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UMAT convergence problem

UMAT convergence problem


I'm simulating the behavior of anisotropic material using a UMAT subroutine. The UMAT works fine when using single element. However, when I use multiple elements (C3D8R), I get convergence problems.

The problem is as follows:
The simulation stops suddenly with error 'Too many attempts.....'. The point where the simulation stops depend on the elements size. When I increase the element size (coarser mesh) I get better convergence, but for finer meshes, the simulation isn't completed.

I've tried to increase the hourglass stiffness, and I found that for higher values, the simulation is completed, but the resulting forces are higher !

Is this problem coming out from the definition of the DDSDDE matrix inside the UMAT, or it's a result of using reduced elements (C3D8R)?

Thank you in advance.

Best Regards.

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