Anbu Mechanical Properties of Dental Materials

7/30/2019 Anbu Mechanical Properties of Dental Materials

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MECHANICALPROPERTIES

OF

DENTAL MATERIALS

Presented byANBU .ILAI yr P.G student


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CONTENTS:

IntroductionForces

Forces in oral cavity

Variants of forces

Stress and Strain

Types of stress

Based on Elastic deformation

Elastic modulus

Shear modulus

Flexibility


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CONTENTS

Strength properties:

STRESS-STRAIN CURVE

Proportional limitElastic limit

Yield strength

Ultimate strengthFracture strength

Flexural strength

Fatigue strength


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CONTENTS

Masticatory forces

Toughness

Fracture toughness

Brittleness

Ductility and Malleability

Surface properties:

Hardness and the tests

BRINELLROCKWELL

KNOOP

VICKERSBARCOL & SHORE


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INTRODUCTION

Laws ofMECHANICS

Based on energy and forces

Static and dynamic

mechanical properties:

Success or failure potentialof any

prostheses is dependent upon theirmechanical

properties.


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deformation:

Reversible : proportional limit, resilience,

elastic modulus

Plastic: hardness, percent elongation

Combination: yield strength, toughness


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FORCE Generated by one body interacting with the

other.

Results in transformation or deforming.

Defined by

1.point of application

2.magnitude

3.direction

Unit Newton (N)


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Forces in oral cavity Occlusal / masticatory

Forces on restorations

Occlusal :

a) Maximum range 200 -3500 N

b) Highest biting force 4337 N - 2secs

c) The average biting forces on permanent teeth

were 665,450, and 220 N on molars, bicuspids,and incisors respectively


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FORCES

On/by the Restoration :

Removable partial denture 65 235 N.(rpd)

FIXED 40% of the natural dentition.

REMOVABLE OR COMPLETE denture only 15%

Of the natural.

Women 90N less than men.


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FORCES - VARIANTS

1. Axial (tensile or compressive)

1. Shear (sliding, rubbing)

1. Bending (bending movement)

1. Tortional (twisting movement)


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FORCES & THEIR DEFORMATION


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TYPES OFFORCES:

compressive,

tensile,

shear


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RESULT OF THE APPLIED FORCE

Compression- two sets of forces directedtowards each other

Tension - two sets of forces directed away

from each other in a straight line

Shear - two sets of forces directed parallel to eachother , but not along the same straight line

Torsion results from the twisting of the body.


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STRESS:

Internal reaction equal in intensity andopposite to the direction of the applied externalload/force

force/area = F/A

within a structure

Dependent on

a. Strain rate

b. Shape


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STRESS:

Denoted by S or

Designated as force per unit area ( =N/m )

Pascal = 1 N / m.

Commonly stress is reported in terms of

mega Pascals(MPa)


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Types of stress

SIMPLE

1.tensile

2.compressive

3.shear

COMPLEX


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Types of stress

TENSILE STRESS :

stretch / elongate

perpendicular to thedirection

Eg: A sticky candy can be used to removecrowns by means of tensile force


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Types of stressSHEAR STRESS: force/cross sectional area

parallel to the direction of theforce

Application of shear force may produceelastic shear strain or plastic shear strain.

FLEXURAL STRESS:

Also called as bending stress.


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FLEXURAL STRESS

THREE UNIT BRIDGE & CANTILEVERBRIDGE


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STRAIN:

change in length/ unit initial length.

Relative deformation of an object that issubjected to stress

It is denoted by

Designated as L / L.


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YOUNGs MODULUS:

relative stiffness (or) rigidity

elastic stress

elastic strain

unit GPa its a constant

unaffected by the elastic (or)plastic stress

Independent of the ductility, heat or


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Enamel - stiffer but

brittle

Dentin - flexible

alsotougher


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DYNAMIC YOUNGs MODULUS

in dynamic state of motion

Velocity of the sound waves

Ultrasonic transducers and receivers

Often higher than values by static measurements

SHEAR modulus: shear stress is induced

38 % of elastic modulus


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POISSONs RATIO:

WITHIN ELASTIC RANGE,

LATERAL STRAIN

AXIAL STRAIN

MORE DUCTILE SOFT GOLD ALLOYS - HIGH

REDUCTION IN CROSS SECTIONAL AREA ANDHIGHER POISSONS RATIO.

