In order to clarify the fatigue behavior in a wide life region, fatigue tests in the life region of N=0.5∿(10)9 were performed for high strength steel of SUJ2, by means of a multi-type testing machine developed here. Thus, it was found that the S-N curve in the medium life region can be straightforwardly extrapolated to the low cycle region and connected to the static strength which provides the fatigue strength at N=0.5. It is another finding that S-N property of this steel in such a wide life region was successfully explained as duplex S-N characteristics corresponding to the surface induced fracture and interior inclusion induced fracture with a fish-eye, respectively. Characteristic fracture surface so-called fine granular area (FGA) was formed in the vicinity at the inclusion at the center of the fish-eye. Consequently, the long life fatigue fracture of this steel was caused through three different processes of (1) formation of FGA, (2) crack propagation to form the fish-eye and (3) rapid crack propagation to cause the catastrophic fracture.
