Effect of delayed crack nucleation under threshold pulse loading

Abstract

The kinetic nature of fracture in solids underlies some features in the behavior of macroscopic cracks at the starting stage of their propagation in a dynamic stress field [1‐3]. Analysis of experimental data demonstrates that one of the dominant factors in this process is the incubation period of preparation for the development of a macroscopic rupture of a material. Among the effects related to the existence of the incubation time, the effect of fracture delay is of special importance. This effect implies that a rupture at a given point of the material can occur at a stage when the magnitude of the local force field is reduced. This phenomenon was clearly observed in experiments involving the spalling of materials [1] and, based on the theory of incubation time, was predicted for samples with macroscopic cracks [2]. In this paper, we report the results of our experimental study, including those confirming the existence of delayed fracture near the crack tip under short-pulse loading. We interpret these results in terms of the incubation-time criterion [3]. The experiments were performed using polymethyl methacrylate samples. The samples had a thin cut imitating a macroscopic crack and were characterized by the following material parameters: c 1 = 1970 m/s, c 2 = 1130 m/s, and = 1.47 MPa m 1/2 , where c 1 and c 2 are the longitudinal and transverse velocities of the elastic waves and is the ultimate stress-intensity factor under static loading. The character of fracture under dynamic loading was determined with the help of a magnetic-pulse method of loading. The scheme and the procedure of loading [4] created a pulsed pressure uniformly distributed over the edges of the cracklike cut. This pressure was formed using the flat current-carry bus. The generator of the electric-current pulses produced an oscillatory mode discharge with a period T ~ 5.5—6 μ s and K Ic