non-linear viscoelastic

Hi all,

I'm using Code-Aster (FE solver) and I need to implement a UMAT subrouting to model a non-linear viscoelastic model in it.

The model is combination of viscoelasticity+ Ogden hyperleastic model. I was thinking of 2 approaches:

1. I wonder do you know where I can find such a subroutine?

2. I've seen linear viscoelastic subroutine and maybe I can find another subroutine for hyperlelastic part, I wonder how can I combine this two models to a non-linear viscoelastic model? 

I really appreciate your help

BME