Abstract
Crack initiation processes in a high-strength steel under torsional fatigue were observed by atomic force microscopy (AFM), and the critical slip distance for crack initiation was discussed. The depth of an intrusion increased linearly with the logarithm of the number of cycles, and the increasing rate of the depth drastically increased with its outgrowth to a crack. The critical distance of slip, however, may not be the same value for plane bending fatigue. The growth rate of shear type cracks was not controlled by the stress intensity range, ΔK_<II>. It was higher for higher stress amplitude. The transition condition from shear to tension type crack propagation was given by the condition that the Mode I crack growth rate is higher than Mode II crack growth rate, and this condition is almost equal to that the value of ΔK_<I> exceeds the value of ΔK_<Ith>.
