Shear banding and fracture in large strain deformation: Understanding the mechanics of flow localization from Zener's time to the present.

Shear banding is an instability in large strain deformation of solids, where otherwise homogeneous flow becomes localized in narrow 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 nearly 75 years since the publication of Zener and Hollomon's pioneering work (C. Zener and J. H. Hollomon, J Appl. Phys., 15:22-32, 1944) which is widely credited with drawing the attention to shear bands and localization phenomena. Since this landmark publication, there has 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, post-initiation. This article summarizes our present understanding of plastic flow dynamics around a shear band and subsequent transition to fracture, with specific focus on the post-instability stage. We begin with a semi-historical look at some of Zener's early ideas and discuss recent advances in full-field experimental methods for mapping the localized flow during band formation, including in situ imaging as well as ex situ/post-mortem analyses. Classical analytical theories are revisited in the light of recently published experimental data. In agreement with Zener's original ideas, we show that shear bands exhibit a wealth of complex flow characteristics that bear striking resemblance to viscous fluid flows and boundary layer phenomena. Finally, new strategies for reproducing shear band formation at low speeds are discussed. 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.