The effect of superposition of small repeating stress on σ_sec the endurance stress under which no crack initiation occurs within 100h, and on the incubation time (time of crack initiation) has been investigated using notched specimens of four kinds of steels (JIS: SKD61, SNCM8, SUS420J2, and 18Ni mamging steel) quenched and tempered at various temperatures. The results obtained are as follows:
(1) For all the steels examined, the endurance stress σ_sec was not varied by the superposition of repeating stress when the stress amplitude, σ_a, was 2.5kg/mmmm², when σ_a was larger than 5kg/mmmm², however, σ_sec decreased with increase of σ_a.
(2) The amount of decrease ofσ_sec by the superposition of repeating stress seemed to be dependent not only on the yield strength of the materials but also on the stability of protective film on the notch surface against repeating stress and the intensity of metal-environment corrosion reaction after the destruction of the protective film by repeating stress. 18Ni maraging steel and SUS420J2 stainless steel with higher value of σ_sec under static stress revealed the smaller decrease of σ_sec under repeating stress than SKD61 and SNCM8 steels.