Mesh refinement

For contact as well as all other types of analyses, the solution improves as the mesh is refined. For contact analyses using a pure main-secondary approach, it is especially important that the secondary surface is refined adequately so that the main surface facets do not overly penetrate the secondary surface. The balanced main-secondary approach does not require high mesh refinement on the secondary surface to have adequate contact compliance. Mesh refinement is generally most important with pure main-secondary contact between deformable and rigid bodies; in this case the deformable body is always the pure secondary surface and, thus, must be refined enough to interact with any feature on the rigid body. Figure 1 shows an example of the penetration that can occur if the discretization of the secondary surface is poor compared to the dimensions of the features on the main surface. If the deformable surface were more refined, the penetrations of the rigid surface would be much less severe.

Figure 1. Example of inadequate secondary surface discretization.

Tie constraints

The tie constraint prevents surfaces initially in contact from penetrating, separating, or sliding relative to one another. It is, therefore, an easy means of mesh refinement. Since any gaps that exist between the two surfaces, however small, will result in nodes that are not tied to the opposite boundary, you must adjust the nodes to ensure that the two surfaces are exactly in contact at the start of the analysis.

The tie constraint formulation constrains translational and, optionally, rotational degrees of freedom. When using tied contact with structural elements, you must ensure that any unconstrained rotations will not cause problems.