Which is better - updating FKN (contact stiffness in the normal direction) each equilibrium iteration, each substep, or never updating FKN?

The answer to the above should be evaluated by reviewing the amount of contact penetration--specifically, one should compare penetration values (in units of length) compared to the local and overall deformations in that direction. (In general, when using penalty-based methods like "Pure Penalty" or "Augmented Lagrange," reviewing the amount of penetration as well as contact pressure distribution is needed.)

The contact stiffness update algorithm may increase *or* decrease FKN, depending on the solution progress. The contact stiffness update algorithm looks at a "maximum allowable penetration" parameter FTOLN (which is automatically set by default, based on mesh density - a finer mesh has a smaller allowable penetration [more accuracy] than a coarser mesh), along with convergence behavior and current contact pressure values to determine whether to increase or decrease contact stiffness. One can override the default FTOLN value through the use of real constant #4. In Workbench Simulation, under the "Solution Information" branch, you can also track contact stiffness, contact penetration, etc. during the course of the solution using the "Result Tracker" feature. That helps to see what the contact algorithm is doing.

While using the contact stiffness update algorithm is generally useful to balance accuracy and computational cost, it is unfortunately not substitute for evaluating the contact penetration that is present to determine whether or not the contact stiffness was too low.

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