Study of Crack Problems by the Optical Method of Caustics

Abstract

In the optical method of caustics, the generatrix curve of the caustic, the so-called initial curve, should lie in the region where plane stress conditions apply. Experiments have shown that plane stress conditions dominate at distances from the tip approximately larger than half the specimen thickness, while in the remaining region the state of stress is three-dimensional. This condition imposes restrictions on the geometrical dimensions of the optical arrangement, specimen dimensions and thickness, crack length and applied loads for the correct determination of stress intensity factors. In the present work the optical method of caustics is critically reviewed, and the limits of its applicability based on the condition that the initial curve should lie in the region of plane stress are studied. The use of optically anisotropic materials is introduced to obtain a double caustic which provides the correct state of stress (plane strain, three-dimensional of plane stress) along the initial curve. When the state of stress is known the proper values of stress-optical constant can be used for the correct determination of stress intensity factors.