Machine elements such as rolling element bearings are widely used in engineering and transportation areas. The life of a bearing is closely related to the stress state in the constituent components. Determining the stress state experimentally is difficult since the contact region is hidden inside the contacting bodies, making it difficult to characterize without altering the contact itself. This paper presents an experimental study to monitor static and dynamic ball-on-flat contacts, by using an ultrasonic reflectometry measuring technique, to demonstrate the concept of monitoring operating ball-bearing conditions in a non-invasive manner. By using an ultrasonic focusing probe and a 64-element ultrasonic array, contacts between a nitrile ball and a Perspex plate as well as contact between a steel ball and a grooved steel plate were characterised under both static and dynamic conditions. Both contact size and distribution of contact stress can be visualized in 2-dimensional plots. In this paper, the capability of ultrasonic reflectometry for non-invasive characterisation of contact conditions are demonstrated, and more importantly the development of multiple measuring mechanisms to realize real-time contact monitoring from static to dynamic conditions is illustrated. The proposed technique in the study is expected to characterise dynamic contacts of bearings in various scenarios from small mechanical systems (e.g., micro motors) to large civil infrastructures (e.g., wind turbines).
Keywords: Contact mechanics; Non-destructive evaluation; Structural health monitoring; Ultrasonic array; Ultrasonic testing.
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