Calculation of plastic strain

Hello everybody,

i am modeling my crosslinked epoxy with a parallel network by using a yeoh model and a linear elastic + Bergstrom-Boyce Network-Dependent Flow. I have a question regarding the material model.

I am trying to model my softening, which is not a lot for my epoxy and an exponential evolution is enough. Is the plastic strain calculated there by using the plastic deformation gradient F_p and calculating the strain ln(V) ?