Effects of temperature and pressure fluctuations on the crack-arrest threshold stress intensity factor for high-strength low-alloy steel in high-pressure hydrogen gas

• Pressure and temperature fluctuations impact hydrogen-induced delayed fracture. • Crack arrest threshold, K THa rises with temperature in constant environments. • Hydrogen trap-site occupancy likely governs K THa under variable conditions. • Crack-tip mechanical state controls subcritical crack propagation behavior. • Provides insights for mitigating embrittlement risks in H2 storage systems. Environment-assisted, time-dependent subcritical cracking is a significant concern in high-pressure storage systems and pipelines, as minor flaws can evolve into critical failures under sustained stress over time. Moreover, hydrogen gas storage facilities are known to experience pressure and temperature fluctuations during pressurization and depressurization. In this study, the impact of such fluctuating conditions on hydrogen-induced delayed fracture was investigated. Constant-displacement bolt-load tests were conducted using low-alloy steel JIS-SCM435H. Threshold stress intensity factors for crack arrest, K THa , were measured in high-pressure hydrogen gas under both constant and fluctuating pressure and temperature conditions. For tests conducted in constant environments, K THa values increased at high temperatures compared with room temperature, which may be associated with lower hydrogen trap-site occupancy at elevated temperatures. Furthermore, under temperature and pressure fluctuations, K THa values appeared to correlate with the hydrogen trap-site occupancy, which in turn was likely affected by the instantaneous combination of temperature and pressure. Consequently, it was revealed that the mechanical state at the crack tip, potentially influenced by yield strength and hydrogen occupancy, could govern the subcritical crack propagation behavior in the material. This study provides valuable insights for mitigating hydrogen embrittlement risks in practical applications and offers a comprehensive understanding of how environmental fluctuations can influence material integrity in hydrogen storage systems.