HI,

I have implemented the nonlinear elastic part (Network A) of the Bergstroem Boyce model (2001) in a UMAT. I am using a forward difference approximation with a perturbation of the deformation gradient for the elasticity tensor (Sun 2008, Miehe 1994). At this stage I am comparing my user material subroutine with the Abaqus built-in Arruda-Boyce model using a 1-element cube. When I apply a prescribed displacement then the two approaches deliver very similar results (stresses).

In a second step I applied a constant load. The simulation using the UMAT did not converge. However looking at the results did not reveal any obvious issues. I then thought that there is a flaw in the formulation of the elasticity tensor. So far I have not found one. I then applied a constant load using the built-in Abaqus model and to my surprise the simulation did not converge either. I am at the moment a bit confused as to what could be wrong.

Has anyone any suggestions?

Thanks a lot,

Andreas