Mechanical properties

A design engineer must have a good concept about the mechanical properties of materials.

1. Notes
on
“Mechanical properties of “Mechanical properties of
materials”
7/27/2018 1Prem Kumar Soni 9755084093

2. Introduction
Amaterial'sproperty(ormaterialproperty)is
anintensivepropertyofsomematerial,i.e.
aphysicalpropertythatdoesnotdependonthe
amountofthematerial.amountofthematerial.
7/27/2018 2Prem Kumar Soni 9755084093

3. Oftenmaterialsaresubjecttoforces(loads)
whentheyareused.Mechanicalengineers
calculatethoseforcesandmaterialscientists
howmaterialsdeform(elongate,compress,howmaterialsdeform(elongate,compress,
twist)orbreakasafunctionofappliedload,
time,temperature,andotherconditions.
7/27/2018 3Prem Kumar Soni 9755084093

4. Various Mechanical Properties
Anelasticity
Ductility
Elasticdeformation
Engineeringstrain
Engineeringstress
Hardness
Modulusofelasticity
Tensilestrength
Toughness
Yielding
Yieldstrength
Fatigue
Creep
StiffnessModulusofelasticity
Plasticdeformation
Poisson’sratio
Proportionallimit
Shear
Stiffness
Strength
7/27/2018 4Prem Kumar Soni 9755084093
Here We will see the change in physical behavior of material
due to external changes on it.

5. Anelasticity
•Herethebehavioriselasticbutnotthestress-
straincurveisnotimmediatelyreversible.It
takesawhileforthestraintoreturntozero.
•Theeffectisnormallysmallformetalsbutcan•Theeffectisnormallysmallformetalsbutcan
besignificantforpolymers.
7/27/2018 5Prem Kumar Soni 9755084093

6. Brittleness
Brittlenessofamaterialindicatesthathoweasily
itgetsfracturedwhenitissubjectedtoaforceor
load.
Whenabrittlematerialissubjectedtoastressit
observesverylessenergyandgetsfracturesobservesverylessenergyandgetsfractures
withoutsignificantstrain.
Brittlenessisconversetoductilityofmaterial.
Brittlenessofmaterialistemperaturedependent.
Somemetalswhichareductileatnormal
temperaturebecomebrittleatlowtemperature.
7/27/2018 Prem Kumar Soni 9755084093 6

7. Ductility
Ductilityisapropertyofasolidmaterial
whichindicatesthathoweasilyamaterialgets
deformedundertensilestress.
Ductilityisoftencategorizedbytheabilityof
materialtogetstretchedintoawirebypullingmaterialtogetstretchedintoawirebypulling
ordrawing.
Thismechanicalpropertyisalsoanaspectof
plasticityofmaterialandistemperature
dependent.Withriseintemperature,the
ductilityofmaterialincreases.
7/27/2018 7Prem Kumar Soni 9755084093

8. TensiletestofanAlMgSialloy.Thelocalneckingandthecupandcone
fracturesurfacesaretypicalforductilemetals.

9. Malleability
Malleabilityisapropertyofsolidmaterialswhich
indicatesthathoweasilyamaterialgetsdeformed
undercompressivestress.
Malleabilityisoftencategorizedbytheabilityof
materialtobeformedintheformofathinsheet
byhammeringorrolling.byhammeringorrolling.
Thismechanicalpropertyisanaspectofplasticity
ofmaterial.
Malleabilityofmaterialistemperaturedependent.
Withriseintemperature,themalleabilityof
materialincreases.
7/27/2018 Prem Kumar Soni 9755084093 9

10. Elastic deformation
•Elasticdeformation.Whenthestressis
removed,thematerialreturnstothedimension
ithadbeforetheloadwasapplied.
7/27/2018 10PremKumarSoni9755084093

11. Engineering strain
•Thereisachangeindimensions,or
deformationelongation,đLasaresultofa
tensileorcompressivestress.Toenable
comparisonwithspecimensofdifferentlength,comparisonwithspecimensofdifferentlength,
theelongationisalsonormalized,thistimeto
thelengthL.Thisiscalledstrain,e.
•e=đL/L
7/27/2018 Prem Kumar Soni 9755084093 11

