Effect of Ultrasonic Surface Treatment on Giga-cycle Fatigue Properties of a Rolling Stock Axle Material

In the past, components under fatigue loading were designed by using an S-N curve and endurance limit according to very high cycle fatigue tests [1-3], fatigue strength to failure at a given life decreases as loading cycles increase. In other words, endurance limit is not observed for most materials, which is true for axle materials for rolling stock. An axle is one of the key components for structural safety of rolling stock. Failure of an axle may lead to derailment, fatal accidents, property loss, etc. During the lifetime of a railway vehicle, about 3 ×106 km mileage, an axle installed in the vehicle experiences very high cycles of fatigue loading (1.0~1.2 giga cycles). However, giga-cycle fatigue properties of axles have not been studied much owing to the restrictions of test time and costs. In this study, an ultrasonic fatigue test equipment with a piezoelectric actuator of frequency of 20 kHz was developed. By using the newly equipment, the effect of ultrasonic surface treatment on the fatigue behavior of an axle material was examined in giga cycle range. Failure mechanism was observed and fatigue strength in giga-cycle range was obtained. It was found that the ultrasonic surface treatment could improve the fatigue strength of the axle material by more than 10 %.