Advances in formation mechanisms and multiscale simulations of adiabatic shear bands in metallic materials

Adiabatic shear bands (ASBs) become an important failure mechanism when the ductile metallic materials are loaded under the dynamic impact. This thermoplastic instability behavior occurring in such a narrow band under the coupling actions of force and thermal effect easily causes a rapid propagation of crack inside the ASB or along its boundary, subsequently leading to a sudden failure. The extremely small scale of adiabatic shear bands in space and time has brought great challenges to experimental investigations. For this reason, scholars have not obtained a comprehensive understanding of this unique mechanical behavior, but a lot of research work has already been done and many breakthroughs have been made in this research field. In the experimental studies, the applications of high-speed and high-resolution testing techniques make people observe the whole process of initiation and propagation of the ASBs more clearly, and further recognize the role of different influencing factors in the emergence of ASBs, more specifically, the causal relationship between the adiabatic temperature rising and plastic localization. Based on a large number of experimental results, scholars have summarized a variety of mechanical explanations in recent years to reveal the formation mechanism of adiabatic shear instability behavior. Moreover, in the aspect of numerical analysis, the scholars have already put forward many kinds of modelling and simulation methods to help discern the formation process of ASB. In addition to the modelling method at a macro scale, multiscale simulation methods consider different kinds of microscopic mechanisms in their constitutive relations. As such, the influences of different microscopic variables on the dynamic mechanical behavior can be analyzed independently via multiscale simulations, which can greatly promote the comprehension of formation mechanisms of adiabatic shear bands. In this review, the latest investigation advances on adiabatic shear instability behavior of the research group from impact dynamics and engineering application laboratory in Northwestern Polytechnical University, as well as a number of published results of other scholars, focusing on the formation mechanisms of ASBs in the metallic materials with different microscopic characteristics, are summarized. Firstly, the most classical thermal softening theory is discussed, and the latest measurement results revisiting the causal relationship between adiabatic temperature rise and localized deformation are reviewed. Then we briefly introduce some other softening mechanisms proposed based on the microstructure characteristics of metallic materials, including the geometric softening theory and dynamic recrystallization softening mechanism. Based on these above instability mechanisms, many simulation models are constructed to reproduce the formation process of ASBs. Therefore, we also systematically elaborate on the simulation studies on adiabatic shear instability behavior via multiscale simulation methods and track their respective progress. Besides the traditional modelling and finite element method at the macro scale, more simulations carried out at the meso/micro scales to reveal the physical essence of this behavior in recent years are stated, such as the crystal plasticity finite element method, phase filed simulation, discrete dislocation dynamics method and even atomistic simulations. Finally, we discuss the problems existing in this research field and future research focuses for the adiabatic shearing localizations.