12. Engineering and Engineering Stress
Whenoneappliesaconstanttensileforcethe
materialwillbreakafterreachingthetensile
strength.
Thematerialstartsnecking(thetransversearea
decreases)butthestresscannotincreasedecreases)butthestresscannotincrease
beyonds
TS.
Theratiooftheforcetotheinitialarea,whatwe
normallydo,iscalledtheengineeringstress.If
theratioistotheactualarea(thatchangeswith
stress)oneobtainsthetruestress.
7/27/2018 Prem Kumar Soni 9755084093 12

13. Hardness
Itistheabilityofamaterialtoresisttopermanentshapechangedue
toexternalstress.
Therearevariousmeasureofhardness–ScratchHardness,
IndentationHardnessandReboundHardness.
ScratchHardnessScratchHardnessistheabilityofmaterialstothe
opposethescratchestooutersurfacelayerduetoexternalforce.opposethescratchestooutersurfacelayerduetoexternalforce.
IndentationHardnessItistheabilityofmaterialstoopposethedent
duetopunchofexternalhardandsharpobjects.
ReboundHardnessReboundhardnessisalsocalledasdynamic
hardness.Itisdeterminedbytheheightof“bounce”ofadiamond
tippedhammerdroppedfromafixedheightonthematerial.
7/27/2018 Prem Kumar Soni 9755084093 13

14. Hardnessistheresistancetoplasticdeformation
(e.g.,alocaldentorscratch).
Thus,itisameasureofplasticdeformation,asis
thetensilestrength,sotheyarewellcorrelated.
Historically,itwasmeasuredonanempirically
scale,determinedbytheabilityofamaterialto
scratchanother,diamondbeingthehardestand
talcthesofter.Nowweusestandardtests,whereatalcthesofter.Nowweusestandardtests,wherea
ball,orpointispressedintoamaterialandthe
sizeofthedentismeasured.
Thereareafewdifferenthardnesstests:
Rockwell,Brinell,Vickers,etc.Theyarepopular
becausetheyareeasyandnon-destructive(except
forthesmalldent).
7/27/2018 Prem Kumar Soni 9755084093 14

15. Hardenability
Itistheabilityofamaterialtoattainthe
hardnessbyheattreatmentprocessing.Itis
determinedbythedepthuptowhichthematerial
becomeshard.
TheSIunitofHardenabilityismeter(similartoTheSIunitofHardenabilityismeter(similarto
length).
Hardenabilityofmaterialisinversely
proportionaltotheweld-abilityofmaterial.
7/27/2018 Prem Kumar Soni 9755084093 15

16. Poisson’s ratio
Materialssubjecttotensionshrinklaterally.
Thosesubjecttocompression,bulge.Theratio
oflateralandaxialstrainsiscalled
thePoisson'sration.thePoisson'sration.
n=e
lateral/e
axial
The elastic modulus, shear modulus and
Poisson's ratio are related byE= 2G(1+n)
7/27/2018 Prem Kumar Soni 9755084093 16

17. Modulus of elasticity
•Intensiletests,ifthedeformationiselastic,thestress-
strainrelationshipiscalledHooke'slaw:
•s=Ee
•Thatis,Eistheslopeofthestress-straincurve.E
isYoung'smodulusormodulusofelasticity.isYoung'smodulusormodulusofelasticity.
7/27/2018 17

18. Elasticity
Abilityofabodytoresistadistortinginfluence
orstressandtoreturntoitsoriginalsizeand
shapewhenthestressisremoved.
Itistheabilityofabodytoresistadistorting
influenceandtoreturntoitsoriginalsizeand
shapewhenthatinfluenceorforceisremoved.shapewhenthatinfluenceorforceisremoved.
Solidobjectswilldeform when
adequateforcesareappliedonthem.Ifthe
materialiselastic,theobjectwillreturntoits
initialshapeandsizewhentheseforcesare
removed.
7/27/2018 Prem Kumar Soni 9755084093 18

