FINITE-ELEMENT STUDY ON THE OPTICAL METHOD OF CAUSTIC FOR MEASURING IMPACT FRACTURE-TOUGHNESS

THE OPTICAL method of caustics for measuring impact fracture toughness, K(Id), is investigated by using two- and three-dimensional elastic finite element methods. It is found that there exists a delay time to detect the crack initiation, and the delay time may result from the fact that it takes a time for the stress wave (due to crack initiation) of sufficient amplitude to have an observable effect on the caustics to reach the location of the initial curve. The delay time may explain to some extent the experimental result that K(Id), depends on the incident wave form and also on the loading rate for a short time-to-fracture. The distribution of K(I)(t) along the crack front is examined and the possible dependence of K(Id) on the shape of a propagating crack front is also discussed.