Components for which deformation is negligible and stress is unimportant may be modeled as rigid bodies using
*CONSTRAINED_NODAL_RIGID_BODY. The elastic constants defined in
*MAT_RIGID are used for contact stiffness calculations. Thus the constants should be reasonable (properties of steel are often used).
Though there are several contact types in LS-DYNA which are applicable specifically to rigid bodies (RIGID appears in the contact name), these types are seldom used. Any of the penalty-based contacts applicable to deformable bodies may also be used with rigid bodies, and in fact, are generally preferred over the RIGID contact types. Rigid bodies and deformable materials may be included in the same penalty-based contact definition. Constraints and constraint-based contacts may not be used for rigid bodies.
Rigid bodies should have a reasonably fine mesh so as to capture the true geometry of the rigid part. An overly coarse mesh may result in contact instability. Another meshing guideline is that the node spacing on the contact surface of a rigid body should be no coarser than the mesh of any deformable part which comes into contact with the rigid body. This promotes proper distribution of contact forces. As there are no stress or strain calculations for a rigid body, mesh refinement of a rigid body has little effect on cpu requirements. In short, the user should not try to economize in the meshing of rigid bodies.
*CONTACT_ENTITY is an altogether different way of defining an analytic, rigid contact surface which interacts with nodes of deformable bodies. For more information