Extraction of normal modes for highly coupled incomplete systems with general damping

A frequency-domain technique to extract the normal modes from a measurement contaminated by noise for highly coupled structures is developed. The relation between the complex frequency response functions (FRFs) and the normal FRFs is derived. An algorithm is developed to calculate the normal modes from the complex FRFs. In this algorithm, only the magnitude and phase data at the undamped natural frequencies are utilised to extract the normal mode shapes so the effect of measurement noise is reduced. In addition, the technique is applicable to either viscous or structural damping. Two experimental examples are employed to illustrate the technique. The results indicate that this technique can extract the normal modes from the experimental FRFs of a highly coupled incomplete system. (C) 1996 Academic Press Limited