3. 2 | P a g e
An Analysis of Material Behavior When
Processed Through Stages of Cold Working
(Part A).
Introduction:
The processthat we are investigatinghere isof the name ‘ColdWorking’. (Asekland,FulayandWright,
2011)The name isderivedfromthe lowtemperature atwhichthe metal isplasticallydeformed,by
exceedingthe yieldstrengthof the material.Withoutthe plasticdeformation,the material wouldrevert
back to itsoriginal shape andnochange will have takenplace. The deformationoccursbelow its
Recrystallization temperature,whichmeansthat new-grainscan’temergeinthe material.
Howeverthe amountof dislocationsthatoccurs,inthe metal’scrystallinestructure,increasesdue to
thisideaof ‘slip’.Slipiswhere planesof atomsina crystal structure slide againsteachother. Which
occurs whena sufficientlyhighenoughshearstressisappliedtothe crystal structure,of whichhasa
dislocation.The resultbeingthatthe tophalf of the crystal structure has movedbya distance of 1 lattice
parameter, relative,tothe lowerhalf.(Asekland,FulayandWright,2011)The bondsacross the slipplane
are broken,andthe atomsbelowthe slipplane establishanew bondwiththe atomsof the dislocation.
Thistype of mechanical deformationisusedin twomaincoldworkingprocessescalled:
‘ColdRolling’.The productbeingachievedissheetsof metal,throughrollingametal flatuntil its
diameterhasdecreased.The metal thatisproducedbythe rolling,ismore compactdue to the
distortionof the grains,inthe directionof the rollingprocess. The yieldstrengthof the metal
needstobe lowerthanthat of the compressive force actinguponthe metal.
‘Wire drawing’. A metal rodis pulledthroughadie,whichissmaller atthe endthanthe
beginning, toreduce the diameterof the metal rod,asit passesthrough.
The metal in thisprocessisdrawn,and therefore atensileforce isappliedhere.The force must
be higherthanthe yieldstrengthof the metal,butonlyenoughtoplasticallydeformthe metal
withoutleadingtofracture,due to the increasedbrittlenessof the metal atthisstage.
A careful selectionof force strengthmustbe made forbothprocessestoachieve the requiredproduct
withoutcausingitto fracture.
4. 3 | P a g e
Results.
Specimen. A B C D E F
% Cold work
received.
0 10 25 35 45 50
Test specimen
cross sectional
areas (m2
) –
before cold
working.
1.5x10-5
15 x10-5
15 x10-5
15 x10-5
15 x10-5
15 x10-5
Test specimen
cross sectional
areas (m2
) –
after cold
working.
15.0 x10-5
1.35 x10-5
1.12 x10-5
9.75 x10-6
8.19 x10-6
7.41 x10-6
Tensile
strengths (MPa)
702 919 1182 1201 1430 1470
Yield strengths
(MPa)
320 524 820 1000 1160 1230
Ductility
(% Elongation)
65 46 29 20 12 10
(Worked examplesand graph summarisingmaterialproperties,aredisplayed on thefollowing pages).
5. 4 | P a g e
Discussion.
The sampleswere all fromone material thatwere subjectedtoincreasingyieldstressuntil the material
reducedinitsthicknesstothe requiredamounts.Therefore the thicknesswasthe aimachievedbythe
independentvariable of yieldload.
Trendsnoticedare therefore talkedaboutasthe thicknessdecreased:
Percentage ColdWorkingincreased,becausemore workmustbe done toreduce the thickness
of the material underload.As%coldwork isa measure of area whichdecreasedfrom1.5x10-5
to the thinnestsample (sample F) 7.41 x10-6
the percentagesof coldworkstartingat 0% and
increasingto50% were expecteddue tothe increaseddifferenceinareasformsample A – F.
Tensile strengthwasseentoincrease throughoutspecimensA – F whichwasexpectedasthe
processisone of a hardeningprocessdue tothe dislocationhinderingeffectthatiscausedby
increasedamountsof newdislocationshinderingotherdislocationsforming.
I expectedthe yieldstrengthtotrendthe same wayas the tensile strengthwhichwasindeed
whatwas showninthe results.FromspecimensA –F increasingbyabout900 MPa. Thisis
usuallythe case as the yieldstrengthmustbe below the tensilestrengthatall times,otherwise
the material wouldfail instantly.
The percentage elongationshowsaconstantdecrease whichsurprisedme asI expectedthe
specimentoelongate asitsthicknessdecreased.Butinfact due to the constantincrease in
dislocationcreatedincoldworkingtheyeventuallyleadtolimitedductility.
