SEPARATION OF CLOSELY SPACED VIBRATION MODES USING HOLOGRAPHIC-INTERFEROMETRY

An experimental method for the determination of real normal mode shapes of continuous structures with proportional damping is presented. Using the established technique of strobed holographic interferometry, it is shown in this paper how individual mode shapes may be found by comparing strobed recordings made using two nearly identical forcing frequencies close to the natural frequency of the mode to be identified. It is shown that it is always possible to exclude completely any single mode from the recording, which allows the eigenvectors of closely coupled modes to be accurately determined. The two excitation frequencies and the timing of the strobe pulse used in the experiments are chosen on the basis of anticipated phase angle behaviour of the various modes, which requires prior knowledge of their damping factors and natural frequencies. This information can be determined from the analysis of a single measurement made for each mode using conventional Fast Fourier Transform based modal analysis techniques. As with other holographic methods, the technique is non-intrusive and records information over the full field of view. Experimental results of nine normal modes of a square clamped steel plate are presented, including three pairs of "degenerate" (very closely spaced) modes. © 1992.