资源描述
Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1 1Preloads in LS-DYNAn nIntroductionIntroductionn nAnalysis Techniques(General)Analysis Techniques(General)Dynamic RelaxationDynamic Relaxationn nExplicitExplicitn nImplicitImplicit Transient Explicit with Mass DampingTransient Explicit with Mass Damping Implicit AnalysisImplicit Analysisn nBolt Preload TechniquesBolt Preload Techniques ThermalThermal Interference ContactInterference Contact Stress in Solid Cross-sectionStress in Solid Cross-section Force in BeamsForce in BeamsPreloads in LS-DYNAIntroductioCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 2 2Preload-Introductionn nSometimes it is important to induce a steady state Sometimes it is important to induce a steady state preload before performing a transient dynamic analysis.preload before performing a transient dynamic analysis.Rotating fan or turbine blades,rotating flywheelsRotating fan or turbine blades,rotating flywheels GravityGravity Pressure vessels or tiresPressure vessels or tires Shrink-fit partsShrink-fit parts Stresses induced by a torqued boltStresses induced by a torqued boltPreload-IntroductionSometimeCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 3 3Explicit Dynamic Relaxation(DR)n nExplicit DR is an Explicit DR is an optional optional transient analysis that takes place in transient analysis that takes place in pseudo-time(precedes regular transient analysis).pseudo-time(precedes regular transient analysis).n nDR is typically used to preload a model before onset of transient DR is typically used to preload a model before onset of transient loading.loading.Preload stresses are typically elastic and displacements are small.Preload stresses are typically elastic and displacements are small.n nIn explicit DR,the computed nodal velocities are reduced each In explicit DR,the computed nodal velocities are reduced each timestep by the dynamic relaxation factor(default=.995).timestep by the dynamic relaxation factor(default=.995).Thus the solution undergoes a form of damping during DR.Thus the solution undergoes a form of damping during DR.n nThe distortional kinetic energy is monitored.When this KE has The distortional kinetic energy is monitored.When this KE has been sufficiently reduced,i.e.,the“convergence factor”has been sufficiently reduced,i.e.,the“convergence factor”has become sufficiently small,the DR phase terminates and the become sufficiently small,the DR phase terminates and the solution automatically proceeds to the transient analysis phase.solution automatically proceeds to the transient analysis phase.n nAlternately,DR can be terminated at a preset termination time.Alternately,DR can be terminated at a preset termination time.Explicit Dynamic Relaxation(DCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 4 4Explicit Dynamic Relaxationn nDR is typically invoked by setting parameter SIDR in a load curve DR is typically invoked by setting parameter SIDR in a load curve(*DEFINE_CURVE)to 1 or 2.(*DEFINE_CURVE)to 1 or 2.n nRamp the load during DR phase and then hold load constant until Ramp the load during DR phase and then hold load constant until solution convergessolution converges Make sure convergence occurs Make sure convergence occurs afterafter 100%of preload is applied 100%of preload is appliedn nMaintain the preload in subsequent transient analysis phase(use Maintain the preload in subsequent transient analysis phase(use separate load curve without the ramp)separate load curve without the ramp)DR convergesSIDR=1(DR phase)SIDR=0(transient phase)Explicit Dynamic RelaxationDR Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 5 5*CONTROL_DYNAMIC_RELAXATION*CONTROL_DYNAMIC_RELAXATION*CONTROL_DYNAMIC_RELAXATION parameters*CONTROL_DYNAMIC_RELAXATION parametersn nIterations between convergence check(default=250)Iterations between convergence check(default=250)Also affects output interval for“d3drlf”Also affects output interval for“d3drlf”n nConvergence tolerance(default 0.001)Convergence tolerance(default 0.001)Ratio of distorsional KE at convergence to peak distorsional KERatio of distorsional KE at convergence to peak distorsional KE Smaller value results in converged solution nearer to steady state Smaller value results in converged solution nearer to steady state but run will take longer to get therebut run will take longer to get theren nDynamic relaxation factor(default=0.995)Dynamic relaxation factor(default=0.995)Reduction factor for nodal velocities each time stepReduction factor for nodal velocities each time step If value is too small,model never reach steady