Issue 38

M. Sokovikov et alii, Frattura ed Integrità Strutturale, 38 (2016) 296-304; DOI: 10.3221/IGF-ESIS.38.40 297 been studied by the methods of statistical physics and thermodynamics of irreversible processes. The results of theoretical and experimental studies suggest that one of the mechanisms of the plastic shear instability and localization of plastic strain at high-velocity perforation is related to structural and kinetic transitions in microshear ensembles. K EYWORDS . Plastic strain localization; Microdefects; Dynamic loading; Numerical modeling. I NTRODUCTION hermoplastic instability has long been considered to be a mechanism responsible for plastic strain instability and localization at high loading velocities, [1]. It has been suggested that heat generated in materials during plastic deformation cannot be removed in a short characteristic time, which causes thermal softening and further increase in plastic deformation. The avalanche-type process that is accompanied by a sudden increase in temperature in the area of plastic strain localization is initiated. It has also been found that the temperatures can reach the values high enough to melt the material. Experimental studies of the microstructure of adiabatic shear bands carried out in some works, e.g. [2], have demonstrated that one of the mechanisms of plastic shear band formation is related to multi-scale instabilities observed in microshear ensembles (mesolevel defects). In the present paper plastic strain instability and localization in the material subjected dynamic loading are considered. The theoretical analysis is based on a previously developed theory, in which the methods of statistical physics and thermodynamics of irreversible processes have been used to study the effect of microshears on the plastic properties of solid bodies, [3, 4]. E XPERIMENTAL STUDY o investigate the behavior of the material under conditions close to pure-shear dynamic loading, we used a Hopkinson-Kolsky bar. During dynamic deformation, the lateral strain localization area was studied using a high- speed infra-red camera CEDIP Silver 450M. A special sample shape was proposed to realize pure shear strain localization state (Fig.1). Samples were made from aluminum alloy AlCu4Mg1. Motivation to use this shape of samples is a necessity to have a plane lateral surface for examining the plastic strain distribution by infrared imaging techniques. Scheme for testing and the obtained results are shown in Figs.1, 2. Figure 1 : Special shaped sample for testing under conditions close to pure shear, and the scheme of loading with the Hopkinson- Kolsky bar apparatus. 1 – input bar, 2 – frame, 3- sample, 4 – output bar (pure shear state is illustrated by shaded areas). T T