Effect of Cycle Frequency on Fatigue Crack Propagation Behavior for Steels in Hydrogen Storage

This paper addresses the characteristics of the fatigue crack propagation behavior of high-strength low-alloy steels in a high-pressure hydrogen gas environment, especially in terms of the role of cycle frequency. Several heats of the steels, with different steel strengths and microstructures, were tested in high-pressure gaseous hydrogen utilizing fracture mechanics specimens. When the cycle frequency decreases, they showed higher crack propagation rates (da/dN) at or above a certain K (= K_max^T) on a K_max-da/dN diagram, while no cycle frequency dependencies were observed on da/dN below K_max^T. This K_max^T and observed da/dN were different among the various steels, and resulted from the difference in hydrogen gas embrittlement susceptibility caused by metallurgical variables (e.g. hardness, microstructure, impurity segregations). It is shown that the values of K_max^T observed in the steels were almost equivalent to K_IH-R (the threshold stress intensity factor for hydrogen-assisted crack growth) of the steels tested which were obtained with slow rising load tests (K̇ = <0.08MPa-m^1/2/s) in high-pressure gaseous hydrogen. Therefore, the increase in da/dN above K_max^T is attributed to the occurrence of hydrogen-assisted crack growth (da/dt), which is well explained by the K_IH-R parameter.