Investigation of mechanism for hydrogen-assisted crack propagation in austenitic stainless steels using acoustic emission method

Hydrogen embrittlement is a critical issue for structural materials used in high-pressure hydrogen systems, where cyclic loading can accelerate fatigue crack propagation. This study investigates the effect of hydrogen on fatigue crack propagation in AISI 304 austenitic stainless steel using acoustic emission (AE) method. Specimens were prepared under uncharged and hydrogen-charged (exposed to high-pressure hydrogen gas) conditions. Fatigue crack propagation tests were conducted under constant stress intensity factor ranges (Δ K ), during which AE signals were continuously recorded, and the DBSCAN algorithm was applied to classify and extract distinct AE event groups. Comparative analysis of the stress phase at AE initiation suggested that hydrogen charging can be associated with a lower initiation phase and a reduced dependency on Δ K . This behavior may be associated with changes in crack-opening-related AE activity and less apparent variation across the three tested ΔK conditions. Furthermore, the frequency characteristics of the AE clusters showed smaller apparent variations among the three ΔK conditions after hydrogen charging. These differences may reflect changes in crack-tip deformation and crack-propagation behavior, although the underlying AE source mechanisms could not be uniquely identified. Meanwhile, verification of the AE RMS voltage indicated that the strain-induced martensitic transformation itself was difficult to detect directly via the AE method under the present experimental conditions.