Abstract
The purpose of this research is to evaluate the effects of surface qualities on crack initiation life as well as fatigue life. The practical method presented by the authors for the detection of fatigue crack initiation using an ion-sputtered film was used to divide the fatigue life into the fatigue crack initiation life and the fatigue crack growth life in three-point bending fatigue tests. Normalized and case-carburized test pieces with crowned round notches were used in the tests where the development of fatigue was monitored from fatigue crack initiation to breakage. The normalized test pieces have different surface roughnesses and the case-carburized test pieces have different surface microstructures on the notch surfaces. The changes in crack initiation life and fatigue life owing to the notch surface qualities were investigated. From the obtained results, the following conclusions were drawn: the fatigue crack initiation life is increased several times by decreasing the surface roughness Ra from 1.27μm to 0.03μm for a normalized middle carbon steel; removal of the non-martensitic layer formed during case-carburized treatment leads to a marked increase in fatigue strength.
