Convergence behavior for contact problems depends strongly on the particular problem. The options listed below are either typical or recommended for most surface-to-surface contact analyses. See the Command Reference for further details.
The time step size must be small enough to capture the proper contact zone. The smooth transfer of contact forces is disrupted if the time step size is too large. The time step size is specified by a number of substeps or the time step size itself. The NSUBST and DELTIM commands adjust these parameters.
Note: A reliable way to set an accurate time step size is to turn automatic time stepping on.
The AUTOTS,ON option is automatically invoked, but may override the time step parameters if needed.
If the contact status changes during the iteration process, discontinuity can occur. To avoid a slow convergence rate and use an updated stiffness matrix, use the NROPT,FULL,,OFF command to set the Newton-Raphson option to FULL.
Also, do not use adaptive descent. Adaptive descent usually does not provide any help for surface-to-surface contact applications, and Ansys recommends turning it off.
In cases where frictional sliding dominates, set the unsymmetric solver option (NROPT, UNSYM,,OFF) to avoid slow convergence or divergence.
Use the NEQIT command to set the number of equilibrium equations to a number that is appropriate for a reasonable time step size. This command defaults to between 15 and 26 iterations, depending upon the physics of the problem.
Because the iterations tend to become unstable for large increments, use the line search (LNSRCH) command to stabilize the calculations.
Turn the predictor-corrector option on with the PRED command, except for dynamic analyses.
Many convergence failures in contact analyses are the result of using too large a value for contact stiffness (real constant FKN). Be sure to follow the recommendations given earlier in this chapter for estimating contact stiffness. If such estimated values lead to a convergence failure, reduce the contact stiffness and restart.
Conversely, if overpenetration occurs in your contact analysis, you probably need a larger value of FKN. In this case, gradually increase the contact stiffness value to an appropriate level by redefining it using RMODIF commands over several load steps in a restart.