UNITLESS.


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STRESS - STRAIN CURVE

THE MECHANICAL PROPERTIES OF THE

MATERIAL.

STRAIN IS PLOTTED OVER X AXIS (horizontal

axis ) IS AN INDEPENDENT VARIABLE.

STRESS PLOTTED OVER Y AXIS(vertical axis).

AS THE STRESS INCREASES STRAIN ALSOINCREASES.


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STRESS STRAIN CURVE


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STRESS -STRAIN CURVE

A. PROPORTIONAL LIMIT & ELASTIC LIMIT

A. YIELD STRENGTH

A. ULTIMATE STRENGTH

A. FRACTURE STRENGTH


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PROPORTIONAL LIMIT (A) :

DEFINED AS THE GREATEST STRESS THAT AMATERIAL WILL SUSTAIN WITHOUT DEVIATIONFROM LINEAR PROPOTIONALITY OF STRESS TOSTRAIN WITHOUT PERMANENT DEFORMATION.

ELASTIC LIMIT:

IS THE GREATEST STRESS TO WHICH A MATERIAL

CAN BE SUBJECTED SUCH THAT IT RETURNS TO ITSORIGINAL DIMENSIONS WHEN FORCE IS RELEASED.

WITHOUT PERMANENT DEFORMATION.


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YIELD STRENGTH/ YIELD POINT /

PROOF STRESS (B):

THE STRESS AT WHICH A MATERIAL

EXHIBITS A SPECIFIED LIMITINGDEVIATION FROM PROPORTIONALITYOF STRESS TO STRAIN.

AMOUNT OF PERMANENT STRAIN ISREFERRED TO AS PERCENT OFFSET


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YIELD STRENGTH:

ELASTIC LIMIT AND YIELD STRENGTH DEFINESTHE TRANSITION FROM ELASTIC TO PLASTICBEHAVIOUR .

BEGINSTO FUNCTION IN A PLASTIC MANNER.

E.g Clasp is bent and the function of it is based

on elastic recovery within the range and provideretention.


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ULTIMATE STRENGTH(C):

COMPRESSIVE STRENGTH :

Compressive stress at the point of fracture

TENSILE STRENGTH:

tensile stress at point of fracture

YIELD STRENGTH IS IMPORTANTTHAN ULTIMATE STRENGTH BECAUSE ITIS AN ESTIMATE OF WHEN A MATERIAL

WILL START TO DEFORM PERMANENTLY


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FRACTURE STRENGTH(D):

THE STRESS AT WHICH A BRITTLE MATERIALFRACTURES

MATERIAL DOES NOT FRACTURE AT THE POINTAT WHICH MAXIMUM STRESS OCCURS

AFTER MAXIMUM TENSILE FORCE - MATERIALELONGATES RESULTING IN NECKING REDUCTION IN CROSS SECTIONAL AREA


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FRACTURE STRENGTH(D):

MATERIALS EXHIBITING NECKING, THE

ULTIMATE & FRACTURE STRENGTH ARE

DIFFERENT.

IN DENTAL ALLOYS AND CERAMICS


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FLEXURAL STRENGTH

MODULUS OF RUPTURE (or) TRANSVERSE. LOADING A SIMPLE BEAM SUPPORTED (NOT

FIXED) AT EACH END, WITH A LOAD APPLIED INTHE MIDDLE.

THREE POINT BENDING TEST .

IN COMPARING DENTURE BASE MATERIALS INWHICH STRESS OF THIS TYPE IS APPLIED WITH

THE MASTICATORY FORCES.


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FATIGUE STRENGTH

A structure subjected to repeated or cyclic stresses

well below its ultimate tensile strength can produceabrupt failure.this phenomenon is called fatiguefailure.

Endurance limit: the maximum stress that can bemaintained without failure over an infinite number ofcycles

brittle with rough surface fail in fewer cycles.

Ceramic brackets with activated wires - static fatiguefailure.


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IMPACT STRENGTH

ENERGY REQUIRED TO FRACTURE A MATERIALUNDER AN IMPACT FORCE

1. CHARPY-TYPE IMPACT TESTER2. IZOD IMPACT TESTER

LOW ELASTIC MODULUS &HIGH TENSILE STRENGTH WILL HAVE

HIGH IMPACT RESISTANCE


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TEAR STRENGTH

Resistance of a material to tearing forces.

