Quasistatic simulation

Dynamic relaxation is not intended for general quasi-static analysis. It’s ok for applying preload when the preload produces only small elastic strains or for initializing a system to a prescribed geometry but it’s not good for much else.

You can do a quasi-static analysis the old way by running a regular explicit simulation, invoking time- and/or mass-scaling as necessary to crank out the results in a reasonable timeframe, but this approach can be tricky. You have to keep an eye on the kinetic energy in the system as you want to minimize the inertial effects. Basically, the kinetic energy should remain small relative to the internal energy. (By time-scaling, I mean applying the load more quickly than in the quasi-static experiment in order to reduce the simulation time.) See the file “mass_scaling” for more on mass-scaling.

Or, you can try an implicit, static analysis using v. 960 of LS-DYNA. See the commands *CONTROL_IMPLICIT_... and Appendix M in the 950 User’s Manual. There are examples of implicit analysis on our user ftp site in the ls-dyna/example directory.

See also:

Mass scaling

Long run times

Note:

Initializing to a prescribed geometry

1. Write a file of nodal displacements from the final state of your first run. To get this data in the necessary format, use LS-TAURUS as follows:ls-taurus g=d3plot < executes 1000 < goes to final state deform < write a file as described above t < termimate LS-TAURUSNote the d3plot does not contain nodal rotations and thus the rotations are written as zero. This could be a real problem for initialization of shells and beams.LS-PREPOST has an option to write the displacements using Output > Nodal Displacements but the output is i8,3e16 rather than the required i8,3e15 and hence suggested use of LS-TAURUS. LS-TAURUS is not available for Windows PCs. It’s free and available for Unix and Linux workstations.

   If you do a regular dynamic relaxation run to get to the initialized state, a file of prescribed displacements and rotations will automatically be written at the conclusion of the DR phase.

2. In your second run, quickly initialize to the prescribed geometry written in step 1. You need to set IDRFLG=2 in *CONTROL_DYNAMIC_RELAXATION and include m = filename on the execution line where filename is the file created in step 1. Before the transient run begins, LS-DYNA will automatically run a precusor analysis of 100 timesteps wherein the nodse are displaced according to the data in filename.

revised 7/21/03 jpd 7/2004