Microstructure-sensitive ductile fracture in micro-cutting of 3J33 maraging steel: Linking dislocation-mediated strain gradients with fracture energy evolution

Material fracture critically influences cutting processes by profoundly affecting chip formation, surface integrity, and tool wear. Nevertheless, how microstructural characteristics govern fracture behavior during micro-cutting remains poorly explored, limiting understanding of ductile fracture mechanisms at micro scales. To address this knowledge gap, this study employed an integrated experimental and computational approach, incorporating strain gradient plasticity theory to account for dislocation effects, to investigate microstructure-sensitive ductile fracture ahead of the tool tip in the micro-cutting of 3J33 maraging steel. Experimental results demonstrated that ductile fracture initiation preceded tool advancement in the localized deformation region ahead of the tool tip, providing compelling evidence for fracture occurrence during material removal. Dislocation density assessments in both the fracture zone and primary shear zone revealed a distinct increase with decreasing uncut chip thickness. A modified constitutive model integrating strain gradient plasticity was developed and implemented in finite element simulations. Analysis of the simulated stress distribution within the deformation zone elucidated the ductile fracture mode ahead of the tool tip. Furthermore, fracture energy was quantified using the J-integral, incorporating dislocation-induced strengthening effects through strain gradient plasticity theory. The results indicated that accounting for strain gradient plasticity led to higher specific fracture energy. Additionally, the size effect of specific cutting energy—a unique phenomenon in micro-cutting—was investigated, offering deeper mechanistic insights. The findings suggest that incorporating strain gradient plasticity reduces the relative contribution of fracture energy to the total specific cutting energy. This study advances the fundamental understanding of microscale cutting mechanics and facilitates the broader industrial application of micro-cutting processes.