Instability of Critical and Geometric Characteristics of the Fracture Zone under Spall Conditions
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ability of Critical and Geometric Characteristics of the Fracture Zone under Spall Conditions Yu. V. Petrova,b,* and A. A. Utkina,b,** a
Institute for Problems in Mechanical Engineering, Russian Academy of Sciences, St. Petersburg, 199178 Russia b St. Petersburg University, St. Petersburg, 199034 Russia *e-mail: [email protected] **e-mail: [email protected] Received January 11, 2020; revised January 15, 2020; accepted January 29, 2020
Abstract—Many features of dynamic fracture caused by shock-wave action are most pronounced under spalling conditions, i.e., when an intense wave pulse is reflected from a free boundary. Therefore, such material tests are one of the main ways to study the fracture processes occurring in a solid under ultrafast dynamic stress. Nevertheless, the geometrical characteristics of the fracture zone formed under spalling conditions are relatively little studied. It was experimentally established that the qualitative appearance of such a multiple fracture region is characterized by strong instability and depends on the parameters of the applied pulse, such as the rate of increase and decrease in stress, amplitude, and duration. Based on an analysis of a number of experimental results, it will be shown below that the unstable behavior of the fracture zone formed in the spalling conditions observed in the experiments can be explained and predicted using the structural-temporal approach based on the concept of incubation fracture time. Keywords: dynamic fracture, impulse loading, spalling, velocity dependence of strength, incubation processes DOI: 10.3103/S0025654420030139
1. INTRODUCTION The spalling phenomenon resulting from the interaction of a compression wave pulse with a free surface and the appearance of a reflected tensile wave tearing the material provides a unique opportunity to study the patterns of fracture of materials during high-speed dynamic stress at extreme strain rates above 103 s–1. Currently, dynamic strength is usually associated only with the rate of deformation of the material in the spall section at the moments preceding fracture, not taking into account other parameters of the temporal profile of the initial pulse, such as, for example, its duration, as well as the presence of various stages. Most of the research in this area is aimed at establishing the mentioned “speed” dependence of strength, which is often interpreted as a function of the material. However, many of the effects found in experiments do not fit into the generally accepted model, which leads to its significant complications. In particular, such are, for example, the effect of “stabilization” of strength [1] and failure at a strain rate in the spall section close to zero [2]. Also little studied is the geometry of the multiple fracture zone formed upon impact. It was found that the qualitative appearance of the fracture region very much depends on the parameters of the applied pulse, such as the rate of increase and decrease in voltage, amplitude, and duration. In this case, the fractured region itself
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