Coulomb Damping using a nonlinear CBUSH Element.
Coulomb damping is usually described as dry friction damping. Once motion has
started, with velocity greater than zero, an opposing friction force is instantly
developed. The friction force is constant and is not dependent on magnitude of
velocity or displacement. The value of friction force is defined by the normal force
times the coefficient of friction, which is independent of the dynamic solution .  If
the direction of the velocity reverses, then the friction force reverses.
This form of damping is nonlinear.  It has two states, each with a linear response at
the two states, but the switching between them gives a non-linearity. There are
techniques to represent this nonlinear state with a series of harmonics to keep in
the linear domain, which will be explored at a later date.
Figure 1 shows the form of the damping force versus velocity. In theory the
switch between positive and negative friction force should be an instantaneous
vertical line. I have put a small slope in for stability of the look up table
Figure 1. Coulomb Damping Force versus velocity relationship.
Coulomb Damping - Page 1