[CIG-SHORT] solver issue, fieldsplit with traction boundary

[Birendra jha]

Dear developers

I am facing problems with fieldsplit+ML preconditioner for fault friction when I apply traction boundary condition on a face. 

When I use the fieldsplit and ML preconditioner, I get following error:

 -- Solving equations.
  0 SNES Function norm 9.795955105332e+01
[0]PETSC ERROR: --------------------- Error Message -------------
[0]PETSC ERROR: Detected zero pivot in LU factorization:
...
...
    def solve(self, *args): return _problems.SolverNonlinear_solve(self, *args)
RuntimeError: std::exception

Without fieldsplit+ML, linear solve takes ~800 iterations. From the norm (~200), it does not look like nonlinear solve will converge.

Without the traction boundary condition, it runs fine. But I need to apply some compression on the fault.

I have attached the two cases.

Thanks a lot for your help.
Birendra
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