Hygroscopic aspects of epoxy.docx
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Hygroscopic aspects of epoxy.docx
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Hygroscopicaspectsofepoxy
Hygroscopicaspectsofepoxy/carbonfibercompositelaminatesinaircraftenvironments
H.S.Choia,K.J.Ahnb,J.-D.NamcandH.J.Chund
aKoreaInstituteofAerospaceTechnology,KoreanAir,Taejon305-390,SouthKorea
bSukKwangCo.,685,SungWonBuilding,YukSam-Dong,KangNam-Ku,Seoul135-080,SouthKorea
cPolymerScienceandEngineering,SungKyunKwanUniversity,Suwon440-746,SouthKorea
dSchoolofElectricalandMechanicalEngineering,YonseiUniversity,Seoul120-749,SouthKorea
Received3November1999;
revised3August2000;
accepted8August2000
Availableonline6March2001.
Abstract
Inthisstudy,varioushygroscopiceffectsofsuchparametersashygrothermaltemperature,matrixvolumeratio(Vm),voidvolumeratio(Vv),specimenthickness,lay-upsequenceandinternalstresswereinvestigatedforepoxy/carbonfibercompositelaminates.Thespecimenthicknessandlay-upsequencehadlittleeffectonthethrough-the-thicknesswaterabsorptionbehaviorofcompositelaminates,buttheotherparametersaffectedthemoistureabsorptionrateandequilibriumwateruptakeindifferentwaysandintensities.Theglasstransitiontemperatureofcompositelaminateswasstronglyaffectedandlinearlydecreasedbythequantityofequilibriumwateruptake.Acharacteristiclengthofmoisturemigrationthroughtheunidirectionallaminateswasproposedasafunctionoffiberangletotheexposedlaminatesurface.Inthisapproach,thefibersimbeddedinthematrixwereassumedtoactasabarriertothepenetratingwatermolecules,andthedevelopedmodelwaswellcomparedwiththeexperimentalresults.
AuthorKeywords:
A.Polymer–matrixcomposites(PMCs);Lay-up;E.Prepreg
ArticleOutline
1.Introduction
2.Theoreticalbackground
2.1.Fick'ssecondlawofdiffusion
2.2.Modelingofdiffusionpathofwatermoleculesintounidirectionalcomposite
3.Experimental
4.Resultsanddiscussion
5.Conclusions
Acknowledgements
References
1.Introduction
Theabsorbedwatermoleculesinpolymercompositematerialsareknowntohavesignificanteffectsontheirphysicalandchemicalpropertiesofmatrixaswellasontheirfinalperformanceofcompositestructuresespeciallyintheirlong-termutilization.TheabsorbedwaterusuallydepressestheglasstransitiontemperatureTgbyplasticizingthepolymernetworkandalsoaffectsmechanicalperformanceandlong-termdurabilityofhigh-performanceapplications[1,2,3,4,5and6].Longitudinalpropertiesdominatedbyfiberpropertiesintheunidirectionalcompositesmaynotdropsonoticeablybutsuchpropertiesascompressionstrengthandintralaminar/interlaminarshearstrengtharesignificantlyaffectedbytheabsorbedmoistureespeciallyathightemperatures.Theprimaryandsecondarycompositestructuresusedinstateoftheartaircraftusuallyexperiencetherepeatedabsorption/desorptionofwaterinawiderangeofhumidityandtemperature.Thistypeofnon-mechanicalfatigueisconsideredtobecloselyassociatedwiththelong-termdurabilityofcompositematerialsespeciallywhenthewaterabsorptionisaccompaniedwithhighenvironmentalvariationsoftemperatureandpressureaswellasmechanicalloadvariationsduringtheserviceusageofaircraft.Accordingly,fortheapplicationsofhighperformancecompositematerialsinaircraftenvironments,thecompositematerialshavebeenstronglyrequiredtosatisfythehot–wetmechanicalpropertiesdesignatedinthematerialspecifications,wherethespecimensaretestedafterbeingexposedtowaterathightemperaturesforspecifiedtime[7].
Achangeintemperatureandmoisturecontentusuallyinduceshygrothermalforcesandmomentresultantsaswellasdimensionalchangesinthecompositebody.Inaddition,thethermalstressesproducedduringthecoolingprocessofcompositelaminateaftercureatelevatedtemperaturecouldbesocombinedwiththosehygrothermalstressesinducedmoistureabsorption[8and9].Theresultinghygrothermalandmechanicalstressescombinedwitheachothermaybesufficientlylargeenoughtoinfluencethefailureofthelaminateandthusshouldnotbeneglectedinmoderndesignanalysisandlifetimeestimation.Furthermore,therecursivechangesofinternalstressesduetowaterabsorption–desorptionprocessesmayinducefatiguedamageintheinter-andintralaminarregionofcompositelaminatesinfluencinglong-termdurabilityandperformanceofcomposite[10].Inaddition,inthefieldofcompositeapplicationsutilizingelectricandelectromagneticcharacteristicsofcarbonfiberreinforcedcomposite,thewaterabsorptionisoneofkeyissuesthatshouldbeaddressed.Thecompositelaminatesexposedtowaterforatimeusuallyswellandthustheinterfacialcharacteristicsandlocalstressconcentrationofmatrixandfiberstendtochangeandthenconsequentlyinfluencetheelectricconductivity.Accordingly,theelectricresistivitycloselyrelatedwithconductivityisknowntobeproportionaltothesquareofthemoistureweightgaininthelongitudinaldirectionofepoxy/carbonfibercompositelaminate[11].Especiallyinsuchapplicationsofaircraftradomefabricatedwithglassfiberreinforcedcompositesorantennawithcarbonfiberreinforcedcomposites,theabsorbedwatercanstronglyinfluencetheaircraftsafetysincethedielectricpropertiesandtheelectromagneticcharacteristicsofthesefunctionalandsafety-relatedcompositestructurescanbeeasilydeterioratedbythemoistureabsorbedinthestructure[12].
