Quantitative détermination of large vibrational amplitudes of a mechanical structure

Abstract

In order to measure great vibrational amplitudes that cannot be approached with the holographic methods or those that bring the properties of the scattered coherent waves into play, two techniques have been developed allowing to reach the constant vibrational amplitude lines of a mechanical structure. The methods of projected fringes are applied when considering that the constant amplitude lines of a vibrating surface can be classed as contour lines of a static tridimensional object. To this end, we have been brought to fix in one or two particular positions the deformation of the fringes system that modulates the repartition of intensity connected with the structure by using a stroboscopic lightning of the latter. The first method consists in visualizing the contour lines of the vibration on the surface by means of an optical filtering method applied to a single system of distorted fringes. The second one leads to the determination of the same contour lines from the variation of the visibility of an interference phenomenon that results from the superposition of two symmetrical systems of distorted fringes. The experimental results bear on the calculations that have been made. Both techniques have allowed to measure with the same accuracy vibrational amplitudes of several millimeters.