Simulating the Impact Behaviour of Composite.docx
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Simulating the Impact Behaviour of Composite.docx
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SimulatingtheImpactBehaviourofComposite
SimulatingtheImpactBehaviourofComposite
AircraftStructures
ErkanKirtil,DieterPestal,AlexanderKollofrath,NilsG‰nsicke,JosefMendler
EADSMilitaryAircraft
81663Munich-Germany
Abstract:
Lightweightcompositestructuresarewidelyusedinaircraftindustries.Withincertificationprocedurestheimpactabsorptioncapabilityofaffectedstructuralpartshavetobeprovedusuallyviatimeandcostconsumingcertificationtests.
Althoughauthoritiesdonotacceptameretheoreticalapproachforimpact-proof,simulationtechniquesofdifferentimpactscenariosgainincreasingsignificance,especiallywithindevelopmentphasesofaircraftstructures.
Ifthosetheoreticalapproachesarefinallyvalidatedbysufficienttestresults,thesimulationtechniquescanalsobeappliedonsimilarstructuresinordertocertifythestructureunderimpactresistantconsiderations.
UnderthisbackgroundbirdimpactanalysesonaircraftCFRPleadingedgesareillustratedusing
ABAQUS/Explicit.Phenomenologicalmicro-mechanicswillbediscussedinordertofocusonhigh-velocityimpactbehaviourofbrittlestructures.Inaddition,theapplicationoftheuser-definedsubroutineVUMATwithinABAQUS/Explicitwillbedemonstrated.
Finally,thereportdealswiththedevelopmentofaspecialimpactresistantcompositedesignfortypicalaircraftleadingedges,combiningtheflexibilityofaramidfabricwiththestiffnessof
CFRPshellstructures.
1.Introduction
Compositematerialsgainincreasingsignificanceintheirapplicationwithinaircraftstructures.Regardingtheimpactcausedbyforeignobjects(e.g.bird,tiredebris,enginefragments,etc.)nocatastrophicflightsituationafterthedamagemayoccur.
Itisessentialduringadevelopmentphaseofaircraftstructurestosimulatethe(highandlowenergy)impacteventsusingtheoreticalapproachesinordertoidentifycriticalimpactscenariosandtoinitiateadequatestructuralimprovements.
Thetheoreticalapproacheshavetobeverifiedbycertificationtestsandareoftensubstantiatedin
anearlyprojectstatusviadevelopmenttests.Thisprocedurehelpsalsotoreducethenumberofnecessaryteststothemostcriticaleventswhicharefiguredoutbymereimpactanalyses.
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Inaddition,thoseactivitiesrequiredetailedknowledgeofthefailuremechanismoftheaffected
structurebutalsotheengineeringcapabilitytosimplifythecompleximpactresponsebehaviourofcompositeswithintheoreticalapproaches.
ThisreportdealswiththeanalysesofabirdimpactonaverticalstabilizerCFRPleadingedge.
TheFE-codeABAQUS/Explicitisusedincombinationwiththeuser-definedsubroutineVUMAT
inordertoperformsingleplyanalysisduringshorttermimpactsimulations.Basedonclassicallaminatetheory(CLT)prerequisiteslinearshellelementsareusedtomodelmonolithicdesign,butalsotoinvestigateaspeciallydevelopedimpactresistantdesigncombiningCFRPwithfabric
aramid.Thereportillustratesalsotheusageandeffectofavailablecontactalgorithmswithin
ABAQUS/Explicitimpactanalysesandprovidesrecommendationsanddiscussionsofspecialparameterswhichdeterminesignificantlytheimpactresponseofcompositestructures.
2.Modellingtheimpactbehaviourofcomposites
2.1MaterialModels
Therearedifferentpossibilitiestodefinecompositematerialsinthefinite-elementprogram
ABAQUS/Explicit.
Onepossibilityistodefinealinearelasticorthotropicmaterialinplanestresswiththedefinition
oftheLAMINAñparameterintheELASTICñoption.Elements,associatedwiththeLAMINAñparametercannotfailinasimulation.Inthiscasefailureofelementswillbejustindicatedbyafailureindex.Thismeans,thatelementsarestillabletotransferloadswithinsubsequenttimesteps.
Inordertoletelementsfail,anotherpossibilityistodefinealinearelasticandisotropicmaterialbehaviourwiththeISOñparameterintheELASTICñoption.Elements,associatedwiththeISOñparametercanfailinasimulation.Thismeans,thatelementsarenotabletotransferloadswithinsubsequenttimesteps.
ThethirdpossibilityisthedefinitionofauserñdefinedmaterialbehaviourinthesubroutineVUMAT,whichallowstouseuserñdefinedmateriallawsandfailurecriteria.Elements,associatedwiththeusersubroutinecanfailinasimulation.
2.1.1OrthotropicMaterial(LAMINA)
Thisapproachcanbeusedwhenthecompositestructurecanbeconsideredasanlinearelasticandorthotropicmaterialinplanestress.Thedefinitionofanorthotropicmaterialallowstheinputof
theelasticmodulusin1ñand2ñdirection,whereasthe1ñdirectionisassumedtoalignwiththefiberdirectionofthematerial.Furthermorethepoisson`sratioandshearmodulushavetobe
defined.Inaddition,theactuallayñupdefinitionofthecompositestructurecanbedefinedinthe
SHELLSECTIONñoptionusingtheCOMPOSITEñparameter.Therefore,eachsingleplyisdescribedbyacorrelatinglaminadataset.ThedensityvalueofonelayerhastobeaddedinthematerialblockusingtheDENSITYñoption.
