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Page|1 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
S A M P L E A N S W E R S
MaterialsTesting:Plastics
IntroductionPlasticsareeverywhere.Theyhaveanextraordinaryrangeofuses,fromsoftdrinkbottlesandpackagingtocarpanelsandbuildingmaterials.Theplasticthatisusedforanobjecthasbeenselectedbecauseofitspropertiesincludingitsstrength,itsflexibility,itsdurabilityanditscost.
Supermarketbagsareextremelyconvenientbutalsoenvironmentallydamaging.Researchersandindustrycontinuetosearchforcost-effectiveenvironmentallyfriendlybiodegradableplastics.Toreplacetraditionalsupermarketbags,thenewbioplasticsmustbeasstrongandresilientastheplasticsusedcurrently.Justhowstrongwillthesenewplasticsneedtobetomatchtheplasticsusedinthecurrentsupermarketbags?Arethebiodegradableandrecyclablebagsbeingusedasgoodasthetraditionalbags?
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Page|2 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
KeyideasForce-apushorapull.
Strength-canbethoughtaboutinmanyways.Inthisactivity,strengthwillbemeasuredeitherastheloadrequiredtostretchandbreakapieceofmaterial,orthepunctureforcetopushanobjectintoorthroughthatmaterial.
Load–Adownwardsforcethatisbeingappliedtoanobject.
Punctureforceistheforcerequiredtopushanobjectintoorthroughamaterial.
Fairtest-Whentestingdifferentmaterialsallthevariablesexcepttheonebeingtestedneedtobekeptthesame.
Variable-Somethingthatcanchange.
Independentvariable-Variablethatisdeliberatelychanged.
Controlledvariables-Variablesthatarekeptconstant.
Dependentvariable-Variablethatchangesinresponsetochangesintheindependentvariableandthatisobservedormeasured.
Equipmentandmaterials• Differentplasticbags(3)• Retortstandandclamparm• Paperclipsorwiretomakehookfromwhichtosuspendweights• Scissors• Stickytape• Icypolesticks• Bambooskewers• Weights• Ruler(and/ormeasuringtape)• Rubberbands• Onebeakerorcup(approx.8-10cmdiameter)• Safetyglasses/goggles
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Page|3 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part1:StrengthInthisactivity,youwillworkoutjusthowstrongtheplasticisindifferentsupermarketbags.Theprinciplesinthetestingprocedureyouwillusearethesameasthoseusedbymaterialsscientistsintheirlabs.
Part1:HazardsThemainhazardwillarisewhensamplesofplasticgivewayunderload.Thingsmayflyinunpredictabledirections.Youmustwearsafetyglasses/goggles,andalsokeepfaceswellawayfromtheplasticswhentheyareheavilyloaded.Keepclearifyouthinktheplasticisabouttosnap.Setupatraywithsomethingsoftinitsotheweightsdonotcrashtothefloorandkeepfeetclear.
Part1:InvestigationInstructionsFromthesamplesofplasticbagsfortesting,cutstrips30cmlongand2cmwide.Windtheplasticstriparoundtheoneicypolestickasshownleaving20cmbetweenthesticks.Usestickytapetoholdtheplasticinplace.Tapeasecondicypolesticktothefirstoneforextrasupport.
Suspendthetopicypolesticksfromaclamponaretortstandasshownbelow.Addweightstoincreasetheloadon50–100gatatimeinitiallyand50gatatimewhenyouthinktheplasticisabouttosnap.Placesomethingsoftunderneaththeweightssothatwhentheplasticbreakstheweightsdonotcrashtotheground.Thesupportsonbrassweightscanbeeasilybroken.Eachtimeyouaddtheloadmeasurethedistancebetweenthetopandbottomicypolesticksi.e.thelengthoftheplastic
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Page|4 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
andalsomeasurethewidthoftheplasticatthemidpointofthelengthofplastic.Continuetoaddweightsuntiltheplasticteststripbreaks.
Aswellasthemeasurements,noteotherobservationsyoumakewhiletheweightsarebeingadded.
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Page|5 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part1:ResultsRecordyourresultsinthespacesbelow.