6. 5 | P a g e
Extended analysis:
Task 1.
If one specimenhadlongerannealingtime tothe otherthantheywouldbe expectedtoshow different
tensile strengthswhentested.
Annealingisthe processof reheatingacoldworkedmetal torestore the original mechanical properties,
microstructure andotherpropertiesforexampletensileandyieldstrengthsplusductility.
Whena metal is annealeditenters3mainstagesof the entire process:
1. Recovery= Reducingthe Internal residual stresswhichhasbeencreatedatthe coldworking
process.Residual stressformswhennotall of the energyputintodeformingthe metal hasbeen
usedinthat formbut insteadremainsinthe metal aftercoldworking.
2. Recrystallisation=Whenthe metal entersahightemperature of annealingnew setsof strain
free softequiaxedgrainsreplace the coldworkedgrainstructure.Thiscanonlyoccur due to the
hightemperaturescausinggreaterlevelsof energymeaningthatthe atomsinthe crystal lattice
are able tomove producingthe new grainstructure.By the endof thisstage the original
microstructure hasbeenrestored.
3. Grain growth= if the metal iskeptat elevatedtemperaturesstrainfree grainscontinuetogrow.
Thisleadsto a deteriorationinthe mechanical factorssuchas yieldstrength,because aslarger
more coarse grainsdevelopatthe expenseof smallergrains,lessgrainboundariesare present
creatinga higherriskof slippage inplanes, due toanincreasedamountof differentangles
createdby multiplegrains.Whichiswhatdefinesthe strengthsof ametal.
Therefore the processof annealingisgenerallyfavoredtobe stoppedatthe endof the recrystallisation
process(recognizedatone hour) plusatemperature will have beencalculatedtoensure thatit
maintainsthe metal below the 3rd
developmentstage butallowingthe first2stagesto occur. This
temperature iscalledthe recrystallisationtemperature andiscalculatedusingthiscalculation:
0.4 × 𝑇𝑚 ( 𝑎𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑚𝑒𝑙𝑡𝑖𝑛𝑔 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒) = 𝑟𝑒𝑐𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑠𝑎𝑡𝑖𝑜𝑛 𝑡𝑒𝑚𝑝𝑒𝑟𝑎𝑡𝑢𝑟𝑒.
For the requirementsof task1 the recrystallisationtemperature is:
0.4 × 14200 𝐶 = 5680 𝐶
If one specimenwasallowedtocontinue annealing,thatspecimenwouldenterthe stage 3and the
mechanical propertiesthatwere restoredwill begintodeterioratedue tothe overtakingof coarse
grainsfrom theirsmallercounterparts.Causingincreasedplane slippage andtherefore reduced
strengthssuchas the yieldstrengthsshowninthe graphbelow.
(Graph followsthispage,showing expected stress-strain curves).
7. 6 | P a g e
Task 2.
From the graph that I createdforthe material propertiesthatoccurreddue tocoldworkingprocessin
the results section,I am able to extrapolate the dataneededforthistask.
I startedby drawinga horizontal line from15% elongationacrosstowhere itmeetsthe elongationdata
line,thenIwas able to draw a secondline vertical fromthispointtowhere itmeetsthe yieldstrength.
Thena secondhorizontal linewasdrawnfrom950 MPa to where itmeetsthe yieldstrengthdataline,
thena vertical line wasdrawndowntothe percentage coldworkedaxis.
These twovertical linesrepresentthe maximumandminimumworkingareaforthe coldworking
processof the die.Therefore Ifoundthe average point,at36.5 % coldworked,anddraw a vertical line
crossingthe percentage elongationat19%, yieldstrengthat1020 MPa, and continuingthe line tothe
tensile strengthgagedat1240 MPa.
These figuresare correctto the requiredspecificationsof the customer.
36.5% CW
19% EL (above 15%)
1020MPa yieldstrength(above 950MPa)
1240MPa tensile strength
ThenI beganto workon the dimensionsneededforthe wire drawingdie.
I knowthat the material needstobe 0.46mm in diameter,therefore Icanreverse the percentage cold
workedequationtocalculate whatthe openingdimensionsof the die needsto be.
1. 0.46xπ = 1.45mm
2
2. (1.44x36.5)/100 = 0.53 mm
2
3. 0.53+1.44 = 1.97mm
2
4. 1.97/π = 0.63mm
2
Finallyaforce needstobe calculatedsothat it isgreat enoughtoplasticallydeformthe metal sothat
the diametercanreduce,butnot so muchthat breakswhilstbeingdrawnunderforce.
