[petsc-users] Hints for using petscfe for plasticity -- how to update/access internal variables?

[Maximilian Hartig]

Hello,

I’m trying to implement plasticity using petscFE but I am quite stuck since a while. Here’s what I’m trying to do:

I have a TS which solves the following equation:
gradient(stress) +Forces = density*acceleration
where at the moment stress is a linear function of the strain and hence the gradient of the displacement. This works fine. Now I want to compare the stress to a reference value and if it lies above this yield stress, I have to reevaluate the stress at the respective location. Then I need to update the plastic strain / yield stress  at this location.
I tried doing that first by solving three fields at the same time: displacements, stresses and yield stress. This failed.
Then, I tried solving only for displacement increments, storing the displacements, stresses and yield stress from the past time step in an auxiliary field. The auxiliary fields are updated after each time step with a second SNES, using the displacement increments from the current, converged time step. This also failed.
In both cases the code had problems converging and when it did, I ended up with negative plastic strain. This is not possible and I don’t know how it happens because I explicitly only increment the plastic strain when the increment is positive.

I am sure there is an easy solution to how I can update the internal variables and determine the correct stress for the residual but I just cannot figure it out. I’d be thankful for any hints.

Thanks,
Max