How does the damping term work in tangential contact force?

[Qihan Xuan]

Hi,

I have a question about the tangential contact force in the below paper.

Fleischmann, Jonathan. "DEM-PM contact model with multi-step tangential contact displacement history." Simulation-Based Engineering Laboratory, University of Wisconsin-Madison, Technical Report No. TR-2015-06 (2015).

Equation 1 shows that F_t has the damping term which is proportional to the relative velocity.

So, the larger V_t, the larger F_t. And there is no upper limit.

However, Figure 1 shows that the F_ has an upper limit because of the Coulomb friction.

My question is: 

How does the damping term work when F_t reaches the Coulomb friction limit?

Is the damping term no longer proportional to relative velocity?

Thanks,

Qihan

  

[Qihan Xuan]

I got the answer.

The Coulomb law limits the total tangential force, which contains the damping term.