Bonded contact (default) prevents sliding or separating, effectively treating contact regions as if they are glued. Any gaps are closed, and initial penetration is ignored. Since the contact length/area does not change with the application of the load, the solution is linear and computationally efficient.

Autodetected contact regions are defined as bonded by default (Connections > Generate Automatic Connection on Refresh = yes). Bonded contact is ideal when no relative motion is expected, such as welded joints. For modeling separation or stress near interfaces, consider using one of the nonlinear contact types: Frictionless, Rough, Frictional, or No Separation, Penetration Only. These handle gaps and more accurately model the true area of contact, but they may increase solution time and cause convergence issues. If convergence problems arise or if determining the exact area of contact is critical, apply a finer mesh (using Sizing control) on the contact faces or edges. Other contact types are listed below with examples of when they may be useful.

No Separation allows the contact pair to slide without friction but not separate (available only for regions of 3D solid faces or 2D plate edges). The solution is linear. Wipers sliding on a windshield is an example of bodies that slide but do not separate.

Bonded, Initial maintains the initial contact state for the entire analysis. If the contact is initially closed, it remains attached. For an open initial state, the contact remains open even if deformation brings the contact pair close together. Choose Bonded, Initial when the initial contact condition is a key factor in the analysis.

Frictionless models standard unilateral contact: normal pressure equals zero if separation occurs. Both separation and frictionless sliding are allowed. Therefore, gaps can form in the model between contact surfaces depending on the loading. The solution is nonlinear because the area of contact may change as the load is applied. A zero coefficient of friction is assumed. The model should be well constrained when using this contact setting. Weak springs are added to the assembly to help stabilize the model and achieve a reasonable solution. Frictionless contact is applicable when parts can come in and out of contact without any frictional resistance, for example lubricated surfaces.

Rough models frictional contact conditions that prevent sliding, corresponding to an infinite friction coefficient. Separation can occur, and the solution is nonlinear. Only applicable for regions of faces (for 3D solids) or edges (for 2D plates). No automatic closing of gaps is performed. Rough contact is an accurate contact type when parts are expected to stay locked together without any relative motion, for example, a tight interference fit.

No Separation, Penetration Only enforces that surfaces must remain in contact without separating once contact occurs. No gaps are allowed to form between the surfaces once they touch, ensuring that they remain in contact or penetrate each other during the analysis.

This contact type is useful for modeling mechanical systems with components that are expected to remain in contact under load, like press-fit joints or gasketed connections. By preventing separation that may impact the load path or stress distribution, this contact type is particularly advantageous when accurate modeling of the contact pressure distribution is required.

Frictional is the most realistic representation of the contact surfaces. The contact pair can separate and slide with friction (you specify the Friction Coefficient—the frictional resistance value between contact surfaces.). The surfaces in contact can carry shear stresses up to a certain magnitude across their interface before they start sliding relative to each other. The pre-sliding state is called "sticking”. Once an equivalent shear stress is reached, sliding begins as a fraction of the contact pressure.

Forced Frictional Sliding is similar to Frictional except that there is no "sticking" state. (Supported only for Rigid Dynamics). A tangent resisting force is applied at each contact point. The tangent force is proportional to the normal contact force.

Asymmetric behavior designates one surface as the contact side and the other as its counterpart, forming a single contact pair. The nodes on the contact surface are prevented from penetrating the target surface. Asymmetric behavior, also referred to as “one-pass contact,” is usually the most efficient way to model face-to-face contact for solid bodies. Behavior must be set to asymmetric if at least one of the contacting bodies has rigid Stiffness Behavior. Contact results are only available on the contact side for asymmetric behavior.

Auto Asymmetric automatically creates an asymmetric contact pair, when possible, which can boost performance. With this setting, the solver selects the best contact face during the solution phase. [In Explicit Dynamics analyses this option is available for Bonded connections; see Bonded Type.]

Symmetric behavior refers to the designation of each interacting surface as both a contact and a target, resulting in the formation of two distinct contact pairs from the same pair of surfaces. The nodes on the contact and target surface are prevented from interpenetrating. Symmetric contact behavior, also referred to as “two-pass contact”, is less efficient than asymmetric contact. However, many analyses will require its use, typically to reduce penetration, specifically when:

Note that as results are available on both sides of the contact pair, symmetric behavior makes results interpretation tricky. The actual contact result is the average of results from the contact surfaces. The symmetric pairs will have the same contact characteristics (using KEYOPT(8)=1) except when the Nonlinear Adaptive Region object is present.

Program Controlled (Default for the Mechanical APDL solver) behavior depends on the Stiffness Behavior of the bodies in contact, as summarized in the following table.

For Rigid-Rigid contact, the behavior property is under-defined for the Program Controlled setting. The validation check is performed at the Contact object level when all environment branches are using the Mechanical APDL solver. If the solver target for one of the environments is other than Mechanical APDL, then this validation check will be carried out at the environment level; the environment branch will become under-defined.

Rename: Enables you to change the contact region name to a name that you type (similar to renaming a file in Windows Explorer).

Rename Based on Definition: Enables you to change the contact region name to include the corresponding names of the items in the Geometry branch of the tree that make up the contact region. The items are separated by the word "To" in the new contact region name. You can change all the contact region names at once by clicking the right mouse button on the Connections branch, then choosing Rename Based on Definition from that context menu. A demonstration of this feature follows.