When you need to force two or more degrees of freedom (DOFs) to take on the same (but unknown) value, you can couple these DOFs together. A set of coupled DOFs contains

a prime DOF, and one or more other DOFs. Coupling will cause only the prime DOF to be retained in your analysis' matrix equations, and will cause all the other DOFs in a coupled set to be eliminated. The value calculated for the prime DOF will then be assigned to all the other DOFs in a coupled set.

Typical applications for coupled DOFs include: 1) maintaining symmetry on partial models, 2) forming pin, hinge, universal, and slider joints between two coincident nodes, and 3) forcing portions of your model to behave as rigid bodies (see this chapter's discussion of constraint equations for more general rigid region capability).

You can use several different methods to create a coupled degree-of-freedom set. These include the CP command and other commands such as CPNGEN, CPINTF,CPLGEN, and CPSGEN. These methods of generating coupled sets are discussed in the following sections.

In addition to the methods discussed here, you can use the internal multipoint constraint (MPC) feature of certain contact elements (CONTA171, CONTA172,CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177) to model coupling constraints. By this method, the program builds MPC equations internally based on the contact kinematics. See Multipoint Constraints and Assemblies in the Contact Technology Guide for more information on how to use this feature.

Instead of coupling coincident nodes, you can use one of these alternative methods to force the nodes to behave in the same way:

If all DOFs are to be coupled for coincident nodes, it is usually more efficient to simply merge those nodes together by using the NUMMRG command (Main Menu> Preprocessor> Numbering Ctrls> Merge Items).

You can connect coincident pairs of nodes by creating 2-node elements between them by using the EINTF command (Main Menu> Preprocessor> Modeling> Create> Elements> Auto Numbered> At Coincid Nd).

To tie together two regions having dissimilar mesh patterns, use the CEINTF command (Main Menu> Preprocessor> Coupling/Ceqn> Adjacent Regions). This operation generates constraint equations that connect the selected nodes of one region to the selected elements of the other region

You can use the internal multipoint constraint (MPC) approach (KEYOPT(2) = 2), in conjunction with certain bonded and no separation contact definitions (KEYOPT(12) = 4, 5, or 6), to define various contact assemblies and kinematic constraints. This capability is available for contact elements CONTA171, CONTA172,CONTA173, CONTA174, CONTA175, CONTA176, and CONTA177. By this method, the program builds MPC equations internally based on the contact kinematics. You can use this method to model the following contact assemblies and surface-based constraints:

Solid-solid assembly - both contact and target surfaces paste onto solid element faces