The fatigue crack growth properties of the Inconel 718 and Udimet 720 Ni-base alloys being used for liquid hydrogen fueled rocket engine materials were investigated as functions of ΔK and frequency in a high pressure hydrogen of 19.7MPa at room temperature. The results obtained are as follows;
(1) The fatigue crack growth rate in hydrogen was larger than that in argon for both Inconel 718 and Udimet 720. The effect of hydrogen on fatigue crack growth rate increased with decreasing the frequency of the cyclic loading. The effect of hydrogen on fatigue crack growth of Udimet 720 was higher than that of Inconel 718.
(2) Above the ΔK which the striations were observed on the fracture surface in argon, hydrogen showed marked effect on the fatigue crack growth, i.e., the fracture along the interfaces between δ phase and γ matrix for Inconel 718 and that the interfaces between γ' phase and γ matrix for Udimet 720 were observed on the fracture surface in hydrogen, respectively.