The Study of ODA's Formative Condition by Fatigue Test in Vacuum of High Strenght Steel
Hiroki KAWANO, Yoshihito KUROSHIMA and Shoji HARADA
Abstract:A high strength steel shows fish-eye failure in very long life fatigue. In this region, the fatigue crack growth in ODA (Optically Dark Area) controls the fatigue process. However, ODA's formative mechanism is unknown. Therefore, to predict fatigue life of fish-eye failure, it is important to know ODA's formative mechanism. So, the present study examined for the influential factors on ODA in the fatigue process through uniaxial push-pull fatigue tests in vacuum as well as computer simulation. In experiment, vacuum environment is employed to simulate fish-eye type crack growth on surface of specimens becase of shutting off supply of air. The test results in vaccum showed that the fatigue life property of surface failure has corresponded to fish-eye fatigue life. And the site of inclusion which has fracture origin size determines whether surface failure or internal failure in vacuum. The results of fractographic observation of crack initiation showed that the rough area of surface fracture in vacuum was formed by mechanism of ODA type crack. Furthermore, the fatigue life of numerical simulation in vacuum showed that the fatigue life which was necessary to form ODA on specimen surface corresponded experimental life. Key Words:High strength steel, ODA, Vacuum, Fish eye, Simulation