19. Tensile strength
Yieldpoint.Ifthestressistoolarge,thestraindeviatesfrom
beingproportionaltothestress.Thepointatwhichthis
happensistheyieldpointbecausetherethematerialyields,
deformingpermanently(plastically).
Yieldstress.Hooke'slawisnotvalidbeyondtheyieldpoint.
Thestressattheyieldpointiscalledyieldstress,andisan
importantmeasureofthemechanicalpropertiesofmaterials.
Inpractice,theyieldstressischosenasthatcausingaInpractice,theyieldstressischosenasthatcausinga
permanentstrainof0.002(strainoffset,Fig.6.9.)
Theyieldstressmeasurestheresistancetoplasticdeformation.
Thereasonforplasticdeformation,innormalmaterials,isnot
thattheatomicbondisstretchedbeyondrepair,butthemotion
ofdislocations,whichinvolvesbreakingandreformingbonds.
Plasticdeformationiscausedbythemotionofdislocations.
7/27/2018 Prem Kumar Soni 9755084093 19

20. Tensilestrength.Whenstresscontinuesinthe
plasticregime,thestress-strainpassesthrougha
maximum,calledthetensilestrength(s
TS),and
thenfallsasthematerialstartstodevelop
aneckanditfinallybreaksatthefracturepoint.
Notethatitiscalledstrength,notstress,butthe
unitsarethesame,MPa.
Notethatitiscalledstrength,notstress,butthe
unitsarethesame,MPa.
Forstructuralapplications,theyieldstressis
usuallyamoreimportantpropertythanthetensile
strength,sinceoncetheitispassed,thestructure
hasdeformedbeyondacceptablelimits.
7/27/2018 Prem Kumar Soni 9755084093 20

21. Toughness
Itistheabilityofamaterialtoabsorbtheenergyand
getsplasticallydeformedwithoutfracturing.Its
numericalvalueisdeterminedbytheamountofenergy
perunitvolume.ItsunitisJoule/m
3
.Valueof
toughnessofamaterialcanbedeterminedbystress-
straincharacteristicsofamaterial.Forgoodtoughness,
materialsshouldhavegoodstrengthaswellasductility.materialsshouldhavegoodstrengthaswellasductility.
Forexample:brittlematerials,havinggoodstrengthbut
limitedductilityarenottoughenough.Conversely,
materialshavinggoodductilitybutlowstrengthare
alsonottoughenough.Therefore,tobetough,a
materialshouldbecapabletowithstandbothhighstress
andstrain.
7/27/2018 PremKumarSoni9755084093 21

22. Yielding
•Forstructuralapplications,theyieldstressis
usuallyamoreimportantpropertythanthe
tensilestrength,sinceoncetheitispassed,the
structurehasdeformedbeyondacceptablestructurehasdeformedbeyondacceptable
limits.
7/27/2018 Prem Kumar Soni 9755084093 22

23. Fatigue
Fatigueistheweakeningofmaterialcausedbythe
repeatedloadingofthematerial.
Whenamaterialissubjectedtocyclicloading,and
loadinggreaterthancertainthresholdvaluebutmuch
belowthestrengthofmaterial(ultimatetensilestrength
limitoryieldstresslimit),microscopiccracksbeginto
formatgrainboundariesandinterfaces.Eventuallytheformatgrainboundariesandinterfaces.Eventuallythe
crackreachestoacriticalsize.
Thiscrackpropagatessuddenlyandthestructuregets
fractured.Theshapeofstructureaffectsthefatiguevery
much.Squareholesandsharpcornersleadtoelevated
stresseswherethefatiguecrackinitiates.
7/27/2018 Prem Kumar Soni 9755084093 23

24. Fatiguelimit,endurancelimit,andfatigue
strengthareallexpressionsusedtodescribeaproperty
ofmaterials:theamplitude(orrange)ofcyclicstress
thatcanbeappliedtothematerialwithout
causingfatiguefailure.
Ferrousalloysandtitaniumalloyshaveadistinctlimit,
anamplitudebelowwhichthereappearstobeno
numberofcyclesthatwillcausefailure.Othernumberofcyclesthatwillcausefailure.Other
structuralmetalssuchasaluminiumandcopperdonot
haveadistinctlimitandwilleventuallyfailevenfrom
smallstressamplitudes.Inthesecases,anumberof
cycles(usually10
7
)ischosentorepresentthefatigue
lifeofthematerial.
FatiguelimitisusedinplottingS-Ncurves(Strength
VsNumberofCycles)
7/27/2018 Prem Kumar Soni 9755084093 24