Theobjectiveofthisstudyistoinvestigatevariousaspectsandconcernsencounteredbywaterabsorptioninthecaseof177°C(350°F)curableunidirectionalcarbonfiber/epoxycompositesystemsthatarewidelyandincreasinglyusedassecondaryandprimarystructuresforaircraftapplications.Focusingonthecompositeapplicationsinaircraftenvironment,asignificantdepressionofglasstransitiontemperatureduetotheabnormalwateruptakebehaviorwasidentifiedinhygrothermaltemperatureconditions.VarioushygroscopicaspectsofcompositelaminateswereinvestigatedbyexaminingtheeffectsofmatrixvolumeratioVm,voidvolumeratioVv,specimenthickness,lay-upsequence,appliedbendingload,etc.Inaddition,thecharacteristicdiffusionlengthofwatermoleculeswasderivedtopredicttheanisotropicdiffusionratesforcompositespecimenswithdifferentfibervolumeratiosandfiberangles.
2.Theoreticalbackground
2.1.Fick'ssecondlawofdiffusion
Forarelativelylargeandthincompositeplate,thewaterconcentrationinthethicknessz-directionmaybegovernedbytheone-dimensionaldiffusionequationofFick'ssecondlaw[13],viz.:
(1a)
(1b)c=ci(0 (1c)c=c∞(z=0,L,t>0)wheretistime;L,thicknessofspecimen;z,distancemeasuredfromthebottomsurface;subscripts‘i’and‘∞’represent‘initial’and‘fullsaturated’states,respectively;andDz,moisturediffusioncoefficientinthez-direction.Themoistureconcentration,c(z,t),isdefinedas (2) Thesolutionof(1a),(1b)and(1c)isobtainedinaseriesformas (3) TheaveragemoistureconcentrationthroughthethicknessLofaspecimenmaybeestimatedbythefollowingintegration. (4) Accordingly,thefollowingformofpercentmoisturecontentfrequentlyappearsinliteratureas (5) whereMiisinitialmoisturecontent,whichshouldbezerowhenthespecimensarecompletelydryintheinitialstageandM∞issaturatedormaximumpercentmoisturecontent.AndMisdefinedby (6) Theseriesformofsolutionmaybedirectlyusedtoanalyzediffusionexperiments,butthefollowingformofapproximatesolutionisoftenusedtodeterminethediffusioncoefficientfromtheexperimentaldataplotandidentifydiffusionmechanism[14]. (7) TheaboveequationisusuallyappliedwhereDzt/L2<0.05(orG<0.5).ApplyingEq.(7)toexperimentaldata,thediffusioncoefficientisobtainedfromaninitialslopeoflinearrelationinGvs. plot. 2.2.Modelingofdiffusionpathofwatermoleculesintounidirectionalcomposite EventhoughtheFick'ssecondlawisusuallyappliedtohomogeneousandisotropicmaterials,itisalsoapplicabletothecompositeswithoutlossofgeneralityincasethatthefibersareuniformlydistributedthroughthebody.Itisknownthattheglassandcarbonfibersabsorblesswatercomparedtopolymericmaterialsusedformatrix.Whentheyareusedasareinforcementofcomposite,thefibersmayplayaroletochangethediffusionpathofwatermoleculesinananisotropicfashionbyhinderingthemfromstraightdiffusion.Therefore,theexperimentalevaluationofanisotropicdiffusioncoefficientofcompositematerialsmaybeapproachedwellbyconsideringtheincreaseddiffusionlengthofwatermoleculesbygeometricarrangementoffibersincomposite. Toseetheeffectofmoisturediffusionlengthonthemoisturediffusioncoefficientofunidirectionalcomposite,specimenshavinganangleθbetweentheaxisoffibersandthesurfacenormalarecutfromthethickunidirectionalcomposite[0100]asshowninFig.1.Inparticular,itshouldbenotedthat,90°−yand90°−zspecimensarecomparativelyusedtoseethechangeofdiffusioncharacteristicsofcompositesresultingfromtheexistenceofdifferentextentofresin-richlayersinthethrough-thicknessandtransversedirections,respectively.Fig.2showstheunidirectionalcompositespecimenswithfiberdistributionanglesof0,θ,and90°withrespecttothesurfacenormaldirection.Fig.3alsoshowscrosssectionalview(seenfromcutA–BinFig.2)andcorrespondingdiffusionpathsofwatermoleculesforthethreecasesofθ=0,45,and90°,respectively.Eventhoughthesespecimenshavethesamethickness,i.e.thesameapparentthicknessLa,itcanbegenerallyacceptedforthespecimens
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