About Identity and Contact Pairs
Identity Pairs
An identity pair (Identity Boundary Pair ()) is a pair that, by default, makes the fields across two connected boundaries (one from each connecting object in an assembly) continuous. This is equivalent to the continuity that is obtained by default on interior boundaries in a geometry created by forming a union. Some physics provide special boundary conditions for identity pairs to model “slit conditions” such as resistive layers. You can specify boundary conditions for these pairs from the Pairs submenu at the bottom of the boundary condition part of the context menu for the physics feature node. The nodes in the Model Builder that represent pair boundary conditions use an icon with a pair symbol in the lower-left corner: . There are also similar Identity Edge Pair () and Identity Point Pair () pair nodes.
Contact Pairs
A contact pair () is a pair that define boundaries that can come into contact with each other under deformation for modeling of mechanical contact and multiphysics contact.
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Contact pair modeling requires the Structural Mechanics Module, MEMS Module, or Multibody Dynamics Module. More details about this pair type can be found in the Structural Mechanics Module user’s guide.
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In a continuity feature, it is possible to select either an identity pair or a contact pair. Selecting a contact pair is only meaningful in conjunction with a structural mechanics interface containing a Contact node in which the contact state for the pair is computed. If the boundaries are in contact, then the continuity is also active.
To add Pairs to any Component:
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In the Definitions toolbar, select features from the Pairs menu (), or
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Right-click the Definitions () node and choose an option from the Pairs submenu.
Go to Pair Name for information about the Settings window Pair name. Also see Settings and Properties Windows for Feature Nodes.
Fallback Boundary Conditions on NonOverlapping Parts
For pairs there can be parts of the boundaries that do not overlap, which typically represent exterior boundaries outside of the overlapping area. For those parts the other boundary conditions under the physics interface will contribute with proper conditions, acting as fallback boundary conditions on the nonoverlapping parts. These boundary conditions have an indicator in the lower-left corner () identifying their status as fallback boundary conditions (see Figure 5-5).
Figure 5-6: An example where the default Free boundary condition of the Solid Mechanics interfaced act as a fallback condition to the default Continuity pair.
For most physics interfaces, the default boundary condition will act as the default fallback condition for all pairs. If you want to use another boundary condition for the nonoverlapping parts, simply add that boundary condition from the context menu of the physics interface or its ribbon section. In the Settings window, assign its selection so that it overlaps with the pair selection where the boundary condition should contribute. In the following illustration, which shows a simple example with two partially overlapping rectangles, there is one identity pair that consists of two boundaries, each with a nonoverlapping part. There can be one boundary condition overlapping with the top boundary and another boundary condition overlapping with the bottom boundary if desired.
Physics features can also be fallback features for multiphysics coupling pair features.
Applicable Pair Region
This section is visible with the Advanced Physics Options enabled in the Show More Options dialog box and only for features that are allowed to contribute to nonoverlapping parts. Here you can modify how a particular features act as a fallback feature on the nonoverlapping parts, using the Allowed region list with the options Fallback and nonpair regions (the default), Nonpair region, and All regions. For the first and default option, the feature has the fallback behavior described above. Select the Nonpair region option if you do not want the feature to act as a fallback feature at all. All parts of the feature’s selection that overlaps with any pair feature will then be excluded from the feature’s selection. Choose the All regions option if you want the feature to contribute to the entire selection, including the parts where the pair is in contact. The feature’s conditions will then contribute also with pair features conditions.