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The mechanism of fatigue failure in the ultra-high cycle regime was studied using a martensitic stainless steel. The effect of internal hydrogen trapped by nonmetallic inclusions on high cycle fatigue behaviour has been discussed by Murakami et al. In order to investigate more in detail the mechanism of the elimination of the conventional fatigue limit and the influence of hydrogen trapped by inclusion, specimens with various artificial small defects were prepared. The fatigue failure of a specimen which contained an artificial hole occurred at Nf ≅108 from a nonmetallic inclusion which is smaller in size than the artificial small surface hole. In the case of the specimen containing two artificial holes connected with a fatigue crack, non-propagation crack was observed at the edge of the artificial hole, and the fatigue limit was in good agreement with the value predicted by the √area parameter model. In the case of the specimen whose fatigue crack originating at an inclusion, the crack continued propagation, and finally the specimen failed. Considering that nonmetallic inclusions trap hydrogen, it is presumed that the elimination of the conventional fatigue limit due to fatigue crack originated at nonmetallic inclusion is caused by synergetic effect of cyclic stress and hydrogen trapped by inclusions.

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P75 Elimination of conventional fatigue limit due to fatigue crack originated at nonmetallic inclusion, and non-propagation of fatigue crack originated at artificial small hole