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Shear bands in materials processing: Understanding the mechanics of flow localization from Zener's time to the present

Shear banding is a material instability in large strain plastic deformation of solids, where otherwise homogeneous flow becomes localized in narrow micrometer-scale bands. Shear bands have broad implications for materials processing and failure under dynamic loading in a wide variety of material systems ranging from metals to rocks. This year marks 75 years since the publication of Zener and Hollomon's pioneering work on shear bands which is widely credited with drawing the attention of the mechanics community to shear bands and related localization phenomena. There has since been significant experimental and theoretical investigation into the onset of shear banding. Yet, given the extremely small length and time scales associated with band development, several challenges persist in studying the evolution of single bands. Recent full-field displacement measurements, coupled with numerical modeling, have only begun to ameliorate this problem. This article summarizes our present understanding of plastic flow dynamics around single shear bands and the subsequent transition to fracture, with special applications to materials processing. We begin with a semi-historical look at some of Zener's early ideas on shear bands and discuss recent advances in experimental methods for mapping localized flow during band formation, including direct \emph{in situ} imaging as well as \emph{ex situ}/post-mortem analyses. Classical theories are revisited in the light of recently published experimental data. Shear bands exhibit a wealth of complex flow characteristics that bear striking resemblance to boundary layer phenomena in fluid flows. It is hoped that these will help further our understanding of shear band dynamics, the subsequent transition to fracture, and lead to practical `control' strategies for suppressing shear band-driven failures in processing applications.