# Damping in cosmos/M geostar

Discussion created by genexxer genexxer on Mar 10, 2007
Hello again gurus...
I just got some rather disappointing news about damping and cosmos/M. "Composite material damping" specified when I use the DAMP material property assigns the value of damping ratio for the material. It isn't the damping coefficient, "c" that is in all the structural dynamics books. That is a bummer. It means I can get whatever response I want out of the simulation, not a prediction of real life response. damping should depend on the mass and stiffness of the system in the form of damping = 1/root(k*m) for a single degree of freedom. So if I use a heavier mass, the response at resonance should increase due to damping decreasing. That doesn't happen unless I also decrease DAMP when I increase density. The response turns out to be identical for increased density and DAMP held constant.

I know some rules of thumb for damping like more damping for denser materials, etc. Please tell all of your rules of thumb you use to change damping for a changing elastic modulus or density(or both). I know that modal damping is a very difficult property to simulate unless you can physically measure the system. But we wouldn't be using FEA then, if we already had the system.