25. 7/27/2018 Prem Kumar Soni 9755084093 25

26. Creep
Creepisthepropertyofamaterialwhich
indicatesthetendencyofmaterialtomove
slowlyanddeformpermanentlyunderthe
influenceofexternalmechanicalstress.
Itresultsduetolongtimeexposuretolarge
externalmechanicalstresswithinlimitofexternalmechanicalstresswithinlimitof
yielding.
Creepismoresevereinmaterialthatare
subjectedtoheatforlongtime.Slipinmaterial
isaplanewithhighdensityofatoms.
7/27/2018 Prem Kumar Soni 9755084093 26

27. 7/27/2018 Prem Kumar Soni 9755084093 27

28. Itisthetendencyofasolidmaterialtomoveslowlyordeform
permanentlyundertheinfluenceofmechanicalstresses.Itcanoccur
asaresultoflong-termexposuretohighlevelsofstressthatarestill
belowtheyieldstrengthofthematerial.Creepismoreseverein
materialsthataresubjectedtoheatforlongperiods,andgenerally
increasesastheyneartheirmeltingpoint.
Therateofdeformationisafunctionofthematerial'sproperties,
exposuretime,exposuretemperatureandtheappliedstructuralload.
Dependingonthemagnitudeoftheappliedstressanditsduration,
thedeformationmaybecomesolargethatacomponentcannothedeformationmaybecomesolargethatacomponentcanno
longerperformitsfunction—forexamplecreepofaturbineblade
willcausethebladetocontactthecasing,resultinginthefailureof
theblade.Creepisusuallyofconcerntoengineersandmetallurgists
whenevaluatingcomponentsthatoperateunderhighstressesorhigh
temperatures.Creepisadeformationmechanismthatmayormay
notconstituteafailuremode.
7/27/2018 Prem Kumar Soni 9755084093 28

29. Stiffness
Abilityofanobjecttoresistdeformationinresponsetoanappliedforce;
rigidity;complementarytoflexibility
7/27/2018 Prem Kumar Soni 9755084093 29

30. Strength
•Itisthepropertyofamaterialwhichopposes
thedeformationorbreakdownofmaterialin
presenceofexternalforcesorload.
•Materialswhichwefinalizeforour•Materialswhichwefinalizeforour
engineeringproducts,musthavesuitable
mechanicalstrengthtobecapabletowork
underdifferentmechanicalforcesorloads.
7/27/2018 Prem Kumar Soni 9755084093 30

31. Resilience
Capacitytoabsorbenergyelastically.Theenergy
perunitvolumeistheareaunderthestrain-stress
curveintheelasticregion.
Resilienceistheabilityofmaterialtoabsorbtheenergy
whenitisdeformedelasticallybyapplyingstressand
releasetheenergywhenstressisremoved.
ProofresilienceisdefinedasthemaximumenergythatProofresilienceisdefinedasthemaximumenergythat
canbeabsorbedwithoutpermanentdeformation.
Themodulusofresilienceisdefinedasthemaximum
energythatcanbeabsorbedperunitvolume
withoutpermanentdeformation.Itcanbedetermined
byintegratingthestress-straincurefromzerotoelastic
limit.Itsunitisjoule/m
3
.
7/27/2018 Prem Kumar Soni 9755084093 31

32. •Capacity to absorb energyelastically.The energy per unit
volume is the area under the strain-stress curve in the
elastic region.
•Theareaunderthelinearportionofastress–straincurveis
theresilienceofthematerial
•Inmaterialscience,resilienceistheabilityofamaterialto
absorbenergywhenitisdeformedelastically,andrelease
thatenergyuponunloading.Proofresilienceisdefinedas
themaximumenergythatcanbeabsorbeduptotheelastic
limit,withoutcreatingapermanentdistortion.Themoduluslimit,withoutcreatingapermanentdistortion.Themodulus
ofresilienceisdefinedasthemaximumenergythatcanbe
absorbedperunitvolumewithoutcreatingapermanent
distortion.
•Itcanbecalculatedbyintegratingthestress–strain
curvefromzerototheelasticlimit.Inuniaxialtension,
undertheassumptionsoflinearelasticity,
7/27/2018 Prem Kumar Soni 9755084093 32

33. 33

34. 7/27/2018 Prem Kumar Soni 9755084093 34

35. Thanks
7/27/2018 Prem Kumar Soni 9755084093 35

36. Any Query
??
Contact me..
7/27/2018 Prem Kumar Soni 9755084093 36