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Bergstroem Boyce Model - hyperelastic part - convergence problems

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Hello,

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

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