[CIG-SHORT] convergence troubles: drucker-prager material with fault

[Eric Lindsey]

Hi guys,

I'm having some trouble getting the model with a fault in an elastoplastic
rheology (plane strain) to converge. I'm pretty sure I've fixed all the
boundary conditions to behave correctly now. The LU method fails entirely
once yielding occurs, and the approximate method (multiplicative fieldsplit
using ml and jacobi for the two domains) does not converge. More details:

The first thing I tried was using (solver04.cfg) from the tutorial, which
is a schur fieldsplit with LU and the custom Pylith preconditioner. In the
case of a fault in a purely elastic material, this procedure is working
fine. The nonlinear solve does take over 100 iterations, I'm not sure if
this is just a function of my mesh; anyway I'm not too worried about
optimization yet.

In the plastic case, whenever stresses near the bends in the fault begin to
exceed the yield criterion (I increase them slowly to this value over
several time steps), I get a Zero Pivot error. But maybe this is not
unexpected for a plastic rheology?

I also tried the solver options for an approximate solution with
multiplicative fieldsplit suggested by Brad (solver08.cfg): The elastic
case still works fine; it uses way more KSP iterates but still finishes in
about the same total time. But the plastic case still throws a Zero Pivot.
When I ran it earlier without increasing snes_max_it, I got a string of
these messages instead:

"Nonlinear solve did not converge due to DIVERGED_FNORM_NAN iterations 0"

In either case, I think I must need a better solver. I've gone back through
the tutorial videos for the solvers, but I'm not very clear on how to apply
the nonlinear options from the driven cavity problem to my situation, or
really where to start. My input files are attached, but I'm happy to send
any additional files or output as needed. Any insight, suggestions, or wild
guesses?

Thanks
Eric
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