Use of Speckle Interferometry and Modal Assurance Criterion for Identification of Component Modes

One of the problems encountered in performing modal analysis is the identification of different modes occurring at close natural frequencies. In this paper it is proposed to face such problem by an original application of the Modal Assurance Criterion, that is typically used to verify the reliability of numerical structural models by evaluating the correspondence between numerical and experimental mode shapes. To properly apply the Modal Assurance Criterion, experimental mode shapes must be retrieved acquiring data at a high number of experimental points suitably distributed on the surface of the structure. Using usual modal analysis techniques based point wise transducers, difficulties can be encountered to retrieve mode shapes with the required spatial resolution. To overcome such difficulties Speckle Interferometry techniques can be used. In this paper it is proposed a procedure based on the application of highly reliable Modal Assurance Criterion evaluated by time average Speckle Interferometry for recognising the modes contributing to the stationary vibration patterns. The results obtained from the modal analysis of a thin square plate have shown that the proposed procedure is capable to point out the component modes.