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Extracting mode shapes from area vibration response measurements on damped structures

Laser techniques are commonly used to measure structural vibration responses with high spatial resolution over an extended area. Forced mode shapes will generally be similar to natural mode shapes and undamped mode shapes, if the damping is not too great nor the modal spacing too close. When this is not the case, the real components of forced mode shapes of close modes, and of the response at an off-resonant frequency, can be combined to reveal the corresponding natural mode shapes. These components are combined in proportions calculated to fit natural mode shape data extracted from frequency responses, measured at a relatively small number of suitable points within the scanned area. Procedures may be varied to suit different measurement techniques; continuous-scan LDV techniques offer the possibility of fitting to line-scan polynomial coefficients instead of the amplitudes at specific points thus reducing the time needed for the data acquisition. All the techniques promise considerable economies in test measurement timescales.