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1、Preloads in LS-DYNA,IntroductionAnalysis Techniques (General)Dynamic RelaxationExplicitImplicitTransient Explicit with Mass DampingImplicit AnalysisBolt Preload TechniquesThermalInterference ContactStress in Solid Cross-sectionForce in Beams,Preload - Introduction,Sometimes it is important to induce
2、 a steady state preload before performing a transient dynamic analysis.Rotating fan or turbine blades, rotating flywheelsGravityPressure vessels or tiresShrink-fit partsStresses induced by a torqued bolt,Explicit Dynamic Relaxation (DR),Explicit DR is an optional transient analysis that takes place
3、in pseudo-time (precedes regular transient analysis).DR is typically used to preload a model before onset of transient loading. Preload stresses are typically elastic and displacements are small.In explicit DR, the computed nodal velocities are reduced each timestep by the dynamic relaxation factor
4、(default = .995). Thus the solution undergoes a form of damping during DR.The distortional kinetic energy is monitored. When this KE has been sufficiently reduced, i.e., the “convergence factor” has become sufficiently small, the DR phase terminates and the solution automatically proceeds to the tra
5、nsient analysis phase.Alternately, DR can be terminated at a preset termination time.,Explicit Dynamic Relaxation,DR is typically invoked by setting parameter SIDR in a load curve (*DEFINE_CURVE) to 1 or 2.Ramp the load during DR phase and then hold load constant until solution convergesMake sure co
6、nvergence occurs after 100% of preload is appliedMaintain the preload in subsequent transient analysis phase (use separate load curve without the ramp),*CONTROL_DYNAMIC_RELAXATION,*CONTROL_DYNAMIC_RELAXATION parametersIterations between convergence check (default=250)Also affects output interval for
7、 “d3drlf”Convergence tolerance (default 0.001)Ratio of distorsional KE at convergence to peak distorsional KESmaller value results in converged solution nearer to steady state but run will take longer to get thereDynamic relaxation factor (default=0.995)Reduction factor for nodal velocities each tim
8、e stepIf value is too small, model never reach steady state due to overdampingOptional termination time for DR (default = infinity)DR will stop if time reaches DRTERM even if convergence criterion not satisfiedTime step scale factor used during DR,Explicit Dynamic Relaxation,*CONTROL_DYNAMIC_RELAXAT
9、ION parametersIDRFLGFlag to activate DR (not required if DR is activated with *DEFINE_CURVE) Set to 2, will invoke a completely different and faster initialization approach Initialization by Prescribed Geometry.Requires supplemental input file containing nodal displacements and rotations (“m=filenam
10、e” on execution line). Such a file drdisp.sif is written at conclusion of standard DR run. If nodal rotations are not included in file, method is invalid for beams and shells.LS-DYNA runs a short transient analysis of 100 timesteps to preload the model by imposing the nodal displacements and rotatio
11、ns.Solution then proceeds with regular transient analysis.Set to 5, activates implicit method for solution of preloaded stateMust also set DRTERM to signal end of DR phase.*CONTROL_IMPLICIT. provide controls on implicit phase.,*CONTROL_DYNAMIC_RELAXATION,Dynamic Relaxation,Output Related to Dynamic
12、Relaxation,ASCII output files are NOT written during DR phase, e.g., glstat, matsum, rcforc, etc. The binary d3thdt file can be used if IDRFLG=-1.Binary database, d3drlf, is written by including command *DATABASE_BINARY_D3DRLF. Set output interval to 1. This will cause a state to be written each tim
13、e convergence is checked during DRPlotting time histories from d3drlf with LS-PrePost allows user to confirm solution is near steady staterelax file is automatically written and contains record of convergence history. Data can be plotted with LS-PrePost. drdisp.sif contains nodal displacements and r
14、otations at conclusion of DR phase.,Dynamic Relaxation,Output Related to Explicit Dynamic Relaxation,Explicit Dynamic Relaxation,Dynamic Relaxation information is writtento the screen. The transient phase starts when the convergence tolerance or a Specified termination time is reached.,Convergence p
15、lot from relax file,Kinetic Energy plot from relax file,Typical Loads During Dynamic Relaxation,Gravity loads and centrifugal loads (spinning bodies) are imposed using *LOAD_BODY_option. LCID and LCIDDR are separate curves for transient phase and DR phase, respectively.Thermal stresses can be impose
16、d using *LOAD_THERMAL_LOAD_CURVE.Parts, e.g., bolts, defined with a coefficient of thermal expansion will have thermal stresses imposed.LCID and LCIDDR are separate curves for transient phase and DR phase, respectively.Other load types or boundary conditions are applied during DR if SIDR in correspo
17、nding *DEFINE_CURVE is set to 1 or 2. Example: *LOAD_SEGMENT, *BOUNDARY_PRESCRIBED_MOTION.*CONTACT_._INTERFERENCE imposes load associated with geometric interference.*INITIAL_. (more on that later),Dynamic Relaxation,Explicit Dynamic Relaxation,Example Gravity Loading on a Tire,g,Contact,Ground is c
18、onstrained,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
19、. This model is used to investigate the behavior of Dynamic Relaxation.,Node Considered,Dynamic Relaxation,Example Gravity Loading on a Tire,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 (*DEFINE_CURVE) set to 1.
