Tensile behavior of PVC-coated woven membrane materials under uni- and bi-axial loads

TensileBehaviorofPVC-CoatedWovenMembraneMaterialsUnderUni-andBi-AxialLoads

ShouhuiChen,1XinDing,1RaulFangueiro,2HongleiYi,3JingNi41CollegeofTextiles,DonghuaUniversity,Shanghai201620,People’sRepublicofChina

ofTextileEngineering,UniversityofMinho,Guimar~aes4800-058,Portugal

CollegeofFashionandArtDesign,JiaxingUniversity,Jiaxing314001,Zhejiang,People’sRepublicofChina4DepartmentofCivilEngineering,ShanghaiUniversity,Shanghai200072,People’sRepublicofChina

2Department3Received30March2007;accepted23August2007DOI10.1002/app.27303

Publishedonline30October2007inWileyInterScience(www.interscience.wiley.com).ABSTRACT:Tensilecharacteristicsarethemostsigni?cantmechanicalpropertiesforcoatedwovenfabricsasmembranematerialsusedinlightweightconstructions.Factorsthatmightaffecttestresultsofthematerialunderuni-andbi-axialtensileloadsareexamined.AfterseriesoftensiletestsonPVC-coatedmembranematerials,itisdemonstratedthat

垂直的(1)tomeasurethestrainsinthetwoperpendiculardirections,延伸仪thecontactmethodbytheneedleextensometerdoesnotinterferethecorrectdatarecording;(2)thepositionswherethestrainsaremeasuredonspecimenshaveagreatin?uenceonthetestresultsofthestiffnessandPoisson’sratioinwarpdirectionunderuni-axialload;(3)toperformbi-axialtensile

teststhesizeofthecruciformspecimeninbi-axialtensiletestcanbemuchsmallerthanthosesuggestedintheliterature.Thetensilebehaviorofcoatedmembranematerialsunderbi-axialloadsareaffecteddramaticallybythestressratiointhewarpand?lldirections.Besidestheresidualstrainsofcoatedmembranematerialsareaffectednotonlybythepropertiesoftheconstituentyarnsandwovenstructurebutalsobyloadingconditionsduringthecoatingprocess.ó2007Wiley

Periodicals,Inc.JApplPolymSci107:2038–2044,2008

Keywords:poly(vinylchloride)(PVC);mechanicalprop-erties;uniaxial;biaxial;coatedwovenmaterials

INTRODUCTION

Inthepasttwodecades,theapplicationofcoatedfabricsasarchitecturalmembranematerialforlight-weightstructureshasgrowndramatically.Asakindof?exiblematerials,coatedmembranematerialshavevirtuallylittlebendingstiffness,andthereforecanonlyresistintensile.Tosustainashape,themembranematerialmustbeintension.Tensilebehaviorsarethereforevitaltoestablishthematerialpropertiesforstructuraldesign.1Totestthetensilebehaviorsofcoatedmembranematerialsunderuni-axialloadthereareseveralinternationalstandards.However,itisknownthatmembranematerialsareundercomplexloadingcon-ditionswhenusedasarchitecturestructures.Uni-axialtest,althoughsimpletoundertake,islessfavorableinevaluatingtheserviceperformanceofthematerial.Thus,thebi-axialtensiletestisrecom-mended.However,presentlythereisnocommonrecognizedtestingmethodonbi-axialtensiletestformembranematerials.Neitherthecon?gurationsofspecimenshapenorthetestingprocedureshave

Correspondenceto:X.Ding(xding@dhu.edu.cn).

Contractgrantsponsor:TheInnovationFundofDonghuaUniversity.

C2007WileyPeriodicals,Inc.VJournalofAppliedPolymerScience,Vol.107,2038–2044(2008)

beenidenti?ed.Sofar,asamatteroffact,various

testmethodshavebeensuggestedbymanufacturers.Toperformbi-axialtensiletestonmembranema-terial,threemethodshavebeenreferredinthelitera-ture,i.e.,theburstingtest,thecylindertestandthein-planebi-axialtest.2–4Amongthem,thein-planebi-axialtest,especiallywithacruciformspecimen,representsthebestresultsforstress-strainrelationofthematerial.Therefore,ithasbeenusedmorepopu-larlysince1980s.

