Applications of holography to fracture mechanics

Abstract

In the development of a theory of fracture mechanics for ductile metals, the role of the zone of constrained plastic deformation which forms in the region of a high stress concentration is of fundamental importance. Consequently, experimental studies of this plastic zone are of considerable interest and have attracted the attention of many investigators using many techniques. The present study represents an effort to extend the interferometric technique through the use of laser holography. In this work, a lens-less picture or hologram of each fracture specimen was exposed while the specimen was under nominal load in the testing frame. After development, the hologram was replaced and illuminated so as to superimpose the reconstructed specimen image on the actual specimen. As a result, subsequent minute deformations of the specimen produced by changes in the applied load result in the formation of readily observable fringe patterns.

Theoretically, if sufficient care is taken in positioning the hologram and if the fringe distribution can be measured with sufficient accuracy, a quantitative three-dimensional determination of surface deformations could be made. The photographs taken in this study, while lacking the rigor demanded by the preceding conditions, nevertheless illustrate the nature of this technique and provide a quantitative one-dimensional measure of out-of-plane surface movement.