Dental polymers in thin sections

eg : Flexible impression materials in interproximalareas, maxillofacial materials and soft liners fordentures.

Rapid removal in alginate impression leads tomaximum tear strength .


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TEAR STRENGTH

Material Tearstrength(kN/m)

Agar duplicating material0.22

Denture liners

2.6-45

Impression materials

Agar

0.99


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FLEXIBILITY

FLEXIBILITY:

maximumflexibility is defined asflexural strain occuringwhen material is

stressed to itsproportional limit.


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ELONGATION:

DEFORMATION THAT RESULTS BY TENSILE FORCE .

WORKABILITYOF THE ALLOY.

%ELONGATION =INCREASE IN LENGTH X 100%

ORIGINAL LENGTH


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ELONGATION:

% ELONGATION INCLUDES BOTH ELASTIC ANDPLASTIC ELONGATION.

PLASTIC ELONGATION IS GREATER.

HIGHER YIELD STRENGTH ,LESS ELONGATION

Alloy % Elongation

Gold (type 3) 34.0

40% Au-Ag-CU 2.0


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COMPRESSION:

AMOUNT OF DEFORMATION A MATERIAL CAN

WITHSTAND BEFORE RUPTURE UNDER

COMPRESSIVE STRESS - % OF COMPRESSION.

DUCTILITY AND MALLEABILITYARE

INDICATED BY ELONGATION AND


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DUCTILITY :


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DUCTILITY:

RELATIVE ABILITY OF THE MATERIAL TO DEFORMPLASTICLLY BEFORE IT FRACTURES.

ABILITY OF A MATERIAL TO DRAWN IN TO WIRES WHENSUBJECTED TO TENSILE FORCES.

RELATED TO THE WORKABILITY OF A MATERIAL .

B.I= DUCTILITY

YIELDSTRENGTH


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Measurement of ductility:

1. Percent elongationafter fracture

1. Reduction in area of the tensile test

specimens

1. Maximum number of the bends in cold blendtest

. More the ductility, higher the metal can beburnished


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MALLEABILITY :

ABILITY OF A MATERIALTO BE HAMMERED ORROLLED IN TO THINSHEETS WITHOUTFRACTURING.


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DUCTILITY AND MALLEABILITY


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RESILIENCE

Relative amount ofelastic energyper unitvolume on unloading is defined as resilience

Area bounded by elastic region

Unit mMN/cubic m

Difference in loading and unloading portion iscalled as hysteresis


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RESILIENCE

defined as the amount ofenergy absorbed

by a structure when it is stressed not toexceed its proportional limit.

Springiness


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TOUGHNESS

IS THE AMOUNT OF ELASTIC AND PLASTIC

DEFORMATION. ENERGY REQUIREDTOFRACTURE A MATERIAL.

ABILITY OF A MATERIAL TO ABSORB ELASTICENERGY AND TO DEFORM PLASTICALLY BEFOREFRACTURE.

AREA UNDER THE ELASTIC AND PLASTICPORTION(TOTAL) OF A STRESS STRAIN CURVE.

GREATER THESTRENGTH

AND HIGHER THEDUCTILITY- TOUGHNESS INCREASES.

FRACTURE TOUGHNESS


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FRACTURE TOUGHNESS

Ability to be plastically deformed withoutfracture.

is a material property & is proportional to the

energy consumed in plastic deformation.

Gives a relative value of a materials ability to

resist crack propagation.

larger the flaw, lower the stress required tocause the fracture.


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FRACTURE TOUGHNESS

Brittle - glass - breaks

Ductile - copper rod - bends

Fillers in resin increases it.

Aging &

storage decreases the fracture toughness


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HARDNESS

RESISTANCE OF THE MATERIAL TO DEFORMPLASTICALLY.

RELATED TO HARDNESS OF MATERIAL

COMPRESSIVE STRENGTH,

PROPORTIONAL LIMIT AND

DUCTILITY.

