Development of S-N Curves for Some Pressure Vessel Steels

In this paper the stress range versus the number of cycles to failure (S-N) curves has been developed for steels used in the pressure vessel and construction industries. These include SA 533 Grade B and C36 Steel. The method used to obtain the S-N curves involves a material parameter based on fatigue crack threshold stress intensity factor range and the endurance limit. This parameter has been used to study fatigue crack growth of short cracks as a means to study fatigue crack initiation in steels and is based on the concept that initiation occurs only when the material ahead of the crack tip is damaged enough by cyclic straining. A separate model has been explored that incorporates a microstructural variable, namely the grain size. For the two materials studied, the initiation and growth of small cracks have been investigated with consideration for crack closure. Using the material parameters, the number of cycles to initiate a fatigue crack has been determined.