Possibilities of reflected caustics due to an improved optical arrangement: some further aspects

Abstract

A theoretical study, based on large field geometric optics and an experimental verification of an improved optical arrangement for recording reflected caustics, was undertaken. Improvement of the typical optical setup, used until now, was made by introducing a convergent lens, (collimator), in the experimental apparatus. It was shown that, by changing either the distance between the specimen and the collimator or between the specimen and the reference screen, the size of the caustic may be drastically and finely changed. Moreover, by passing the critical point of zeroing of the generatrix curve of the caustic, the shape of the caustic may also be changed. When an edge-cracked plate is loaded in simple tension, that is, the KI mode of fracture, the shape of the caustic may be changed from a typical cusped shape to an almost circular one. For purely elastic conditions in close vicinity of the crack tip, the initial curve for these shapes of the caustic is a circle. Introduction of plasticity in the limited region where the initial curves of the caustic lie causes changes in the caustic’s shapes. Using the proposed optical arrangement, it is possible to determine, through the shape variations of the caustics, the size and extent of deviations of their respective initial curves from their circular shapes. In this way the collimating arrangement for caustics constitutes a sensitive gauge for evaluating such phenomena as plasticity and their region of extension.