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Multiaxial Fatigue Behaviour of Unidirectional Composites Damage Mechanics and Modeling  
Multiaxial Fatigue Behaviour of Unidirectional Composites Damage Mechanics and Modeling
資料類型: PDF文件
關鍵詞: Fatigue  Damage  Composites  Unidirectional  
資料大小: 218K
所屬學科: 通用高分子材料
來源: 來源網絡
簡介:
Theincreasinguseofcompositematerialsforstructuralapplicationsrequiresdedicatedresearcheffortsaimedtothedefinitionofstandardcriteriaandproceduresfordesign,stillmissinginmanyimportantfeaturesconcerningcompositematerials.Inparticularthemultiaxialfatiguebehaviourofcompositeshasreceivedonlyalittleattentionsofar,andreliabledesigncriteria,suitabletodealwithcomplicatedstressconditions,havenotbeenyetassessed.Someofthecriteriaavailableintheliteraturehavebeencomparedtoexperimentaldatain,revealingtheabovementionedlackofreliability.Thesameworkhighlightsalsothealmostcompleteabsenceofquantitativeinformationaboutthedamagemechanicsofadvancedcompositeundermultiaxialcyclicloading.AdedicatedresearchprogramwasestablishedfewyearsagoatDTG-UniversityofPadovawiththeaimtoprovideacontributiononthissubjectandthepresentlectureprovidesabriefoverviewontheactivitydonesofarfromtheexperimentalandmodelingpointofview.Whendealingwiththedamagemechanics,thematrixandinterfacedominatedbehaviourisofprimaryinterest,sincethedamageprocessofamultidirectionallaminateischaracterizedbythenucleationandpropagationofintralaminarcracksforabigportionofthefatiguelife.Thein-planestresscomponentsleadingtoanon-fiber-dominatedresponsearemainlythetransversestressa2andtheshearstressa6;theratioofthesequantityhasbeendefinedasbiaxialityratioAl2=U6/U2.Hereispresentedasummaryoftheextensiveexperimentalinvestigationonthenon-fiber-dominatedbehaviourofglass/epoxyunidirectional(UD)tubes,subjectedtocombinedtension-torsionfatigueloading.Theactivityismainlyfocusedontheinfluenceofthebiaxialityratio\12onthelifetocrackinitiation,thefatigueCrackGrowthRate(CGR)andthedamagemodesatamicroscopiclevel.Tubularspecimenshavebeentestedusingthreedifferentlay-ups([904],[0/903],[0/903/0]).S-NcurveshavebeenobtainedthehighlightinginfluenceofAl20nthefirstcrackinitiation,asillustratedinFigurel(a).ThecrackpropagationhasbeenanalyzedandtherelevantParis-IikecurvesshowonceagainastronginfluenceofthebiaxialityratioFigurel(a).SEManalysisonthefracturesurfacesrevealsthatthedamagemodesaresignificantlydependentonAl2.Theseresults,obtainedfromtheapplicationofexternalmultidirectionalloads,areeventuallycomparedtothoseon[0/602/0/-602]Slaminatesmadeofthesameprepregandtestedunderuniaxialfatigueloading(inthiscasethemultiaxialityofthestressstateisoriginatedbythematerialorthotropicbehaviour).Thecomparisonindicatestheequivalence,intermsofcrackinitiationandpropagationofthetwoloadingconditions,whenthelocalbiaxialityratioisthesame.Thisequivalenceisofextremeimportanceinviewofimprovingtheknowledgeaboutdamageevolutionundermultiaxialloadingconditions:easy-to-doaxialtestscaninfactbeusedtosupportthemoreexpensiveandcomplicatemultiaxialfatiguetesting.Withreferencetothepredictivecriteria,itisrathereasytoobservethatthemajorityofstaticandfatiguefailuremodelsforunidirectional(UD)compositesarebasedonmacroquantitiesasthenominalstressesinthematerialcoordinatesystem,whichareofteninvolvedinapolynomialexpressiondescribingthefailurecondition.Tsai-HillandTsai-Wuareclassicalexamplesforstaticbehavior.Similarapproacheshavebeenproposedforfatigue,butallthesecriteriadonotaccountforthedifferentdamagemodesactingatamicroscopiclevel,whicharethereasonforthedegradationprocessbringingacompositelaminatothefatiguefailure.Sincethefirstmechanismtakingplaceduringthefatiguelifeofamultidirectionallaminateisthenucleationofintralaminarcracks,theunderstandingandthedescriptionofthenon-fiber-dominatedbehaviourofUDlaminaeareoffundamentalimportance.AnattemptinthisdirectionhasbeenproposedbyPlumtreeandCheng,consideringthelocalstressesactinginthematrix.Theyproposedacriterioninvolvingthelocalstressesactingonthefractureplane,definedastheplanenormaltothefiberdirection.Thisisindeedthefinal(ormacroscopic)separationplane,whilethelocal(ormicroscopic)fractureplaneshouldbeconsideredsincethedegradationprocessoccursatamicroscopiclevel,asdescribedinFigure2.Hence,anewcriterionforthecrackinitiationhasbeendevelopedatDTG,basedontheconsiderationthatthelocalfractureplaneisindeedthatnormaltotheMaximumPrincipalStressatlocal(microscopic)scale(LMPS)cs].Asaconsequence,theLMPSistheparametertobeusedasequivalentstressforthedefinitionoftheS-Ncurvestocrackinitiationanditisprovedtosummariseverywellthefatiguedataforthedifferentmultiaxialityconditionsobtainedduringtheexperimentalcampaign.Thenewapproachdoesnotworkwellonlyforfatiguedatanearthepuretransversetensionconditionsbutthisisnotsurprising,sinceAspetal.[6]alreadyindicatedthat,inthecaseofpuretransversetension,thelocalstressstateinthematrixishighlyhydrostatical,suchthatthedilatationalenergydensityisasuitableparameterforthepredictionofthestatictransversestrength.IntheseconditionsthefirstinvariantofthestresstensorIimaybeusedasequivalentparameterfortheinitiationfatiguecurves,resultingonceagainingoodagreementwithexperimentaldata.
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上傳時間: 2013-06-21 14:30:34
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