Load-controlled triaxial test simulation in ABAQUS using clay plasticity material model

[danieljgalles]

I am attempting to simulate a triaxial test in ABAQUS/Standard using the clay plasticity model in the ABAQUS material database. I am using a 3-D cylindrical specimen which contains a single element (3D stress - C3D10) . For my initial BCs, I applied a zero-displacement BC in the vertical direction on the bottom of the specimen. My first step is a geostatic step, in which I apply the confining pressure. In my second step, I apply a pressure at the top of the specimen. The geostatic step converges in a single increment, which tells me my initial stress is correct. For my second step, I converge as long as my stress state is within the yield surface. However, if I apply a pressure that causes the stress state to reach the yield surface, the simulation will not converge.

I then removed the pressure from step 2 and replaced it with a displacement-controlled BC. The ensuing simulation converged with no problems.

I'm wondering if anyone has a suggestion how I can achieve convergence using a load-controlled BC? I've tried using stabilization techniques described in the ABAQUS manual, but the ratio of my static dissipation energy is too high (roughly 20% of the internal energy). Other things I've tried (but had no success) are specifying nonlinear geometry and running a soil analysis using pore fluid/stress elements. This seems like a very simple problem - perhaps I'm over-looking something. Has anyone else encountered this problem?

Thanks,
Daniel

 

[csk62]

I think you're correct in stating that the 20% is probably an indication of excessive damping energy in comparison to internal strain energy.
I would think that NLGEOM would have to be specified in an analysis like this where the stress state exceeds the yield surface, and large deformations can occur.
What information is given in the .msg file?