Forthespeci?cationsofthecruciformspecimen,thereareanumberofexamplesandsuggestionsinthepublishedliterature.Thedimensionsofthecruci-formspecimen,particularlythecrossarea,variedfrom1603160mm2to3003300mm2.5–9Theslitsinthearmswerereportedhelpfulforauniformstressdistributioninthecrossareaofthespeci-men.2,5Thetransferef?ciencyofthestressfromtheholdingclamptothecrossareawouldbegreatlyimprovedwiththeassistantoftheseslits.Inaddi-tion,theshapeofthecrosscornersofthespecimenmeantcertaineffectonthestressdistribution.Com-paredtothestraightcornerortriangularcorner,theroundedcorner,witharadiusof5–15mm,hadshownitsbene?ttothetransmissionofstress.2,6Forbi-axialtensiletestoncoatedmembranemate-rials,alargedeformationcouldbeexpected.Tomea-surethetensilestrainofthematerialtherearetwokindsofmethod,thatis,contactandnoncontact

TENSILEBEHAVIOROFPVC2039

TABLEI

Speci?cationsofPVC-CoatedPolyesterFabrics

SampleWeaveYarncountYarnsdensityMassThicknessofcoated

AveragethicknessBreakingstrengthcodepattern(Tex)(warp/?ll)(ends/cm)

(g/m2)S1Basket111.1112/12900S2Basket144.4413/131100S3

Basket

111.11

12/12

1000

method.Needlemethodisoneofthecontactmeth-ods.6,8Thestrainismeasuredwithanextensometerattachedtothecenterofthetestsamplewithtwoneedles.Withthismethod,theneedleshavetopene-tratethespecimen,whichmightinterferethecorrecttests.Toovercomethepossibleinterferencesofthecontactmethod,2,10,11noncontactmethods,suchasphoto-graphmethodandlasermethod,12havebeenusedwidely.Becauseoftheavailabilityapairofneedleextensometerswasusedinthepresentstudy.Therefore,beforeconductingbi-axialstrainstest,itwouldbeinterestingtoknowtheperformanceoftheneedleextensometer.

Inthepresentstudy,severalfactorsthatmightinterferecorrecttensiletestswereexamined.Inaddi-tiontotheperformanceevaluationoftheneedleex-tensometer,thein?uenceofthepositionsonspeci-menwherestrainsweredetectedwasinvestigated.Thepossibilityofusingsmallsizecruciformspeci-menswasexamined.Besides,theeffectofloadingconditions,suchastensilecyclesandstressratiosbetweenthewarpand?lldirectionswerealsodis-cussed.Ithopesthatthestudywouldprovideaddi-tionalinformationforthosewhowanttoevaluatethetensileperformancesofcoatedmembranemateri-alsbymeansofbi-axialtensiletests.

EXPERIMENTAL

Abi-axialtensiletester,modelZ010/TH2AbyZwickGmbH,wasused.Thetesterconsistsoftwoloadframescrosswiseconnected.Theloadframeconsistsoftwoguidepro?les,twomovingcross-headsandaheadplate.Theheadscrewshavetowcounterrotationthreadssothatthemovingcross-headsmovesynchronouslyinoppositedirections.13Twopairsofspecimenholdersarecontrolledbycompressedairsothatspecimenscanbeheldasrig-orouslyaspossible.

PVC-coatedpolyesterfabricswithasurfacetreat-mentofpolyvinylidene?uoride(PVDF)wereusedinthecurrentresearch.TableIliststhespeci?cationsofthesamples.

Thecon?gurationofthespecimenforbi-axialten-siletestsisshowninFigure1.Thecrossareaofthespecimenis60360mm2andtheshapeofthecrosscornerisroundedwitharadiusof15mm.Thereare

fabric(mm)

ofPVClayer(mm)

(warp/?ll)(N/5cm)

0.740.313900/35001.000.425500/50000.90

0.40

4400/4000

threelongslitsineachtensilearm.Forthereasonofcomparison,uni-axialtensiletestswerealsocon-ductedonthesametester.Thedimensionsofthespecimenforuni-axialtestsasastripshapearecom-patiblewiththoseofbi-axialtests.

