Early-fatigue-damage evaluation in AISI 304 steel by nonlinear ultrasonic testing: methodology and mechanism interpretation

Timely detection of early-stage fatigue microdamage remains a challenge in structural health monitoring (SHM) of safety-critical components. Here, we present a nonlinear ultrasonic (NLU) detection approach to assess early-stage microstructural degradation in cold-rolled AISI 304 stainless steels under high-cycle fatigue. An NLU system with controllable excitation and harmonic isolation was developed, aiming to correlate ultrasonic nonlinearity responses accumulation with microstructural evolution through the nonlinear metric β’. NLU measurements revealed a bimodal trajectory of β’ during fatigue progression, with peaks at approximately 30% and 70% of fatigue life. Correlative metallographic characterisations validated the NLU response and elucidated its physical origin. Fatigue-induced changes, including dislocation activity, PSB formation, and microdamage accumulation, aligned closely with the β’ trajectory. Among them, dislocation (geometrically necessary dislocations, GNDs) dynamics played a dominant role in nonlinearity response fluctuations. A framework is proposed to major attribute early β’ evolution to lattice-elastic nonlinearity, while at later stages, crack-initiation-induced contact nonlinearity serves as an additional amplifier, boosting β’ towards its global maximum. These findings demonstrate the efficacy of harmonic-based NLU for capturing microstructural deterioration and early-damage, offering a scalable path towards physics-informed SHM in safety-critical metallic materials (as stainless steel) components.