Abaqus单元选择、压力、接触和网格生成(abaqus软件公司北京代表处).ppt

ABAQUS/Standard基础教程,ElementSelectionCriteria,Appendix1,ABAQUS/Standard基础教程,内容提要,ElementsinABAQUSStructuralElements(ShellsandBeams)vs.ContinuumElementsModelingBendingUsingContinuumElements用实体单元模拟弯曲StressConcentrations应力集中Contact接触IncompressibleMaterials不可压缩材料MeshGeneration网格生成SolidElementSelectionSummary,ABAQUS/Standard基础教程,ElementsinABAQUS,ABAQUS/Standard基础教程,ElementsinABAQUS,ABAQUS单元库中提供广泛的单元类型，适应不同的结构和几何特征ThewiderangeofelementsintheABAQUSelementlibraryprovidesflexibilityinmodelingdifferentgeometriesandstructures.Eachelementcanbecharacterizedbyconsideringthefollowing:单元特性：Family单元类型Numberofnodes节点数Degreesoffreedom自由度数Formulation公式Integration积分,ABAQUS/Standard基础教程,单元类型（Family）Afamilyoffiniteelementsisthebroadestcategoryusedtoclassifyelements.同类型单元有很多相同的基本特。Elementsinthesamefamilysharemanybasicfeatures.同种类单元又有很多变化：Therearemanyvariationswithinafamily.,ElementsinABAQUS,special-purposeelementslikesprings,dashpots,andmasses,continuum(solidelements),shellelements,beamelements,rigidelements,membraneelements,trusselements,infiniteelements,ABAQUS/Standard基础教程,ElementsinABAQUS,Numberofnodes节点数(interpolation)Anelementsnumberofnodesdetermineshowthenodaldegreesoffreedomwillbeinterpolatedoverthedomainoftheelement.ABAQUSincludeselementswithbothfirst-andsecond-orderinterpolation.插值函数阶数可以为一次或者两次,ABAQUS/Standard基础教程,ElementsinABAQUS,自由度数目DegreesoffreedomTheprimaryvariablesthatexistatthenodesofanelementarethedegreesoffreedominthefiniteelementanalysis.Examplesofdegreesoffreedomare:Displacements位移Rotations转角Temperature温度Electricalpotential电势,ABAQUS/Standard基础教程,公式FormulationThemathematicalformulationusedtodescribethebehaviorofanelementisanotherbroadcategorythatisusedtoclassifyelements.Examplesofdifferentelementformulations:Planestrain平面应变Planestress平面应力Hybridelements杂交单元Incompatible-modeelements非协调元Small-strainshells小应变壳元Finite-strainshells有限应变壳元Thickshells后壳Thinshells薄壳,ElementsinABAQUS,ABAQUS/Standard基础教程,积分Integration单元的刚度和质量在单元内的采样点进行数值计算，这些采样点叫做“积分点”Thestiffnessandmassofanelementarecalculatednumericallyatsamplingpointscalled“integrationpoints”withintheelement.数值积分的算法影响单元的行为Thenumericalalgorithmusedtointegratethesevariablesinfluenceshowanelementbehaves.ABAQUS包括完全积分和减缩积分。ABAQUSincludeselementswithboth“full”and“reduced”integration.,ElementsinABAQUS,ABAQUS/Standard基础教程,Fullintegration:完全积分Theminimumintegrationorderrequiredforexactintegrationofthestrainenergyforanundistortedelementwithlinearmaterialproperties.Reducedintegration:简缩积分Theintegrationrulethatisoneorderlessthanthefullintegrationrule.,ElementsinABAQUS,ABAQUS/Standard基础教程,ElementsinABAQUS,Elementnamingconventions:examples单元命名约定,B21:Beam,2-D,1st-orderinterpolation,CAX8R:Continuum,AXisymmetric,8-node,Reducedintegration,DC3D4:Diffusion(heattransfer),Continuum,3-D,4-node,S8RT:Shell,8-node,Reducedintegration,Temperature,CPE8PH:Continuum,Planestrain,8-node,Porepressure,Hybrid,DC1D2E:Diffusion(heattransfer),Continuum,1-D,2-node,Electrical,ABAQUS/Standard基础教程,ElementsinABAQUS,ABAQUS/Standard和ABAQUS/Explicit单元库的对比Bothprogramshaveessentiallythesameelementfamilies:continuum,shell,beam,etc.ABAQUS/Standardincludeselementsformanyanalysistypesinadditiontostressanalysis:热传导,固化soilsconsolidation,声场acoustics,etc.AcousticelementsarealsoavailableinABAQUS/Explicit.ABAQUS/Standardincludesmanymorevariationswithineachelementfamily.ABAQUS/Explicit包括的单元绝大多数都为一次单元。例外:二次单元和四面体单元and二次beamelementsManyofthesamegeneralelementselectionguidelinesapplytobothprograms.,ABAQUS/Standard基础教程,StructuralElements(ShellsandBeams)vs.ContinuumElements,ABAQUS/Standard基础教程,StructuralElements(ShellsandBeams)vs.ContinuumElements,实体单元建立有限元模型通常规模较大，尤其对于三维实体单元如果选用适当的结构单元(shellsandbeams)会得到一个更经济的解决方案模拟相同的问题，用结构体单元通常需要的单元数量比实体单元少很多要由结构体单元得到合理的结果需要满足一定要求：theshellthicknessorthebeamcross-sectiondimensionsshouldbelessthan1/10ofatypicalglobalstructuraldimension,suchas:ThedistancebetweensupportsorpointloadsThedistancebetweengrosschangesincrosssectionThewavelengthofthehighestvibrationmode,ABAQUS/Standard基础教程,ShellelementsShellelementsapproximateathree-dimensionalcontinuumwithasurfacemodel.