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
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