Sourceofplastic
Plastic1 Averystretchyplasticbag
Plastic2
Plastic3
PlasticBag1 PlasticBag2 PlasticBag3
Load Length Width Length Width Length Width
0g 27.5cm
Mostplasticswouldnotstretchsomuch
100g 28.5cm
200g 29.0cm
300g 29.0cm
400g 29.3cm
500g 125cm
600g 136cm
700g 152cm
800g 162cm
900g 179cm
1000g 192cm
1100g failure
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Page|6 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part1:AnalysisCalculatethepercentagechangeinlengthversusload.
Plastic1:percentagechangeinlengthversusloadLoad Length Changeinlength Percentage
changeinlength
0g 27.5cm
omitted
300g 29.0cm 0.0cm 5
400g 29.3cm 0.3cm 7
500g 125cm 95.7cm 355
600g 136cm 11cm 395
700g 152cm 16cm 453
800g 162cm 10cm 489
900g 179cm 17cm 551
1000g 192cm 13cm 598
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Page|7 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Plastic2:percentagechangeinlengthversusloadLoad Length Changeinlength Percentage
changeinlength
Plastic3:percentagechangeinlengthversusloadLoad Length Changeinlength Percentage
changeinlength
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Page|8 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Drawgraphsofthechangeinlengthversusloadandpercentagechangeinlengthversusload.Putyourdifferentgraphsonthesamesetofaxes.
0
100
200
300
400
500
600
700
0 200 400 600 800 1000 1200
%ch
angeinlength(cm)
Load(g)
Part1:DiscussionandConclusionsHowdidtheplasticscompare?Whichoneisstrongest?Whatistheevidence?
Thestrongertheplasticthesmallerthepercentagechangeinlengthwithincreasingloadandthegreatertheload/weightneededtobreaktheplasticteststrip.
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Page|9 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Howdideachoftheplasticsbehaveastheloadwasincreased?Weretheredifferences?
Onlydatafromaverystretchyplastichasbeenshowninthesesampleanswers.
Mostplasticswouldhavesimilarfeaturesonthegraph:flatregion,sharprise,followedbysteadyincreaseuntilitbreaks.Formostplasticssomeoftheseregions(usuallythesharprise)isverysmallandmight
notbeeasilyobservable.
Representwhatyouthinkishappeningattheparticle(molecular)levelastheplasticisstretchedandeventuallybreaks.Representtheelasticphase,theplasticphaseandthepointoffailure.
Therearesomegoodpicturesatthewebsite,TheHandBookonMechanicalMaintenance,compiledby:KPShah,<http://practicalmaintenance.net/?p=1135>
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Page|10 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Aretherevariablesyouhavenotcontrolledinyourtests?Howmightthesevariablesaffectyourconclusions?
Possiblevariablesnotcontrolledarethicknessofplasticsamplesandtimeweightsareleftsuspendedfromplasticbeforemeasurementistaken.Inawell-designedexperiment,allvariablesexcepttheindependentvariable(thedifferentplasticbags)shouldbekeptthesametoensureanychangesobservedinthedependentvariableareduetotheplasticitselfandnotthethicknessofthesampleortimeweightislefthangingfromtheplastic.
Identifytwoimprovementstothemethodtoensureitisawell-controlledexperiment.
• Plasticsamplestobesamethicknesstostartoffwith.
• Allthreeplasticsamplestobetestedatthesametimeafterweighthasbeenadded.
Supposeyouhadtoincreasethestrengthofthehandlesoftheplasticbag—whatcouldyoudo?Describeatestyoucoulddotogainevidenceforyourproposal.
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Page|11 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Wecouldaddmorethanonelayerofplasticwhenmakingthehandle.
Repeattheexperimentvaryingthenumberofpiecesofplasticsuspendedbetweentheicypolesticks.ThiswillbetheIndependentVariable.Couldstartoffusingagreaterloadanddeterminethenumberof50-100gweights/loadneededtobreaktheteststrip.
