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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
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ElasticRegion
PlasticRegion
StrainHardening Fracture
ultimatetensilestrength
Elastic regionslope =Youngs (elastic)
modulusyield strength
Plastic regionultimate tensile strengthstrain hardeningfracture
necking
yieldstrength
UTS
y
E
E
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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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