materials testing: plastics - deakin university blogs...2017/07/23 · page | 2 sample answers –...

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?


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


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


andalsomeasurethewidthoftheplasticatthemidpointofthelengthofplastic.Continuetoaddweightsuntiltheplasticteststripbreaks.

Aswellasthemeasurements,noteotherobservationsyoumakewhiletheweightsarebeingadded.


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


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



changeinlength


changeinlength


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.


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>


Aretherevariablesyouhavenotcontrolledinyourtests?Howmightthesevariablesaffectyourconclusions?

Possiblevariablesnotcontrolledarethicknessofplasticsamplesandtimeweightsareleftsuspendedfromplasticbeforemeasurementistaken.Inawell-designedexperiment,allvariablesexcepttheindependentvariable(thedifferentplasticbags)shouldbekeptthesametoensureanychangesobservedinthedependentvariableareduetotheplasticitselfandnotthethicknessofthesampleortimeweightislefthangingfromtheplastic.

Identifytwoimprovementstothemethodtoensureitisawell-controlledexperiment.

• Plasticsamplestobesamethicknesstostartoffwith.

• Allthreeplasticsamplestobetestedatthesametimeafterweighthasbeenadded.

Supposeyouhadtoincreasethestrengthofthehandlesoftheplasticbag—whatcouldyoudo?Describeatestyoucoulddotogainevidenceforyourproposal.


Wecouldaddmorethanonelayerofplasticwhenmakingthehandle.

Repeattheexperimentvaryingthenumberofpiecesofplasticsuspendedbetweentheicypolesticks.ThiswillbetheIndependentVariable.Couldstartoffusingagreaterloadanddeterminethenumberof50-100gweights/loadneededtobreaktheteststrip.

Part2:CuttingandtearingNotonlyareitemspurchasedfromthesupermarketheavyrequiringsupermarketbagstobestrongbuttheyalsooftencomeinpackageswithsharpedgesandcorners.Bagsmaybepuncturedorcutandeventuallytear.

Inthisactivity,youusethematerialsandyourownimprovedversionofthesuggestedtestingtechniqueprovidedtocomparetheresistanceofthebagstopuncturing.

Part2:HazardsTherearehazardsinthisactivityfromsharpobjects.Careshouldbeusedwithscissorstokeepfingersclearwhilecutting.Bothscissorsandthebambooskewersaresharpandcarethattheyarenotpokedintoskinoreyes.Weareyeprotection.

Part2:InvestigationInstructionsFromthesamplesofplasticbagsfortesting,cut15cmsquares–bigenoughtobestretchedoverthebeaker/cupandheldinplacewitharubberband.


Stretchasheetofplasticbagoverthetopofaplasticcuporbeakerandsecureitwitharubberbandasshown.Dropabambooskewer,pointdown,ontothestretchedplastic,fromdifferentheights.

Part2:PreliminaryObservationsandResultsWhatdoyouobservewhenyoudropabambooskewer,pointdown,fromaheightofabout1cmabovetheplasticbagontotheplastic?

Bambooskewerreboundslikeaballbouncingonthegroundandtheplasticstretchesslightlyastheskewerhitsit.

Repeatthisfrom20cmabovethestretchedplasticbag.Whatdoyouobservethistime?

Skewerpierces/puncturestheplasticanddoesnotreboundoffit,butstaysembeddedwithintheplastic.

Usingthistechniquecomparethreedifferentplasticbagsfortheirresistancetopuncturing.Whatneedstobedonetoensurethatthetestsarefair?


Therearemanypossibilities.Justsomeinclude:• Sametype,sizeandopeningdiameterplasticcup/beakerusedfor

eachplasticsample.• Eachplasticsampleshouldbestretchedandsecuredthesameway

overtheplasticcup/beaker.• Skewerreleasedthesamewayfromfingerswithnoforceexerted

onskewerbyfingers.• Measurementofheightofskewerfromcup/beakertakenthesame

way.


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.


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).


Part2:AnalysisSummariseyourresultsinaformthatvisuallydisplaysthedifferencesinresistancetopuncturing.

Thetest(s)oneachplasticyieldsasingleaveragenumber.Hencetheresultforeachplasticcouldbeplottedonanumberline.

Ifweputaseriesofnumberlinessidebysideinaverticalformat,wegetabargraphtoshow:

Horizontalaxis:averagevariousplasticsamplesasseparate“entries”

Verticalaxis:averagedroppingheightofskewer(cm)thatcausesplastictopuncture

Thereareotherpossibilities.

Whydidyouchoosetousethisparticularrepresentation?Hint:whataretheadvantagesofusingthiswayofvisuallysummarisingyourresults?

• Usingsomekindofgraphorpictorialrepresentationmakesiteasytocomparesetsofdatabetweendifferentgroupsataglance.

• clarifytrendsbetterthandotables.


Part2:DiscussionandConclusionsHowdidtheplasticscompare?Whichoneisstrongest?Whatistheevidence?

Strongestplasticwillbetheonethathastheskewerlargestdroppingheightneedtopunctureit.

Doyouthinkyourresultsarereliablewhenmakingajudgementaboutwhichplasticbagwillresistcuttingbysharpobjects?Whatarethestrengthsandweaknessesofthetestingprocedureyouused?

Reliabilityisthedegreetowhichanassessmenttoolproducesstableandconsistentresults.Repeatinganexperimentreducestheeffectoferrorsandhenceincreasesthereliabilityofanexperiment.Ensuringexperimentiswellcontrolledalsoassistshere.

Strengths:Wellcontrolledexperimentwithonlyonevariable.

Weakness:Couldquestionifaskewerisagoodmodelofasharpobjectthatcouldpierceaplasticbag.Couldtestavarietyofsharpobjectsandcouldtestforceappliedbysharpobjectratherthandroppingheight.


Aretherevariablesyouhavenotcontrolledinyourtests?Howmightthesevariablesaffectyourconclusions?

Dependsonexperimentaldesign.Thepurposeofacontrolledexperimentistoensureanychangesmeasured/observed(piercingheight)areduetoindependentvariable(differentplasticsample)andnootheruncontrolledvariable.Tobeabletoattributeallchangestoindependentvariablebeingtested.

Identifytwoimprovementstothemethodtoensureitisawell-controlledexperiment.

Part3:ReflectionsWhatotherinvestigationscouldyouundertakewiththeplasticsandequipmentused?


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.

materials testing: plastics - deakin university blogs...2017/07/23 · page | 2 sample answers –...

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