高效率的模拟面内弯曲Modelbendingandin-planedeformationsefficiently.Ifadetailedanalysisofaregionisneeded,alocalthree-dimensionalcontinuummodelcanbeincludedusingmulti-pointconstraintsorsubmodeling.如果需要三维实体单元模拟细节可以使用子模型,Shellmodelofahemisphericaldomesubjectedtoaprojectileimpact,StructuralElements(ShellsandBeams)vs.ContinuumElements,ABAQUS/Standard基础教程,StructuralElements(ShellsandBeams)vs.ContinuumElements,Beamelements用线简化三维实体。Beamelementsapproximateathree-dimensionalcontinuumwithalinemodel.高效率模拟弯曲，扭转，轴向力。提供很多不同的截面形状截面形状可以通过工程常数定义,linemodel,framedstructuremodeledusingbeamelements,3-Dcontinuum,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,PhysicalcharacteristicsofpurebendingTheassumedbehaviorofthematerialthatfiniteelementsattempttomodelis:纯弯状态：Planecross-sectionsremainplanethroughoutthedeformation.保持平面Theaxialstrainxxvarieslinearlythroughthethickness.Thestraininthethicknessdirectionyyiszeroif=0.Nomembraneshearstrain.Impliesthatlinesparalleltothebeamaxislieonacirculararc.,xx,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,Modelingbendingusingsecond-ordersolidelements(CPE8,C3D20R,)二次单元模拟Second-orderfull-andreduced-integrationsolidelementsmodelbendingaccurately:Theaxialstrainequalsthechangeinlengthoftheinitiallyhorizontallines.Thethicknessstrainiszero.Theshearstrainiszero.,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,Modelingbendingusingfirst-orderfullyintegratedsolidelements(CPS4,CPE4,C3D8)Theseelementsdetectshearstrainsattheintegrationpoints.Nonphysical;presentsolelybecauseoftheelementformulationused.Overlystiffbehaviorresultsfromenergygoingintoshearingtheelementratherthanbendingit(called“shearlocking”).,Donotusetheseelementsinregionsdominatedbybending!,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,Modelingbendingusingfirst-orderreduced-integrationelements(CPE4R,)Theseelementseliminateshearlocking.However,hourglassingisaconcernwhenusingtheseelements.Onlyoneintegrationpointatthecentroid.Asingleelementthroughthethicknessdoesnotdetectstraininbending.Deformationisazero-energymode(有应变形但是没有应变能的现象called“hourglassing”).,Changeinlengthiszero(impliesnostrainisdetectedattheintegrationpoint).,Bendingbehaviorforasinglefirst-orderreduced-integrationelement.,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,Hourglassingisnotaproblemifyouusemultipleelementsatleastfourthroughthethickness.Eachelementcaptureseithercompressiveortensileaxialstrains,butnotboth.Theaxialstrainsaremeasuredcorrectly.Thethicknessandshearstrainsarezero.Cheapandeffectiveelements.,Hourglassingcanpropagateeasilythroughameshoffirst-orderreduced-integrationelements,causingunreliableresults.,Fourelementsthroughthethickness,Nohourglassing,ABAQUS/Standard基础教程,ModelingBendingUsingContinuumElements,DetectingandcontrollinghourglassingHourglassingcanusuallybeseenindeformedshapeplots.Example:Coarseandmediummeshesofasimplysupportedbeamwithacenterpointload.ABAQUShasbuilt-inhourglasscontrolsthatlimittheproblemscausedbyhourglassing.Verifythattheartificialenergyusedtocontrolhourglassingissmall(0.475).RubberMetalsatlargeplasticstrainsConventionalfiniteelementmeshesoftenexhibitoverlystiffbehaviorduetovolumetriclocking,whichismostseverewhenthesematerialsarehighlyconfined.,ABAQUS/Standard基础教程,IncompressibleMaterials,Thecauseofvolumetriclockingisthateachintegrationpointsvolumemustremainalmostconstant,overconstrainingthekinematicallyadmissibledisplacementfield.Forexample,inarefinedthree-dimensionalmeshof8-nodehexahedra,thereisonaverage1nodewith3degreesoffreedomperelement.