Hi all, 

I'm relatively new to user subroutines in general - I have written a UHYPER and a UANISOHYPER_STRAIN, but I have little experience with UMATs. I was hoping someone could share some of their experience with me.

How could I implement linear viscoelasticity to a preexisting non-linear elastic material (which is based upon a strain energy function i.e. hyperelastic)?

Could I do this by adding linear viscoelasticity to the material separately from the subroutine? My constitutive model, for example, uses the internal variables method, so if this is the case how does Abaqus solve the viscous effects?

With UHYPER or UANISOHYPER you don't explicitly calculate the stress matrix. As such, I assume (perhaps wrongly?) that UHYPER or UANISOHYPER cannot be used to incorporate viscoelasticity. However, since you do explicitly calculate the stress matrix in a UMAT, will I have to write the viscoelasticity algorithm (internal variables method) in the UMAT to directly calculate the stress matrix?

I should note that I am currently using S4R elements, the material is incompressible, and the model is in the finite-strain regime.

Any guidance would be greatly appreciated :) 

Abaqus ​​​​​​​