20、 The load is ramped in curve LCIDDR over 2000 time steps. The *CONTROL_DYNAMIC_RELAXATION parameters are all set to default and the deck is the same as before.,A No DRB With DR,Dynamic Relaxation,Example Gravity Loading on a Tire,Three different settings of the convergence tolerance, DRTOL, were tri
21、ed: 1e-3 (default), 1e-4 and 1e-6. The tolerance is the only change in the model.,The value of DRTOL offers a tradeoff between run time and amplitude of residual dynamic oscillation.,Transient Stress Initialization,As an alternative to using DR, in some cases the preload can be established in the ea
22、rly part of the regular transient simulation.Use *initial_velocity_generation_start_time for problems whose transient response is driven by initial velocity.Delays onset of “initial” velocity.Ramp up preload quasi-statically and then hold steady.Use time-dependent mass damping (*DAMPING_GLOBAL) to i
23、mpose near-critical damping until preload is established.Drop damping constant to zero after preload is established and transient loading is ready to be applied. Apply transient loads AFTER preload is established. Use nonzero birthtime or arrival time for transient loads,Transient Stress Initializat
24、ion,Load,Time,Preload Transient Load,Mass Damping,Coef,Time,Load,Time,t1,t1,t2,t2,Preload via Implicit Analysis,Recall that true static analysis is possible by invoking implicit analysis in LS-DYNA. Static analysis is well-suited to inducing preload. However, no rigid body modes can be present for a
25、 static analysis. One has the option of dynamic implicit combined with an extended loading period.Implicit analysis is invoked via the command *CONTROL_IMPLICIT_GENERAL.Other implict-related commands often used are:*CONTROL_IMPLICIT_AUTO automatically adjusts step size based on ease or difficulty in
26、 achieving convergence.*CONTROL_IMPLICIT_DYNAMICS can make the implicit solution dynamic rather than static.Invoking dynamics can ease convergence.Step size has units of time if dynamics is invoked.,Preload via Implicit Analysis,Approach 1: Two separate analyses.Make an implicit (or explict) simulat
27、ion of the preload. In the input deck specify *INTERFACE_SPRINGBACK_LSDYNA. This creates an ASCII file called dynain when the simulation is finished. The dynain file contains keyword commands describing the deformed geometry, stresses, and plastic strains. Merge these commands into the original deck
28、, deselect the implicit cards, modify the loads, and run a second, explicit simulation.The dynain file does not include contact forces nor does it contain nodal velocities. Thus these quantities from the preload analysis do not carry over to the second analysis.Using only data from the d3plot databa
29、se, LS-PrePost can output a dynain file via Output Format: Dynain Ascii Write.,Preload via Implicit Analysis,Approach 2: Single, switched analysis.Use one input deck where switching between implicit and explicit is determined by a curve. The abscissa of the curve is time and the ordinate is set to 1
30、.0 for implicit and to 0.0 for explicit (curve is a step function). This switching is activated by setting IMFLAG at *CONTROL_IMPLICIT_GENERAL to -|curve ID|. Switching from one analysis to the other is seamless and has no CPU or I/O overhead.Approach 3: Implict DR (mentioned previously).,Bolt Prelo
31、ad,Iterative Loading TypesRequire multiple runs to tune load in order to give desired bolt stress*LOAD_THERMAL_LOAD_CURVE*CONTACT_INTERFERENCENon-iterative Loading TypesBolt stress is specified directly.*INITIAL_STRESS_SECTIONSolid elements only*INITIAL_AXIAL_FORCE_BEAMType 9 beams only,*LOAD_THERMA
32、L_LOAD_CURVE,Idea is to shrink the bolt by cooling it. As bolt contracts during DR phase, preload is induced.Coefficient of thermal expansion (CTE) must be given for bolt material, e.g., via *MAT_ADD_THERMAL_EXPANSION.Negative temperature is prescribed using *LOAD_THERMAL_LOAD_CURVE.LCID = curve of
