Viewing a structure with damage as a time varying system, the concept of instantaneous vibration modes has been introduced and the instantaneous modal parameters, i.e. the instantaneous natural frequency and the normalized instantaneous mode shape, can be effectively used to monitor the structural health condition. The instantaneous modal parameters have been successfully identified for both of sudden stiffness loss and progressive stiffness degradation by using wavelet based approaches in the authors' previous work. The paper addresses some important issues in identification of the instantaneous modal parameters, particularly the effect of the system nonlinearity. A multi-degree of freedom system with bilinear components, subjected to both impulse and harmonic excitation, is considered in this study. The simulated response data are analyzed by wavelet based approaches, i.e. the continuous wavelet transform (CWT) in conjunction with the Hilbert Transform (HT), to identify the instantaneous modal parameters of the system. It is illustrated that change in the identified instantaneous modal parameters may have different patterns for the linear system, the system with slow stiffness degradation, and the system with bilinear behavior. The results may help to distinguish change in the instantaneous modal parameters caused by structural damage or by structural nonlinear behavior and therefore to improve the accuracy of structural health monitoring.
展开▼
