Hello Soroush:

In Abaqus software, after building your model and specifying material properties, etc. do the following;

1- Create a step, pick linear perturbation choice

2-Under procedure type, pick up frequency

3- When you get to frequency dialog box, you can pick the number of the first eigenvalues that you would like to solve for or the frequency range that you would like to cover. As a rule of thump and since frequency range is not known, then pick up the values of eignvalues. Leave the default eignsolver in the dialog box as the Lanczos solver. And of course you can go to Abqus documentation online for further help.

BR

Mohamed,

Hi Soroush

Depending on your analysis purpose, you may need to consider the use of an advanced material model, e.g. using Biot theory.

The reason for this is simply that a foam carries load both from its structure matrix and from gas entrapped in the foam. Flow inside the foam and structure matrix relaxation may affect response due to damping. The behaviour is different also between open and closed cell foam.

I am no expert on these matters but perhaps the enclosed links can provide some suggestions on where to looka and what to consider.

http://www.diva-portal.org/smash/get/diva2:14135/FULLTEXT01.pdf

http://www.matelys.com/publications/CBRB+10.pdf

http://rsta.royalsocietypublishing.org/content/364/1838/89.full

Hi Soroush

Natural frequency extraction is a linear step therefore all sources of nonlinear behavior are going to be neglected.  The following is taken directly from  Abaqus documentation:

Section 6.3.5 Natural frequency extraction:

The density of the material must be defined (“Density,” Section 21.2.1). The following material properties are not active during a frequency extraction: plasticity and other inelastic effects, rate-dependent material properties, thermal properties, mass diffusion properties, electrical properties (although piezoelectric materials are active), and pore fluid flow properties—see “General and linear perturbation procedures,” Section 6.1.3.

I hope this helps.

David

Thanks all for your replies.