ABAQUS/Explicit provides two distinct algorithms for modeling contact: general contact (defined with the *CONTACT option) and contact pairs (defined with the *CONTACT PAIR option).
General contact interactions allow you to define contact between many or all regions of a model; contact pair interactions describe contact between two surfaces or between a single surface and itself.
General contact can be defined in the model or history part of the input file; contact pairs are defined in the history part of the input file.
Surfaces are defined using the *SURFACE option. Individual nodes can be included in a contact pair by using the TYPE=NODE parameter on the *SURFACE option. Analytical rigid surfaces are assigned to a rigid body by using the ANALYTICAL SURFACE parameter on the *RIGID BODY option. The parameters TYPE=SEGMENTS, CYLINDRICAL, or REVOLUTION on the *SURFACE option specify the type of analytical rigid surface.
Surfaces used with the general contact algorithm can span multiple unattached bodies. More than two surface facets can share a common edge. In contrast, all surfaces used with the contact pair algorithm must be continuous and simply connected.
Single-sided surfaces on shell, membrane, or rigid elements must be defined so that the normal directions do not “flip” as the surface is traversed.
ABAQUS/Explicit does not smooth rigid surfaces; they are faceted like the underlying elements. Coarse meshing of discrete rigid surfaces can produce noisy solutions with the contact pair algorithm. The general contact algorithm does include some numerical rounding of features.
Tie constraints are a useful means of mesh refinement.
ABAQUS/Explicit adjusts the nodal coordinates without strain to remove any initial overclosures prior to the first step. If the adjustments are large with respect to the element dimensions, elements can become severely distorted.
In subsequent steps any nodal adjustments to remove initial overclosures induce strains that can potentially cause severe mesh distortions.
The ABAQUS Analysis User's Manual contains more detailed discussions of contact modeling in ABAQUS. “Contact interaction analysis: overview,” Section 21.1.1 of the ABAQUS Analysis User's Manual, is a good place to begin further reading on the subject.