Effect of mean stress and notch radius on fatigue fractured surface in theearly second stage in notched SM400 steel specimen

The fatigue strength of a specimen with a sharp notch has a tendency to dec rease when the mean stress applied to the specimen is increased. To find ou t some changes of the feature in a fractured surface related to the effect of a mean stress on a fatigue strength, the early second stage of a crack m ust be observed because the first stage crack can not be affected by the me an stress. In this report, some changes of the early second stage crack caused by the mean stress in a notched SM400 steel specimen is investigated by using a sc anning electron microscopy. The other hand, for the example that there is n o effect of a mean stress on a fatigue strength, the fractured surface of t he specimen with a large radious notch is additionally observed. The results obtained are as followed. Under the condition that the stress a mplitude is kept at a constant value, the region size of the fractured surf ace involving with small plane parts is changed by the effect of the mean s tress when the mean stress is small. When the mean stress is large, there i s no change of the region size. There is no change of the resion size in a specimen with a large radius notch.