Understanding strain localization in metallic materials: a review of high-resolution digital image correlation and related techniques

Plastic deformation in metallic materials is generally governed by highly localized and intrinsically heterogeneous deformation processes, including crystallographic slip banding, deformation twinning, phase transformation and grain-boundary sliding. These mechanisms operate at the sub-grain scale where they are competing, interacting, and are sometimes incompatible for short-range transmission due to deformation confinement within individual grains. The heterogeneous nature of irreversible deformation at the microstructure scale also applies at the mesoscale, i.e. the scale of the crystalline aggregate. Capturing experimentally the discrete and heterogeneous deformation processes at the microstructure scale is essential to understand elementary deformation processes involved for specific loading conditions, quantifying their intensity to finally achieve a better dialogue with numerical models of crystal plasticity for the prediction of mechanical behavior and the lifetime of parts. High-resolution digital image correlation (HR-DIC), implemented on scanning electron microscopy images, has emerged as a key technique to quantify these phenomena by providing full-field measurements of in-plane displacement and strain at sub-micron spatial resolution over statistically representative fields of view. This review outlines the experimental foundations, data-processing strategies, and correlative analysis frameworks that underpin the use of HR-DIC for studying strain localization in metals.