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Dr. Alagiriswamy A A, (M.Sc, PhD, PDF)Asst. Professor (Sr. Grade),
Dept. of Physics, SRM-University,Kattankulathur campus,
Chennai
UNIT VLecture 2
MECHANICS OF MATERIALS
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Features of ductile/brittle materials
Destructive testing & explanations
Fundamental mechanical properties
Stress-strain relation for different engineering materials
Examples
Outline of the presentation
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Ductility; the property of a metal by virtue of which it can be drawn into an elongated state before RUPTURE takes place.
Percentage of elongation =
100length Original
length in Increase
Stress measures the force required to deform or break a material = F/A
Strain measures the elongation for a given load = (L-Lo)/Lo
MaterialsPercentage of ElongationLow-Carbon -37%Medium-Carbon 30%High-Carbon- 25%
A ductile material is one with a large Percentage of elongation before failure
Ductility increases with increasing temperature.
Easily drawn into wire
Moldable,
Easily stretchable without any breakage
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Issues of ductile material
Ductility is the ability of a metal to ________ before it breaks.
A: Bend B: Stretch or elongate C: Be forged D: Be indented
Quiz time
A brittle material is one with a low % of elongation before failure
Brittleness increases with pressure
≤ 5 % elongation
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Features of Brittle material
Grey cast iron (example)A specified amount of stress
applied to produce desired strain
Dislocations/defects/imperfections could be the probable reasons
Fundamental Mechanical Properties
(i)Tensile strength
(ii) Hardness
(iii) Impact strength
iv) fatigue
(v) Creep
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(i)Tensile strength (Alloy steel ; 60-80 kg/mm2)
provides ultimate strength of a material
maximum withstandable stress before breakage
just an indication of instability regime
provides the basic design information to the test of engineers
i. Yield strength (elastic to plastic deformation)
ii.Ultimate strength (maximum stress that can withstand)
iii.Breaking strength (strength upto the rupture)
Destructive testing
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(ii) Hardness factor Ability of a material to resist
before being permanently damaged
Direct consequences of atomic forces exist on the surface
This property is not a fundamental property (like domain boundary)
Measure of macro/micro & nano-hardness factors provide the detailed analyses
• Rockwell hardness testRockwell hardness test• Brinell hardnessBrinell hardness• VickersVickers• Knoop hardnessKnoop hardness• ShoreShore
Hardness Measurement Methods
Yes, you could use AFM tip as a nanoindenter
Destructive testing
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• Brinell, Rockwell and Vickers hardness tests;
to determine hardness of metallic materials to check quality level of products, for uniformity of sample of metals, for uniformity of results of heat treatment.
Knoop Test;
relative micro hardness of a materialRock well hardness;
a measure of depth of penetrationShore scleroscope ;
in terms of the elasticity of the material.
Destructive testing
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Microhardness test involves using a diamond indenter to make a microindentation into the surface of the test material, the indentation is measured optically and converted to a hardness value
HV = 1.854(F/D2); F is the force applied, d2 is the area of the indentation
Vickers hardness tests
MetalographyMetalography; viewing of samples through high powerful microscopes
The _______ type hardness test leaves the least amount of damage on the metals surface.
A: Rockwell B: Brinell C: Scleroscope D: Microhardness
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Impact StrengthThe ability of a material
to withstand shock loading
Try to pull it -- tensile strength
Try to compress it -- compressional strength
Try to bend (or flex) it -- flexural strength
Try to twist it -- torsional strength
Try to hit it sharply and suddenly --(as with a hammer)
impact strength
Affected by the rate of loading, temperature variation in heat treatment, alloy content
Destructive testing
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(i)Fatigue Fatigue is the name given to failure
in response to alternating loads (as opposed to monotonic straining
expressed in terms of numbers of cycles to failure (S-N)
Occurs in metals and polymers but rarely in ceramics.
Also an issue for “static” parts, e.g. bridges.
Destructive testing
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(i)Fatigue
Repeated/cyclic stress applied to a material
An important mode of a failure/disaster
Loss of strength/ductility Increased uncertainty in service
SEM Fractograph (Aluminum alloy)
Destructive testing
Will you be embarrassed by reviving “Who you are??????????”
You are the message (based on several consequences)
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Factors affecting FatigueFactors affecting Fatigue Surface roughness/finishing
thermal treatment
Residual stresses
Strain concentrations
What causes fatigue?Fatigue is different for every person. Here are
some causes of fatigue:Chemotherapy/Pain Sleep problems/Radiation Certain medicines/Lack of exercise Surgery/Not drinking enough fluids Not being able to get out of bed/Nausea Eating problems
Creep
property of a material by virtue of which it deforms continuously under a steady load
slow plastic deformation (slip) of material
occurs at high temperatures.
Iron, nickel, copper and their alloys exhibited this property at elevated temperature.
But zin, tin, lead and their alloys shows creep at room temperature.
Adopts this kind of relationship
Undergo a time-dependent
increase in length
1) Primary creep is a period of transient creep. The creep resistance of the material increases due to material deformation. Predominate at low temperature test such as in the creep of lead at RT.
2) Secondary creep provides a nearly constant creep rate. The average value of the creep rate during this period is called the minimum creep rate.
3) Tertiary creep shows a rapid increase in the creep rate due to effectively reduced cross-sectional area of the specimen
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Logarithmic Creep (low temp)Logarithmic Creep (low temp) Recovery Creep (high temp)Recovery Creep (high temp) Diffusion Creep (very high Diffusion Creep (very high
temperatures)temperatures)
Different stages of creep
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Factors affecting CreepFactors affecting CreepHeat Treatment
Alloying
Grain size
Types of stress applied
Dislocations
Slips
Grain boundaries
Atomic diffusion
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Types of Fracture Brittle Fracture Ductile Fracture Fatigue Fracture Creep Fracture
Fracture; a disaster occurs after the application of load,
Local separation of regions
Origin of the fracture (in two stages): initial formation of crack and spreading of crack
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Depending on the ability of material to undergo plastic deformation before the fracture two fracture modes can be defined - ductile or brittle
• Ductile fracture - most metals (not too cold):
Extensive plastic deformation ahead of crack
Crack is “stable”: resists further extension unless applied stress is increased
• Brittle fracture - ceramics, ice, cold metals:
Relatively little plastic deformation
Crack is “unstable”: propagates rapidly without increase in applied stress
Ductile fracture is preferred in most applications
Fracture
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Different stages of Fracture
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=
Where, e is half of the crack length, is the true surface energy E is the Young's modulus. the stress is inversely proportional to the square
root of the crack length. Hence the tensile strength of a completely brittle
material is determined by the length of the largest crack existing before loading.
For ductile materials (additional energy term p involved, because of plastic deformations
e
E2
Equation governing fracture mechanisms
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Surface energy increases as temperature decreases.
The yield stress curve shows the strong temperature dependence
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On recalling/revisiting
Roughness/ductility/Brittleness/hardnessIsotropy/anisotropy/orthotropy/elasticityResilience/enduranceBrittle fracture Corrosion fatigue CreepDislocation/slipDuctile fracture Ductile-to-brittle transition Fatigue /Fatigue life Fatigue limit/Fatigue strength
Make sure you understand language and concepts: