LARGE PLASTIC FLOW AND SHEAR BANDING: MECHANISM, MECHANICS, AND CONTROL

Large plastic flow in metals, rocks, and polymers can appear as shear bands, folds, or superplastic flow under specialized conditions. Among these, shear bands localize extreme deformation and are ubiquitous yet remain difficult to study due to inconclusive formation mechanisms and challenges in isolating local parameters. Using a metal cutting apparatus and a marker-based technique with both microstructural and inscribed markers, shear bands are studied fundamentally in Naval brass, 4140 steel, and Ti-6Al-4V titanium alloy. Results show that shear bands can form at low temperatures (approximately 35 °C in cold-worked Naval brass), indicating that thermal softening is not mandatory. Detailed microstructural evidence such as localized displacement steps in the band, voids on the sheared surface extending outward, and infrequent voids inside the band points to a void-formation-and-closure mechanism for shear banding. Marker profiles reveal a common macroscopic shape that is well modeled as a classical boundary layer from fluid mechanics, showing excellent agreement. Boundary layers are also observed on external surfaces such as tool–workpiece interfaces and indentation subsurface. Characteristic features of the proposed mechanism are also seen in two pure metals, a metallic glass, a polymer, and rocks, suggesting a universal mechanism of large plastic flow. Based on this understanding, two control strategies for large plastic flow are proposed, focusing on (i) process design and (ii) material design. The study’s implications span materials science, solid mechanics, and geology.