Rotating bending fatigue property and fractography for high strength steels and carbon steel over ultra wide life region

In order to clarify the effect of strength level on the fatigue property in a wide life region, fatigue tests in the life region of N=0.5∼109 were performed for high strength steels of SUJ 2, SNCM 439 and the carbon steel of S 35 C with low strength. S-N characteristics of respective steels were compared with each other and the fatigue mechanisms were discussed from a view point of fractography. Thus, it was found that the S-N curve for SUJ 2 steel in the medium life region can be straightforwardly connected to the static strength level, but the S-N curves for SNCM 439 and S 35 C steels in the low cycle region tends to be horizontal so that the S-N curves have the reversed S-shape. It was also found that S-N property for high strength steels of SUJ 2 and SNCM 439 were explained as duplex S-N characteristics corresponding to the surface induced fracture and interior inclusion induced fracture with a fish-eye, respectively. However, the S-N property for the low strength steel of S 35 C does not have such an aspect, but it has the simple S-N characteristics corresponding to the surface induced fracture. Consequently, the duplex S-N property of metallic materials in the ultrawide life region appears depending on the strength level.