Effect of notch shape and absorbed hydrogen on the fatigue fracture below fatigue limit

It has been a common understanding that fatigue failure would not occur if all stress amplitudes are within the fatigue limit diagram. However, it was shown that fatigue failure occurred in some special cases of loading condition even when all stress amplitudes were kept within fatigue limit diagram in the case of small-notched specimen. The cause of such a phenomenon was examined using two-step loading pattern.In the case of constant stress amplitude loading, non-propagating crack was formed only at low mean stress and not formed at high mean stress. In the case of two-step loading in which the first step stress was chosen with zero mean stress and the second step stress was with high mean stress, a non-propagating crack was formed by the first step stress, which caused the fatigue fracture below fatigue limit. In this study, the effect of notch size, especially small notch, and root radius was examined. It was shown that a very small notch as small as 10μm deep notch caused the reduction of fatigue limit. The effect of absorbed hydrogen was also investigated. Absorption of 0.3ppm hydrogen caused more reduction of fatigue limit.