Factors Influencing the Formation of ODA in Ultralong Fatigue Regime

摘要

The mechanism of fatigue failure in the ultra-high cycle regime was studied using a bearing steel, JIS SUJ2 (SAE 52100). In the case of tension-compression fatigue testing, the S-N curve for high strength steels consisted of a single straight line in contrast to the dual S-N curve observed in rotating bending. The effect of internal hydrogen trapped by nonmetallic inclusions on high cycle fatigue behavior has been discussed by Murakami et al. In order to investigate further the influence of hydrogen trapped by various types of inclusions, four materials of different chemical compositions were prepared. Those specimens having a longer fatigue life had a particular morphology designated as ODA (Optically Dark Area) surrounding the inclusion at the fracture origin. In order to investigate the growth of ODA with cycling, fatigue tests involving multi-step loading were carried out, and the appearance of the ODA was compared with that of specimens tested under a constant amplitude loading. A specimen which endured N = 2 X 10~8 under a stress was fractured by tensile test at -190 deg C. The fracture origin in this tensile test was at a subsurface, nonmetallic inclusion. If the fatigue test had not been interrupted, the potential fatigue life of the specimen was estimated from the size of the inclusion and the ODA using the area~(1/2) parameter model. The model was also used to estimate the fatigue limit in the ultra-high cycle range. The condition for the critical size of ODA for the start of conventional fatigue crack growth was analyzed using fracture mechanics. The critical stress intensity factor range DELTA K_(ODA) for the critical size of the ODA can be correlated with the threshold stress intensity factor range DELTA K_(th) expressed by the area~(1/2) parameter model for small cracks. A fatigue design method for ultra-high fatigue life regime was proposed based upon the growth behavior of the ODA, the statistics of extremes of inclusions, and the area~(1/2) parameter model.