Failure assessment of crack propagation in as-built LPBF 17–4PH stainless steel under rolling contact fatigue

This work presents an integrated experimental–numerical study on the rolling contact fatigue (RCF) behaviour of as-built Laser Powder Bed Fused (L-PBF) 17–4PH stainless steel lubricated with water and oil. Controlled twin-disc tests, finite element analyses and modelling were combined to clarify the influence of lubrication on crack initiation and propagation mechanisms. Water-based lubrication promoted accelerated crack growth due to hydraulic pressurization and enhanced shear stresses, while oil lubrication provided a stable elastohydrodynamic separation, delaying crack initiation and mitigating branching. A novel predictive framework was formulated by incorporating elastohydrodynamic similarity parameters into a generalized crack-growth law, enabling the quantification of mechanical–tribological coupling effects. The proposed model exhibited strong agreement with experimental data (R2 = 0.75, RMSE = 0.195), confirming the synergistic contribution of stress intensity and viscous dissipation in controlling fatigue damage evolution. Furthermore, the Failure Assessment Diagram (FAD) methodology was extended to account for finite-life and lubrication effects, successfully delineating the transition from crack arrest to propagation across pressure regimes. The developed approach provides a unified, physically consistent basis for assessing lubrication-dependent fatigue performance in additively manufactured steels, offering enhanced predictive capability for RCF design of AM components.