state due to If value is too small,model never reach steady state due to overdampingoverdampingn nOptional termination time for DR(default=infinity)Optional termination time for DR(default=infinity)DR will stop if time reaches DRTERM even if convergence criterion DR will stop if time reaches DRTERM even if convergence criterion not satisfiednot satisfiedn nTime step scale factor used during DRTime step scale factor used during DRExplicit Dynamic Relaxation*CONTROL_DYNAMIC_RELAXATION*COCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 6 6*CONTROL_DYNAMIC_RELAXATION parameters*CONTROL_DYNAMIC_RELAXATION parametersn nIDRFLGIDRFLG Flag to activate DR(not required if DR is activated with*DEFINE_CURVE)Flag to activate DR(not required if DR is activated with*DEFINE_CURVE)Set to 2,will invoke a completely different and faster initialization Set to 2,will invoke a completely different and faster initialization approach approach Initialization by Prescribed GeometryInitialization by Prescribed Geometry.n nRequires supplemental input file containing nodal displacements and rotations Requires supplemental input file containing nodal displacements and rotations(“m=filename”on execution line).(“m=filename”on execution line).Such a file Such a file drdisp.sifdrdisp.sif is written at conclusion of standard DR run.is written at conclusion of standard DR run.If nodal rotations are not included in file,method is invalid for beams If nodal rotations are not included in file,method is invalid for beams and shells.and shells.n nLS-DYNALS-DYNA runs a short transient analysis of 100 timesteps to preload the model by runs a short transient analysis of 100 timesteps to preload the model by imposing the nodal displacements and rotations.imposing the nodal displacements and rotations.n nSolution then proceeds with regular transient analysis.Solution then proceeds with regular transient analysis.Set to 5,activates Set to 5,activates implicitimplicit method for solution of preloaded state method for solution of preloaded staten nMust also set DRTERM to signal end of DR phase.Must also set DRTERM to signal end of DR phase.n n*CONTROL_IMPLICIT.provide controls on implicit phase.*CONTROL_IMPLICIT.provide controls on implicit phase.*CONTROL_DYNAMIC_RELAXATION*CONTROL_DYNAMIC_RELAXATIONDynamic Relaxation*CONTROL_DYNAMIC_RELAXATION paCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 7 7Output Related to Dynamic RelaxationOutput Related to Dynamic Relaxationn nASCII output files are NOT written during DR phase,e.g.,ASCII output files are NOT written during DR phase,e.g.,glstatglstat,matsummatsum,rcforcrcforc,etc.The binary,etc.The binary d3thdtd3thdt file can be used if file can be used if IDRFLG=-1.IDRFLG=-1.n nBinary database,Binary database,d3drlfd3drlf,is written by including command,is written by including command*DATABASE_BINARY_D3DRLF.Set output interval to 1.This*DATABASE_BINARY_D3DRLF.Set output interval to 1.This will cause a state to be written each time convergence is will cause a state to be written each time convergence is checked during DRchecked during DR Plotting time histories from Plotting time histories from d3drlfd3drlf with LS-PrePost with LS-PrePost allows user to allows user to confirm solution is near steady stateconfirm solution is near steady staten nrelaxrelax file is automatically written and contains record of file is automatically written and contains record of convergence history.Data can be plotted with LS-PrePost.convergence history.Data can be plotted with LS-PrePost.n n drdisp.sifdrdisp.sif contains nodal displacements and rotations at contains nodal displacements and rotations at conclusion of DR phase.conclusion of DR phase.Dynamic RelaxationOutput Related to Dynamic RelaCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 8 8Output Related to Explicit Dynamic RelaxationOutput Related to Explicit Dynamic RelaxationExplicit Dynamic RelaxationDynamic Relaxation information is writtento the screen.The transient phase starts when the convergence tolerance or a Specified termination time is reached.Convergence plot from relax fileKinetic Energy plot from relax fileOutput Related to Explicit DynCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 9 9Typical Loads During Dynamic RelaxationTypical Loads During Dynamic Relaxationn nGravity loads and centrifugal loads(spinning bodies)are imposed Gravity loads and centrifugal loads(spinning bodies)are imposed using*LOAD_BODY_option.using*LOAD_BODY_option.LCID and LCIDDR are separate curves for transient phase and DR LCID and LCIDDR are separate curves for transient phase and DR phase,respectively.phase,respectively.n nThermal stresses can be imposed using Thermal stresses can be imposed using*LOAD_THERMAL_LOAD_CURVE.