Discussion on the method of caustics in conjunction with the method of birefringent coatings for the solution of fracture mechanics problems

Raftopoulos et al. (1995) recently presented a new method for the experimental measurement of stress intensity factors in opaque materials. They suggested that a thin layer of a transparent material is bonded onto the surface of the body under consideration, so that samples do not need to be polished to obtain surfaces of high reflectivity for the application of the method of caustics. A Similar technique was developed previously by Edwards and Ahmad (1993). The aim was to measure the stress intensity factor at the tip of a crack lying along an interface between epoxy and aluminium (Ahmad et al., 1996). As there is a significantly difference in reflectivity between aluminium and epoxy, a "coating adhesion" technique was developed. In this article, we discuss the differences between the two techniques. We believe that the coating adhesion technique offers several advantages over the use of a birefringent coating, in that it is experimentally and analytically simple, and requires no additional analysis of the results relative to results obtained from a reflective sample.