I have implemented a linear viscoelastic model using the user subroutine Usermat in Ansys.
I have used the classical paper: Formulation and implementation of three-dimensional viscoelasticity at small and finite strains of Kaliske & Rothert
and it works fine for small deformations.
It is written in the documentation that in the case of a simulation with geometric nonlinearity (large deflection), one must rotate the state variables.
Do I have to rotate these state variables if I use solid elements?
I would like to simulate bending of layers (like a shell, but made of solid elements with only displacement as dof).
Do I have to rotate the state variables because of the possible large bending even if the local deformation (strain) is small (I activate the option nonlinear geometric in Ansys) or can I leave the state variables as in the paper.
If so, how should I do that. At the end of each time step, I just multiply the tensor of state variables with the rotation matrix and save then for the next time step? Do I have to rotate any other tensor or the old state variables at the beginning of the new time step?
If I implement a viscoelastic model with a hyperelastic model, I dont have to rotate any tensor, if I define all the quantities in the reference configuration.