Reply To: Initial Oscillation in LS Dyna Following Static Structural Analysis

[Reno Genest]

Hello Ali,

The problem here is that you are using the Ansys solver to find the static equilibrium position under gravity and constant pressure and the LS-DYNA solver for the explicit dynamics impact simulation. The Ansys solver and the LS-DYNA solver are different solvers with different element technologies, material models, etc. and so results with the same mesh may differ a little bit between the 2 solvers. So, the static equilibrium position of 0.0573mm that you find with the Ansys solver is probably a little bit different if you were to do the same implicit static simulation in LS-DYNA. In many cases, the difference in solution is small and acceptable for many applications. But, in your case, it seems you are looking for more accuracy.

So, one solution is to find the static equilibrium under gravity and constant pressure using Dynamic Relaxation in LS-DYNA. This way, you would use the LS-DYNA solver for both the static equilibrium and the explicit dynamics  impact. This way, you should minimize the oscillations that you observe. You can do explicit Dynamic Relaxation which uses the explicit solver and some sort of damping to damp out the vibration and find the static equilibrium position. Explicit Dynamic Relaxation (DR) will usually result in some oscillation left at the end so so Explicit DR may not be the best approach for you. You could use implicit Dynamic Relaxation in LS-DYNA which uses the implicit solver in LS-DYNA to find the equilibrium position. This should give a result similar to the Ansys Static Structural solution. Note that Dynamic Relaxation (DR) in LS-DYNA occurs before time zero such that the model is initialized with deformation and stresses at time=0 due to the applied load (gravity and pressure) during the DR phase. You will find more information about DR here:

https://ftp.lstc.com/anonymous/outgoing/support/FAQ_docs/preload.pdf

You can do Dynamic Relaxation in Mechanical using the Dynamic Relaxation object:

You will need to have only one LS-DYNA system in the WB project schematic. No need for Static Structural or LS-DYNA Restart:

 

Let me know how it goes.

 

Reno.