# 8.13. Common Contact Algorithmic Issues

This sections describes some of the common problem issues that may be encountered when setting up contact.

Contact constraints are one type of constraint among many possible types of constraints. Other constraint types include MPCs (Section 7.16), kinematic constraints (Section 7.4), and welds (Section 7.16.3). Contact constraints can potentially conflict with these other constraint types causing unexpected behavior. Refer to Appendix D for information on how conflicting constraints are handled.

Tied contact that occurs over a substantial distance is not guaranteed to preserve energy or angular momentum at the interface. The issue becomes more problematic as the distance between tied objects becomes a substantial fraction of the contact face size. It is recommended that default search tolerances are used so that only objects in close proximity are tied.

Lofted objects that are large compared to the element size are problematic. For example, lofting of shells that are thicker than they are wide or beams with a large radius compared to their element length.

The speed of contact is based primarily on the number of nodes and faces in the contact surfaces and, to a lesser extent, on the number of interactions specified. Choosing excessively large search tolerances will greatly increase the run time required by contact.

Contact performs best when objects in contact are on the same processor. It is strongly recommended to use a proximity based mesh decomposition method such as RCB or inertial for contact problems. If large motions occur during the problem solution, dynamic mesh rebalancing may be needed to maintain good performance, see Section 6.8.

The glued friction model is enforced by a velocity constraint. Over a very large number of steps, small integration errors can build up and potentially allow for some slip at the glued interface.

Large local mesh motions may cause inversion of the contact interface. It is recommended that shape metric based element death or other mechanisms be used to prevent the large errors that can occur when computing contact on tangled surfaces. Ensuring the nodal Jacobian ratio of all elements is greater than zero will usually ensure the integrity of the contact surface definition.

Even when using frictionless interaction laws, contact interfaces will tend to be energy absorbers. This is due to contact interfaces behaving plastically rather than storing impact energy.

Initial overlap of contacting bodies may cause major instabilities in explicit contact runs. The remove initial overlap capability can be used to remove small overlap. Large overlaps must be removed by modifying the underlying geometry or running a preload step.

The implicit and explicit contact solves behave subtly different. If an implicit static preload is used and that state handed off to an explicit analysis, there will usually be a small equilibrium imbalance that needs to ring down after the transfer.