Ultrasound as an experimental tool for investigating engineering contacts

Within the science of Tribology, there are few tools which practically and effectively allow the measurement of conditions at an interface. This paper describes an ultrasonic technique for measuring interface parameters. Ultrasound is an ideal measurement medium, as it has good transmission through most solids, is unaffected by surface oxide layers, is non-destructive, safe and portable. If an interface is modelled as a stiff spring, an equation can be derived which predicts the frequency profile of the reflected ultrasound. By performing a Fourier transform upon the ultrasonic echo from the interface, the stiffness of that interface can be calculated. The stiffness can be used to infer the behaviour of asperity contacts within the interface. In this work the stiffness of a metal interface over 10 loading cycles was measured, showing the effects of plasticity and adhesion. Results are also presented showing how first plastic loading cycles exhibit asperity creep and how over a number of cycles shakedown is achieved. Finally, a Hertzian ball-on-flat contact was scanned and the ultrasonic reflection profile obtained.