The Effect of the Changing State of Stress Around the Crack Tip on the Determination of Stress Intensity Factors by the Method of Caustics

Abstract The state of stress in the vicinity of the crack tip changes from plane strain at the tip to plane stress at a critical distance (about half the specimen thickness) from the tip through an intermediate region where the state of stress is three-dimensional. In the optical method of caustics, the caustic is the image of a circle on the specimen centered at the crack tip, and, therefore, yields information on the stress state along this circle. In the method conditions of plane stress are assumed along this circle. This condition imposes restrictions on the geometrical dimensions of the optical arrangement, specimen thickness, crack length and applied loads for the correct determination of stress intensity factors. In the present work the limits of applicability of the method of caustics are studied. The use of optically anisotropic materials is introduced to obtain a double caustic which provides the correct state of stress (plane strain, plane stress or three-dimensional) along the initial curve. When the state of stress is known the proper values of stress-optical constants can be used for the correct determination of stress intensity factors