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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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