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Material Science

Unit 5 Mechanical properties of materials

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Terminology outlineStressStrainYield strength

Tensile strengthTensile toughnessDuctility

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You will sometimes see the symbol (lower case sigma) used torepresent stress.

Its units are.............Tensile, Compressive and shear stress

You will sometimes see the symbol (lower case sigma) used torepresent stress.

Its units are.............Tensile, Compressive and shear stress

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As suggested earlier,stress produces

strain.

Strain is defined as the change in dimension (length) divided by theoriginal dimension.

This makes it the change in length per unit length.What are the units of strain?

It has no units. The two units cancel out. It is said to be

dimensionless.

Strain is defined as the change in dimension (length) divided by theoriginal dimension.

This makes it the change in length per unit length.What are the units of strain?

It has no units. The two units cancel out. It is said to be

dimensionless.

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Elastic Strain

Plastic Strain

The symbol for strain is .

The symbol for strain is .0

l

ll 0

0ll

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Stress Strain CurveYoungs ModulusStrain RateViscous material

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Stress Strain Curve

Tensile and compressive stresses arenormal stresses. A normal stress ariseswhen the applied force acts perpendicular

to the area of interest.A shear stress arises when the appliedforce acts in a direction parallel to the areaof interest

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Stress Strain Curve for non

linear materials

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Stress-Strain Diagram

Strain ( ) ( L/Lo)

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2

3

5

ElasticRegion

PlasticRegion

StrainHardening Fracture

ultimatetensilestrength

Elastic regionslope =Youngs (elastic)

modulusyield strength

Plastic regionultimate tensile strengthstrain hardeningfracture

necking

yieldstrength

UTS

y

E

E

12

y

E

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Strain Hardening- If the material is loaded again from Point 4, the

curve will follow back to Point 3 with the sameElastic Modulus (slope).

- The material now has a higher yield strength of Point 4.

- Raising the yield strength by permanently strainingthe material is called Strain Hardening.

Stress-Strain Diagram (cont)

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Tensile Strength (Point 3)- The largest value of stress on the diagram is called

Tensile Strength(TS) or Ultimate Tensile Strength(UTS)

- It is the maximum stress which the material cansupport without breaking.

Fracture (Point 5)- If the material is stretched beyond Point 3, the stressdecreases as necking and non-uniform deformationoccur.

- Fracture will finally occur at Point 5.

Stress-Strain Diagram (cont)

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The Tensile Test: Use of the Stress

Strain Diagram

The tensile test measures the resistanceof a material to a static or slowly

applied force.

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Stress-Strain Testspecimen

machine

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Tensile Test

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Important Mechanical Propertiesfrom a Tensile Test

Young's Modulus : This is the slope of thelinear portion of the stress-strain curve, it isusually specific to each material; a constant,known value.

Yield Strength : This is the value of stress atthe yield point, calculated by plotting young'smodulus at a specified percent of offset (usuallyoffset = 0.2%).Ultimate Tensile Strength : This is the highestvalue of stress on the stress-strain curve.Percent Elongation : This is the change ingauge length divided by the original gaugelength.

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Engineering tensile strain,

Engineering

tensilestress,

smaller %EL(brittle if %EL5%)

Another ductility measure:100% x

A

A A AR

o

f o

Ductility may be expressed as either percent elongation (% plasticstrain at fracture) or percent reduction in area . %AR > %EL is possible if internal voids form in neck.

Lo Lf Ao

Af

100% xl

ll EL

o

o f

Ductility, %ELDuctility is a measure of theplastic deformation that has beensustained at fracture:

A material thatsuffers very littleplastic deformationis brittle .

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ToughnessLower toughness: ceramics

Higher toughness: metalsToughness is the ability toabsorb energy up tofracture (energy per unitvolume of material).

A tough material hasstrength and ductility .

Approximated by the areaunder the stress-straincurve.

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For an aluminum alloy rode of 5cm , if uniform force is applied, Beforebreaking the length extend up to 5.488 cm and the initial diameter1.263 cm convert in to final diameter of 0.995 cm calculate ductility of the alloy

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% elongation=9.7%% reduction in area=37.9%

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F

bondsstretch

return toinitial

1. Initial 2. Small load 3. Unload

Elastic means reversible.

F Linear-elastic

Non-Linear-elastic

Elastic Deformation

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1. Initial 2. Small load 3. Unload

Plastic means permanent.

F

linearelastic

linearelastic

plastic

planesstillsheared

F

elastic + plastic

bondsstretch& planesshear

plastic

Plastic Deformation (Metals)

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Temperature EffectsAs temperatureincreases:

Ductility andtoughness increase.Yield stress and themodulus of elasticitydecrease.

Temperature also affectsthe strain-hardening

exponent of most metals.

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Impact energy The energy required to fracture astandard specimen when the load is appliedsuddenly.

Impact loading Application of stress at a very highstrain rate .Impact test Measures the ability of a material toabsorb the sudden application of a load withoutbreaking. The Charpy and Izod tests are commonly

used impact tests.Impact toughness Energy absorbed by a material,usually notched, during fracture, under theconditions of the impact test.

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Properties Obtained from

the Impact TestDuctile to Brittle Transition Temperature(DBTT):- The ductile to brittle transitiontemperature is the temperature at which the failure

mode of a material changes from ductile to brittlefracture.A material subjected to an impact blow duringservice should have a transition temperature.

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ImpactTesting consists of placinga notched specimen in animpact tester andbreaking it with aswinging pendulum.

Impact or DynamicLoading

CharpyTest -- Specimensupported at both ends.

Izod Test -- Specimensupported at one end.

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ImpactImpact Toughness -- The energydissipated in breaking the specimen

may be obtained from the amount ofswing in the pendulum.Useful in determining the ductile-brittletransition temperature of materials.

High Impact Resistance High Strength High Ductility High Toughness

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Viscous materialA viscous material is one in whichthe strain develops over a period of

time and the material does not returnto its original shape after the stress isremoved

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