[QUOTE=Jorgen,4042]Yes, you can do that. The VUMAT needs to calculate the stress in an intermediate configuration. If you use U as F, then things are OK by default. If you use F, then you need to rotate your calculated stress back to the right configuration using R.

In other words, Abaqus/Explicit takes care of the rotations for you if you use U.

-Jorgen

Dear users,

I have two little questions to fix my ideas. The questions concern the use of the deformation gradient F when implementing a Vumat for hyperelastic material (Arruda Boyce).

If one need to use this tensor (F) directly when implementing the model (without using stretch tensor U), one needs to rotate the stress back to corotational frame (Abaqus documentation).

I think the first step is to calculate the rotation tensor R (not provided in the Vumat) using the relation: F=R*U (R=f*U-1), then apply the relation: TR*Stress*R (TR is the transposed matrix of R). Am I right?

Is there any way to do rotate the stress back using the relative spin increment Dt*(W-Omega) already provided in the Vumat?

Thank you very much.