A combination optical method of lateral shearing interferometry and caustics

A novel experimental technique is presented that combines the optical metho d of caustics and the full-field lateral shearing interferometer of coheren t gradient sensing (CGS) in one setup. Collimated light transmitted through a deformed specimen is separated into two legs. One is processed by the la teral shearing interferometer and is recombined with the other on a single image plane. By adjusting the optical path length of each leg, a combinatio n lateral shearing interferogram/caustic image is generated. When this tech nique is applied to a fracture specimen, information about the stress inten sity factor can be obtained from the caustic and the CGS interference fring es for exactly the same deformation field. This allows for a direct compari son between the two methods when investigating phenomena such as K-dominanc e, three-dimensional deformation and so on. In this paper, the technique is presented in detail, and its feasibility is tested by application to mode I and mixed-mode crack tip deformation fields.