Multiscale characterization of Lüders band propagation in ferritic steel using digital image correlation with multiple microscopy techniques

Lüders band propagation is a key phenomenon linked to the characteristic yield drop and discontinuous yielding in ferritic steels. This study presents a comprehensive multiscale analysis of Lüders band propagation in ultra-low-carbon ferritic steel using digital image correlation (DIC) combined with multiple microscopy techniques. Three scales of DIC were employed: macroscopic (Macro-DIC) using 3D digital imaging, mesoscopic (Meso-DIC) with optical microscopy, and microscopic (Micro-DIC) utilizing scanning electron microscopy (SEM). Macro-DIC revealed smooth Lüders band propagation along the specimen length at constant velocity, accompanied by nearly uniform (or slightly nonuniform) plastic strain accumulation characteristic of yield elongation. At the mesoscale, the Lüders band spanned several grains with a width of approximately 1–2 mm. Despite the continuous macroscopic strain gradient, localized strain heterogeneities and curved band fronts indicated grain-scale instability and heterogeneous plastic deformation. Micro-DIC measurements further elucidated the intragranular strain distribution, showing slip lines consistent with crystallographic slip systems and strain gradients governed by crystallographic anisotropy. The Schmid factor correlated strongly with the grain-level plastic strain, demonstrating that deformation heterogeneity arises from crystallographic orientation. These microscopic variations persist even after the passage of Lüders band, contributing to residual strain inhomogeneity and may be related to subtle surface features occasionally referred to as stretcher strains. The findings clarify that Lüders band propagation, while macroscopically uniform, involves complex unstable and anisotropic plastic deformation at smaller scales. Understanding these multiscale deformation mechanisms provides valuable insight for controlling Lüders band behavior, ultimately improving the surface quality and mechanical performance of thin steel sheets in industrial applications.