Multiple Frequency Ultrasonic Detection of Subsurface Near-Race Inclusions for Improved Fatigue Life Performance

The importance of steel cleanliness for the performance of tapered roller bearings has been clearly established and has led to on-going improvements in steel production methods. The presence of non-metallic impurities within the steel can result in hard/brittle inclusions that may serve as initiation sites for damage due to sub-surface rolling contact fatigue (RCF) if the inclusions lie within the near-race of the bearing components due to the high mechanical stress present. Current inspection standards define steel cleanliness with respect to bulk inclusion morphology, which limits inspection to a small area that may or may not be representative of the entire steel heat. In this presentation, ultrasonic surface wave methods are described for detecting subsurface inclusions directly on finished bearing inner and outer rings. We expand on our previous work to exploit the different inspection depths that can be achieved with different measurement frequencies. The impact of the different inspection depths is quantified through simulated service life testing with heavy axle loading conditions. For this study, bearing components were first subjected to ultrasonic surface wave testing at three different frequencies to identify near-race inclusions. The simulated service life testing was then used to assess the onset and propagation of RCF failure. RCF spall initiations correlated highly with the positions identified by the ultrasonic inspections suggesting that this approach has a predictive potential. However, additional research is needed to establish the specific criteria needed for such predictions with respect to the inclusion location along the race, the depth from the race surface, the inclusion morphology and the inclusion mechanical properties. This work is anticipated to improve the understanding of RCF damage initiation which will lead a higher level of safety for railroad operations.