每个单元平均只有1个有三个自由度的节点Thevolumeateachintegrationpointmustremainfixed.Fullyintegratedhexahedrause8integrationpointsperelement;thus,inthisexamplewehaveasmanyas8constraintsperelement,butonly3degreesoffreedomareavailabletosatisfytheseconstraints.每个单元有8个约束，以至于产生体积锁死。Themeshisoverconstrainedit“locks.”Volumetriclockingismostpronouncedinfullyintegratedelements.Reduced-integrationelementshavefewervolumetricconstraints.Reducedintegrationeffectivelyeliminatesvolumetriclockinginmanyproblemswithnearlyincompressiblematerial.,ABAQUS/Standard基础教程,IncompressibleMaterials,Fullyincompressiblematerialsmodeledwithsolidelementsmustusethe“hybrid”formulation(elementswhosenamesendwiththeletter“H”).Inthisformulationthepressurestressistreatedasanindependentlyinterpolatedbasicsolutionvariable,coupledtothedisplacementsolutionthroughtheconstitutivetheory.Hybridelementsintroducemorevariablesintotheproblemtoalleviatethevolumetriclockingproblem.Theextravariablesalsomakethemmoreexpensive.TheABAQUSelementlibraryincludeshybridversionsofallcontinuumelements(exceptplanestresselements,wheretheyarenotneeded).,ABAQUS/Standard基础教程,Hybridelementsareonlynecessaryfor:以不可压缩材料为主的网格，如橡胶材料。Allmesheswithstrictlyincompressiblematerials,suchasrubber.精密的网格，使用减缩积分仍然有locking的网格，比如弹塑性材料完全进入塑性阶段Refinedmeshesofreduced-integrationelementsthatstillshowvolumetriclockingproblems.Suchproblemsarepossiblewithelastic-plasticmaterialsstrainedfarintotheplasticrange.即使使用了hybrid单元一次三角形或者四面体单元仍然有过度约束。因此建议这类单元使用的比例要小，可以作为六面体单元的“填充物”使用。Evenwithhybridelementsameshoffirst-ordertrianglesandtetrahedraisoverconstrainedwhenmodelingfullyincompressiblematerials.Hence,theseelementsarerecommendedonlyforuseas“fillers”inquadrilateralorbrick-typemesheswithsuchmaterial.,IncompressibleMaterials,ABAQUS/Standard基础教程,MeshGeneration,ABAQUS/Standard基础教程,MeshGeneration,Quad/Hexvs.Tri/TetElementsOfparticularimportancewhengeneratingameshisthedecisionregardingwhethertousequad/hexortri/tetelements.Quad/hexelementsshouldbeusedwhereverpossible.Theygivethebestresultsfortheminimumcost.Whenmodelingcomplexgeometries,however,theanalystoftenhaslittlechoicebuttomeshwithtriangularandtetrahedralelements.,Turbinebladewithplatformmodeledwithtetrahedralelements,ABAQUS/Standard基础教程,MeshGeneration,First-ordertri/tetelements(CPE3,CPS3,CAX3,C3D4,C3D6)arepoorelements;theyhavethefollowingproblems:Poorconvergencerate.Theytypicallyrequireveryfinemeshestoproducegoodresults.Volumetriclockingwithincompressibleornearlyincompressiblematerials,evenusingthe“hybrid”formulation.Theseelementsshouldbeusedonlyasfillersinregionsfarfromanyareaswhereaccurateresultsareneeded.,ABAQUS/Standard基础教程,Equivalentnodalforcescreatedbyuniformpressureonthefaceofaregularsecond-ordertetrahedralelement,MeshGeneration,“Regular”second-ordertri/tetelements(CPE6,CPS6,CAX6,C3D10)cannotbeusedtomodelcontact.Underuniformpressurethecontactforcesaresignificantlydifferentatthecornerandmidsidenodes.Forsmall-displacementproblemswithoutcontacttheseelementsprovidereasonableresults.,ABAQUS/Standard基础教程,MeshGeneration,Modifiedsecond-ordertri/tetelements(C3D10M,etc.)alleviatetheproblemsofothertri/tetelements.Goodconvergencerateclosetoconvergencerateofsecond-orderquad/hexelements.Minimalshearorvolumetriclocking.Canbeusedtomodelincompressibleornearlyincompressiblematerialsinthehybridformulation(C3D10MH).Theseelementsarerobustduringfinitedeformation.Uniformcontactpressureallowstheseelementstomodelcontactaccurately.Usethem!,ABAQUS/Standard基础教程,MeshGeneration,MeshrefinementandconvergenceUseasufficientlyrefinedmeshtoensurethattheresultsfromyourABAQUSsimulationareadequate.Coarsemeshestendtoyieldinaccurateresults.Thecomputerresourcesrequiredtorunyourjobincreasewiththelevelofmeshrefinement.Itisrarelynecessarytous