Strain Localization, Damage Evolution, and Fracture Mechanisms in Sheet-Metal Bending: A Review

Sheet-metal bending produces pronounced through-thickness strain gradients and localized deformation. The resulting damage and fracture depend on microstructure, specimen geometry, loading path, and failure criterion. This review examines bending configurations, stress and strain distributions, neutral-layer evolution, bendability measures, and multiscale characterization methods, and compares damage behavior in steel, aluminum, copper, magnesium, and titanium alloys. It then considers damage initiation and crack propagation in relation to surface roughening and strain localization, slip and shear bands, second phases and interfacial damage, and twinning and tension–compression asymmetry. Most studies describe bending damage in different metals through related stages of heterogeneous microscale deformation, strain localization, damage initiation, and crack propagation. Microstructural heterogeneity and initial defects govern local deformation compatibility, while geometry and processing conditions affect strain gradients, constraint, and damage accumulation. Limit bend angle, minimum bend radius, and load-drop point are apparent measures determined jointly by the material, specimen, and process; none directly measures intrinsic fracture resistance. Bendability should therefore be evaluated using local strain fields, crack evidence, and microstructural evolution together. The review provides guidance for material selection, bending-test design, model calibration, and the definition of safe processing windows, while identifying in situ multiscale characterization, microstructure-informed damage modeling, AI-assisted damage prediction, and standardized bendability assessment as priorities for future research.