Hello,
I am solving the problem of deformation of an energy-absorbing device for a railway car in two stages:
1st stage - static compression and a long period of relaxation
2nd stage - cargo drop
for the first stage, you need to use an implicit solver
for the second stage, you need to use an explicit solver
I tried to use the LS-DYNA program (Revision: R13.0-365-gf8a97bda2a) and the external library of materials PolyUMod.
This problem was solved only by using an implicit solver for the entire problem.
But this method turned out to be unprofitable due to the long solution time.
I tried to use a "full restart", but it turned out that when the model uses the external library of materials PolyUMod, LS-DYNA cannot use the results obtained at the first step for the second step (restart).
LS-DYNA reads the solution results from the first step, but then they are "zeroed out".
Maybe someone could find a way to solve similar problems?
*** I also tried to solve this problem using the material model 318, implemented in the LS-DYNA program (Revision: R14.1.1-4-gaf1eb871e8 | AnLicVer: 2024 R2 (20240501+dl-65-gb272478), but unfortunately I could not solve even the first stage of this problem.
It is very likely that the implemented material model 318 has some "disadvantages"...
Regards Valery
Great question to a pretty complicated problem. I believe the limitation you're encountering with PolyUMod and restart files is a known constraint with external material models in LS-DYNA. The "zeroing out" behavior you describe is typical when external libraries don't properly support state variable transfer during restart.
Sounds like using *Mat_318 is causing instabilities. Maybe try using a test material like *Mat_elastic or MAT_006 or MAT_076, as a test to see if you can get the simulation pipeline working?
As an alternative approach to using implicit for the first stage, you could try using explicit with dynamic relaxation? This would eliminate the need for a restart between stages. Your restart approach might be better for your needs but you'll need to fix the material problem you are having.
https://ftp.lstc.com/anonymous/outgoing/jday/bolt_preload3.pdf