ls-dyna预应力加载介绍相当详细课件

PreloadsinLS-DYNAIntroductionAnalysisTechniques(General)DynamicRelaxationExplicitImplicitTransientExplicitwithMassDampingImplicitAnalysisBoltPreloadTechniquesThermalInterferenceContactStressinSolidCross-sectionForceinBeamsPreload-IntroductionSometimesitisimportanttoinduceasteadystatepreloadbeforeperformingatransientdynamicanalysis.Rotatingfanorturbineblades,rotatingflywheelsGravityPressurevesselsortiresShrink-fitpartsStressesinducedbyatorquedboltExplicitDynamicRelaxation(DR)ExplicitDRisanoptionaltransientanalysisthattakesplacein‘pseudo-time’(precedesregulartransientanalysis).DRistypicallyusedtopreloadamodelbeforeonsetoftransientloading.Preloadstressesaretypicallyelasticanddisplacementsaresmall.InexplicitDR,thecomputednodalvelocitiesarereducedeachtimestepbythedynamicrelaxationfactor(default=.995).ThusthesolutionundergoesaformofdampingduringDR.Thedistortionalkineticenergyismonitored.WhenthisKEhasbeensufficientlyreduced,i.e.,the“convergencefactor”hasbecomesufficientlysmall,theDRphaseterminatesandthesolutionautomaticallyproceedstothetransientanalysisphase.Alternately,DRcanbeterminatedatapresetterminationtime.*CONTROL_DYNAMIC_RELAXATION*CONTROL_DYNAMIC_RELAXATIONparametersIterationsbetweenconvergencecheck(default=250)Alsoaffectsoutputintervalfor“d3drlf”Convergencetolerance(default0.001)RatioofdistorsionalKEatconvergencetopeakdistorsionalKESmallervalueresultsinconvergedsolutionnearertosteadystatebutrunwilltakelongertogetthereDynamicrelaxationfactor(default=0.995)ReductionfactorfornodalvelocitieseachtimestepIfvalueistoosmall,modelneverreachsteadystateduetooverdampingOptionalterminationtimeforDR(default=infinity)DRwillstopiftimereachesDRTERMevenifconvergencecriterionnotsatisfiedTimestepscalefactorusedduringDRExplicitDynamicRelaxation*CONTROL_DYNAMIC_RELAXATIONparametersIDRFLGFlagtoactivateDR(notrequiredifDRisactivatedwith*DEFINE_CURVE)Setto2,willinvokeacompletelydifferentandfasterinitializationapproach…InitializationbyPrescribedGeometry.Requiressupplementalinputnodaldisplacementsandrotations(“m=”onexecutionline).Suchafiledrdisp.sifiswrittenatconclusionofstandardDRrun.Ifnodalrotationsarenotincludedinfile,methodisinvalidforbeamsandshells.LS-DYNA®runsashorttransientanalysisof100timestepstopreloadthemodelbyimposingthenodaldisplacementsandrotations.Solutionthenproceedswithregulartransientanalysis.Setto5,activatesimplicitmethodforsolutionofpreloadedstateMustalsosetDRTERMtosignalendofDRphase.*CONTROL_IMPLICIT...providecontrolsonimplicitphase.*CONTROL_DYNAMIC_RELAXATIONDynamicRelaxationOutputRelatedtoDynamicRelaxationASCIIoutputfilesareNOTwrittenduringDRphase,e.g.,glstat,matsum,rcforc,etc.Thebinaryd3thdtbeusedifIDRFLG=-1.Binarydatabase,d3drlf,iswrittenbyincludingcommand*DATABASE_BINARY_D3DRLF.Setoutputintervalto1.ThiswillcauseastatetobewritteneachtimeconvergenceischeckedduringDRPlottingtimehistoriesfromd3drlfwithLS-PrePost®allowsusertoconfirmsolutionisnearsteadystaterelaxautomaticallywrittenandcontainsrecordofconvergencehistory.DatacanbeplottedwithLS-PrePost.

drdisp.sifcontainsnodaldisplacementsandrotationsatconclusionofDRphase.DynamicRelaxationTypicalLoadsDuringDynamicRelaxationGravityloadsandcentrifugalloads(spinningbodies)areimposedusing*LOAD_BODY_option.LCIDandLCIDDRareseparatecurvesfortransientphaseandDRphase,respectively.Thermalstressescanbeimposedusing*LOAD_THERMAL_LOAD_CURVE.Parts,e.g.,bolts,definedwithacoefficientofthermalexpansionwillhavethermalstressesimposed.LCIDandLCIDDRareseparatecurvesfortransientphaseandDRphase,respectively.OtherloadtypesorboundaryconditionsareappliedduringDRifSIDRincorresponding*DEFINE_CURVEissetto1or2.Example:*LOAD_SEGMENT,*BOUNDARY_PRESCRIBED_MOTION.*CONTACT_..._INTERFERENCEimposesloadassociatedwithgeometricinterference.*INITIAL_...(moreonthatlater)DynamicRelaxationExplicitDynamicRelaxationExample–GravityLoadingonaTiregContactGroundisconstrainedOneofthetiresfromNCAC’sFord250wasusedinthisexamplebutwithoutthecontrolvolume.Agravityloadisappliedinthetransientphaseasaconstantcurve,whichmakesthetirebounceduringthesimulation(time=1)asseenwhenplottingtheZ-displacementforanodeonthetirerim.ThismodelisusedtoinvestigatethebehaviorofDynamicRelaxation.NodeConsideredDynamicRelaxationExample–GravityLoadingonaTireDynamicRelaxationwasaddedtothemodelusingarampedloadcurvefortheDRphase,i.e.,loadcurveLCIDDR(*LOAD_BODY_Z)hasSIDR(*DEFINE_CURVE)setto1.TheloadisrampedincurveLCIDDRover2000timesteps.The*CONTROL_DYNAMIC_RELAXATIONparametersareallsettodefaultandthedeckisthesameasbefore.ANoDRBWithDRTransientStressInitializationAsanalternativetousingDR,insomecasesthepreloadcanbeestablishedintheearlypartoftheregulartransientsimulation.Use*initial_velocity_generation_start_timeforproblemswhosetransientresponseisdrivenbyinitialvelocity.Delaysonsetof“initial”velocity.Rampuppreloadquasi-staticallyandthenholdsteady.Usetime-dependentmassdamping(*DAMPING_GLOBAL)toimposenear-criticaldampinguntilpreloadisestablished.Dropdampingconstanttozeroafterpreloadisestablishedandtransientloadingisreadytobeapplied.ApplytransientloadsAFTERpreloadisestablished.UsenonzerobirthtimeorarrivaltimefortransientloadsTransientStressInitializationLoadTimePreload

