Plasticity and Damage Mechanics of Gradient Microstructure Steel Produced by Surface Severe Plastic Deformation: Theoretical and Experimental Study

This study investigates the plasticity and damage mechanics of gradient microstructure steel produced by surface severe plastic deformation (S2PD) processing, developing a continuum damage mechanics constitutive framework that captures the deformation and failure behaviour of the layered heterogeneous microstructure system. Gradient microstructure steel, in which a nano-grained surface layer transitions to coarser grained interior regions, offers a synergistic combination of high surface hardness and ductile core toughness not achievable in homogeneous microstructures. Steel specimens are processed by ultrasonic shot peening to produce characterised gradient microstructures with surface grain sizes below 100 nm transitioning to the original 20 micrometre grain size at depth. The gradient microstructure is characterised by XRD, EBSD, and TEM at five depth levels. Mechanical gradient profiles are determined by instrumented nanoindentation. Tensile and fatigue experiments are conducted to establish the stress-strain response and fatigue endurance of gradient specimens versus untreated controls. A continuum damage mechanics model with spatially varying elastic-plastic constitutive parameters, calibrated to the depth-resolved nanoindentation data, is implemented in Abaqus to simulate tension and fatigue crack initiation. The model is validated against measured macroscopic stress-strain curves and fatigue crack initiation site observations by SEM. The study provides an original theoretical and experimental contribution to the mechanics of gradient microstructure metallic materials. Keywords: gradient microstructure, severe plastic deformation, damage mechanics, fatigue, nanostructured steel.