Part2:CuttingandtearingNotonlyareitemspurchasedfromthesupermarketheavyrequiringsupermarketbagstobestrongbuttheyalsooftencomeinpackageswithsharpedgesandcorners.Bagsmaybepuncturedorcutandeventuallytear.
Inthisactivity,youusethematerialsandyourownimprovedversionofthesuggestedtestingtechniqueprovidedtocomparetheresistanceofthebagstopuncturing.
Part2:HazardsTherearehazardsinthisactivityfromsharpobjects.Careshouldbeusedwithscissorstokeepfingersclearwhilecutting.Bothscissorsandthebambooskewersaresharpandcarethattheyarenotpokedintoskinoreyes.Weareyeprotection.
Part2:InvestigationInstructionsFromthesamplesofplasticbagsfortesting,cut15cmsquares–bigenoughtobestretchedoverthebeaker/cupandheldinplacewitharubberband.
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Page|12 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Stretchasheetofplasticbagoverthetopofaplasticcuporbeakerandsecureitwitharubberbandasshown.Dropabambooskewer,pointdown,ontothestretchedplastic,fromdifferentheights.
Part2:PreliminaryObservationsandResultsWhatdoyouobservewhenyoudropabambooskewer,pointdown,fromaheightofabout1cmabovetheplasticbagontotheplastic?
Bambooskewerreboundslikeaballbouncingonthegroundandtheplasticstretchesslightlyastheskewerhitsit.
Repeatthisfrom20cmabovethestretchedplasticbag.Whatdoyouobservethistime?
Skewerpierces/puncturestheplasticanddoesnotreboundoffit,butstaysembeddedwithintheplastic.
Usingthistechniquecomparethreedifferentplasticbagsfortheirresistancetopuncturing.Whatneedstobedonetoensurethatthetestsarefair?
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Page|13 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Therearemanypossibilities.Justsomeinclude:• Sametype,sizeandopeningdiameterplasticcup/beakerusedfor
eachplasticsample.• Eachplasticsampleshouldbestretchedandsecuredthesameway
overtheplasticcup/beaker.• Skewerreleasedthesamewayfromfingerswithnoforceexerted
onskewerbyfingers.• Measurementofheightofskewerfromcup/beakertakenthesame
way.
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Page|14 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part2:PlanningMoreDetailedInvestigation(s)Yourtaskistousethepreliminaryinvestigationabovetoworkoutwhichoftheplasticshasthegreatestresistancetopuncturewiththeskewer.Workwithyourpartneroryourgrouptodecidewhichvariablesyouwillkeepthesame(controlledvariables)whichvariable/syouwillchange(independentvariable)andwhichvariableyouwillmeasure(dependentvariable).
Controlledvariables
Theseareessentiallymanyofthevariablesinthepreviousbox:• Sametype,sizeandopeningdiameterplasticcup/beakerusedfor
eachplasticsample.• Eachplasticsampleshouldbestretchedandsecuredthesameway
overtheplasticcup/beaker.• Skewerreleasedthesamewayfromfingerswithnoforceexerted
onskewerbyfingers.• Measurementofheightofskewerfromcup/beakertakenthesame
way.
Independentvariable
Foreachplastic:• heightfromwhichskewerisdropped.
Oncethetestsarecompleted,wecancomparetheresults,butthechoiceofplasticisnotconsideredtobeanindependentvariable.
Dependentvariable(s)
Foreachplastic:
Theobservationofwhetherskewerpuncturestheplasticandpotentiallythedepthofpenetration.
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Page|15 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Recordyourtestingprocedure.Howmanytestswillyoudooneachplastic?
Thiscouldbeapicturesimilartotheoneafewpagesback,butwithmorelabelsandannotations.
Otherstudentsmighthaveadot-pointprocedure.
Sincethepuncturehole(damagetotheplastic)isverylocalised,itispossibletorepeatthetestforeachplasticseveraltimes.
ToparaphraseIanFleming'sJamesBondnovelGoldfinger:
Onceisanaccident.Twiceisacoincidence.Threetimesismuchmorecertain.
Scientistsandengineersnormallyrepeatatestatleastthreetimes.
