An experimental investigation of the dynamic fracture of a brittle material

Abstract

A new method of experimentally investigating the behavior of a crack in a stress wave environment has been developed and used to study the initial stages of dynamic crack propagation in Homalite 100, a polyester. The experimental method, which employs an electromagnetic loading device, permitted the application of pressure pulses to the surfaces of an 18 inch crack. The amplitude (51 psi to 1020 psi) and duration (~ 200 µsec) of the pulses were highly repeatable. The experimental configuration simulates, in the vicinity of the crack tip, a tension wave impacting a stationary semi-infinite crack in an infinite two-dimensional body where the wave front is parallel to the crack. A high speed framing camera, synchronized with the loading device, was used to record the time required for the crack to begin to propagate and its subsequent extension and velocity. The experimental results were analyzed within the bounds of linearly elastic fracture mechanics and a correlation was made between the dynamic stress intensity factor and the time at which the cracks began to propagate. It was found that the critical stress intensity factor increased dramatically with increasing loading rates at very high rates, which contrasts with quasi-static loading where the critical stress intensity factor decreases with an increasing loading rate. A simple model suggests that temperature effects at the crack tip may account for the observed increase in the critical stress intensity factor at high loading rates. Crack branching of running cracks was observed but unlike previous reports of branching, the cracks in this study branched mostly into three separate fast-running cracks.