Discussion of a New Experimental Method in Measuring Fracture Toughness Initiation at High Loading Rates by Stress Waves

Abstract

An experimental method is described for measuring the fracture initiation properties of metals and alloys over a wide range of loading rates, which can cover over six orders of magnitude in K˙I (1 MPam s−1 ≤ K˙I ≤ 106 MPam s−1). With some modification of the standard compact tension specimen, a large series of screening tests can be performed in the high loading region at a relatively low cost. At the lower loading rates a standard closed loop testing machine can be used. To evaluate fracture initiation at a very high loading rate, a special arrangement of the split Hopkinson pressure bar has been proposed. Specimens of the same geometry as those used in quasi-static tests are placed between the Hopkinson bars. Since the wedge is attached to the incident bar, and the specimen is backed by the transmitter bar (Fig. 2), the course of specimen loading and fracturing can be exactly monitored by recording the incident, reflected and transmitted longitudinal waves. Using this technique, fracture initiation of the prefatigued specimen has been achieved within ∼ 20 μs after the beginning of specimen loading. The effects of inertia acting on the specimen and an error introduced by friction are both considered. Experiments performed on some aluminum alloys as well as on medium carbon steel revealed a complicated pattern of the fracture toughness behavior. Generally, for the strain rate sensitive materials a substantial decrease in fracture toughness was observed under high loading rates.