to apply a stress into the structure inexcess of yield strength if the material is to be permanently bent or adapted
59
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ULTIMATE STRENGTH
bull Ultimate tensile strengthstress (UTS) is defined as the maximum stress that amaterial can withstand before failure in tension
bull Ultimate compressive strengthstress (UCS) is the maximum stress that a materialcan withstand in compression
bull The ultimate strength stress is determined by dividing the maximum load intension (or compression) by the original cross-sectional area of the test specimen
60
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MECHANICAL PROPERTIES BASED ON PLASTIC DEFORMATION
bull If a material is deformed by the stress at a point above the proportional limit beforefracture and upon removal of the applied force the stress will reduce to 0 but theplastic strain (deformation) remains
bull Thus the object will not return to its original shape when the force is removed
bull It remains bent stretched or compressed ie it becomes plastically deformed
61
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
COLD WORKING (Strain HardeningWork Hardening)
bull When metals are stretched beyond the proportional limit hardness and strengthincreases at the area of deformation but their ductility decreases
bull Repeated plastic deformation of the metal during bending of orthodontic wire canlead to brittleness of the deformed area of the wire which may fracture on furtheradjustment
62
bull To minimize the risk of brittleness is todeform the metal in small increments so asnot to plastically deform the metalexcessively
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull Defined as the force per unit area at the instant of fracture in a test specimensubjected to flexural loading
bull Also known as modulus of rupture
bull It is a strength test of a bar supported at each endunder a static load
63
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FLEXURAL STRENGTH
bull For a bar with a rectangular cross-section subjected to a 3 point flexure the flexuralstrength can be calculated as
64
σ = 3PL2 wt2
σ Max flexural stress (Mpa)P Load at fracture (N)119871119871 Distance btw 2 supports (mm)119908119908 Width of specimen (mm)t Thickness of specimen (mm)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Most prosthesis amp restoration fractures develop progressively over many stresscycles after initiation of a crack from a critical flaw and subsequently bypropagation of the crack until a sudden unexpected fracture occurs
bull This phenomenon is called fatigue failure
65
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
IMPACT STRENGTH
bull Defined as the energy required to fracture a material under an impact force
66
bull Measured using a Charpy Impact tester where apendulum is released that swings down to fracturethe center of a specimen supported at both ends
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull A moving object possesses a known kinetic energy
bull If the struck object is not permanently deformed it storesthe energy of collision in an elastic manner
bull This ability is due to the resiliency of the material and ismeasured by the area under the elastic region of the stress-strain curve
67
bull Thus a material with low elastic modulus and high tensile strength is moreresistant to impact forces
bull A material with low elastic modulus and low tensile strength has low impactresistance
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Elastic Modulus (Gpa)
Tensile Strength (Mpa)
Composite 17 30 ndash 90
Porcelain 40 50 ndash 100
Amalgam 21 27 ndash 55
Alumina ceramic 350 ndash 418 120
Acrylic 35 60
68
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
TOUGHNESS
bull It is the ability of a material to absorb elastic energy andto deform plastically before fracturing
bull Measured as the total area under a plot of the tensile stressvs strain
bull It can be defined as the amount of elastic and plasticdeformation energy required to fracture a material
bull Toughness increases with increase in strength and ductility
bull ie Greater the strength and higher the ductility the greateris the toughness
69
TOUGHNESS
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
FRACTURE TOUGHNESS
bull It is the mechanical property that describes the resistance of brittle materials to thepropagation of flaws under an applied stress
70
bull The longer the flaw the lower is the stress needed to cause fractureThis is because the stress which would normally be supported by thematerial are now concentrated at the tip of the flaw
bull The ability of a flaw to cause fracture depends on the fracturetoughness of the material
bull Fracture toughness is a material property and is proportional to theenergy consumed in plastic deformation
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Material Fracture Toughness
Enamel 07 ndash 13
Dentin 31
Amalgam 13 ndash 16
Ceramic 12 ndash 30
Composite 14 ndash 23
Porcelain 09 ndash 10
71
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRITTLENESS
bull It is the relative inability of a material to sustain plastic deformation before fractureof a material occurs
bull Eg amalgam ceramics composites are brittle at oral temperatures
bull They sustain nolittle plastic strain before they fracture
bull ie a brittle material fractures at or near its proportional limit
bull Dental materials with low or 0 elongation such as amalgam composite ceramicsetc will have little or no burnishability because they have no plastic deformationpotential
72
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
DUCTILITY
bull It is the ability of a material to sustain a large permanentdeformation under a tensile load upto the point of fracture
bull Eg a metal can be drawn readily into long thin wire is said tobe ductile
bull Ductility is the relative ability of a material to be stretchedplastically at room temperature without fracturing
bull Its magnitude can be assessed by the amount of permanentdeformation indicated by the stress-strain curve
73
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
Methods to determine ductility are
bull Percent elongation after fracture
bull Reduction in area of tensile test specimens
bull Maximum number of bends performed in a cold bend test
74
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
MALLEABILITY
bull It is the ability of a material to sustain considerable permanent deformation withoutrupture under compression as in hammering or rolling into a sheet
bull Gold is the most ductile and malleable pure metal followed by silver
75
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
HARDNESS
bull It is the resistance of a material to plastic deformation which is typically producedby an indentation force
bull In mineralogy the relative hardness of a material is based on its ability to resistscratching
76
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 77
CLASSIFICATION OF HARDNESS TEST
Method of Application Size of the indenter Amount of load applied to the indenter
Static Loading ndash slowly applied
Macro-indentation- Large indenter tip
Macrohardness- ˃ 1kg load
Dynamic Loading ndash rapidly applied
Micro-indentation- Small indenter tip
Microhardness- lt 1kg load