INDICATIVE OF EASE OF FINISHING OF ASTRUCTURE AND ITS RESISTANCE TO

SCRATCHING


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TYPES OF HARDNESS TESTS

MACRO TEST :

BRINELL

ROCKWELL

MICRO TEST :

VICKERsKNOOP


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SHAPE OF THE INDENTER

1


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BRINELL HARDNESS TEST

OLDEST

SMALL STEEL OR TUNGSTEN CARBIDE BALL 1.6mm DIAMETER

LOAD OF 123N30 sec CONTACT WITH SPECIMEN

DIAMETER OF INDENTATION IS MEASRURED

BRINELL HARDNESS NUMBER (BHN)=LOAD/SURFACE AREA OF INDENTATION

FOR METALS AND ALLOYS

1


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ROCKWELL HARDNESS TEST

RAPID -superficialmethod

29.4N -294N 10 min

BALL OR METAL CONEINDENTER

DEPTH OF INDENTATIONMEASURED WITH A

SENSITIVE DIALMICROMETER

GOOD FOR TESTINGVISCOELASTIC MATERIALS

DISADVANTAGES:

1


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KNOOP HARDNESS TEST

MICRO INDENTATION TEST DIAMONDINDENTING TOOL WITH A PYRAMID SHAPE

LENGTH OF THE DIAGONAL IS MEASURED

USED FOR THIN PLASTIC OR METAL SHEETS OR

BRITTLEMATERIALS.

ENAMEL ,DENTIN ,METALS (VARY IN HARDNESS)

1


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VICKERS HARDNESS TEST

SQUARE BASED DIAMOND INDENTER WITH136o POINT ANGLE

DIAGONALS OF THE INDENTATIONMEASURED

VHN= LOAD/AREA OF PYRAMIDAL

IMPRESSION

10-1200N

1


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BARCOL HARDNESS TEST

USED FOR DEPTH OF CURE OF RESINCOMPOSITES

SPRING LOADED NEEDLE INDENTER WITHDIAMETER OF 1mm

10% DECREASE = 20% DECREASE IN

FLEXURAL STRENGTH

1

SHORE A HARDNESS TEST


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SHORE A HARDNESS TEST

SHORE A DURAMETER

USED FOR ELASTOMERS

BLUNT POINTED INDENTER 0.8mm DIAMETERTAPERS TO A CYLINDER OF 1.6mm.

INDENTER ATTACHED BY A LEVER TO A SCALETHAT IS GRADUATED FROM 0 100 UNITS. IFINDENTER COMPLETELYPENETRATES THESPEICMEN, READING OF 0 IS TAKEN.

1


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FRICTION

Friction is the resistance to the motion of onematerial body over the other.

Depends on function

composition

surface finish

lubrication

sliding mechanism in ortho when wire is passedthrough bracket designed for translation

1


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WEAR

Wear is loss of material

By removal and relocation

Through contact of two or more material

Types: AdhesiveCorrosive

Surface fatigue

Abrasive

1

DIAMETRAL COMPRESSION TEST


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DIAMETRAL COMPRESSION TESTFOR TENSION

In this method, the compressive load is placed by a flatplate against the side of a short cylindrical specimen.

The vertical compressive force along the side of the diskproduce as a tensile stress that is perpendicular to the

vertical plane that passes through the center of thedisk. Fracture occur along the vertical plane. In thissituation the tensile stress is directly proportional to thecompressive load applied.

1

DIAMETRAL COMPRESSION TEST


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DIAMETRAL COMPRESSION TESTFOR TENSION

BRAZILIAN TEST

1. Only for the

materials whichexhibitpredominantlyelastic deformation

2. in brittle materialstensile loadingcauses fracture

3. Amalgam,cements,

ceramics,stone

TENSILE STRESS =2P/ DT

D-DIAMETER,T-THICKNESS,

P-APPLIED LOAD

1


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DIAMETRAL TENSILE STRENGTH

1


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COMPRESSSIVE PROPERTIES

Comparison of amalgam, resin ,cements.

Under compressive force.

Length twice that of the diameter -satisfactory

Iftoo short, cone formation in the ends occurs

Iftoo long, buckling occurs

1


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SHEAR STRENGTH

maximum stress that a material can withstandbefore failure in a shear mode of loading.

tested using the punch or push out method.

to study the interface between the two

materials Eg: Porcelain fused to metal.

