In general, the procedure-level force and displacement convergence checks (CNVTOL command) are adequate for achieving solution accuracy. However, in cases of contact chattering (changes in contact status during iterations within a substep), the resulting contact forces may oscillate, and the global solution could possibly converge to an unstable configuration if the procedure-level force convergence tolerance is not tight enough.
In addition to the global force convergence check, you can activate a local convergence check by specifying a convergence tolerance value as contact element real constant TFOR. The local convergence check verifies equilibrium on a per-contact-pair basis, as follows:
For a contact pair, the pair-based force convergence norm is the absolute difference of the norm of the contact forces from the current to the previous iteration.
The pair-based force convergence criterion is the product of the convergence tolerance (TFOR) and the larger of these two values: the global force convergence criterion or the pair-based force convergence norm.
The local convergence check is met if the pair-based force convergence norm is less than the pair-based force convergence criterion.
Specifying this local convergence check can prevent oscillations of the contact reaction force and improve the accuracy of results. In some cases, adding a few iterations to force solution accuracy helps solution convergence for subsequent substeps. The local convergence check is activated only if the pair-based convergence tolerance, real constant TFOR, is specified. A tolerance value between 0.001 to 0.1 is recommended.
You can monitor the pair-based force norm and criterion via the NLHIST and NLDIAG commands. For the purpose of monitoring, the program uses a default tolerance value of 0.1 to calculate the pair-based force convergence norm and pair-based force convergence criterion. This is not a check for local convergence. It is for monitoring purposes only and is useful for nonlinear contact diagnostics.