Determination of K sub I and K sub II by Using a Least-Squares Method With Caustics

Current techniques that utilize the method of caustics to determine stress intensity factors at a crack tip use only a limited amount of data from the caustic image. A non-linear least-squares technique that has been developed to extract K sub I , K sub II and the initial curve radius r sub 0 from an overdetermined set of data are presented. Since the crack tip is obscured by the caustic, the technique presented treats the crack tip location as an unknown. The tip location is determined numerically during an iteration process. The diffraction effect is considered using a digital image-analysis system, EYECOM III. Thin plexiglass (PMMA) specimens with two parallel edge cracks are loaded in tension and used to illustrate the technique.