Quantitative ultrasonic characterization of subsurface inclusions in tapered roller bearings

Initiation sites for rolling contact fatigue (RCF) in bearing components are often associated with subsurface inclusions. The presence of such defects within the region of maximum shear stress just below the contact surface can increase the local state of stress. Eventually, the high-stress region may initiate microcrack formation, leading to raceway spall that is detrimental to bearing performance. Thus, bearing steel cleanliness is critical, particularly in the near-race region. In this article, recent ultrasonic surface wave inspection methods are described with which subsurface inclusions can be identified and characterized. Our past work has shown that such an approach can be used to identify subsurface inclusions that spall when subjected to simulated service life tests. Here, the quantitative aspects of this research will be highlighted. The surface wave penetration depth and the reflection amplitude from an inclusion are both frequency dependent. Thus, multifrequency ultrasonic inspection procedures have been developed to exploit this behavior to identify the size and depth of inclusions. Reference measurements are based on samples created with artificial defects that have a known size and depth. Finite element (FE) modeling is used to identify inclusion sizes and depths that are deemed critical to bearing performance from which the ultrasonic inspection criteria are generated. This work is expected to impact the field of nondestructive testing for bearing components subjected to RCF.