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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: