PHOTOELASTIC ANALYSIS OF A 3-DIMENSIONAL SPECIMEN BY OPTICAL SLICING AND DIGITAL IMAGE-PROCESSING

The authors show a nondestructive method for obtaining the isochromati c and isoclinic fringes in a three-dimensional photoelastic specimen. The basic idea is to delimit a slice between two plane laser beams. Th e properties of polarization of the scattered light (Rayleigh's law) a nd the interference possibilities of the diffused beams are used. By i ntroducing speckle pattern properties, the correlation factor of the t wo scattered beams is similar to the illumination given in a plane pol ariscope for the investigation of a slice (in a classical frozen-stres s technique). The authors use a monochromatic laser beam, a CCD camera and a personal computer. Because they cannot obtain the correlation f actor directly, they do a statistical analysis of the speckle patterns . The variance (function of the correlation factor) is computed from t he light intensities of three images corresponding to the speckle patt ern for plane 1 alone, plane 2 alone, and both planes together.