Output-Only Analysis of Coupled-Mode Structures

In the framework of the operational modal analysis, several approaches have been developed for the estimate of the modal parameters, i.e. natural frequencies, damping ratios, and mode shapes. Specifically, a technique capable to estimate the biased Frequency Response Functions, FRFs, has been proposed. Assuming that only the responses of the structure are disposable, the technique allows one to estimate biased FRFs starting from the Power Spectral Densities, PSDs, and applying the Hilbert Transform. This paper deals with the estimates of the modal analysis parameters: mode shapes, natural frequencies and damping ratios. It is possible to obtain each single mode shape, from the singular vectors achieved by applying the Singular Value Decomposition to the FRF matrix evaluated at the spectral line corresponding to the selected natural frequency. A special attention will be devoted to structures with coupled modes. Once the FRFs have been obtained, the natural frequencies and damping ratios could be achieved either in the frequency domain or in the time domain. Experimental tests, carried out on beams, plates and on the AB-204 helicopter blade, will be presented.