mprovement in the threshold stress intensity factor by tensile overload in large crack was investigated. The crack retarded zone and the non-propagating crack size was validated based on the fracture surface observation. It was observed that crack propagation was retarded by the overload during the fatigue tests. The size of non-propagating cracks and the crack retarded zone corresponded well to the calculated value. Moreover, in order to evaluate the overload effects on the small crack like defect, bending fatigue tests were carried out and discussed the effect of crack size on the improvement of the ΔKth.