Therefore againIlookedatthe graph, previouslydrawn,between the lowestyieldstrength(320MPa)
and lowesttensile strength(702).The average betweenthe twovaluescame to521MPa and to then
calculate a force required,the calculationforstress=force/area,canbe usedtofindthe force.
Force (N) = stress(MPa) x area(mm2
)
1008 = 521 x 1.97
Therefore the requiredforce neededtoreduce the die diameterto0.46mm is1008N.
8. 7 | P a g e
Conclusion.
Usingthe experimental datatoevaluate the changesinmechanical propertieswassuccessful.Withthe
aidof a graph to explainthe yieldandtensile strengthchangesand percentage elongation atvaluesof
percentage coldwork.All the dependentvariables helpedme toexplain whathadoccurredand assisted
me to visualize the data,to aidmy understandingof whatchangesoccurred.Withaidfromtextbooks
and othersources,Iwas able to determinewhetherthe changesthatoccurred were meanttooccur or
was unexpectedforanexperimentof thiskind.
Whenit came to a more indepthlookat the changesthat had occurred,I foundoutabout the
microstructural changesthatoccurred due to coldworkingandthe followingprocessessuchas
annealing. Therefore Iwasable tomake a more detailedconclusionstothe changesthatoccurred by
applyingthe learntknowledgeof the microscopy.
But whatI foundto be most funand helpful wastobe able toapplymy data to practical situations,this
helpstomake a betterpicture of whatisoccurring,and to reallyunderstandthe sciencethatistaking
place inthe experiments.
9. 8 | P a g e
An Analysis of the Creep and Creep
Recovery Behavior of Two
Thermoplastics (Part B).
Introduction:
The investigationhere isbaseduponthe subjectof Creepandthereforetobe able todelve furtherinto
the investigationandstartto applytheoriestonumbersandgainvalidresults,one mustunderstand
whatcreepis. Creepisa plasticdeformationthat occursinthermoplasticmaterialsdue toaconstant
stressor constantloadapplied,withhightemperatures,overasustainedperiodof time.Eventhough
the stressmay be lowerthanthe yieldstrengthof the material,the material canfracture.Lastlycreep
recovery isanotherpart we will be lookingatinthisinvestigation.Simplyitstatesinthe name whatit
means.Butin slightmore detail,itmeansthat,the deformationoccurreddue tocreephas been
removeddue tothe removal of the stress or loadthat was applied.
The way that we draw upconclusionaboutthe creepand creeprecoveryof these twothermoplastics,is
by usingthe data collected fromthe deflectionof the materialswhenlefthangingoutof the vice.Where
gravitational force and the weightedloadputonthe material understress.Thenthe materialsare leftto
recoverbyremovingthe weightedload.A useful wayof demonstratingcreepandcreeprecovery isto
draw springanddashpotcomponents.These componentswere useddue tothe similaritybetweenthe
polymerbehaviorandthe springandthe liquid-likecomponentbeingrepresentedbythe dashpot.
These drawingsare foundintask1.
11. 10 | P a g e
Discussion:
As the graphshave beendrawnbaseduponthe data found,forcreepand creeprecovery.Iam able to
see clearerthe differencesinthe twomaterials.Itiseasierthentosee thatGP polystyrene hasthe
greaterstrengthof the two materials,althoughitdoesdeformto12.3mm, the deflectionthatoccursis
instantanddoesn’tcontinue toincrease,suchlike polypropylene.Polypropylenesdelfectionvalue rises
up to 24mm after34.8 seconds,pluspolypropylenesmeasure of delfectionstartsata highervalue than
that of GP polystyrene. The same trend,forpolypropylene,istherefore showninthe true straingraph
showingaconstant increase after loadedtothe pointof loadrelease.Whatoccursinthe springand
dashpotcomponentsshowsthatthe springincreasesinlengthaswell asthe dashpot.
Whenthe loadis releasedforbothspecimens,the deflectionvaluesdecreasesrapidly,showingelastic
deformationoccurred,astheyare able to returnto theiroriginal state.Polysytrene showsthe best
elasticityof bothspecimens.Thisisdemostratedwell inthe dashpotandspringcomponents,bymeans
of the springretractingandbringingthe dashpotbackup towardsit original value.Assoonasthe loadis
releasedthe delfectionoccurredreducesdramaticallyby11.8mm inthe first0.2 secondsand bythe
time a secondhaspast the material hasretractedto its original length.Thisoccursdue to the structure
of the material.The material althoughtheyare notfullycrystalline,theydisplaysome of the associated
features,hence whythese materialsare namedsemi-crystalline.Forexample,asstatedinAskelands
engineeringbook,betweenthe regionsof crystalline lamellae andspherultiesthereare amorphous
regions.These differentregionsare ‘tied’togetherbypolymerchains.Itiswhenthe loadisappliedthat
the crystalline regionsbegintoslide pastone another.Eventually,if enoughtime isallowedandthe load
isn’ttoogreat to breakthe material,the chainsof crystalline regionsbecome alignedwiththe direction
of load.If the loadis releasedsufficientlybeforethese regionsbegintoentangle,andcause failure,they
can slide pasteachother,returningtotheiroriginal form.Whilstthe amorphousregionsthatwere
stretched,allowingthe crystalline regionstoalign,canalsoreformtotheirunstrecthedstate.
