I was trying to simulate the unloading process of clay consolidation in Abaqus using clay hypoplasticity umat (https://soilmodels.com/download/plaxis-umat-hypoplas-zip/). But I had a convergence issue showing in Fig.1.
The model was meshed with one element (C3D8). The parameters for clay are showing in the Fig.2, initial void ratio was 1.6, calculated using equation in Fig.2 when OCR=1 and p = 1. As Fig.3 shows, the circumferential surfaces are restricted to move in either x or z direction, and the bottom surface is restricted in y direction. In Step 1, 25KPa pressure was applying on the top of the specimen and removed in step 2, linearly with time in both steps. These boundary conditions are expected to make a consolidation process.
However, when the pressure on the top started to decrease in the Step 2, there was a convergence issue. I checked the output files that were used to record message from umat, error was 0 and umat seemed working good. Intergranular strain concept and small stiffness concept were not included.
Did anyone encounter a similar problem? I think this clay umat can be used to simulate unloading process but I can’t make it run. I appreciate any help from you.