Substructuring is a technique that condenses a group of finite elements into a single element represented as a matrix. The single-matrix element is called a superelement. You can use the superelement in an analysis as you would any other element type.
Substructuring requires three passes:
A generation pass, where the group of elements are condensed down to generate the superelement.
A use pass, where the superelement is used in the analysis. In our case, in the multibody analysis.
An expansion pass, where the results of the superelement in the use pass are expanded to the original group of elements so that their displacements, forces, strains, and stresses are recovered.
In the use pass, Mechanical APDL allows the superelement to rotate with arbitrarily large rotations.
In the generation pass, you define master degrees of freedom, the degrees of freedom that the superelement uses to interface with, or connect to, the other bodies or joints.
Because the flexible body analysis occurs within a dynamic analysis, you must include the dynamic (mass) effects. Use component mode synthesis (CMS) to augment the superelement static stiffness with mode shapes that characterize the dynamic behavior, much as you would when performing a mode-superposition transient dynamic analysis.
CMS is a form of substructure analysis allowing you to derive the dynamic behavior of the entire assembly from its constituent components. For more information, see Component Mode Synthesis in the Substructuring Analysis Guide.