Part2:MoreObservationsandResultsRecordyourresults.Whatunitswillyoubeusing?
Unitswouldmostprobablybecentimetres(cm)orperhapsmillimetres(mm)ormetres(m).
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Page|16 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part2:AnalysisSummariseyourresultsinaformthatvisuallydisplaysthedifferencesinresistancetopuncturing.
Thetest(s)oneachplasticyieldsasingleaveragenumber.Hencetheresultforeachplasticcouldbeplottedonanumberline.
Ifweputaseriesofnumberlinessidebysideinaverticalformat,wegetabargraphtoshow:
Horizontalaxis:averagevariousplasticsamplesasseparate“entries”
Verticalaxis:averagedroppingheightofskewer(cm)thatcausesplastictopuncture
Thereareotherpossibilities.
Whydidyouchoosetousethisparticularrepresentation?Hint:whataretheadvantagesofusingthiswayofvisuallysummarisingyourresults?
• Usingsomekindofgraphorpictorialrepresentationmakesiteasytocomparesetsofdatabetweendifferentgroupsataglance.
• clarifytrendsbetterthandotables.
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Page|17 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part2:DiscussionandConclusionsHowdidtheplasticscompare?Whichoneisstrongest?Whatistheevidence?
Strongestplasticwillbetheonethathastheskewerlargestdroppingheightneedtopunctureit.
Doyouthinkyourresultsarereliablewhenmakingajudgementaboutwhichplasticbagwillresistcuttingbysharpobjects?Whatarethestrengthsandweaknessesofthetestingprocedureyouused?
Reliabilityisthedegreetowhichanassessmenttoolproducesstableandconsistentresults.Repeatinganexperimentreducestheeffectoferrorsandhenceincreasesthereliabilityofanexperiment.Ensuringexperimentiswellcontrolledalsoassistshere.
Strengths:Wellcontrolledexperimentwithonlyonevariable.
Weakness:Couldquestionifaskewerisagoodmodelofasharpobjectthatcouldpierceaplasticbag.Couldtestavarietyofsharpobjectsandcouldtestforceappliedbysharpobjectratherthandroppingheight.
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Page|18 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Aretherevariablesyouhavenotcontrolledinyourtests?Howmightthesevariablesaffectyourconclusions?
Dependsonexperimentaldesign.Thepurposeofacontrolledexperimentistoensureanychangesmeasured/observed(piercingheight)areduetoindependentvariable(differentplasticsample)andnootheruncontrolledvariable.Tobeabletoattributeallchangestoindependentvariablebeingtested.
Identifytwoimprovementstothemethodtoensureitisawell-controlledexperiment.
Part3:ReflectionsWhatotherinvestigationscouldyouundertakewiththeplasticsandequipmentused?
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Page|19 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part4:Extensions
StressvsstrainWhatextrameasurementsdoyouneedMaketheappropriatemeasurementsandcalculatetheengineeringstressandstrainatdifferentloadstillfailure.Designatableandrecordyourmeasurements.Plotastressvsstraincurveforeachplastictested.
Howdotheshapesofthecurvescompare?
Whatdotheshapesofthegraphssuggestabouttheplasticsusedinsupermarketbags?
IndependentinvestigationWhatquestiondoyouintendtoinvestigate?
Doyouhaveahypothesis?Brieflystateit.
Outlinetheprocedureforconductingyourtest.
Recordyourmeasurements.
Presentdata.
Analyseyourdata.
Whatconclusionscanyoudraw?Didtheexperimentproduceevidencetosupportyourhypothesis?Arethereimprovementsyouwouldmakeinyourexperimentalmethod?
CopyrightandCreativeCommonsThemoralrightsoftheauthors,StuartPalmer,IanBentley,PetaWhite,RussellTytler,KieranLim,JohnLong,andMaryVamvakashavebeenassertedundertheAustralianCopyrightAct1968(Cth).Exceptinglogos,trademarksorotherthird-partycontentasindicated,thisresourceisdistributedunderaCreativeCommons‘Attribution-NonCommercial-ShareAlike’4.0InternationalLicense.