vct(虛擬裂紋擴展分析技術)

1、Vcct(Virtualcrackclosuretechnique)<Skin-stiffenerdebondpredictionbasedoncomputationalfractureanalysis>Interlaminarfracturemechanicscharacterizestheonsetofdelaminationsincomposites.Shearloadingcausesthepaneltobuckleandtheresultingout-of-planedeformationsinitiateskin/stringerseparationatthelocat

2、ionofanembeddeddefect.METHOD:FiniteElementAnalysisELEMENT:thepanelandsurroundingloadfixtureweremodeledwithshellelements.Asmallsectionofthestringerfootandthepanelinthevicinityoftheembeddeddefectweremodeledwithalocal3Dsoilmodel.1.BACKGROUNDAerospacesstructuresaremadeofflatorcuredpanelswithco-curedorad

3、hesivelybondedframesandstiffenerscomputationalstressanalysistodeterminethelocationoffirstmatrixcracking.Anartificialdefectwasplacedattheterminationofthecenterstiffener.Thestiffenedpanelissubjectedtopureshearloadingwhichcausesthepaneltobuckle.ANALYSIS:nolinearfiniteelementanalysisStrainenergyreleaser

4、atesandmixedmaderatioswerecomputedusingthevirtualcrackclosuretechnique.2.METHODOLOGY2.1InterlaminarfracturemechanicsThetotalstrainenergyreleaserateGTThemodeIcomponentduetointerlaminartensionGIThemodeIIcomponentduetointerlaminarslidingshearGIIThemodeIIIcomponentduetointerlaminarscissoringshearGIIIPXI

5、nterlaminartensionModelPXInterlaminartensionModelQ*IriterlaminarslidingshearModeIIFigure2.FractitreModes.InterlaminfirscissoringshearModeIIPurpose:topredictdelaminationonsetorgrowthfortwo-dimensionalproblems,thesescalculatedGcomponentsarecomparedtointerlaminarfracturetoughnesspropertiesmeasuredovera

6、rangeofmodemixitiesfrompuremodeIloadingtopuremodeIIloading.Aquasistaticmixed-modefracturecriterionisdeterminedbyplottingtheinterlaminarfracturetoughness,Gc,versusthemixed-moderatio,gii/gt,determinedfromdatageneratedusingpureModeI(Gn/Gt0)DoubleCantileverBending(DCB)pureModeII(Gn/Gt1)fourpointEndNotch

7、edFlexure(4ENF),andMixedModeBending(MMB)testsofvaryingratioforIM7/8852carbonepoxymaterial.GCGICGIICGICIIGtGicandGiicarethefracturetoughnessdataformodeIandIIisafactordeterminedbythecurefit.Showninfigure3inthisarticle.GII/Gt,Failureisexpectedwhen,foragivenmixedmoderatiothecalculatedtotalenergyreleaser

8、ate,Gt,exceedstheinterlaminarfracturetoughness,Gc.VCCTrequiresforceanddisplacementinput,whichisobtainedfromcontinuum(2-D)andsolid(3-D)finiteelementanalysesofthecracked(2-D)ordelaminated(3-D)component.GiandGiiarecalculatedforfour-nodedelementsGIZiwlwl*2aGiiXiuiuaaisthelengthoftheelementsatthecrackfro

9、nt;XiandZiaretheforcesatthecracktip(nodalpointi);Therelativedisplacementsbehindthecracktiparecalculatedfromthenodaldisplaces(a):VirtualCrackClosureTechnique(VCCT)tbi!bta-nodedelement.Forgeometricnonlinearanalysiswherelargedeformationsmayoccur,bothforcesanddisplacementsobtainedintheglobalcoordinatesy

10、stemneedtobetransformedintoalocalcoordinatesystem(x,z)whichoriginatesatthecracktip.Forthetwo-dimensionaleight-nodedquadrilateralelementwithquadraticshapefunctionsthisyieldsG|wlwl*GiiUlul*ThetotalenergyreleaserateGtiscalculatedfromtheindividualmodecomponentsaslocalcracktipsystemdeformedstateglobalsys

11、temX.Y.Z:forcesu,v,w：displacementsG,=-Vr/(2Aa)On=-X(-u'i*)/()undeformedstateAa(outline)(h):CrackCiosnreTechnique(VCCT)fbigeoiuetncullyHonlineurdiidlysis.Figure4:CtackClosureTechniquefortwa-diniensionaianalysisInafiniteelementmodelmadeofthree-dimensionalsolidelementsthedelaminationoflengthaisrepr

12、esentedasatwo-dimensionaldiscontinuitybytwosurfaces.(whydoesitisdiscontinuity?)ThemodelI,modeII,andmodeIIIcomponentsofthestrainenergyreleaserateGi,Gn,andGmarecalculatedasG|Gngiii2AZLiwLlwli*2AXLiuLluLl*2AYLiVLlVLl*Aab.HereAistheareavirtuallycloses,aisthebisthebisthelengthoftheelementsatthedelaminati

13、onfront,andwidthoftheelements.localcracktipiz'1w'1Z,systemA辺r/eir辺r/eirdelaminationfrontintactareaglobalsystemdelaminatedareaa(b).Topi?tnr&fuppersurface怡$areomittedfordavity)Figure5.VirtualCrackClosureTechnique/oreightnodedelemeiirs.Alocalcracktipcoordinatesystemisneeded.2.2.2.AGlobal/Lo

