I am trying to write a VUMAT to implement the Arruda-Boyce viscoplastic model. After assuming linear elasticity at the beginning of the analysis, the total deformation gradient (F) can be easily obtained from defgradNew.
I am a little bit confused about how to decompose F into its elastic and plastic components. Should I obtain the plastic components from the state variables and then, of course, find the elastic part via the product of F and the inverse of the plastic part?
How about the plastic flow rate, the rate of the plastic deformation, time-derivative of the deformation gradient, and the strain softening. How to incorporate these to capture the material response?
My reference is Chapter 8 of your book (Mechanics of Solid Polymers).
Thank you very much,