PSI - Issue 20

A.A. Alexeev et al. / Procedia Structural Integrity 20 (2019) 254–258 A.A. Alexeev et al. / StructuralIntegrity Procedia 00 (2019) 000 – 000

255

2

transition of a crack from straight propagation to branching mode is the critical value of the crack propagation velocity V* (limiting velocity), and V* < V R ( V R is the wave speed Rayleigh), and is not equal to a certain part of V R , but depends on the material. There are two fundamentally different approaches for explaining the crack branching mechanism when V* is reached. Crack branching pointed by Yoffe (1951), Finkel (1970), Sharon and Fineberg (1996), Bediy (1991), Naimark and Uvarov (2000) occurs mainly during the inertial restructuring of the stress field, and, therefore, depends on the crack attaining a certain velocity associated with the characteristic wave velocities in the material. However, experimental facts show that there is no correlation between V* and characteristic wave velocities in the material, V* strongly depends on the composition of the material, the experimentally measured V* values are well below the Yoffe threshold. In models proposed by Ravi-Chandar and Knauss (1984), Kobayashi (1985), based on the evolution of the crack formation zone, crack branching is associated with a change in the behavior of the material near the crack tip when V* is reached, which depends on the material properties within the crack formation zone. In accordance with these models, the branching of a crack occurs as a result of wave interaction between microcracks and the main crack and is a stochastic process, whereas its determination is experimentally observed. Thus, at present, the physical mechanism for the transition of a crack from straight propagation to branching mode, explaining the existence of an experimentally observed maximum crack propagation velocity V* , can be

considered not fully established. 2. Equipment and materials

To study crack branching, measuring crack speed at various stages of crack propagation and branching by breaking conductive strips, an installation was created based on a precision signal converter of resistance thermometers and thermocouples “Ter с on” connected to a com puter (Fig. 1).

Fig. 1. Scheme of the measurement installation.