materials testing: plastics - deakin university blogs...2017/07/23 · page | 2 sample answers –...
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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?
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
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
Page|4 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
andalsomeasurethewidthoftheplasticatthemidpointofthelengthofplastic.Continuetoaddweightsuntiltheplasticteststripbreaks.
Aswellasthemeasurements,noteotherobservationsyoumakewhiletheweightsarebeingadded.
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
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
Page|7 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Plastic2:percentagechangeinlengthversusloadLoad Length Changeinlength Percentage
changeinlength
Plastic3:percentagechangeinlengthversusloadLoad Length Changeinlength Percentage
changeinlength
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.
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>
Page|10 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Aretherevariablesyouhavenotcontrolledinyourtests?Howmightthesevariablesaffectyourconclusions?
Possiblevariablesnotcontrolledarethicknessofplasticsamplesandtimeweightsareleftsuspendedfromplasticbeforemeasurementistaken.Inawell-designedexperiment,allvariablesexcepttheindependentvariable(thedifferentplasticbags)shouldbekeptthesametoensureanychangesobservedinthedependentvariableareduetotheplasticitselfandnotthethicknessofthesampleortimeweightislefthangingfromtheplastic.
Identifytwoimprovementstothemethodtoensureitisawell-controlledexperiment.
• Plasticsamplestobesamethicknesstostartoffwith.
• Allthreeplasticsamplestobetestedatthesametimeafterweighthasbeenadded.
Supposeyouhadtoincreasethestrengthofthehandlesoftheplasticbag—whatcouldyoudo?Describeatestyoucoulddotogainevidenceforyourproposal.
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.
Page|12 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Stretchasheetofplasticbagoverthetopofaplasticcuporbeakerandsecureitwitharubberbandasshown.Dropabambooskewer,pointdown,ontothestretchedplastic,fromdifferentheights.
Part2:PreliminaryObservationsandResultsWhatdoyouobservewhenyoudropabambooskewer,pointdown,fromaheightofabout1cmabovetheplasticbagontotheplastic?
Bambooskewerreboundslikeaballbouncingonthegroundandtheplasticstretchesslightlyastheskewerhitsit.
Repeatthisfrom20cmabovethestretchedplasticbag.Whatdoyouobservethistime?
Skewerpierces/puncturestheplasticanddoesnotreboundoffit,butstaysembeddedwithintheplastic.
Usingthistechniquecomparethreedifferentplasticbagsfortheirresistancetopuncturing.Whatneedstobedonetoensurethatthetestsarefair?
Page|13 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Therearemanypossibilities.Justsomeinclude:• Sametype,sizeandopeningdiameterplasticcup/beakerusedfor
eachplasticsample.• Eachplasticsampleshouldbestretchedandsecuredthesameway
overtheplasticcup/beaker.• Skewerreleasedthesamewayfromfingerswithnoforceexerted
onskewerbyfingers.• Measurementofheightofskewerfromcup/beakertakenthesame
way.
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.
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).
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.
Page|17 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Part2:DiscussionandConclusionsHowdidtheplasticscompare?Whichoneisstrongest?Whatistheevidence?
Strongestplasticwillbetheonethathastheskewerlargestdroppingheightneedtopunctureit.
Doyouthinkyourresultsarereliablewhenmakingajudgementaboutwhichplasticbagwillresistcuttingbysharpobjects?Whatarethestrengthsandweaknessesofthetestingprocedureyouused?
Reliabilityisthedegreetowhichanassessmenttoolproducesstableandconsistentresults.Repeatinganexperimentreducestheeffectoferrorsandhenceincreasesthereliabilityofanexperiment.Ensuringexperimentiswellcontrolledalsoassistshere.
Strengths:Wellcontrolledexperimentwithonlyonevariable.
Weakness:Couldquestionifaskewerisagoodmodelofasharpobjectthatcouldpierceaplasticbag.Couldtestavarietyofsharpobjectsandcouldtestforceappliedbysharpobjectratherthandroppingheight.
Page|18 SampleAnswers–MaterialsTesting:Plastics–ReMSTEP(DeakinUniversity)
Aretherevariablesyouhavenotcontrolledinyourtests?Howmightthesevariablesaffectyourconclusions?
Dependsonexperimentaldesign.Thepurposeofacontrolledexperimentistoensureanychangesmeasured/observed(piercingheight)areduetoindependentvariable(differentplasticsample)andnootheruncontrolledvariable.Tobeabletoattributeallchangestoindependentvariablebeingtested.
Identifytwoimprovementstothemethodtoensureitisawell-controlledexperiment.
Part3:ReflectionsWhatotherinvestigationscouldyouundertakewiththeplasticsandequipmentused?
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.
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