Consideration on dominant factors for extremely low cycle fatigue properties

Extremely low cycle fatigue (ELCF) tests were carried out at room temperature under the strain controlled condition on the notched low carbon steel specimens with root radii of 20, 2 and 0.4 mm, which were denoted as R20, R2 and R0.4 series respectively. The fracture mode transition that crack initiation site changes from the surface to the internal of the specimen with increasing the applied plastic strain range was found to occur in R20 series. In R2 and R0.4 series, not the fracture mode transition but the surface cracking fracture mode was observed. In order to elucidate the reason why the fracture mode transition occurred, dominant factors for ELCF properties were considered. Based on the Coffin's theoretical concept, it is assumed that ELCF properties are evaluated by the value of equivalent plastic strain range (Δ ε_Peq) where fracture occurs, which is estimated by finite element method (FEM) analysis. As a result of applying the assumption to the fatigue test results, the assumption would be valid to the fracture mode transition and fatigue life evaluation. It is concluded that the value of Δ ε_Peq where fracture occurs is the dominant factors for ELCF properties.