Modeling the transition from ductile to brittle fracture induced by hydrogen-assisted mechanical degradation in quenching and partitioning (Q&P) steel

Hydrogen embrittlement (HE) critically threatens the structural integrity of advanced high-strength steels (AHSS), particularly quenching and partitioning (Q&P) steels, due to their high susceptibility to hydrogen-assisted fracture. This study presents a unified fracture modeling framework that quantitatively captures the ductile-to-brittle transition in hydrogen-charged Q&P steel under diverse stress states. The model combines a Hosford–Coulomb (HC) criterion for ductile failure with a hydrogen-sensitive maximum principal stress (MPS) criterion for brittle fracture, each accounting for stress triaxiality and Lode angle effects. The framework is embedded within a finite-strain chemo-mechanical formulation, incorporating stress-assisted hydrogen diffusion, reversible trapping, and lattice dilation, which are systematically implemented via a user-defined element (UEL) subroutine in ABAQUS. Model calibration and validation are performed using slow strain-rate tensile (SSRT) tests across distinct specimen geometries, capturing a broad range of stress states. Fractographic analyses confirm the model's ability to reproduce experimentally observed transitions from ductile to brittle fracture. The model accurately predicts fracture initiation sites, modes, and hydrogen-concentration-dependent degradation in both monotonic and non-monotonic loading scenarios, including step-load conditions. The unified criterion offers a computationally efficient and mechanistically grounded tool for evaluating hydrogen-assisted fracture in high-strength steels, with direct implications for structural design in hydrogen-exposed environments.