In the paragraph above the referenced equation, udot is defined as the relative sliding velocity.
As shown in the image in your post, FRICMOD=4 is a friction model with different static and dynamic coefficients of friction.
A3 is therefore the relative sliding velocity where the change between static and dynamic friction occurs. For relative sliding velocities less than or equal to A3, the static coefficient (defined by A1) is used. For relative sliding velocities greater than A3, the dynamic coefficient (defined by A2) is used.
Hey Jim, I have managed to get my solution to 94% converged and it is crushing/collapsing correctly (as seen in previous 90tn press crush tests). There is however some element penetration toward the end of the crush that I cannot seem to get rid of. Could you please have a look at my sim/fem files attached? Run a solve and have a look at the results and provide some feedback? Thanks