Hi

I am using a FORTRAN FE code for 4 quad elements with a viscoelastic material and with contact analysis. The routine follows a Newton Raphson iteration scheme and which makes the residual zero for convergence. I am using direct sparse matrix solver from the FORTRAN math library.

When I use relatively lesser number of elements, about 1500 the solution converges and results are as expected. However, for the same analysis if I use more elements say about 4000, then the analysis starts to diverge midway. The residual starts to increase exponentially and the solution doesn't converge.

What could be the reason for this and how can I overcome it? Any suggestions would be helpful.

  The difference to the common model used is the f,g terms. The 3D FEM formulation has been derived and a new fortran program is written. The derivative matrix is symmetric. But the calculation result does not converge. I have tried every means to get a converging result. But it doesn't work, the residual norm always increases.