Inthebi-axialtests,theloadingspeedinonedirectionwasset1.0N/sandthatintheperpendic-ulardirectionfollowedthestressratiosdetermined.Forexample,forthestressratio1:2(warp:?ll),theloadingspeedinwarpand?lldirectionwassetat0.5and1.0N/s,respectively.Foruni-axialtensileteststheloadingspeedwaskeptat2.0N/s.

Threecyclesofloading-unloadingwereconducted,accordingtotheusualpracticeofbi-axialtensiletestsforcoatedmembranematerials.5,7,9,14Therewasnosuspensionbetweentheloadingcycles.Themaxi-mumstressintheloadingcyclewas15%oftheulti-matetensilestrengthofthecorrespondingspecimen.ForsamplesS1,S2,andS3listedinTableI,themax-imumstresswassetat125,140,and150N/cm,respectively.

Foreachtest,threespecimensweretested.

Figuretests.

1Dimensionsofspecimensforbi-axialtensileJournalofAppliedPolymerScienceDOI10.1002/app

2040CHENETAL.

Figure2Tensilestrainsrecordedbybothneedleandphotographmethods.(a)Uni-axialtestsinwarpdirection.(b)Uni-axialtestsin?lldirection.(c)Bi-axialtestswithastressratio1:1(warp:?ll).

RESULTSANDDISCUSSION

Straintestingbyneedlemethod

Beforeactualtests,therecordingperformanceofneedlemethodswasexaminedbycomparingthetestingresultswiththosebyphotographmethod.Forcomparison,threegroupsofexperiments,underbothuni-andbi-axialtests,werearrangedonsam-pleS1.Theywereuni-axialtestsinbothwarpand?lldirectionsandbi-axialtestswithastressratio1:1(warp:?ll).

Asaphotographmethod,adigitalcamerawas?xedabovethecenterareaofthespecimen.Photosweretakeninevery75sduringthewholetestpe-riodtorecordthedisplacementofprearrangedmarksonthesurfaceofspecimen.

Bypenetratingneedles,apairofresistanceextens-ometerswasattachedonthetopandbottomsidesofspecimens,respectively,torecordthestrainsinper-JournalofAppliedPolymerScienceDOI10.1002/app

pendicularloadingdirections.Therecordeddatawerethentransmittedtoadataacquisitionsystemforfurtherprocessing.

Thetestingresults,bybothphotographandnee-dlemethods,areshowninFigure2.Foreasyidenti-?cation,onlythethirduploadingcurvesaredis-played.Itisnoticedthatthedatarecordedfromthetwomethodsareingeneralagreement.Theneedledmethoddoesnotintroduceunacceptableerrorsinthepresentrangeofmeasurement.Infact,thedatarecordedbytheneedlemethodaremoreconsistentandaccuratethanthephotographone.PositionsofstrainmeasurementonspecimenTostudytheeffectofthepositionwheretensilestrainsweremeasuredonaspecimen,experimentswerearrangedonsampleS2atthreepositions:A,B,andC,asillustratedinFigure3.

TENSILEBEHAVIOROFPVCFigurespecimen.

3PositionsoftensilestrainmeasurementonFigure4showsthetensilestiffnessandPoisson’sratiomeasuredatdifferentpositionsonspecimeninuni-axialtests.Differencesareobservedwhenthemeasurementisatdifferentpositionswhenloadinginwarpdirection.AtpositionA,thetensilestiffnessismuchhigherthanthatinpositionBorC.SimilarvariationsofPoissonratiocanalsobeobserved.Itindicatesthatthestressdistributionperpendiculartotheloadingdirectionisnotasuniformasonemayexpect.Itisthereforerecommendedthat,toobtainaconsistentrecording,thepositionofmeasurementshouldbealongthecenterlineofloadingtoavoidanypossiblepositioneffects.

InFigure4itmayalsobenoticedthatthepositioneffectislesssensitivewhenloadingin?lldirection.Thereasonforthedifferentbehaviorsofpositionsensitivitycanbeattributedtothedifferentlevelsofyarncrimpsduemainlytothecoatingprocess,inwhichwarpdirectionistensionedresultinginagreatdegreeof?llcrimpbecauseofcrimpinter-changeofwovenstructure.

