[CIG-SHORT] Elastoplastic material question

[Eric Lindsey]

I'm having trouble understanding the DruckerPragerPlaneStrain material. I'm
using a simple homogeneous domain with no faults, and a simple Dirichlet BC
imposing shear on the top/bottom. I've imposed an initial isotropic
compression, then I add the shear displacement gradually until it should
exceed the yield stress. On the sides I have a Neumann condition to
maintain the normal stress; for the moment I just set the shear tractions
to zero, but this value doesn't affect the results I'm getting. I expected
to see uniform plastic shear throughout the domain, but instead get the
message:

RuntimeError: Infeasible stress state - cannot project back to yield
surface.

If I include "allow_tensile_yield = True", the model runs, but instead of
homogeneous strain I'm getting the attached result. This is strange because
at no point should the material be under absolute tension; the maximum
shear stress in the elastic case is never larger than the magnitude of
compressive stress. I think I am misunderstanding the setup of this
material somehow; hopefully it's a simple error? Input files are attached,
the elastic case works just fine. Any suggestions would be much appreciated.

Thanks,
Eric

Relevant lines from the spatial databases:
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