Abstract
Rotating bending fatigue tests were carried out and typical factors to govern the fatigue life were experimentally examined in the long life region. Fracture mode in the long life region was an interior inclusion induced fracture with a fish-eye. At the center of the fish-eye, a non-metallic inclusion was necessarily observed, and a fine granular area (FGA) was also necessarily observed in the vicinity around the inclusion on the fracture surface. It was found that the fatigue lives were related to following three factors; (1) the stress amplitude at the inclusion, (2) the inclusion size, and (3) the FGA size. These factors can be connected to the parameters in the formula of Paris' law. Based on this aspect, the fatigue life of this steel in the above fracture mode was formulated from a viewpoint of fracture mechanics. Thus, it was found that the period forming the FGA occupied more than 90% of the total fatigue life. It was another finding that the effective factor to expand the fatigue life was smallness of the inclusion size.
