In order to clarify an influence of hydrogen on the fatigue crack propagation in ultra-low frequency region, we investigated the frequency dependence of the crack propagation rate of S10C in hydrogen and nitrogen atmosphere. In the hydrogen atmosphere, crack propagation rate decreased with decrease in frequency. In particular, the reduction in crack propagation rate was markedly low in the ultra-low frequency (0.001 Hz). However, crack propagation rate in the nitrogen atmosphere also decreased in the ultra-low frequency region. Hence, we concluded that the decrease in crack propagation rate was concluded to stem from carbon diffusion as well as hydrogen diffusion.
