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2010 Contents


Vibration Excitation Methods Applied to Digital Shearography and ESPI.(2010)

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.

Complete Paper (480kB)


Shearography - in identifying the presence and subsequent measurement of Residual stresses.(2010)

ABSTRACT

Residual stresses are known to influence the performance of components in structures usually in a detrimental/damaging manner, however sometimes their presence can be advantageous, especially if they are compressive, thus inhibiting fatigue failure. It is recognized that residual stresses are induced during manufacturing processes and as such they are uncontrolled and unknown.

This paper deals with the feasibility study of using Digital Shearography to identify the presence of Residual Stresses and subsequently quantify them. Experiments were performed on mild steel rectangular bars where a compressive residual stress was introduced in one of the surfaces of the specimens. The results are encouraging in that Digital Shearography could be developed as a residual stress measuring tool thus prompting further parametric studies. The testing method also appears to yield the value of the Young’s modulus of the material

Complete Paper (480kB)