DuctilefractureExperimentsandcomputations

Ductilefracture:Experimentsandcomputations韧性断裂：实验与计算Abstract：Numerouscriteriahavebeendevelopedforductilefracture(DF)predictioninmetalplasticdeformation.FindingawaytoselecttheseDFcriteria(DFCs)andidentifytheirapplicabilityandreliability,however,isanon-trivialissuethatstillneedstobeaddressedingreaterdepth.Inthisstudy,severalcriteriaunderthecategoriesof‘uncoupleddamagecriterion’andthe‘coupleddamagecriterion’,includingthecontinuumdamagemechanics(CDM)-basedLemaitremodelandtheGurson–Tvergaard–Needleman(GTN)model,areinvestigatedtodeterminetheirreliabilityinductilefailureprediction.Tocreatediversestressandstrainstatesandfracturemodes,differentdeformationscenariosaregeneratedusingtensileandcompressiontestsofAl-alloy6061(T6)withdifferentsamplegeometriesanddimensions.Thetwocategoriesofcriteriaarecodedintofiniteelement(FE)modelsbasedontheunconditionalstressintegrationalgorithmintheVUMAT/ABAQUSplatform.Throughphysicalexperiments,computationsandthreeindustrialcasestudies,theentirecorrelationpanoramaoftheDFCs,deformationmodesandDFmechanismsisestablishedandarticulated.Theexperimentalandsimulationresultsshowthefollowing.(1)ThemixedDFmodeexistsineverydeformationofconcerninthisstudy,eveninthetensiletestoftheroundbarsamplewiththesmallestnotchradius.Adecreaseofstresstriaxiality(g-value)leadstoareductionintheaccuracyofDFpredictionbythetwoDFCcategoriesofDFCs,duetotheinterplaybetweentheprincipalstressdominantfractureandtheshear–stressdominantfactor.(2)Fordeformationswithahigherg-value,bothcategoriesofDFCspredictthefracturelocationreasonablywell.Forthosewithalowerorevennegativeg-value,theGTNandCDM-basedcriteriaandsomeoftheuncoupledcriteria,includingtheC&L,AyadaandOyanemodels,providerelativelybetterpredictions.OnlytheTrescaandFreudenthalmodelscanproperlypredictthesheardominantfracture.ThereliabilitysequenceoffracturemomentpredictionisthustheGTNmodel,followedbytheCDM-basedmodelandtheuncoupledmodels.(3)TheapplicabilityoftheDFCsdependsontheuseofsuitabledamageevolutionrules(voidnucleation/growth/coalescenceandshearband)andconsiderationofseveralinfluentialfactors,includingpressurestress,stresstriaxiality,theLodeparameter,andtheequivalentplasticstrainorshearstress.Theseparametersdeterminethedeformationmode(sheardominantormaximumprincipalstressdominantdeformation)and,further,theDFmechanism(dimplefracture/shearfracture/mixedfracture).众多的标准已经发展为韧性断裂（DF）金属塑性变形的预测。寻找一种方法来选择这些DF标准（DFCS）并确定其适用性和可靠性，然而，是一个不平凡的问题，还需要更深入地加以解决。在这项研究中，“非耦合损伤准则”和“耦合损伤准则”类别下的几个标准，包括连续损伤力学（CDM）基于Lemaitre模型和––GursonTvergaard内德勒曼（GTN）模型，研究确定延性破坏预测的可靠性。创建不同的应力应变状态和断裂模式，不同的变形情况使用6061铝合金的拉伸和压缩试验（T6）产生不同的样品几何形状和尺寸。两类标准编码为有限元（FE）基于无条件的应力积分算法在VUMAT/ABAQUS平台模型。通过物理实验、计算和三个产业的案例研究，对控制整个相关的全景，变形模式和DF机制的建立和表达。实验和仿真结果表明以下。（1）混合DF模式中存在的研究中关注每一个变形，甚至在与缺口半径最小的圆棒试样的拉伸试验。减少应力三维度（G值）导致的用两DFC类DF的预测精度降低，由于主应力和剪应力断裂为主–主导因素之间的相互作用。（2）具有较高的G值变形，细胞这两类预测骨折的位置相当好。对于那些具有较低甚至为负值，GTN和CDM的基础标准和一些非耦合的标准，包括C和L，Ayada和Oyane模型，提供相对较好的预测。只有Tresca和弗罗伊登塔尔模型可以正确地预测剪切断裂为主。因此，断裂的时刻预测的可靠性序列GTN模型，其次是基于CDM模型和耦合模型。（3）控制的适用性取决于使用合适的损伤演化规则（孔洞形核/生长/合并和剪切带）和考虑的几个影响因素，包括压应力，应力三维度，Lode参数，和等效塑性应变或剪切应力。这些参数确定的变形模式（剪切主导或最大主应力变形），此外，DF机制（骨折/混合断口韧窝断裂/）。Conclusions：BasedontheanalysisofthedeformationbehaviorandDFmechanismsoftheAl-alloyundervariousstressandstrainstates,thisstudyhasfurtheredourunderstandingoftheapplicabilityandreliabilityofbothcategoriesofDFCsforDFpredictioninthePDprocess.Aseriesofmacro/meso-scaledplasticdeformationcases,includingtensileandupsettingtestswithdifferentsamplegeometriesandshapesandindustrialcasestudies,havebeencarriedouttogeneratediversefracturemodesunderdiversestressandstrainstatesandunevendeformationconditions,coveringawiderangeofgandf.ByemployingAlalloy6061(T6)andcomparingtheresultsofcomputationswithbuilt-inDFCswiththoseofexperiments,thecorrelationbetweenDFCsandmacroscopicDFphenomenahasbeenestablished.Thefollowingconclusionscanbedrawn:(1)ThemixedDFmodeexistsforallofthedeformationsconsideredinthisstudy,eventhatintheroundbartensiletestwiththesmallestnotchradius.Thegdeterminesthedeformationmode,andthusinducesvariousDFmodes,includingmaximumprincipalstressdominantDF,mixedDFandshearDF,whentheg-valuegoesfrompositivetonegative(thereisacutoffvalueofnegative-1/3).Whentheg-valueexceeds1/3,ductilitybecomesmuchmoresensitivetothevoidgrowtheffect.Whentheg-valueislessthanzero,shearDFoccurswithcompressiondeformation.Whengisintherangeof[0,1/3],bothshear-mechanizedDFandvoidgrowth-basedDFcoexistandcompetewitheachother.Thefracturestraininthesheardeformationandupsettingtestsinthisresearchwaslargerthanthatinthetensiletestswithroundbarsamples.(2)Adecreaseinstresstriaxiality(g-value)reducestheaccuracyofDFpredictionforbothcategoriesofDFCsduetotheinterplaybetweenprincipalstressdominantandshear-stressdominantfracture.Fordeformationwithahighergvalue,bothcategoriesareabletopredictthefracturelocationreasonablywell.Fordeformationwithalowerorevennegativeg-value,theGTNandCDM-basedcriteria,aswellassomeoftheuncoupledcriteria,includingtheC&L,AyadaandOyanemodels,providerelativelyreasonablepredictions.OnlytheTrescaandFreudenthalmodelsc