Direct structural damping identification method using complex FRFs

Most of the identification methods are based only on the viscous damping model and uses modal data. In this paper, a new FRF-based direct structural damping identification method is proposed. The proposed method is a direct method and identifies structural damping matrix explicitly. As the new method is a FRF-based method, it overcomes the problem of closely spaced modes for damping identification. The accuracy of identified structural damping matrix depends upon the accuracy of finite element model. In this paper, FRF-based model updating method is used to obtain accurate mass and stiffness matrices. Thus, the procedure to obtain accurate structural damping matrix is a two-step procedure. In the first step, mass and stiffness matrices are updated and in the second step, structural damping matrix is identified using updated mass and stiffness matrices, which are obtained in the previous step. The effectiveness of the new method is demonstrated by three numerical examples and one experimental example. The numerical studies of lumped mass system, fixed-fixed beam and L-shaped frame structure are carried out. The effects of coordinate incompleteness, ill-conditioning and robustness of method under presence of noise are investigated. The proposed method is able to predict FRFs accurately for the frequency range covering the modes considered. However, beyond the considered modes, the predicted FRFs do not match the experimental FRFs. It is suggested in this work that ill-conditioning problem should be dealt by considering all the modes in the frequency range of interest. The performance of the proposed method is investigated for cases of light, medium and heavily damped structures. The numerical studies are followed by experimental case study of cantilever beam structure. The effectiveness of the proposed method is evaluated by comparing the predicted and the experimental FRFs. The results have shown that the proposed method is able to predict accurately the experimental FRFs with all levels of damping in the system.