Abstract:

The mode superposition method is an effective way to solve the dynamic response problem of force excited linear structures. But this method cannot be used directly for base motion excited structures in several commercial CAE softwares, such as MSC.Nastran, ANSYS, etc. The large mass method and large stiffness method, which transfer these structures to force excited structures, are two commonly preprocessing procedures for the mode superposition method. However, additional elements should be added to the original finite element model and the physical parameters of these elements are chosen arbitrarily in these two methods, and the natural vibration characteristics are changed slightly. A new fixed boundary mode superposition method is proposed to transfer the base motion excited degree of freedoms (DOFs) to fixed boundary DOFs, transfer the base motion excitation to the equivalent loads of nodes which lie in the same elements with the base motion excited DOFs, and then solve the dynamic response problem with the traditional mode superposition procedure. The natural vibration characteristics of the original base motion excited model are not changed in the fixed boundary transfer model, and no additional elements need to be added and no arbitrary parameters need to be selected. Furthermore, the transfer load vector can be calculated more precisely by refining the local elements at base motion excited DOFs. Finally, the performance of the proposed fixed boundary mode superposition method is illustrated by solving the frequency response problem of a plane stress structure and the transient response problem of the shell element model of a light truck.

Key words: fixed boundary, mode superposition, transfer load vector, base motion excited