Component Mode Synthesis of a Vehicle System Model Using the Fictitious Mass Method

Abstract

A way to shorten the repeated analysis process is by modal coupling of the modal properties of several substructures. The main difficulty of the classical modal-coupling approach is that the modes of freeboundary structural components normally do not yield adequate results when coupled with each other. The main reason is that local deformations near the interface points that connect the components are not represented in the low-frequency mode shapes. Modal coupling methods that include special static constraint modes for the interface degrees of freedom, such as that of Craig and Bampton 1 , might be difficult to apply when only the modal properties are obtained by standard structural analysis codes. Another major disadvantage of such modal-coupling methods is that it might be very difficult to apply the necessary boundary conditions and verify the interface-related modes in structural tests. A new procedure for dynamic analysis of complex structures, based on modal coupling of substructures, is presented. Normal modes of separate components are calculated by finiteelement analysis with the interface coordinates loaded with fictitious masses that generate local boundary deformations in the low-frequency modes. The fictitious-mass coupling method is extended to include three types of component interconnections: displacement constraints, connection elements, and structural links. The connection elements allow the introduction of springs and dampers between the interface points without adding structural degrees of freedom. The structural links facilitate the inclusion the discrete finite-element representation of typically small components in the coupling equations. This allows a convenient treatment of loose elements and the