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1、Preloads in LS-DYNA,Introduction Analysis Techniques (General) Dynamic Relaxation Explicit Implicit Transient Explicit with Mass Damping Implicit Analysis Bolt Preload Techniques Thermal Interference Contact Stress in Solid Cross-section Force in Beams,Preload - Introduction,Sometimes it is importan
2、t to induce a steady state preload before performing a transient dynamic analysis. Rotating fan or turbine blades, rotating flywheels Gravity Pressure vessels or tires Shrink-fit parts Stresses induced by a torqued bolt,Explicit Dynamic Relaxation (DR),Explicit DR is an optional transient analysis t
3、hat takes place 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
4、relaxation factor (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
5、proceeds to the transient 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
6、converges Make sure convergence occurs after 100% of preload is applied Maintain the preload in subsequent transient analysis phase (use separate load curve without the ramp),*CONTROL_DYNAMIC_RELAXATION,*CONTROL_DYNAMIC_RELAXATION parameters Iterations between convergence check (default=250) Also af
7、fects output interval for “d3drlf” Convergence tolerance (default 0.001) Ratio of distorsional KE at convergence to peak distorsional KE Smaller value results in converged solution nearer to steady state but run will take longer to get there Dynamic relaxation factor (default=0.995) Reduction factor
8、 for nodal velocities each time step If value is too small, model never reach steady state due to overdamping Optional termination time for DR (default = infinity) DR will stop if time reaches DRTERM even if convergence criterion not satisfied Time step scale factor used during DR,Explicit Dynamic R
9、elaxation,*CONTROL_DYNAMIC_RELAXATION parameters IDRFLG Flag 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 di
10、splacements and rotations (“m=filename” 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 imposi
11、ng the nodal displacements and rotations. Solution then proceeds with regular transient analysis. Set to 5, activates implicit method for solution of preloaded state Must also set DRTERM to signal end of DR phase. *CONTROL_IMPLICIT. provide controls on implicit phase.,*CONTROL_DYNAMIC_RELAXATION,Dyn
12、amic Relaxation,Output Related to Dynamic 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. Thi
13、s will cause a state to be written each time convergence is checked during DR Plotting time histories from d3drlf with LS-PrePost allows user to confirm solution is near steady state relax file is automatically written and contains record of convergence history. Data can be plotted with LS-PrePost.
14、drdisp.sif contains nodal displacements and rotations at conclusion of DR phase.,Dynamic Relaxation,Output Related to Explicit Dynamic Relaxation,Explicit Dynamic Relaxation,Dynamic Relaxation information is written to the screen. The transient phase starts when the convergence tolerance or a Specif
15、ied termination time is reached.,Convergence plot 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 phas
16、e, respectively. Thermal stresses can be imposed 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 corresponding *DEFINE_CURVE is set to 1 or 2. Example: *LOAD_SEGMENT, *BOUNDARY_PRESCRIBED_MOTION. *CONTACT_._INTERFERENCE imposes load associated with g
