Output-only full-field modal testing

Operational modal analysis has become the focus of much research attention in the last two decades. Instead of an artificial force, the ambient excitation is considered as white-noise input to the structure and modal properties are calculated only from measured responses. In terms of the measurement technique, full-field optical methods, for example: electronic speckle pattern interferometry and digital image correlation have become popular and there is now much interest in applying these methods in structural dynamics. In this case the generated data is a full displacement map of the object, therefore there is no necessity to select specific measurement locations in order to visualise the deformation. However, there are generally large volumes of data to be processed, which makes the computation expensive and time-consuming, especially for engineering structures with large surface areas. Thanks to image decomposition techniques, huge amounts of data can be compressed into tens of shape descriptors with acceptably small distortion. In this paper, operational modal analysis and full-field methods are combined together, and the analysis is done in the shape descriptor domain to reduce the required computation time. Simulated responses from a finite element model of a clamped plate (under random excitation) serve to illustrate the methodology. Several different operational modal analysis methods are applied to analyse the data, and results are provided for purposes of comparison.