Evaluation of the stress-optical coefficients of transparent solid plates using interferometry

A theoretical analysis that leads to a method for the evaluation of the stress-optical coefficient. If a monochromatic collimated light beam impinges at the tip of the crack, of a plate which is under stress the light rays are deviated in such a way that they form two caustic surfaces, the transmitted and the reflected. The observable quantity is the cross section of this surface with an opaque screen which is of an almost circular shape called briefly caustic. While loading of the specimen with no crack, a displacement of fringes is observed due to the change in optical path. The ratio of the size of the two caustics as they are observed on the screen and the number of displaced fringes per unit of applied stress are the only parameters encountered in the presented analysis in contrast to other methods that require additional knowledge of the elastic parameters of the solid. Experimental data taken in accordance with this method verify its validity.