Stepwise S-N curve and fish-eye failure in gigacycle fatigue

Fatigue failure is normally initiated at the surface of a material. For som e materials, failure can be initiated both at the surface and the interior. This twofold materials behaviour in fatigue is represented by a stepwise s hape in the S-N curve. An internal failure mode is especially important for fatigue life in the gigacycle range, as this mode is predominant at low st ress ranges. Materials with a hardened surface fail from the surface only at high stress es, and at low stresses from the inside, forming a fish-eye facet on the fr acture surface. Exactly the same behaviour can be observed for materials wi thout a hard surface, even at elevated temperatures. This paper displays so me of the results obtained at NRIM and discusses possible interpretations.