Thisexplainswhathappensbutwhyithappenedlike thisisdue tothe difference betweenthe glass
transitiontemperaturesbetweenthe materialstested.
PolypropylenehasaTg of -25⁰C to -20⁰C,whichmeansthat throughoutthe whole processof testingthe
material,itisabove the Tg by about40⁰C, due to testingatroom temperature(20⁰C).Whenapolymeris
above or at theirTg the uncoilingof the chainscan occur allowingthe crystalline regionstoalignand
eventuallytheyare easilyabletoreformthe randomlycoiled formtheynaturallyexsistin.Where asthe
GP polystyrene whichhasa Tg of 85⁰C to 125⁰C, whichmeansthat the temperature atwhichthe testing
isdone is well belowthisrange.Thismeansthatthere isntthe recommenedamountof energyforthe
uncoilingtobe done succesfully,andthe crystalline regionstoslippasteachotherand thenreformthe
original shape.The amorphousregionsare hardand‘glass’like,hence whythe crystallineregionsfindit
difficulttomove,astheironce rubberyamorphousregions,whichwere usedinthe uncoilingprocess,
are unable tomove well astheyare stiff. Thiswe can determine fromthe graphanddata as theyshow
for GP polystyrenethatthe amountof deflectionthatoccursslowlyincreases,whilstloadisonand
slowlydecreasesreturningthe crystalline regionstotheirrandomstate fromthe orderedstate when
the loadis removed.
12. 11 | P a g e
Flexural creepmodulusisameasure of stressdividedbystrain.Itshows thatovertime the ratio
betweenstress:strain, showsthe tendancyforthe material tobend.Whilstthe loadison,itcauses
deflectionwhichdecreasesquiterapidlyafter5seconds.Afterthatthe rate beginstosteady,whilststill
decreasingforthe next25 secondsof load time.There isa constantdecrease inthe tendancyforthe
material tokeepbending,due tothe ratiobetweenstressandstrainincreasing,asthe stressisconstant
but true strainincreasesthroughout.The true strainwill continuetoincrease becausethe material is
constanlybeingputunderpressure,as the load dragsitdownwards.
The use of the materialstestedwere forthe correctionof mis-placedteeth.Therefore the material that
the orthodonticappliance ismade outof mustnot deformwithtime.Whichiswhypolypropylene would
be a poormaterial touse,as not onlydoesitdeformwithloadbutdue to the Tg beingclose tothat of
the operatingtemperature the amorphousregionsare more able tohelpthe alignmentof the crystalline
regionswhichcausescreep.Because of thisthe material will deformovertime,plusaswe have seenin
fromthe experimental data,whenpolypropylene isdeformedittakesalongtime to reformback to the
original shape if ever(atthe endof the experimental timepolypropylene hadnotreformedcompletely,
showingpossible plasticdeformation).
Whichis why, polystyrene isamore suitable material because whenloadisplacedonthe material the
material doesindeedaccomodate the loadthroughdeflection,butwhenthe loadistakenoff itreverts
to itsoriginal shape.Thereforethismaterialismore acceptable asitisshowingmore elastic
deformation,whichisexactlywhatisneededforthe appliance itwill be usedfor.
13. 12 | P a g e
Extended Analysis:
Task 1.
The graph, which followsonthe nextpage, demonstratesthe fall andrise of the dashpotsdue tothe
springaction,causedbythe elasticityof the polypropylene.The spring,whenforcedtoelongate due to
the gravitational force appliedwiththe 0.3kgweighttothe polypropyleneforcingittodeflect
downwards,alsoforcesthe dashpottoelongate inlength(fromthe bottomof the measuredline inthe
dashpotto the bottomof the dashpot). Creepisallowedtooccur as the temperature of the experiment
is40⁰C above itsTg, therefore the amorphousregionsare rubberyandcan stretchthe crystalline
lamellae forcingthemtoalign,inthe directionof the force.Itisthe covalentbondsof the monomer
chainsthat are beingstretchedatthispoint.Asmultiplemonomermolecules make upa polymer(hence
whyit isknownas a macromolecule),thesechainsare whatintertwinethroughthe crystalline and
amorphousregions,attachingthemtogether.
