11.5. Troubleshooting Mortar Contacts

11.5.1. Relax the Penalty Stiffness of Mortar contacts

Relaxing the penalty stiffness of the Mortar contacts by decreasing the scale tracked-side stiffness factor SFSA on the *CONTACT card often provides a smoother response from the model and improves convergence.

This may be an effective remedy when the convergence is "erratic", with residual norms (see Figure 11.1: Tracking convergence information in the mes0* files, Figure 11.2: Examples of convergence histories) "jumping up and down" between a low value and a significantly higher value for every other two iterations.
Changing the penalty stiffness can sometimes help to reveal other problems with the analysis. In most cases, modifying the default contact penalty settings is unnecessary.
The Mortar contact reports both relative as well as absolute values of the maximum penetrations during the simulation in the mes0* files, before each implicit step begins, for example:
```
Contact sliding interface 1010
Number of contact pairs 2784
Maximum penetration is 0.1122220E-02 between
elements 14606 and 10263 on this processor
Maximum relative penetration is 0.9978402E-01 % between
elements 14606 and 10263 on this processor
```
Based on this, you can judge what is acceptable in terms of penetration distance.
The Mortar contacts issue warnings in the mes0* files if the penetration becomes too big to handle for the penalty-based approach. Search for:
```
 *** Warning Penetration is close to maximum before release
```
In case the contact is released, a very unphysical configuration may result, making convergence impossible.
If initial penetrations are present, try minimizing them.
Try increasing the ramp-up of the penalty stiffness by setting (for example) IGAP = 5, 10, 50, 100 (etc.) see also Figure 6.2: Mortar penalty force as a function of penetration (schematic)..
Also, increasing the contact penalty stiffness may help to reduce risk of contact release.

11.5.2. Check Mortar Contacts with Nonlinear Solver

Starting at R11 of LS-DYNA, Mortar contacts can also interact with the nonlinear solver. Should very large penetrations occur during the iterations, the Mortar contact can cause the solver to retry the implicit time step with a smaller time increment. The following message is then printed in the d3hsp file (for example):

Contact algorithm rejected current iterate.
list of element pairs affected follows (at most 10):
   Element #      488758 vs      696147
-----------------------------------------------------
    automatic time step size DECREASE, RETRY step:
  dt(old) = 1.00000E-02     dt(new) = 4.641159E-03
-----------------------------------------------------

In this example, the elements 488758 and 686147 are listed as problematic. Inspect the model using the d3iter and d3plot files in the vicinity of the listed elements to determine the cause of the contact problems. It is also very possible that contact problems occur only a few times, allowing the simulation to successfully complete with satisfactory results.

Starting in R16 of LS-DYNA, you can prevent Mortar contacts from causing the nonlinear solver to reject an iteration. Set the second field of the seventh card on *CONTROL_CONTACT to 1, as shown in the following example.

*CONTROL_CONTACT
…
$Card#7    noreject    TIEOPT
                  1         1

You can use this technique during the troubleshooting process to see if the solution can progress further and if more information from the solution can be found.

11.5.3. Switch to Mortar Tied Contacts

If you suspect that contacts are causing the convergence problems, try switching to Mortar tied contacts (*CONTACT_AUTOMATIC_..._MORTAR_TIED_ID). If this improves convergence, the issue may be with the contacts, for example contact release (see above) or rigid body modes in the model. Switching from sliding to tied contacts can reduce or remove rigid body modes by creating couplings between parts.

Note that switching to Mortar tied contact is not to be seen as a final solution to convergence issues. Rather, it is a possible step in the troubleshooting process. Switching to tied contacts may advance the analysis progress further, revealing other problems with the model.

11.5.4. Avoid shell elform 2 for Mortar contacts

Do not use shell elform 2 for parts involved in Mortar contacts if shell thickness update is active (ISTUPD ≠ 0 on *CONTROL_SHELL). Use the fully integrated formulation 16 instead.

See Element Types for recommendations regarding element formulations.