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2.1.2IsotropicMaterial(ISO)
Thisapproachcanbeusedwhenthecompositestructurecanbeconsideredasalinearelastic,homogeneousandquasi-isotropicmaterial.Thedefinitionofanisotropicmaterialallowstheinput
ofjustoneelasticmodulusandonepoisson`sratio.Inthiscasetheresultantmaterialdataofacompositestructure,whichdependontheelasticdataandthelayñupofthestructure,are
calculatedwiththeCLTandanaverageelasticmodulusofthewholelaminatehastobedefined.Thedensityvaluecorrespondstotheresultantdensityoftheentirecompositestructure.
2.2DefaultFailureCriteriainABAQUS/Explicit
2.2.1LAMINAñFAILSTRESS/FAILSTRAIN
Elements,associatedtotheLAMINAñoptioncannotfailduringasimulation.OnepossibilityistoindicatefailurewiththeFAILSTRESSñandFAILSTRAINñoption,whichcanbeaddedtothematerialblock.TheFAILSTRESSñoptionprovidesfourstressñbasedfailurecriteria.Forthesefailuretheoriesstressñbasedtensile,compressiveandshearlimitshavetobedefined.Thefailurecriteriaprovidedare:
∙Maximumstresstheory
∙TsaiñHilltheory
∙TsaiñWutheory
∙AzziñTsaiñHilltheory
TheFAILSTRAINñoptionprovidesonestrainñbasedfailurecriterion.Forthisfailuretheorystrainñbasedtensile,compressiveandshearlimitshavetobedefined.Thefailurecriteriais:
∙Maximumstraintheory
2.2.2ShearFailure
ForanisotropicmaterialelementfailureiscontrolledbytheSHEARFAILUREñoption,whichdescribestheequivalentplasticstrainatfailure.ThisoptionneedsinadditionthePLASTICñoption,whichdefinestheyieldstress.Duetothefact,thatcompositematerials,suchasCFRPorGFRP,donothavesignificantplasticdeformation,theallowedequivalentplasticstrainatfailure
ofthestructurehastobekeptlow.
2.3VUMATApproach
ThisapproachcanbeusedtodefineauserñdefinedmaterialbehaviourwithintheusersubroutineVUMAT.UnliketotheusageofthepredefinedLAMINAñoption,theusageofthissubroutinehastheadvantageofbeingabletodefineanorthotropicmaterialbehaviourincludingelementfailure.
IfincludingtheVUMATintoananalysisbytheUSERMATERIALñoption,theentirematerialbehaviourandfailurecriteriahavetobedefinedwithinthesubroutineitself.Therefore,theCLTforplanestresswasimplementedinthesubroutineVUMAT,whichisvalidforlinearelasticand
2003ABAQUSUsersíConference3
orthotropiccompositestructures.Inaddition,theactuallayñupdefinitionofthecomposite
structurecanbedefinedintheSHELLSECTIONñoptionusingtheCOMPOSITEñparameter.Therefore,allmaterialvaluesinthematerialblockdefinedbytheUSERMATERIALñoption
refertoasingleplyanalysisbasedonlaminainputdata.Inaddition,theDEPVARñoptionhasto
bedefinedinthematerialblockinordertoincludeasolutionñdependantstatevariable.The
DELETEñparameterofthisoptiondefinesthestatevariable,whichcontrolselementfailureduring
asimulation.TheTsaiñWutheoryandamodifiedMaximumñStresstheoryaretheappliedfailuretheories,whichcontrolthevalueofthestatevariable.Thesetheoriesarealsoimplementedwithinthesubroutine.WhenusingthesubroutineVUMAT,thefiniteelementprogram
ABAQUS/Explicitisnotabletocalculatethetransverseshearstiffnessofthematerial.Therefore,
ithastobedefinedbyusingtheTRANSVERSESHEARSTIFFNESSñoption,whichhastobeaddedtotheSHELLSECTIONñoption.AnexampleofapossibleABAQUS/Explicitñentryisgivenbelow.
*SHELLSECTION,ELSET=STRUCTURE,COMPOSITE,ORIENTATION=OID1
0.625,3,CFRP,0.
0.625,3,CFRP,90.
0.625,3,CFRP,0.
*TRANSVERSESHEARSTIFFNESS
10000.,10000.,0.
*MATERIAL,NAME=CFRP
*DENSITY
1.8E-9,
*USERMATERIAL,CONSTANTS=9
136000.,3600.,2100.,0.3,1360.,110.,800.,80.,
45.
*DEPVAR,DELETE=1
1
3.ContactAlgorithm
Inimpactanalysestwodifferentcontactalgorithmscanbeused.Thefollowingchapterspointoutthefundamentaldifferencesoftheavailablealgorithms.
3.1KinematicContact
WiththeKinematicalgorithmthecontactpenetrationofaslavenodeintoamastersurfaceiseliminatedattheendofeachtimeincrementi.Afterpredictionofthepenetrationdistancedpredanaccelerationcorrectionisappliedtothenodestoresultinacomplianceoftheslavenodeswiththemastersurface.Thecontactforceiscalculatedbytheproductofmassmtimesacceleration
dpred/∆t×(Figure1).SincethenodescometolieexactlyonthemastersurfacetheKinematiccontactcanbephysicallyinterpretedasalocalplasticcontact.
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