Whenthe springis allowedtorecover,asthe loadisremoved,itrecoilsupwardsbringingthe dashpot
withit,demonstratingcreeprecoveryasthe material returnstoitsoriginal form. Thisrecoveryoccurs
because the crystalline regionsdon’tlike beinglinedup,theyprefertobe ina randomlycoiledstate.
Therefore thatiswhyit takes energy(the load) tostretchthem,butwhenthe loadisremovedthere is
nothingtoholdthemin place,andtheywill recoil once again.
I usedthe Voigtelementgraphtodemonstrate these changestothe materialscurvature,asthe springis
neededtobe connectedtothe dashpotto show creeprecovery,asa dashpoton itsowncan’t show
recovery.Polypropylenedoestoacertainextentshow creeprecovery,butnotfully,therefore
suggestinganelementof plasticdeformationoccurred.
(Graph and Voigtgraph withdashpotand spring components followson nextpage,explaining worded
findings).
14. 13 | P a g e
Task 2.
Suitabilityforthe orthodonticapplication.
GP polystyrene wouldbe amore effectiveandappropriate material of the twotested.Due tothe length
at whichGP polystyrene stayswhenplacedunderweightedloadconditions,causingdeflectiononthe
material.The deflectionthatoccursisminimal comparedtothatof polypropylene.Whenthe loadis
released,almostinstantlythe material GPpolystyrenereturnstoitsnatural randomlyorderedsemi-
crystalline state.Thisshowsthatelasticdeformationhasoccurred,andthat the yieldstrengthwasnot
exceededaspermanentdeformationhasnotoccurred.Whereaspolypropylene still hasn’treturnedto
itsnatural state bythe time given,afterthe loadwasreleased.Therefore thismaterial isn’tsuitable as
whenloadisapplied,andthiswill happeninthe real situationinthe oral environment,the material
takestoo longif everto returnto the normal structure.Plusthe material itself deformsmore thanGP
polystyrenedid.
The material shouldbe allowedtodeform whenputunderloadinthe mathsto accommodate pressure,
but shouldreverttoitsconstructedshape sothat the correctional processof the teethcontinues.
Issue regardingboisterouschildren.
The constant harshload placedonthe appliance mayresultinitsfailure.GPpolystyrene becauseof its
highglasstransitionrange,meansthatat lowertemperatures,suchas36⁰C - average oral temperature -
it isbrittle due tothe lack of fluidityof the amorphousregions.Thismeansthatthe crystalline regions
are lessable tomove aroundand align,allowingdeflectionanddeformationtooccurwhilstina solid
state.Because of thisthe material isbrittle andwhenexcessiveforce isappliedtothe appliance itcould
break.If thisexcessive force isconstantlyappliedtothe material itcouldresultinplasticdeformation
occurring,as the crystalline regionsare unable to untangleandalignwhenahighstrainrate isapplied,
the material will snap.
As an alternative material topolystyrene.
I wouldsuggestthat6, 6-nylonandPolyethyleneterephthalate weretwopossible materialsin
contentionforbeingbetterdue toTg’scloserto that of the temperature usedinthe experimentandthe
temperature of the oral environment.The Tgmay still be above thatof the operatingtemperature but
at leastit iscloserand therefore itwillbe lessbrittle thanpolystyrene,due tomore ‘rubbery’
amorphousregions.
Temperature beingacrucial factorin the testingdone.
I wouldincrease the temperature closertothat of the oral environment,whichisabout36⁰C althoughit
isvariable.Whenthisincrease of temperature wasusedinthe same methodof testingthathasbeen
done,Iwouldexpectthe resultstoshowa greateramountof deflectionthatresultsgainedinthe
previoustest.Thisisbecause whenthe temperature isincreasedthe amoutof energyinthe material
increasesmeaningthatthe amorphousregionsare able tobecome more rubberyandthereforethe
crystalline lamellae andspherultiesinthe crysalline regionsare able toslide more fluidlypassedeach
otherto become ordered.
15. 14 | P a g e
Bibliography:
DonaldR. Askeland,PradeepP.Fulay,WendelinJ.Wright(2011). The science andengineeringof
materials.Stamford:Cengage Learning.
JohnF. McCabe, AngusWalls(2008) AppliedDental Materials.Oxford:Blackwell Publishers.
