Contact problems are highly nonlinear and require significant computer resources to solve. It is important that you understand the physics of the problem and take the time to set up your model to run as efficiently as possible.

Contact problems fall into two general classes: rigid-to-flexible and flexible-to-flexible. In rigid-to-flexible contact problems, one or more of the contacting surfaces are treated as rigid (that is, it has a much higher stiffness relative to the deformable body it contacts). In general, any time a soft material comes in contact with a hard material, the problem may be assumed to be rigid-to-flexible. Many metal forming problems fall into this category. The other class, flexible-to-flexible, is the more common type of contact. In this case, both (or all) contacting bodies are deformable (that is, they have similar stiffnesses). An example of a flexible-to-flexible contact is bolted flanges.

Contact problems present two significant challenges. First, you generally do not know the regions of contact until you've run the problem. Depending on the loads, material, boundary conditions, and other factors, surfaces can come into and go out of contact with each other in a largely unpredictable and abrupt manner. Second, most contact problems need to account for friction. There are several friction laws and models to choose from, and all are nonlinear. Frictional response can be chaotic, making solution convergence difficult.

In addition to these two considerations, many contact problems must also address multi-field effects, such as the conductance of heat, electrical currents, and magnetic flux in the areas of contact.

If you do not need to account for friction in your model, and the interaction between the bodies is always bonded, you may be able to use the internal multipoint constraint (MPC) feature (available for certain contact elements) to model various types of contact assemblies and surface-based constraints (see Multipoint Constraints and Assemblies for more information). Another alternative is to use constraint equations or coupled degrees of freedom instead of contact to model these situations (see Coupling and Constraint Equations in the Modeling and Meshing Guide for more information). The external constraint equations or coupling equations are only suitable for small strain applications.