Dr. Jorgen,

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,

Warm regards,

Melly