Unlikethoseinuni-axialtests,thetensilebehav-iorsofspecimenunderbi-axialtestsarelesssensi-tivetothepositionofstrainmeasurement,asshowninFigure5.ThiscanbeexpectedbyreferringtoFig-ure6byBridgens12thatthereisarangewithinthecrossareaofthecruciformspecimen,inwhichthe

Figuresurement4positionsTestingresultsinuni-axialofsampletests.S2withdifferentmea-2041

Figurewithdifferent5Tensilemeasurementbehaviorpositionsofsample(stressS2inratiobi-axial1:1).

testsstressdistributionisratheruniform.Whenthemea-surementpositionisoutsidethisrange,asigni?cantdecreaseoftensilestresswouldbeexpected.Tensilebehaviorsunderuni-andbi-axialloadsSamplesS2andS3weretestedunderbothuni-andbi-axialloads.ForallteststhestrainsweremeasuredbytheneedlemethodatpositionBidenti?edinFig-ure3.TheresultsofthethirduploadingcurvesareshowninFigure7.WandFrepresentthebi-axialtensilecurveinwarpand?lldirections,respectively,followedbythestressratiobetweenwarpand?lldirections.W-uniandF-uniaretheuni-axialtensile

Figure30N/cm6loadsStressindistributionbothofloadingofcruciformdirection.specimen,with12JournalofAppliedPolymerScienceDOI10.1002/app

2042CHENETAL.

Figure7Tensilebehaviorsunderuni-andbi-axialloads.(a)SampleS2and(b)SampleS3.

curveswhenloadinginthewarpandthe?lldirec-tion,respectively.

Referringtothetestsreportedintheliteraturewithlargersizeofcruciformspecimens,suchasItoh’s9withacrossareaof1603160mm2andKato’s7with3003300mm2,itcouldbefoundthatthetensilebehaviorswithdifferentstressratioswerecompatible.Thisindicatesthatthecrossareaofthespecimenforbi-axialtensiletestscouldbereduced,suchas60360mm2inthepresentstudy.

Incoatedmembranematerials,sincetheYoung’smodulusofthepurePVCmembraneisaslowas6.7Mpa,thepolyesterfabricsplayamoreimportantroleinthetensilecharactersofthecoatedmembranematerialsunderbi-axialloadsespeciallyunderhigherstresses.7ThenthedifferencebetweenthetensilebehaviorofS2andS3underbi-axialloading(seeinFig.7)ismostlyresultingfromtheyarnsandthefabricstructures(seeinTableI).

Becauseofthedifferentlevelofthecrimpandtheloosenessinthewarpandthe?lldirectionsinthecoatedmembranematerialsresultingfromthepro-ducingofwovenfabricandcoatingprocess,theunbalanceddeformationsinthewarpand?lldirec-tionshappenaccordingtodifferentstressratios(seeinFig.7).Whenthestressratioislessthanorequalto1(forsampleS3orS2),negativestrainshappeninthewarpdirectionandpositivestrainshappeninthe?lldirectionduetotheunbalancedstructureofthematerials.Thestateoftheshrinkageinthewarpdirectionandthestateoftheextensioninthe?lldirectioncanresultinthesituationofunbalanceddeformationsofthestructuremembrane.Besides,becauseofthedecreaseoftheYoung’smodulusinthewarpdirectionwithstressratioslessthan1,the

JournalofAppliedPolymerScienceDOI10.1002/app

useef?ciencyofthematerialsespeciallyinthewarpdirectionwillbereduceddramatically.Therefore,inthedesignandtheapplicationofthemembranestructures,thepotentialdamage(forexample,theshrinkageofthesurface)resultingfromthespecialstressdistributionsshouldbeavoided.Residualstrainsafterbi-axialloadingcyclesFigure8showstheresidualstrainsaftereachload-ingcycleinbi-axialtestswithdifferentstressratiosforsampleS2andS3,respectively.Tomakethe?guremuchclear,eachvalueisthemeanvaluefromthreespecimenssincethepercenterrorsareassmallaslessthan5%.Thestressratioof10representsthebi-axialtestwithastressratioof1:0(warp:?ll).

ComparedtotheresultsofS3,theresidualstrains(positiveandnegativeones)ofS2aremuchsmallerinboththewarpandthe?lldirections.Byreferringtothesample’sspeci?cationsinTableI,itisknownthat,withhigher-strengthyarnsandtighterwovenstructure,sampleS2showsbetterelasticrecoverywithcycle-loadsthansampleS3.