Shear strength = F/dh

1

BOND STRENGTH


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BOND STRENGTH

TESTS HAVE BEEN DEVELOPED TO MEASURETHE BOND STRENGTH BETWEEN TWO

MATERIALS

CERAMIC METAL,CEMENT TO METALRESIN COMPOSITES AND ADHESIVES TO ENAMEL AND DENTIN.

THESE BOND STRENGTH VALUES MAY NOT SIMULATE

THE CLINICAL SITUATION BECAUSE OF DIFFERENCES

BETWEEN THE GEOMETRY OF THE TEST SPECIMENS AND

CLINICAL APPLICATION

1

BOND STRENGTH


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BOND STRENGTH

BOND STRENGTH VALUES TYPICALLY

OVERESTIMATE THE BOND STRENGTH OBTAINEDIN CLINICAL USAGE .

BOND STRENGTH PERFORMED IN TENSION

CANNOTBE DIRECTLY COMPARED TO THOSEDONE IN SHEAR

1


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STRESS CONCENTRATION FACTORS

Flaws or defects

stress intensity increases with the length of the flaw

Surface flaws are associated with higher stress

Minimized by : polished surface

increased size

design shouldnot vary abruptly

brittle material with lower E value

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VISCOELASTICITY


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VISCOELASTICITY

VISCOSITY:RESISTANCE OF A FLUID TO FLOW

Viscosity =shear stress / shear strain rate

MATERIALS THAT HAVE MECHANICAL

PROPERTIES DEPENDENT ON LOADING RATEARE TERMED AS VISCOELASTIC.

WITH THE PROPERTIES OF ELASTIC SOLID ANDVISCOUS FLUID.

FLUIDS : NEWTONIAN

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VISCOELASTICITY

NEWTONIAN: constant viscosity

independent of shear rate

e.g., cements

PSEUDOPLASTIC: viscosity decreases withincreasing shear rate

e.g., endodontic cements

DILATANT: viscosity increases with increasing

shear rate

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VISCOELASTICITY

MOST DENTAL MATERIALS BEGIN TO SET AFTERTHE COMPONENTS HAVE BEEN MIXED ANDTHEIR VISCOSITY INCREASES WITH TIME.

EXCEPTION, ZOE NEEDS MOISTURE TO SET.

VISCOSITY INCREASES WITH INCREASING

TEMPERATURE

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VISCOELASTICITY

STRESS RELAXATION IS THE REDUCTIONINSTRESS IN A MATERIAL SUBJECT TOCONSTANT STRAIN .

CREEP IS THE INCREASE IN STRAIN IN AMATERIAL UNDER CONSTANT STRESS.

CREEP COMPLIANCE (J) IS DEFINED AS STRAINDIVIDED BY STRESS AT THE GIVEN TIME.

J= STRAIN

STRESS

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STRESS ANALYSIS AND DESIGN


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STRESS ANALYSIS AND DESIGN

DESIGN SHOULD NOT RESULT IN STRESS ANDSTRAIN THAT EXCEED THE STRENGTHPROPERTIES OF THE MATERIAL.

TECHNIQUES:

Strain gauges

Brittle coatings analysis

Holography

2D , 3D photoanalysis

Finite element analysis

For inlays, crowns, dentures (fixed /partial/

complete) endodontic posts and implants

11SUMMARY


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PHYSICAL AND MECHANICAL PROPERTIES OFDENTAL MATERIALS SHOULD WITHSTAND

THE STRESSES OF MASTICATION

APPLIANCES SHOULD BE DESIGNED SOTHAT THE RESULTING FORCES OF

MASTICATION ARE DISTRIBUTED ASUNIFORMLY AS POSSIBLE

THREE INTERRELATED FACTORS FOR LONG TERM FUNTION OF DENTAL MATERIALS ARE

1.MATERIAL CHOICE

2.COMPONENT GEOMETRY (to minimizestress concentration )

3.COMPONENT DESIGN (to distribute

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REFERENCES

CRAIGs restorative dental materials12th & 13th ed.

PHILLIPs science of dental materials

11th ed.JOHN F. McCABE ,ANGUS W.G. WALLS :

Applied dental materials. 8th ed.

JOHN J MANAPPALLIL basic dental materials2nd ed.

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Anbu Mechanical Properties of Dental Materials