What is the difference between a rigid part and a rigid body constraint?

Similarly, you can create a rigid body constraint in the Interaction module. Rigid body constraints allow you to constrain the motion of regions of the assembly to the motion of a reference point. The relative positions of the regions that are part of the rigid body remain constant throughout the analysis. In addition, you can select regions from a part instance and use a rigid body constraint to specify an isothermal rigid body for a fully coupled thermal-stress analysis. For detailed instructions on defining rigid body constraints and assigning a rigid body reference point, see Defining rigid body constraints.

You do not have to create a reference point for a part, even if the part type is discrete or analytical rigid. However, if you do not create a reference point for a rigid part, every instance of the part in the assembly must be included in a rigid body constraint.

Rigid parts are associated with parts; rigid body constraints are associated with regions of the assembly. For example, if you define a part to be rigid, every instance of the part in the assembly is rigid. In contrast, if you define a part to be deformable, you can use rigid body constraints to make only some of the instances rigid. If you do not create a reference point in the Part module, you cannot create a rigid body reference point by associating an instance of the rigid part with a reference point created in the Assembly module. However, you can associate the instance with a rigid body constraint and a reference point created in the Assembly module.

If you define a part to be rigid, you can use the Model Tree to change the part type to be deformable. To check that your basic model is correct, you might run a quick analysis with a part defined as rigid and then change the type to deformable. Similarly, if you define a part to be deformable and apply a rigid body constraint to an instance of the part in the assembly, you can easily remove the constraint at a later time. You can run your quick analysis with a rigid body constraint applied to the part instance and then remove the constraint and run a full analysis with the part instance acting as a deformable body. The two approaches are very similar.