ls-dyna预应力加载简绍-相当详细.ppt
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ls-dyna预应力加载简绍-相当详细.ppt
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PreloadsinLS-DYNA,IntroductionAnalysisTechniques(General)DynamicRelaxationExplicitImplicitTransientExplicitwithMassDampingImplicitAnalysisBoltPreloadTechniquesThermalInterferenceContactStressinSolidCross-sectionForceinBeams,Preload-Introduction,Sometimesitisimportanttoinduceasteadystatepreloadbeforeperformingatransientdynamicanalysis.Rotatingfanorturbineblades,rotatingflywheelsGravityPressurevesselsortiresShrink-fitpartsStressesinducedbyatorquedbolt,ExplicitDynamicRelaxation(DR),ExplicitDRisanoptionaltransientanalysisthattakesplaceinpseudo-time(precedesregulartransientanalysis).DRistypicallyusedtopreloadamodelbeforeonsetoftransientloading.Preloadstressesaretypicallyelasticanddisplacementsaresmall.InexplicitDR,thecomputednodalvelocitiesarereducedeachtimestepbythedynamicrelaxationfactor(default=.995).ThusthesolutionundergoesaformofdampingduringDR.Thedistortionalkineticenergyismonitored.WhenthisKEhasbeensufficientlyreduced,i.e.,the“convergencefactor”hasbecomesufficientlysmall,theDRphaseterminatesandthesolutionautomaticallyproceedstothetransientanalysisphase.Alternately,DRcanbeterminatedatapresetterminationtime.,ExplicitDynamicRelaxation,DRistypicallyinvokedbysettingparameterSIDRinaloadcurve(*DEFINE_CURVE)to1or2.RamptheloadduringDRphaseandthenholdloadconstantuntilsolutionconvergesMakesureconvergenceoccursafter100%ofpreloadisappliedMaintainthepreloadinsubsequenttransientanalysisphase(useseparateloadcurvewithouttheramp),*CONTROL_DYNAMIC_RELAXATION,*CONTROL_DYNAMIC_RELAXATIONparametersIterationsbetweenconvergencecheck(default=250)Alsoaffectsoutputintervalfor“d3drlf”Convergencetolerance(default0.001)RatioofdistorsionalKEatconvergencetopeakdistorsionalKESmallervalueresultsinconvergedsolutionnearertosteadystatebutrunwilltakelongertogetthereDynamicrelaxationfactor(default=0.995)ReductionfactorfornodalvelocitieseachtimestepIfvalueistoosmall,modelneverreachsteadystateduetooverdampingOptionalterminationtimeforDR(default=infinity)DRwillstopiftimereachesDRTERMevenifconvergencecriterionnotsatisfiedTimestepscalefactorusedduringDR,ExplicitDynamicRelaxation,*CONTROL_DYNAMIC_RELAXATIONparametersIDRFLGFlagtoactivateDR(notrequiredifDRisactivatedwith*DEFINE_CURVE)Setto2,willinvokeacompletelydifferentandfasterinitializationapproachInitializationbyPrescribedGeometry.Requiressupplementalinputfilecontainingnodaldisplacementsandrotations(“m=filename”onexecutionline).Suchafiledrdisp.sifiswrittenatconclusionofstandardDRrun.Ifnodalrotationsarenotincludedinfile,methodisinvalidforbeamsandshells.LS-DYNArunsashorttransientanalysisof100timestepstopreloadthemodelbyimposingthenodaldisplacementsandrotations.Solutionthenproceedswithregulartransientanalysis.Setto5,activatesimplicitmethodforsolutionofpreloadedstateMustalsosetDRTERMtosignalendofDRphase.*CONTROL_IMPLICIT.providecontrolsonimplicitphase.,*CONTROL_DYNAMIC_RELAXATION,DynamicRelaxation,OutputRelatedtoDynamicRelaxation,ASCIIoutputfilesareNOTwrittenduringDRphase,e.g.,glstat,matsum,rcforc,etc.Thebinaryd3thdtfilecanbeusedifIDRFLG=-1.Binarydatabase,d3drlf,iswrittenbyincludingcommand*DATABASE_BINARY_D3DRLF.Setoutputintervalto1.ThiswillcauseastatetobewritteneachtimeconvergenceischeckedduringDRPlottingtimehistoriesfromd3drlfwithLS-PrePostallowsusertoconfirmsolutionisnearsteadystaterelaxfileisautomaticallywrittenandcontainsrecordofconvergencehistory.DatacanbeplottedwithLS-PrePost.drdisp.sifcontainsnodaldisplacementsandrotationsatconclusionofDRphase.,DynamicRelaxation,OutputRelatedtoExplicitDynamicRelaxation,ExplicitDynamicRelaxation,DynamicRelaxationinformationiswrittentothescreen.ThetransientphasestartswhentheconvergencetoleranceoraSpecifiedterminationtimeisreached.,Convergenceplotfromrelaxfile,KineticEnergyplotfromrelaxfile,TypicalLoadsDuringDynamicRelaxation,Gravityloadsandcentrifugalloads(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),DynamicRelaxation,ExplicitDynamicRelaxation,ExampleGravityLoadingon
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