The contact detection algorithm

Surfaces must meet two requirements to be identified by the automatic contact detection tool:

  • The surfaces must be separated by a distance less than or equal to the specified separation tolerance.

  • The surfaces must be intuitively opposed, as defined below.

Abaqus/CAE defines the separation between two surfaces as the distance between the points of closest approach on the surfaces. This distance is reported in the Separation column of the contact pair candidates table. The separation tolerance is the primary input used during the contact detection search. You should specify a separation tolerance that encompasses the separation distances between all of the potentially contacting surfaces in your model. For more information, see Choosing a separation tolerance and extension angle below.

Note:

The value reported in the Separation column may not correspond exactly to the separation used by Abaqus/Standard during an analysis. Certain automatic surface enhancements applied during the analysis to improve contact robustness (such as main surface smoothing and surface extensions) can lead to slight discrepancies between the separations calculated in the Abaqus/CAE preprocessor and the Abaqus/Standard analysis. For more details on automatic surface enhancements and contact formulations, refer to Defining Contact Pairs in Abaqus/Standard.

Two surfaces are considered intuitively opposed if the two surface normals constructed at the points of closest approach lie between 135° and 225° of each other (see Figure 1). In other words, the surfaces must be offset from each other by less than 45° at the points of closest approach. It is not possible to adjust or ignore the surface orientation requirement.

Figure 1. The relative orientation of the normals determines whether or not the surfaces are intuitively opposed.

Figure 2 illustrates a simple example of the contact pair requirements.

Figure 2. Two bodies involved in potential contact. The bodies are rendered in two dimensions for simplicity.

The dashed line represents the separation tolerance as calculated from surface X. Surface B, which is parallel to surface X, is identified as part of a contact pair because it is both within the separation tolerance and intuitively opposed to surface X. Similarly, surface C meets both of these criteria. Surface D, although it is intuitively opposed to surface X, does not lie within the separation tolerance at any point; surface D is not considered for inclusion in a contact pair. Surface A, although it is within the separation tolerance, is not intuitively opposed to surface X; therefore, surface A is also excluded from any contact pair definition. The connected surfaces (A, B, C, and D) do not form contact pairs with each other. By default, Abaqus/CAE only searches for surfaces on separate part instances. However, even if you were to enable searching within the same instance (see Defining contact within the same instance and self-contact below), these surfaces would not meet the orientation requirements.