A. A. Utkin

Effect of delayed crack nucleation under threshold pulse loading

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

Time dependence of the spall strength under nanosecond loading

The problem of spall fracture is considered using an incubation time criterion. Some experimental fracture effects are discussed. The spall strength is found to depend substantially on the pulse parameters, in particular, the rate of decrease of the load. The strain-rate and time dependences of spall strength are shown to be considered as calculated characteristics rather than as functions of a material.

On the similarity of the initial stage of failure of solids and liquids under impulse loading

Cavitation is the disturbance of continuity of a liquid (the initial stage of failure) in the field of tensile stresses; it is accompanied by the growth of vapor-gas bubbles on the cavitation nuclei that are always present in liquid media as microbubbles of a free gas, or microparticles, or both [1, 2]. One of the parameters that characterize the cavitation strength of water is the cavitation threshold, which is understood as a negative pressure, the excess above which causes an intense growth of cavitation nuclei and, as a consequence, a steep change in the dynamics of the free surface of the liquid [3], in the intensity of light scattering [1, 3], etc. Depending on the measuring technique and quality of water purification, the cavitation threshold varies from units [2, 3–6] to several hundred atmospheres [4–6], and its statistical dispersion, based on standard measuring procedures, reaches 50–100% [5, 6] and is determined by the size dispersion of cavitation nuclei, fluctuations in the nucleus distribution, nonlinear dynamics of microbubbles, and by the measuring technique.

On some principal features of data processing of spall fracture tests

A method for processing the results of dynamic spall fracture tests, based on the exact solution of the wave equation, and its commonly used simplified version based on the assumed unique relation between the free surface velocity drop and the ultimate medium fracture stress, are analyzed. Using the considered exact solutions of the wave technique, tensile stress pulses during spalling are determined. The obtained stress levels at the fracture point are compared with the spall strength calculated by the velocity drop technique. The cases of agreement and disagreement of the results obtained using both techniques are shown. By the example of differently shaped loading pulses, possible scenarios of sample fracture are presented, in particular, the probability of the fracture delay effect is shown, which can be lost in the simplified processing method.

Failure-delay effect in destruction of steel samples under spalling conditions

Dynamic spalling tests have been run on two batches of 30KhN4M steel samples. Experimental data have been processed with the classical technique based on solution of the elastic wave equation. Three samples have been revealed that demonstrated the failure-delay effect under testing. The incubation-time criterion has been used to show the conditions of emergence of failure delay with the example of triangular loading pulses. A rate strength curve has been constructed for the other samples. It has been shown that the limiting strengths under dynamic loads considerably differ for samples from different batches despite the same chemical composition and static strength.

Fracture of spheroplastic under static and dynamic stressing

The fracture of a composite material, a spheroplastic consisting of a polyester resin matrix and glass microspheres as a filler, is studied experimentally and theoretically under static and dynamic stressing. A shock is generated by a pulsed magnetic field. The fracture type in relation to the shock parameters and material structure is analyzed. A method for testing the dynamic behavior of the material based on the incubation time accumulation is suggested.

INVESTIGATION OF FRACTURE OF SPHEROPLASTICS UNDER STATIC AND DYNAMIC LOADING

An experimental study of static and dynamic fracture properties of a spheroplastic that has a matrix of polyester resin containing a filler of glass microspheres was conducted. Crack propagation was investigated under loading conditions generated by a pulse magnetic field. Microstructure features of dynamic fracture were analyzed.