TransientLoad

MassDamping

CoefTimeLoadTimet1t1t2t2PreloadviaImplicitAnalysisRecallthattruestaticanalysisispossiblebyinvokingimplicitanalysisinLS-DYNA®.Staticanalysisiswell-suitedtoinducingpreload.However,norigidbodymodescanbepresentforastaticanalysis.Onehastheoptionofdynamicimplicitcombinedwithanextendedloadingperiod.Implicitanalysisisinvokedviathecommand*CONTROL_IMPLICIT_GENERAL.Otherimplict-relatedcommandsoftenusedare:*CONTROL_IMPLICIT_AUTOautomaticallyadjustsstepsizebasedoneaseordifficultyinachievingconvergence.*CONTROL_IMPLICIT_DYNAMICScanmaketheimplicitsolutiondynamicratherthanstatic.Invokingdynamicscaneaseconvergence.Stepsizehasunitsoftimeifdynamicsisinvoked.PreloadviaImplicitAnalysisApproach2:Single,switchedanalysis.Useoneinputdeckwhereswitchingbetweenimplicitandexplicitisdeterminedbyacurve.Theabscissaofthecurveistimeandtheordinateissetto1.0forimplicitandto0.0forexplicit(curveisastepfunction).ThisswitchingisactivatedbysettingIMFLAGat*CONTROL_IMPLICIT_GENERALto-|curveID|.SwitchingfromoneanalysistotheotherisseamlessandhasnoCPUorI/Ooverhead.Approach3:ImplictDR(mentionedpreviously).BoltPreloadIterativeLoadingTypesRequiremultiplerunstotuneloadinordertogivedesiredboltstress*LOAD_THERMAL_LOAD_CURVE*CONTACT_INTERFERENCENon-iterativeLoadingTypesBoltstressisspecifieddirectly.*INITIAL_STRESS_SECTIONSolidelementsonly*INITIAL_AXIAL_FORCE_BEAMType9beamsonly*LOAD_THERMAL_LOAD_CURVEIdeaistoshrinktheboltbycoolingit.AsboltcontractsduringDRphase,preloadisinduced.Coefficientofthermalexpansion(CTE)mustbegivenforboltmaterial,e.g.,via*MAT_ADD_THERMAL_EXPANSION.Negativetemperatureisprescribedusing*LOAD_THERMAL_LOAD_CURVE.LCID=curveoftemperaturevs.timefortransientphase(constantT).LCIDDR=curveoftemperaturevs.timeforDRphase.SIDR=1in*DEFINE_CURVE.RampTandthenholdconstant.TemperatureT(orCTE)toproduceatargetboltstressscanbeestimated.s=E*CTE*-TAdjustT(orCTE)insubsequentruntofinetuneboltstressExample:*CONTACT_..._INTERFERENCETimeTimeDynamicrelaxation(LCID1)+TransientPhase(LCID2)TransientPhaseOnly(LCID2)ifLCID1=0TimeContactStiffnessScaleFactorORContactStiffnessScaleFactorContactStiffnessScaleFactor1.01.01.0*CONTACT_..._INTERFERENCEFourboltsclamptwo,1.0”thicksolidringstogether.Meshisdefinedsoeachboltheadandeachnutoverlap(penetrate)thesolidringsurfaceby0.003”.Trialoverlapbasedlooselyontargetboltstress/(boltlength*E)*CONTACT_SURFACE_TO_SURFACE_INTERFERENCEdefinedbetweenoverlappingsurfaces.ContactstiffnessisrampedupovertimeduringDRphase.Overlapcanbeadjustedinsubsequenttrialstofinetuneboltstress.Example:PreloadingaSolidCross-sectiontoaKnownStress*INITIAL_STRESS_SECTIONwillpreloadacross-sectionofsolidelementstoaprescribedstressvaluePreloadstress(normaltothecross-section)isdefinedvia*DEFINE_CURVE(stressvs.time)ThiscurveistypicallyflaggedwithSIDR=1,sothatdynamicrelaxationisinvokedforapplyingthepreloadStressshouldberampedfromzeroPhysicallocationofcross-sectionisdefinedvia*DATABAS