33、temperature vs. time for transient phase (constant T).LCIDDR = curve of temperature vs. time for DR phase.SIDR=1 in *DEFINE_CURVE.Ramp T and then hold constant.Temperature T (or CTE) to produce a target bolt stress s can be estimated.s = E * CTE * -T Adjust T (or CTE) in subsequent run to fine tune
34、bolt stressExample: http:/,*CONTACT_._INTERFERENCE,Developed for modeling shrink-fit parts. Define the initial geometry to include finite initial penetration between parts. Parts are initially in an unstressed state.The initial penetration check is not done for ths contact type.To avoid sudden, larg
35、e contact forces, the contact stiffness is scaled with time using LCID1 (DR phase) and LCID2 (Transient phase). Shell thickness offsets are considered.Segment orientation is important. Orient the normals correctly facing against opposing contact surface.Specify the contact using segment sets.Types:*
36、CONTACT_NODES_TO_SURFACE_INTERFERENCE*CONTACT_ONE_WAY_SURFACE_TO_SURFACE_INTERFERENCE*CONTACT_SURFACE_TO_SURFACE_INTERFERENCE,*CONTACT_._INTERFERENCE,Contact Stiffness Scale Factor,Contact Stiffness Scale Factor,1.0,1.0,1.0,*CONTACT_._INTERFERENCE,Four bolts clamp two, 1.0” thick solid rings togethe
37、r.Mesh is defined so each bolt head and each nut overlap (penetrate) the solid ring surface by 0.003”. Trial overlap based loosely on target bolt stress/(bolt length * E)*CONTACT_SURFACE_TO_SURFACE_INTERFERENCE defined between overlapping surfaces.Contact stiffness is ramped up over time during DR p
38、hase.Overlap can be adjusted in subsequent trials to fine tune bolt stress.,Example:http:/,Preloading a Solid Cross-section to a Known Stress,*INITIAL_STRESS_SECTION will preload a cross-section of solid elements to a prescribed stress valuePreload stress (normal to the cross-section) is defined via
39、 *DEFINE_CURVE (stress vs. time)This curve is typically flagged with SIDR=1, so that dynamic relaxation is invoked for applying the preloadStress should be ramped from zeroPhysical location of cross-section is defined via *DATABASE_CROSS_SECTIONA part set, together with the cross-section, identify t
40、he elements subject to the prescribed preload stressContact damping (VDC) and/or *DAMPING_PART_STIFFNESS may be required to attain convergence during the dynamic relaxation analysis,*INITIAL_STRESS_SECTION,Four bolts clamp two, 1.0” thick solid rings together.The four bolts are given a prestress of
41、20,000 psi using *INITIAL_STRESS_SECTION.The sections being preloaded are defined by a plane through the middle of the bolts.The direction of prestress is normal to the plane.,Example: http:/,*INITIAL_STRESS_SECTION,Example of preloaded bolts,Target bolt stress is achieved without multiple trial sim
42、ulations.,Initial Forces in a Beam,*INITIAL_AXIAL_FORCE_BEAM will preload beam elements to a prescribed axial force.The preload curve (axial force vs. time) is defined with *DEFINE_CURVE.The curve is typically flagged with SIDR=1 so preload is applied during a DR phase.Curve should ramp up beam forc
43、e to ease convergence.The beam to be loaded is given by a SET_BEAM.Beam formulation (ELFORM) must be set to 9 (spot weld beam).Use with *MAT_SPOTWELD.The spot weld beams initialized in this manner will not be excluded from automatic contacts. For models with contact, damping in the contact (VDC=20)
44、is recommended. *DAMPING_PART_STIFFNESS may promote convergence during DR phase.,Initial Forces in a Beam,The bolt is modeled with a type 9 beam and *MAT_100. The deformable bolt beam is attached to the plates being bolted by rigid beams. The bolt is preloaded with a force of 0.05 using *INITIAL_AXI
45、AL_FORCE_BEAM. The load curve is applied in DR phase with a ramp function.No additional load is applied in subsequent transient phase.,Example: http:/,Deformable plates,Bolt,Rigid beams,Bolt beam,Initial Forces in a Beam,Stress at conclusion of DR phase due to bolt preload.,Example of preloaded bolt,Axial force in bolt is successfully initialized,