14、calShell3DModelingTechniqueComputedmixedmodestrainenergyreleaseratecomponentsdependonmanyvariablessuchaselementorderandsheardeformationassumptions,kinematicconstraintsintheneighborhoodofthedelaminationfront,andcontinuityofmaterialpropertiesandsectionstiffnessinthevicinityofthedebondwhendelaminations

15、ordebondsaremodeledwithplateorshellfiniteelements.3. FINITEELEMENTMODELING3.1.GlobalShellModelofStringerStiffenedPanelTheglobalmodelincludesthesteelloadframeandattachments,thepanelmadeofgraphite/epoxyprepregtapeandthestringersmadeofgraphite/epoxyfabric.Theoutersteelloadframeandtheattachmentboltswere

16、modeledwithbeamelementsavailableintheelementsoftwareABAQUST.heinnersteelloadframewhichoverlapsthepaneledgewasmodeledwithstandardshellS4elements.Theshellelementsateconnectedbybeamelementsdesignedtoenforceplatetheoryconstraints.Inthesectionscontainingtheartificialdefectsthebeamelementswerereplacedbyga

17、pelements.Inpreparationfortheglobal/localmodelingapproachshellelementsrepresentingthefootofthestiffenerandthepanelwereremovedfromtheoriginalshellmodelaroundthecenterstringerterminationasshowninFigure9.Theshellelementsusedtomodelthestiffenerwebandhatwerekeptinplace.AttheboundariesshelledgesinABAQUASw

18、eredefinedasshownwhichwereusedtoconnecttheshellmodelwiththelocal3DinsertmodelusingtheshelltosolidcouplingoptioninABAQUSwhichallowstheconnectionbetweennon-conformingshellandsolidmodels.3.2Local3DInsertModelforSolidModelingofStringerFootandPanelSkinThelocal3DinsertmodelwasgeneratedusingC3D8Isolidbrick

19、elementsandconsistedofanintactsectionandadelaminatedsectionwithafinemesharoundthedelaminationfront.SurfacesweredefinedontheouterfacesoftheinsertmodeltoprovideaconnectionwiththeglobalshellmodelusingtheshelltosolidcouplingoptioninABAQUS.Theinitialdefectislocatedatthebondlinebetweenstringerfootandthepa

20、nel.Thisdefectwastreatedasadelaminationandmodeledasadiscretediscontinuityusingtwounconnectednodeswithidenticalcoordinatesoneoneachsideofthedelamination.Arefinedmeshwasusedalongthestringerboundaryinordertocaptureedgeeffects.UsingthefiniteslidingoptionavailableinABAQUScontactwasmodeledbetweenthedelami

21、nationsurfacestoavoidinterpenetrationduringanalysis.3.3.CombinedGlobal/LocalShell/3DModelofStringerStiffnedPanelUniformdisplacementsu,vwereappliedatonecornernodetointroduceshearasshowninFigure11a.Theinplanedisplacementsu,vweresuppressedatthediagonallyoppositecornerandtheoutofplanedisplacementwweresu

22、ppressedalongallfouredgesacrosstheentirewidthoftheinnerandoutersteelloadframe.Theglobalshellmodelwasconnectedtothelocal3DinsertmodelusingtheshelltosolidcouplingoptioninABAQUSwhichallowstheconnectionbetweennon-conformingshellandsolidmodels.Fortheentireanalysesthenon-linearsolutionoptionwasusedinABAQU

23、SA.totalofeightdelaminationlengthsweremodeled.Additionallengthswerechosentostudythechangeinenergyreleaseratedistributionacrossthewidth(b)ofthestringerwithincreasingdelaminationlength(a).4. ANALYSISRESULTS4.1DeformedPanelThelongercausedachangeinthestiffnesswhichresultedinanalteredbucklingpattern.Earl

24、yintheanalysis(increment5)amodeIopeningwasobservedonlynearoneedge.Withincreasingappliedexternaldisplacementthedeformationchangedlocallyandforincrement15modeIdisappearedcompletelyandthedelaminationappearedclosedovertheentiredelaminatedlength.Asmallscissoringshear(modeIII)couldbeobserved.Furtherincrea

25、singtheexternaldisplacementresultedinasmallmodeIopeningacrosstheentirewidthofthestringerasobservedforincrement20.Forthelaststepoftheanalysis(increment41)aftertheentireexternaldisplacementu=v=6.35mmhadbeenappliedmodeIopeningwasobservedacrosstheentirewidthofthestringerovertheentiredelaminatedlength.Th

26、efigurerevealsthatnottheentiredelaminatedsectionopensundermodeI.Afterinitialopening,thesectionbelowthewebterminationclosesandthedelaminatedsurfacescontact.Thisclosingiscausedbyachangeinthelocalbucklingpattern,duetostiffnesschangescausedbythelongerdelamination,asdiscussedabove.Itwasobservedthatthelocalbucklingpatternintheimmediatesurroundingofthedelaminatedstringerisdependentonthedelaminationlengthmodeled,whichmadeconvergencedifficult.4.2CalculationofMixed-ModeStrainEnergyReleaseRatesandFailureIndicesVCCTwasusedtocalculatethemodecontributionsGI，GIIandGIIIthetotalener