I'm running a code analyzing the creep deformation of a compact tension specimen under a substantial dead load. I'm calling your hybrid model UMAT in Abaqus 6.7. I find that the evolution of stress and chain strain at the notch tip are not similar with CPE4 and CPE4R elements. Specifically, the CPE4 elements predict a chain stretch slightly above that required for failure (e~1.1) after 5000 seconds, and around that time the code aborts due to too many iteration attempts after reducing the step time in abaqus/standard. However, CPE4R elements do not reach such an extreme state even after 500,000 seconds, pseudo-asymptotically reaching about chain strain e=0.99 (e~0.875 at 5000 seconds).
My question is: what do you think about the divergence in solutions between the various element types using your UMAT (I note some variance in your benchmarks runs)? Given that the difference is that between failure and survival, I can't ignore the discrepancy.
Is there reason to believe that the creep behavior is better predicted by one element over another?
I am currently running the simulation with a finer mesh and an incrememtal creep strain tolerance of 1e-6 to see if it improves.