FromFigure8,itcanbenoticedthatforsampleS2andS3duringthethreeloadingcycles,whenthestressratioislessthan1theresidualstrainsinwarpdirectionarestillnegative.However,inthe?lldirec-tiontheresidualstrainsarepositive.Thisphenom-enonemphasizesthatthestressdistributionwithstressratioslessthan1shouldbeavoidedintheapplicationofthestructuremembrane.

Figure8alsoshowsthat,whenthestressratioismorethan1,theresidualstrainsinbothwarpand?lldirectionsarelessthan2.0%andthedifferenceaftereachloadingcycleislessthan0.5%.Itshowsa

TENSILEBEHAVIOROFPVC2043

Figure8Residualstrainsaftereachloadingcycleunderbi-axialtests.(a)InwarpdirectionforsampleS2.(b)In?lldirec-tionforsampleS2.(c)InwarpdirectionforsampleS3.(d)In?lldirectionforsampleS3.

goodelasticrecoveryofthecoatedmembranemate-rialswhenloadinginwarpdirectionishigherthanthatin?lldirection.However,whenthestressratioislessthan1,theresidualstrainin?lldirectionisasgreatas5%andthedifferenceaftereachloadingcycleisaslargeas1.0%,althoughtheresidualstrainsinwarpdirectionisstilllessthan1.0%.Thedifferentresultsofthebi-axialtestsbetweenapairof‘‘symmetrical’’stressratios,suchas1:2and2:1,shouldattributetotheinherentunbalancedstructureofthecoatedfabrics,15inwhichthecrimpofyarnsin?lldirectionismuchhigherthanthatinwarpdirection.Thedifferentresidualstrainsbetweenthewarpand?lldirectionscanleadtothedifferenceofrelaxationinthetwoperpendiculardirections,andthencanresultinthedif?cultytomaintenanceofastableandsmoothsurfaceintensilestructures.

Therefore,suitablestressratiosshouldbedeter-minedinthedesignoftensilestructuretoobtainuniformtensilebehaviorsofmembranematerials.Itisalsostronglysuggestedthatpropertensionshouldbeappliedtothe?lldirectionofwovenfabricdur-ingthecoatingprocesstoreducethedegreeof?llyarncrimp.

CONCLUSIONS

Factorsthatmightaffecttensiletestingresultsofcoatedwovenfabricasmembranematerialsunderuni-andbi-axialtensileloadsareexaminedandsomemeasurementtechniquesarediscussed.

Withinthetestingrangeofthepresentstudy,theneedledmethod,althoughtwoneedlespenetratingthespecimen,doesnotintroducenoticeableerrors.Itisapreferablemethodifthereisnomoresophisti-catednoncontactmethodavailable.

Foruni-axialtensiletestswiththeneedlemethod,theattachmentoftwopenetratingneedlesonthespecimenshouldbealongthecenterlineofloadingtoavoidanypossiblepositioneffectsbecauseofthenonuniformityofthestressdistributionperpendicu-lartotheloadingdirection.Forbi-axialtensiletests,however,thepositioneffectislessnotablebecausethestressdistributionwithinthecrossareaofcruci-formspecimenisratheruniform.

Itisshownthatinbi-axialtensiletestswithsmallsizespecimensareingoodagreementwiththosewithlargeronessuggestedintheliterature.Furtherstudiesarerequiredtoidentifythemostsuitablesizeforthecruciformspecimens.

JournalofAppliedPolymerScienceDOI10.1002/app

2044ThestrainsandresidualstrainsofPVC-coatedmem-branematerialsunderbi-axialcyclicloadsareaffectedgreatlybythedifferentdegreesofyarncrimpinwovenstructure.Becauseoftheessentialunbalanceofyarncrimpinthewovenstructure,suitablestressratiosshouldbedeterminedtomaintainastableandsmoothsurfaceintensilestructures.However,toobtainauni-formtensilebehaviorofmembranematerialsitisstronglysuggestedthatpropertensionshouldbeappliedtothe?lldirectionofwovenfabricduringthecoatingprocesstoreducethedegreeof?llyarncrimp.

WeexpressourgratefulacknowledgmenttoProf.LuoRen’an(ShanghaiUniversity)forhiscooperationonthebi-axialapparatusandShanghaiShendaKobondNewMaterialsCo.,Ltdforthematerialsprovided.

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