The S-N technology used in the ASME Code and similar design criteria includes both crack initiation and crack propagation phases of fatigue failure. Some of the pressure vessel construction codes have used the simplified fatigue initiation criterion based on stresses calculated at a distance of 0.05 mm from the surface crack. In this work such techniques have been assessed for pressure vessels in light of the theory of critical distances. This method has been used to generate S-N curves for typical materials such as medium carbon steels, nickel chrome steels and stainless steels. Separate techniques have been highlighted for obtaining the number of cycles to nucleate a crack from a free surface, and also the number of cycles to drive the nucleated surface crack to propagate to a length equal to the critical distance parameter related to the process zone. The two estimates on the number of cycles when combined constitute a reasonable estimate of the number of cycles to initiate a crack and form the basis of the S-N design curves. The method requires the material property information on elastic-plastic fracture toughness and short crack propagation characteristics including crack closure effects.

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