Bending fatigue life tests with notched plane specimens up to 107 cycles in 3-70 MPa hydrogen gas and fatigue crack growth tests with CT specimens in 0.7-90 MPa hydrogen gas were carried out for SCM435 low alloy steel. The fatigue crack growth rate depended on the hydrogen gas pressure and test frequency. The fatigue crack growth rate was 34 times higher than that in air at the maximum. The equation for fatigue life estimation was derived from these results and provided good fit to the experimental data of bending fatigue life tests of sharp notched and pre-cracked specimens at low-cycle fatigue range. It was deduced that this equation would be actually useful for the fatigue life estimation of various high-pressure hydrogen systems such as storage cylinder and piping which can not avoid the defects on the surface of an inner side. In addition, fatigue life tests showed that fatigue limit was the same as that in air. Non-propagating cracks were observed at the notch root of non fractured specimens at 107 cycles both in air and 70 MPa hydrogen. It was deduced that fatigue crack closure was probably a key factor for the fatigue limit in hydrogen gas and air. Hardness affected fatigue limit in hydrogen gas and air, whereas chemical components of S, Mo and V in materials did not affect fatigue life in hydrogen gas in this study.
