## 3.5. Designating Contact and Target Surfaces

Contact elements are constrained against penetrating the target surface. However, target elements can penetrate through the contact surface. For rigid-to-flexible contact, the designation is obvious: the target surface is always the rigid surface and the contact surface is always the deformable surface. For flexible-to-flexible contact, the choice of which surface is designated contact or target can cause a different amount of penetration and thus affect the solution accuracy. Consider the following guidelines when designating the surfaces:

• If a convex surface is expected to come into contact with a flat or concave surface, the flat/concave surface should be the target surface.

• If one surface has a fine surface mesh and, in comparison, the other has a coarse mesh, the fine mesh should be the contact surface and the coarse mesh should be the target surface.

• If one surface is stiffer then the other, the softer surface should be the contact surface and the stiffer surface should be the target surface.

• If higher-order elements underly one of the external surfaces and lower-order elements underly the other surface, the surface with the underlying higher-order elements should be the contact surface and the other surface should be the target. However, for 3-D node-to-surface contact, the lower-order elements should be the contact surface. The higher-order elements should be the target surface.

• If one surface is markedly larger than the other surface, such as in the instance where one surface surrounds the other surface, the larger surface should be the target surface.

• In the case of 3-D internal beam-to-beam contact modeled by CONTA176 (a beam or pipe sliding inside another hollow beam or pipe), the inner beam should be considered the contact surface and the outer beam should be the target surface. However, when the inner beam is much stiffer than the outer beam, the inner beam can be the target surface.

These guidelines are true for asymmetric contact; however, asymmetric contact may not perform satisfactorily for your model. The following section details the difference between asymmetric and symmetric contact and outlines some of the situations that require symmetric contact.

### 3.5.1. Asymmetric Contact vs. Symmetric Contact

#### 3.5.1.1. Background

Asymmetric contact is defined as having all contact elements on one surface and all target elements on the other surface. This is sometimes called "one-pass contact." This is usually the most efficient way to model surface-to-surface contact. However, under some circumstances asymmetric contact does not perform satisfactorily. In such cases, you can designate each surface to be both a target and a contact surface. You can then generate two sets of contact pairs between the contacting surfaces (or just one contact pair; for example, a self-contact case). This is known as symmetric contact (or "two-pass contact"). Obviously, symmetric contact is less efficient than asymmetric contact. However, many analyses will require its use (typically to reduce penetration). Specific situations that require symmetric contact include models where

• The distinction between the contact and target surfaces is not clear.

• Both surfaces have very coarse meshes. The symmetric contact algorithm enforces the contact constraint conditions at more surface locations than the asymmetric contact algorithm.

If the meshes on both surfaces are identical and sufficiently refined, the symmetric contact algorithm may not significantly improve performance and may, in fact, be more "expensive" in CPU time. In such circumstances, pick one surface to be the target and the other the contact surface.

For a symmetric contact definition, ANSYS may find one side of a contact surface as closed and the other side of the surface as closed. In this case, it can be difficult to interpret the results. The total contact pressure acting on both sides is the average of the contact pressures on each side of the surface.

#### 3.5.1.2. Using KEYOPT(8)

When there are several contact pairs involved in the model, and the graphical picking of contact and target surfaces is difficult, you can just define the symmetric contact pairs and, by setting KEYOPT(8) = 2, ANSYS will internally select which asymmetric pair is to be used at the solution stage based on the guidelines mentioned above in Designating Contact and Target Surfaces.

 Note: In any contact model, you can mix different types of contact pairs: rigid-to-flexible or flexible-to-flexible contact; symmetric contact or asymmetric contact. However, only one type can exist with a contact pair.