Hello,
I am trying to solve a nonlinear contact problem using penalty method. The
tangent stiffness matrix is of the form K_e + c * K_p, where K_e and K_p
are the elastic and penalty contributions and c is the penalty parameter,
which is a large number (in the order of 10^2 to 10^3 in our case). The
part of the problem that carries out the linear solve for the incremental
displacement using Newton Raphson method is below:
****************************************************************************************
PETScWrappers::MPI::Vector distributed_incremental_solution(
locally_owned_dofs, mpi_communicator);
distributed_incremental_solution = incremental_solution;
SolverControl solver_control(20000, 1e-5);
PETScWrappers::SolverCG solver(solver_control, mpi_communicator);
PETScWrappers::PreconditionBlockJacobi preconditioner(system_matrix);
solver.solve(system_matrix,
distributed_incremental_solution,
system_rhs,preconditioner);
incremental_solution = distributed_incremental_solution;
hanging_node_constraints.distribute(incremental_solution);
solution.add(1.0,incremental_solution);
****************************************************************************************
I receive the following error upon running the code:
------
Iterative method reported convergence failure in step 33. The residual
in the last step was 0.000778898.
This error message can indicate that you have simply not allowed a
sufficiently large number of iterations for your iterative solver to
converge. This often happens when you increase the size of your
problem. In such cases, the last residual will likely still be very
small, and you can make the error go away by increasing the allowed
number of iterations when setting up the SolverControl object that
determines the maximal number of iterations you allow.
The other situation where this error may occur is when your matrix is
not invertible (e.g., your matrix has a null-space), or if you try to
apply the wrong solver to a matrix (e.g., using CG for a matrix that
is not symmetric or not positive definite). In these cases, the
residual in the last iteration is likely going to be large.
-------
For my problem, I don't require a relative error of the order of say
10^(-12) as is often used in the tutorials and a relative error of 10^(-4)
probably should also work. I increased the relative error to 10^(-4) and
still got a similar error with the final residual a little more than
10^(-4). I further increased the relative error to 10^(-3), but the
accuracy of the results got compromised. Let me know if there is any way to
go around this issue.
Also please suggest if there is a way to NOT make the code stop if such a
situation arises and instead accept the result as it is and move ahead?
Finally, do you recommend any other solver in this case .
Thanks,
Sabyasachi
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