Vibration Excitation Methods Applied to Digital Shearography and ESPI

Abstract

Digital Shearography (DS) and Electronic Speckle Pattern Interferometry (ESPI) are laser based optical interference techniques used amongst other to inspect materials and manufactured components for defects. DS captures the rate of surface displacement and ESPI the displacement of an object in response to an applied stress. There are a number of ways to stress the object during the inspection process, the most common being the use of thermal heating or vacuum and pressure chambers. These forms cause whole field object responses, which generate the equivalent fringe patterns. Within these fringe patterns fringe anomalies revealing the presence of a defect are sometimes masked due to the object’s whole field displacement. As an alternative stressing technique, this paper investigates the use of vibration excitation methods. The intention with this approach is that the friction produced in an object’s defect area generates localised thermal gradients which in turn should be revealed in the produced fringe pattern. This paper describes the above mentioned inspection techniques and examines the results obtained using Digital Shearography and ESPI when applied to selected samples. In particular the results obtained using vibration excitation are compared with results obtained using thermal stressing techniques, which ultimately attempts to determine the suitability of vibration excitation methods to inspect objects for defects.