Full Field ESPI Measurements on a Plate: Challenging Experimental Modal Analysis

ABSTRACT Electronic Speckle Pattern Interferometry (ESPI) technology gives the opportunity nowadays to measure high spatially defined displacement maps in a wide frequency range by means of optical non-contact techniques. Light structures easily exhibit high modal density with complex patterns, where the shapes might be resolved with a fine grid of experimental dofs and without adding the sensor masses to the structure. Coherent light interferometry techniques, like ESPI/Holography, were initially developed to detect displacement maps in the sub-micron range at a single dynamic event, such as a shock, a single frequency vibration or a static deflection, but they may challenge other experimental techniques also in the vibration field. Here is proposed an approach to bridge the ESPI technology to the Experimental Modal Analysis environment. Starting from a review of the advantages and drawbacks of this optical technology, a procedure is here outlined to extract FRFs and contouring data from ESPI vibration measurements. Since the number of measuring dofs available is much greater than in traditional accelerometric tests, here is performed an evaluation of the impact of dofs number on the quality of the estimated modal model. The whole approach is tested on an aluminum rectangular plate with 3D full field measurements, and reported in detail.