ASURFACEDEFECTPREDICTIONMETHODFORMETALFORMIN GPROBLEM¥Peng,Yinghong;Ruan,Xueyu(ShanghaiR esearchInstituteofTool&DieTechnology,Shangh aiJiaoTongUniversity,shanghai200030)Zuo,Tie yong(DepartmentofScience&Technology,StateEd ucationCommission,Beijing100816)Abstract:On thebasisoftherigidviscoplasticFEMtheories,a surfacedefectpredictionforplasticformingpro cessbyFEMnumericalsimulationhasbeendeeplyst udied.Sometechnicaltreatmentsandalgorithmso fdefectpredictionareproposed.Usingthismetho d,theprocessesofdefectinitiationanddevelopm entduringCONFORMprocessarepredictedsuccessf ully,andsomecriticaltechnologicalparameters areobtained.Therefore,itisalsoanextensionfo rapplicationfieldsofrigidviscoplasticFEMtop redictformingdefect.Keywords:continuousextr usionformingsurfacedefectpredictionmethodri gidviscoplasticFEM1INTRODUCTIONItisanimport antsubjectinindustryproductiontobuildaquali tyanalysisandprotectionsystem.Howtopredictt heinitationanddevelopmentofdefectsisakeysto neofthesubject.Meanwhile,itisalsoanimportan tbasistoavoiddefectappearance.Thereareinter iorandsurfacedefectsinplasticformingproblem s.Manyfactorscouldinitiatethesedefects,fore xample,theinteriorstructuresofmaterials,def ormingways,speedandtemperatureandstructures ofdiesetcinfluenceontheappearanceofdefects. Inthispaper,westudythepredictionmethodofsur facedefectsinitiatedbyunreasonablediestruct ureandgettingwayofcriticalstructureparamete rofdies.TherigidviscoplasticFEMisaneffectiv etooltodealwiththelargedeformationproblem,a ndithasbeenwidelyusedinsimulatingmanyplasti cdeformationtechnologies[1-3].Metalformingp rocesswithdieisadistinguishingprocessbetwee nthedeformingbodyandtheboundaryofdie,whichi salsoanatureofFEMnumericalsimulationtechniq ues.Therefore,theautodistinguishingtechniqu esbetweendeformingbodyandboundaryofdieandth edefectpredictionalgorithmsareproposedinthi sstudy.Usingdefectpredictionmethod,thedefec tinitiationanddevelopmentduringtheCONFORMpr ocesshavebeensimulatedsuccessfully,andthecr iticaltecknologicalparametersunderwhichthed efectscouldnotappearhavebeenpresented.Thisw orkprovidesanimportanttheoreticalbasisforCA D/CAMofdie&moldandtechnologyanalysis.Itisan extensionofapplicationfieldsforrigidviscopl asticFEMtopredictdefects,andalsoasupplement forCONFORMtechnologycriterion.2BASICTHEORIE SOFRIGIDVISCOPLASTICFEMTherigidviscoplastic formulationhasbeenextensivelyusedfortheanal ysismaterialresponseduringmetalformingproce sses.Thecompleteformulationcanbefound[2].Th ebasicequationsthatReceivedMar.18,1994;acce ptedinrevisedformMay9,1994havetobesatisfied duringaformingprocessare:(1)Equilibriumequa tion:(2)Strainratedefinition:(3)Constitutiv erelation:(4)Incompressibilitycondition:(5) Boundaryconditions:wtereσij,εijandviarethes tress,strainrateandvelocity,respectively.Vi sthevolumeofthedeformingbody,andSf,Svarefor cesurfaceandvelocitysurface,respectively.Th efieldequationsgivenabovecanbesolvedbyavari ationalprinciplecxpressedas:whereKisalargep ositiveconstanttopenalizevolumechange.Theva riationalfunctionalcanbeconvertedtonon-line aralgebraicequationsbyutilizingtheFEMdiscre tizationprocedure.Thesolutionofnon-linearsi multaneousequationscanbeobtainedbyNewton-Ra phsonmethod.3SURFACEDEFECTPREDICTIONTECHNIQ UESItisthenatureofmetalformingprocessthatme talsflowwiththerestrictionofdiesoastosatisf ythephysicandgeometricdemandsofproducts.The refore,thenatureofunsteadyplasticformingpro cessisdistinguishingprocessbetweendeforming bodyandboundaryofdie.Theinitiationanddevelo pmentofsurfacedefectsaccompanywiththisdisti nguishingprocess.Inaway,itisanimportantbasi sforsimulatingunsteadyplasticformingproblem stobuildanaccuratedistinguishingsystem.Mean while,itisalsoakeystonefordefectprediction. Thedistinguishingtechniquescontains:(Ⅰ)math ematicsdescriptionofdieconfiguration;(Ⅱ)con tactjudgmentofboundarynodes;(Ⅲ)separatingju dgmentofcontactnodefromdie;(Ⅳ)correctionofc ontactnode.Thecomputerdescriptionofdieconfi gurationshouldsatisfy(Ⅰ)therestrictioncharacteristicsofdieondeformingbodycanbecontainedcompletely;(Ⅱ)thecomplicatedmathematicsexpressionshouldbeavoided;(Ⅲ)thegeneralization
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