| Summary: | Contacts of rough surfaces are present in almost all machines and mechanical components. Friction at the rough interface cause energy dissipation, wear and damage of surfaces. Engineers are interested in knowing the frictional conditions at contact interfaces. Despite friction being such a fundamental phenomenon, it is surprisingly difficult to measure reliably as results depend on the test method measurement environment. Methods have been developed to measure the friction and sliding contact tribometers are devised mostly in a laboratory environment. Their applications in measuring friction in-situ in a real contact is a challenge. Therefore, the aim of this research is to develop an ultrasonic method to measure friction and friction coefficient in-situ in a contact interface. Ultrasonic methods developed for non-destructive testing have been used to measure tribological parameters, such as oil film thickness, viscosity and pressure, in-situ bearings and machines. In conventional ultrasonic techniques, pulses are low power and when they strike an interface they do not result in a change in the contact state. The process is linear and elastic. However, high power sound waves can cause opening or closing of an interface, or interfacial slip; this is non-linear. Recently Contact Acoustic Nonlinearity (CAN) has drawn interest due to its potential in the non-destructive evaluation. When high power bulk shear ultrasound propagates through a compressed rough contact interface, higher order frequency components, higher odd order harmonics (3f, 5f, etc.) are generated in both transmitted and reflected waves. The nonlinear nature of the stick-slip phenomenon in friction may be the source of nonlinearity. In this study, nonlinearity due to the interaction of a shear ultrasonic wave with a frictional interface has been initially investigated numerically. A one-dimensional numerical model has been employed to understand contact nonlinearity generation and its dependence on incident ultrasonic amplitude, contact pressure and friction coefficient. The third harmonic increases and then decreases when contact stress rises, which suggests that nonlinearity generation due to the 'stick-slip' motion occurs at low contact stress and is restricted at high contact pressure. Harmonic generation at the contact was secondly investigated experimentally using a high frequency nonlinear ultrasonic technique. Methods were developed to separate the contact nonlinearity from the measured ultrasonic nonlinearity. Contact nonlinearity originating from a rough interface are assessed under various test conditions. Experimental measurement shows good agreement with the numerically computed nonlinearity. Two strategies were developed to estimate the friction coefficient using experimentally measured contact nonlinearity in conjunction with the numerical computation. The ultrasonically measured friction coefficient agrees reasonably with the sliding test results and published data. Using the contact nonlinearity, the ultrasonic method shows the usefulness in measuring the friction coefficient in-situ in a contact interface.
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