The use of automated projection interferometry in monitoring aerofoil buckling

Projection moiré is frequently used to examine the out-of-plane displacement of thin-walled shells during buckling. One way of implementing this technique is to use double exposure photography to superimpose the initial and deformed images of a grating projected onto the surface of the specimen. This generates a pattern of fringes representative of points of equal displacement, thus presenting a snapshot of the full-field buckling behavior.This paper outlines a technique to extend this method to provide a computer generated real-time fringe pattern throughout the whole buckling and post-buckling process. This is achieved by using a CCD camera and specially developed processing software to continuously superimpose the initial image of the grating (i.e., the first frame of the captured video) onto subsequent frames in which this grating is deformed due to the displacement. This method produces series of fringes in digital format, which are ideal for further processing.