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Various hardness tests include
bull Brinell Testbull Rockwell Testbull Vickerrsquos Testbull Knooprsquos Test
bull Selection of the test should be done on the basis of the material being tested
78
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Used extensively for determining the hardness of metals and metallic materials usedin dentistry
bull Related to the proportional limit and ultimate tensile strength
bull The methods depends on the resistance to the penetration of a small steel balltypically 16mm diameter when subjected to a specified load
79
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
BRINELL TEST
bull Methodbull A hardened steel ball is pressed under a specified load
into a polished surface of the material to be tested
bull Load remains in contact with the material for a fixedtime of 30s
bull After 30s it is then removed and the indentationdiameter is measured
80
bull Load value is then divided by area of projected surface of indentation and thequotient is referred to as the Brinell Hardness Number (BHN)
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Since the brinell test yields a relatively large indentation area this test is good fordetermining average hardness value and poor for determining very localized values
bull Thus for a given load the smaller the indentation the larger is the number andharder is the material
81
ADVANTAGES DISADVANTAGESBest suited for testing ductile materials
Hardness of cold-worked materialsare difficult to measure
Not suitable for brittle materials
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
ROCKWELL HARDNESS TEST
bull This test was developed as a rapid method for hardness determinations
bull Here instead of a steel ball a conical diamond point is used
bull The depth of the penetration is directly measured by a dial gauge on the instrument
bull This test is not suitable for testing brittle materials
82
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
The value is the Rockwell HardnessNumber (RHN)
83
ADVANTAGES DISADVANTAGESDirect reading of the depth of indentation
Not suitable for brittle materials
Rapid testing time
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
VICKERrsquoS HARDNESS TEST
bull This test uses a square based pyramidalindenter
bull The impression obtained on the material isa square
bull The method is similar to Knooprsquos andBrinell tests
84
bull The load value divided by the projected area ofindentation gives the Vickerrsquos Hardness Number(VHN)
bull The lengths of the diagonals of the indentations aremeasured and averaged
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull Used for testing dental casting gold alloys as well as toothstructure
bull Suitable for determining the hardness of brittle materials
85
ADVANTAGES DISADVANTAGESUseful for measuring the hardness of small areas and hard materials
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
KNOOPrsquoS HARDNESS TEST
bull This test was developed mainly to fulfill the needs of a micro-indentation testmethod
bull Suitable for testing thin plastic or metal sheets or brittle materials where theapplied force does not exceed 35N
bull This test is designed so that varying loads may be applied to the indentinginstrument
bull Therefore the resulting indentation varies according to the load applied and thenature of the material tested
86
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull The impression is rhomboid in outline and the lengthof the largest diagonal is measured
bull The load value is then divided by the projected area toget the Knooprsquos Hardness Number (KHN)
87
ADVANTAGES DISADVANTAGESUseful for very brittle materials or thin sheets
Material requires highly polished surface
Longer time required to complete the test
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew 88
MATERIAL KHN
Enamel 343
Dentin 68
Cementum 40
Denture Acrylic 21
Zinc Phosphate Cement 38
Porcelain 460
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
STRESS CONCENTRATION EFFECTS
bull The cause of strength reduction is the presence of small microscopic flaws ormicrostructural defects on the surface or within the internal structure
bull These flaws are especially critical in brittle materials in areas of tensile stressbecause tensile stress tends to open cracks
89
bull Stress at the tips of these flaws is greatlyincreased which leads to crack inititationand broken bonds
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull When a brittle or a ductile material is subjected to compressive stressit tends to close the crack and this stress distribution is more uniform
90
bull When a ductile material is subjected to tensile force it tends toopening of the flaw and only plastic deformation has occurred
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
bull 2 important aspects of the flaws
1 Stress intensity increases with the length of the flaw2 Flaws on the surface are associated with higher stresses than flaws of same size
in the interior region
Therefore the surface of brittle materials such as ceramics amalgams etc areextremely important in areas subjected to tensile stress
91
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CAUSES FOR AREAS OF HIGH STRESS CONCENTRATION AND METHODS TO MINIMIZE THEM
1 Surface defects such as porosity grinding roughnessbull Polish surface to reduce the depth of the defects
2 Interior flaws such as voidsbull Little can be done about the interior flaws but to
ensure highest quality of the structure or toincrease the size of the object
92
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
3 Marked changes in contour ndash sharp internal line angle ataxio-pulpal line angle
bull Design of the prosthesis should vary gradually thanabruptly
bull Notches should be avoidedbull All internal line angles should be rounded
4 A large difference in elastic moduli or thermal expansioncoefficient across bonded surface
bull The elastic moduli of the 2 materials should be closelymatched
bull Materials must be closely matched in theircoefficients of expansion or contraction
93
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
5 A Hertzian load (load applied to a point on the surface of a brittle material)bull Cusp tip of an opposing crown or tooth should be well rounded such that the
occlusal contact areas in the brittle material is large
94
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
CONCLUSION
bull While designing a dental appliance or a restorative material it should haveadequate mechanical properties to withstand the stress and strain caused by theforces of mastication
bull All the methods must be employed to minimize stress concentration so that therestorative material or the appliance is in harmony with the different types of forcesoccurring in the oral cavity
95
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
Mechanical Properties of Dental Materials - Dr Nithin Mathew
REFERENCES
bull Philliprsquos Science of Dental Materials ndash 10th amp 12th Edition
bull Craigrsquos Restorative Dental Materials ndash 13th Edition
bull Dental materials and their selection William JrsquoO Brien ndash 3rd Edition
bull Materials Used in Dentistry S Mahalaxmi ndash 1st Edition
96
97
97