Hi everyone here!
I have been working for a while on UMATs with anisotropic stiffness tensors, and since I couldnt make it converge I decided to go simpler and modeled a simple cube strained in tension around 1% using Abaqus Elastic Anisotropic material with full 21 different constants. This simple model does not converge when the large deformation option NLGEOM is activated, but works well without it. Does anyone know why this can happen?
Interestingly, when I use the original UMAT without the NLGEOM option, it does not converge, not even in the first increment, but have no idea why.
One important fact is that I verified that the eigenvalues of the stiffness matrix are all possitive, so it is possitive definite.