Learning Forum

[wissem chakchouk]

I would like to add damping coefficients to my materials in LS-DYNA. I tried incorporating them through the elastic material module by using the alpha and beta coefficients, as I had done previously in ANSYS using an implicit scheme. I also attempted to use the Damping Part Stiffness module, but the results did not show any significant change. Do you have any suggestions on how to effectively incorporate these damping coefficients, considering that I am working with an explicit scheme?

[gcongdon]

Damping is a very complex topic and more information is needed to help you. In general, I usually use either DAMPING_PART_STIFFNESS and DAMPING_PART_MASS (Rayleigh Damping) or I use DAMPING_FREQUENCY_RANGE_DEFORM. The frequency damping provides constant (ish) damping between a range of specified frequencies. It only works well for low damping and you need to read the manual (and notes) carefully for this card.

[Tonmoy]

Hi Wissem!

Just as an info, if you are intending to use Rayleigh Damping in MDOF system—which is commonly used in structural dynamics for its simplicity—and you have previously applied it using the ANSYS implicit scheme, please note that the LS-Dyna explicit solver (after release 960) handles Rayleigh Damping differently. Unlike traditional formulations, especially for the *DAMPING_PART_STIFFNESS, LS-Dyna’s explicit solver does not use stiffness matrices. Instead, stiffness is computed from the element level. That’s why if you implement Rayleigh damping for the SDOF systems, they perfectly overlays, but with the MDOF system they differs a lot. According to the devs:

The LS-DYNA implementation of Rayleigh damping for standard, nonlinear analysis is done at the element level, as you stated. This is done for numerical convenience, since in the explicit method we don’t need to form the stiffness matrix K. Instead, we compute internal forces by simply integrating stresses over the element area. The Rayleigh damping terms are implemented as corrections to these stresses.

(source: https://www.dynasupport.com/howtos/general/damping)

However if you are doing implicit analysis using dyna with any version, you’ll find that due to the presence of the stiffness matrices, the damped responses will match with the theoretical rayleigh's formulation.

We did some verifications analysis, and later discussed those with LS-Dyna devs in the forum. You can see more information in the following url:

Rayleigh Damping for explicit dynamics