Problem calculating time derivative of plastic strain

Hi,

I have encountered a problem when calculating the time derivative of a internal variable solid.epeGp (plastic strain) in elastoplastic model. The model is aimed to investigate the elastoplastic deformation and yield regions in the plate caused by the movement of the triangle-shape seamount.

(1)

I am trying to impose a time-healing function as a global quation: mod1.epe_efft+mod1.epe_eff/tau-d(mod1.solid.epeGp,TIME). Effective plastic strain (epe_eff) is the new variable I want to solve for further coupling in the initial yield stress equation to replace plastic strain in the future.

But it can't evaluate the time derivative somehow...
Error message is like:
Failed to evaluate variable.
- Variable: mod1.solid.epeGp
- Global scope
Failed to evaluate expression.
- Expression: mod1.epe_efft+mod1.epe_eff/tau-d(mod1.solid.epeGp,TIME)

Does anyone know what I could try for this problem?

(2)

I also notice that when I try to plot d(solid.epeGp,TIME) in my previous models, it always equals to zero although solid.epeGp shows up correctly. And I can't plot prev(solid.epeGp,1) or bdf(solid.epeGp,1), neither.

I am also curious why this would happen. Is there anyone who knows?

Thank you a lot!
Min