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
Dear Daniel
for 3D simulation soil, rock ,.., we have some Simplification. did u checked your boundary condition and interface between your soil and cylinder? in the lab when we wanna measuring triaxial stress, first of all we have compaction then we select normal pressure and back pressure with different amount, i think the difference is about 0.2kg/cm3. then we start to internal lateral and vertical stress. it means, first you need compacting your sample in simulation by statistically method and then you starting to apply your force in different time and reading the displacement. make sure your boundary condition is correct. you need select the dashpot boundary and determine what is the max distance for your boundary. also you need make sure the displacement is zero before apply the force after compaction. I hope you achieved to your results soon.
B R
Parsa
A couple of things strike me about your problem. Firstly you are using a single solid element. This might be okay if the stress field is fairly simple, e.g., constant but if not then you might want to try a more refined mesh. Secondly, and this is more likely the problem you are facing, once the cylinder has reached yield it can then, in the absence of hardening, take no more load and so cannot converge - this behaviour is correct. As you note if the problem is displacement driven then this is not a problem but it is with the force driven case.
Hello Daniel, i have used same boundary and loading conditions. I have used C3D8R elements. Simulation run but i don't know how to plot (σ1-σ3) vs axial strain and volumetric strain vs axial strain. Can you help me in this regard??
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