PSI - Issue 47

Mikhail Sokovikov et al. / Procedia Structural Integrity 47 (2023) 693–697 Author name / Structural Integrity Procedia 00 (2019) 000–000

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1. Intrudiction The phenomenon of plastic strain localization, namely, the formation of small areas of plastic flow where the level of plastic strain is much higher than that in the surrounding material is of critical theoretical and practical significance. Plastic strain localization in metals at different loading rates is a complex process that depends on the rate and magnitude of deformation and temperature, as well as on the structure evolution of the material. This phenomenon was investigated by Giovanola J. H. (1988), Marchand А. & Du ffy J. (1988), and Rittel D. et al. (2008). Nowadays, there are two main concepts of the strain localization mechanism: the thermoplastic instability, which is realized at high strain rates, and the structural evolution-associated mechanisms, which may occur in a wide range of loading rates. Together with the thermoplastic instability which manifests itself at high loading rates, the role of structural transitions in the ensembles of mesolevel defects is also important. The main objective of this study is to provide evidence of the existence of one of the plastic strain localization mechanisms associated with the jumpwise processes in the defect material structure, Naimark O.B. (2003). 2. Experimental study The mechanisms of plastic strain localization in the material subjected to dynamic loading in a split Hopkinson pressure bar were investigated using the specimens made of aluminum alloy AMg6, which exhibits the "tendency" to plastic flow instability. In studying plastic strain localization, the inclined (skewed) cylindrical specimens were used. The application of these specimens made it possible to estimate the tendency of materials to localization under shear and to realize sufficiently high strain rates. Furthermore, in the dynamic loading tests on such specimens, the surface quality does not affect plastic strain localization . The energy dissipation-driven temperature fields observed in the specimens subjected to deformation in the Hopkinson pressure bar apparatus with intent to study the localized shear fracture evolution were investigated in-situ using a CEDIP Silver 450M high-speed infrared camera, Figure 1, Bilalov D. A. at al. (2018), Sokovikov M.A. at al. (2020). The main characteristics of the camera are as follows: sensitivity not less than 25 mK at 300°K, spectral range 3 5 μm, maximum frame size 320x240 pxl, coordinate resolution ("pixel size") ~ 0.2 mm, time resolution ~ 0.25 ms. During deformation, the temperature fields were visualized “in situ”. Figure 1 presents the graph of dependence of temperature on the system coordinate at a selected instant of time, the sketch of the specimen, the infrared image of the A М g6 specimen obtained during its testing and the scheme of the experiment. The temperature of the plastic strain localization area does not exceed ~150 0 С . This has led to the conclusion that thermal softening does not play a decisive role in the localized shear mechanism under these loading conditions The study of a high-speed loading process with the infrared camera in real-time mode indicates that, in the case of the АМ g6 alloy investigated under a given load, conditions for the implementation of the thermoplastic instability mechanism are absent. 3. Structural studies The structure analysis was performed for longitudinal (cut in the longitudinal direction) skewed specimens using a FEI PHENOM G2 ProX scanning electron microscope at an accelerating voltage of 15 kV and a magnification of 1,000-15,000x. The results of the structural analysis of the specimen after testing in the split Hopkinson pressure bar are shown in Fig.2. It has been found that the central part of the general stress concentration zone is the site of the occurrence of plastic deformation involving characteristic stretching and bending of both large grains and the assembly of small grains of the original structure. In the structural units (grains), in which the conditions of massive plastic deformation are realized, one can observe the formation of structural elements with characteristic features of localized shear bands shown by the arrows in Figure 2a. The peripheral region is characterized by the formation of microdefects in the form of microcracks, at the tips of which the formation of structural elements with characteristic features of localized shear bands was recorded (Fig.2b).