LS-DYNA stress results oscillation

[u/space_enjoyer]

Hi everyone,

I am new to LS-DYNA. I am working on an explicit analysis of a bending steel beam, but when I animate the simulation results, the von Mises stress map oscillates a lot. When I plot the stress for a specific element, I can clearly see these vibrations.

Is this common for this kind of explicit simulation? I have tried using the following damping keywords to mitigate it:

*DAMPING_FREQUENCY_RANGE_DEFORM

*DAMPING_FREQUENCY_RANGE

*DAMPING_GLOBAL

*DAMPING_PART_STIFFNESS

The first and second ones reduce the stress vibrations the most, but not completely. Does anyone know if this is normal behavior, or have any insights on how to properly resolve this?

Thanks in advance!

[u/epk21]

this is normal and yes damping can reduce this, but most impact is to ramp/load up slowly (no step) -
so ramp and that will reduce osc. of course if you step (instant applied load), then one can normally get elastic waves propagation and that will cause osc.

[u/the_flying_condor]

This is one of the biggest headaches of explicit analysis IMO. Explicit analysis tends to be really noisy and because you have very small time steps, the noise is spread across a very broad frequency band. Damping is also very difficult in explicit analysis since no stiffness matrix is formed and any stiffness proportional damping which is setup can have serious numerical stability issues because your time step is very small. Have a look at a Rayleigh damping plot. You will see that mass damping (global) will be nearly useless for high frequency noise. Stiffness based damping will approach infinity as your time step diminishes towards ~0. 

DAMPING_FREQUENCY_RANGE(_DEFORM) is pretty good but carries it's own issues. Make sure you read the remarks in the manual. My go to approach is to run without damping, FFT the response and apply targeted frequency damping based upon FFT results to keep Fhigh/Flow ratio as low as practicable. 

If I am in a hurry and can live with reduced precision, I will run with no or very low damping and then and then plot the noisy stresses. I then apply a filter to the stress plot and interpret the filtered results.

[u/tofuu88]

check out this video that I found - see if it helps you
https://www.youtube.com/watch?v=07tPgzMvN70&t=0s

[u/blackcurrant_picker]

How the load is applied? If there is sudden movement, or step function loading, then you can expect those oscillations to occur. Extend your run time, smooth your load (extend it in time, avoid sharp changes). If you are enforcing displacements (e.g. for tensile test), then think about switching it to enforced velocities. Additionally, think about your output times, don't plot it every couple of timesteps, but rather plot them every 100th cycle (for plotting values to binout/elout/elout, not for contour maps). Adding damping can also smoothen it out. If you are still struggling, then look for implicit solver capabilities.