## Chapter 9: Multipoint Constraints and Assemblies

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

• Shell-shell assembly - both contact and target surfaces paste onto shell element faces

• Shell-solid assembly - the contact surface pastes onto shell element faces and the target surfaces paste onto solid element faces

• Rigid surface constraint - the contact nodes are constrained to the rigid body motion defined by the pilot node (similar to the CERIG command)

• Force-distributed constraint - the forces or displacements applied to the pilot node are distributed to contact nodes, in an average sense, through shape functions (similar to the RBE3 command)

• Beam-solid assembly - one beam end-node is the pilot node which connects to the solid or shell surface (use the rigid surface constraint or force-distributed constraint type of MPC)

The internal MPC approach can overcome the drawbacks of the traditional contact algorithms and other multipoint constraint tools. For example:

• Degrees of freedom of the contact surface nodes are eliminated, reducing the wave front size of the system equation solver.

• No contact stiffness is required as input. For a small deformation problem, this represents true linear contact behavior; no iterations are needed to solve the system of equations. For large deformation problems, the MPC equations are updated during each iteration, overcoming the small strain restriction in conventional constraint equations.

• Both translational and rotational degrees of freedom can be constrained.

• Generation of internal MPC is simple because it uses contact pair definitions.

• Shape functions are taken into account automatically; no weight factor is needed for a force-distributed multipoint constraint (which is similar to the RBE3 command) if you use higher order elements or axisymmetric elements. In addition to forces, displacements can be applied on the pilot node for this type of MPC.

Before using the internal MPC feature, be sure to review the last topic listed above on restrictions and recommendations.