*LOAD_THERMAL_LOAD_CURVE.Parts,e.g.,bolts,defined with a coefficient of thermal expansion will Parts,e.g.,bolts,defined with a coefficient of thermal expansion will have thermal stresses imposed.have thermal stresses imposed.LCID and LCIDDR are separate curves for transient phase and DR LCID and LCIDDR are separate curves for transient phase and DR phase,respectively.phase,respectively.n nOther load types or boundary conditions are applied during DR if Other load types or boundary conditions are applied during DR if SIDR in corresponding*DEFINE_CURVE is set to 1 or 2.Example:SIDR in corresponding*DEFINE_CURVE is set to 1 or 2.Example:*LOAD_SEGMENT,*BOUNDARY_PRESCRIBED_MOTION.*LOAD_SEGMENT,*BOUNDARY_PRESCRIBED_MOTION.n n*CONTACT_._INTERFERENCE imposes load associated with*CONTACT_._INTERFERENCE imposes load associated with geometric interference.geometric interference.n n*INITIAL_.(more on that later)*INITIAL_.(more on that later)Dynamic RelaxationTypical Loads During Dynamic RCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1010Explicit Dynamic RelaxationExample Gravity Loading on a TireExample Gravity Loading on a TiregContactGround is constrained One of the tires from NCACs Ford 250 was used inthis example but without the control volume.A gravity load is applied in the transient phase as a constant curve,which makes the tire bounce during the simulation(time=1)as seen when plotting the Z-displacement for a node on the tirerim.This model is used to investigate the behavior of Dynamic Relaxation.Node ConsideredExplicit Dynamic RelaxationExaCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1111Dynamic RelaxationExample Gravity Loading on a TireExample Gravity Loading on a Tiren nDynamic Relaxation was added to the model using a ramped load curve for the Dynamic Relaxation was added to the model using a ramped load curve for the DR phase,i.e.,load curve LCIDDR(*LOAD_BODY_Z)has SIDR DR phase,i.e.,load curve LCIDDR(*LOAD_BODY_Z)has SIDR(*DEFINE_CURVE)set to 1.The load is ramped in curve LCIDDR over 2000 time(*DEFINE_CURVE)set to 1.The load is ramped in curve LCIDDR over 2000 time steps.The*CONTROL_DYNAMIC_RELAXATION parameters are all set to default steps.The*CONTROL_DYNAMIC_RELAXATION parameters are all set to default and the deck is the same as before.and the deck is the same as before.A No DRB With DRDynamic RelaxationExample GrCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1212Dynamic RelaxationExample Gravity Loading on a TireExample Gravity Loading on a Tiren nThree different settings of the convergence tolerance,DRTOL,were tried:1e-3 Three different settings of the convergence tolerance,DRTOL,were tried:1e-3(default),1e-4 and 1e-6.The tolerance is the only change in the model.(default),1e-4 and 1e-6.The tolerance is the only change in the model.nThe value of DRTOL offers a tradeoff between run time and amplitude of residual dynamic oscillation.DRTOLDRTOL1e-31e-31e-41e-41e-61e-6Elapsed Time(sec)Elapsed Time(sec)38083808503250321375513755Dynamic RelaxationExample GCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1313Transient Stress Initialization n nAs an alternative to using DR,in some cases the preload As an alternative to using DR,in some cases the preload can be established in the early part of the regular can be established in the early part of the regular transient simulation.transient simulation.Use*initial_velocity_generation_start_time for problems whose Use*initial_velocity_generation_start_time for problems whose transient response is driven by initial velocity.transient response is driven by initial velocity.n nDelays onset of“initial”velocity.Delays onset of“initial”velocity.Ramp up preload quasi-statically and then hold steady.Ramp up preload quasi-statically and then hold steady.Use time-dependent mass damping(*DAMPING_GLOBAL)to Use time-dependent mass damping(*DAMPING_GLOBAL)to impose near-critical damping until preload is established.impose near-critical damping until preload is established.n nDrop damping constant to zero after preload is established and Drop damping constant to zero after preload is established and transient loading is ready to be applied.transient loading is ready to be applied.Apply transient loads AFTER preload is established.Apply transient loads AFTER preload is established.n nUse nonzero birthtime or arrival time for transient loadsUse nonzero birthtime or arrival time for transient loadsTransient Stress InitializatioCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1414Transient Stress Initialization Load TimePreload Transient Load Mass Damping Coef TimeLoad Timet1t1t2t2Transient Stress InitializatioCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1515Preload via Implicit Analysisn nRecall that true static analysis is possible by invoking implicit Recall that true static analysis is possible by invoking implicit analysis in LS-DYNAanalysis in LS-DYNA.Static analysis is well-suited to inducing.Static analysis is well-suited to inducing preload.However,no rigid body modes can be present for a preload.However,no rigid body modes can be present for a static analysis.One has the option of static analysis.One has the option of dynamicdynamic implicit implicit combined with an extended loading bined with an extended loading period.n nImplicit analysis is invoked via the command Implicit analysis is invoked via the command*CONTROL_IMPLICIT_GENERAL.*CONTROL_IMPLICIT_GENERAL.n nOther implict-related commands often used are:Other implict-related commands often used are:*CONTROL_IMPLICIT_AUTO automatically adjusts step size based on*CONTROL_IMPLICIT_AUTO automatically adjusts step size based on ease or difficulty in achieving convergence.ease or difficulty in achieving convergence.*CONTROL_IMPLICIT_DYNAMICS can make the implicit solution*CONTROL_IMPLICIT_DYNAMICS can make the implicit solution dynamic rather than static.dynamic rather than static.Invoking dynamics can ease convergence.Invoking dynamics can ease convergence.Step size has units of time if dynamics is invoked.Step size has units of time if dynamics is invoked.Preload via Implicit AnalysisRCopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1616Preload via Implicit Analysisn nApproach 1:Two separate analyses.Approach 1:Two separate analyses.Make an implicit(or explict)simulation of the preload.In the input Make an implicit(or explict)simulation of the preload.In the input deck specify*INTERFACE_SPRINGBACK_LSDYNA.This creates an deck specify*INTERFACE_SPRINGBACK_LSDYNA.This creates an ASCII file called ASCII file called dynaindynain when the simulation is finished.The when the simulation is finished.The dynaindynain file contains keyword commands describing the deformed file contains keyword commands describing the deformed geometry,stresses,and plastic strains.Merge these commands into geometry,stresses,and plastic strains.Merge these commands into the original deck,deselect the implicit cards,modify the loads,and the original deck,deselect the implicit cards,modify the loads,and run a second,explicit simulation.run a second,explicit simulation.n nThe The dynaindynain file does not include contact forces nor does it contain file does not include contact forces nor does it contain nodal velocities.Thus these quantities from the preload analysis do nodal velocities.Thus these quantities from the preload analysis do not carry over to the second analysis.not carry over to the second analysis.n nUsing only data from the Using only data from the d3plotd3plot database,LS-PrePost database,LS-PrePost can output a can output a dynaindynain file via Output Format:Dynain Ascii Write.file via Output Format:Dynain Ascii Write.Preload via Implicit AnalysisACopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1717Preload via Implicit Analysisn nApproach 2:Single,switched analysis.Approach 2:Single,switched analysis.Use one input deck where switching between implicit and explicit is Use one input deck where switching between implicit and explicit is determined by a curve.The abscissa of the curve is time and the determined by a curve.The abscissa of the curve is time and the ordinate is set to 1.0 for implicit and to 0.0 for explicit(curve is a ordinate is set to 1.0 for implicit and to 0.0 for explicit(curve is a step function).This switching is activated by setting IMFLAG at step function).This switching is activated by setting IMFLAG at*CONTROL_IMPLICIT_GENERAL to -|curve ID|.Switching from*CONTROL_IMPLICIT_GENERAL to -|curve ID|.Switching from one analysis to the other is seamless and has no CPU or I/O one analysis to the other is seamless and has no CPU or I/O overhead.overhead.n nApproach 3:Implict DR(mentioned previously).Approach 3:Implict DR(mentioned previously).Preload via Implicit AnalysisACopyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload Copyright 2003-2011 by LIVERMORE SOFTWARE TECHNOLOGY CORPORATION Preload 1818Bolt Preload n nIterative Loading TypesIterative Loading Types Require multiple runs to tune load in order to give desired bolt Require multipl
展开阅读全文