Dear Jorgen and all,
Recently I have been working on a subroutine (VUMAT) for finite strain elastoplasticity which utilizes generalized strain and stress measures.
I have done my 1 element analysis with error-free results. Then I have started to solve more complicated ones.
For 3D analysis, although the code works quite well until certain deformation levels, there comes a point where the analysis looses stability and crashes.
As an example, for a 3D necking problem (triggered by a geometrical imperfection in the case of Simo and colleagues), I have quite good results
which preserves symmetry, however then after certain point the symmetry is lost, as far as the distribution of equivalent plastic strains are concerned.
I also see that the loss of symmetry is always in a certain direction. I mean if the circular bar has a x-section located on 1-2 axis and longitudinal axis is 3, the accumulation of equivalent plastic strains occur on axis-1 at the face.
What do I have to suspect at this point. Can you help me with solutions or can you suggest me some benchmarks?
Thank you for your time,