When two rough surfaces touch, only the peaks of the surface roughness (or asperities) come into contact. The real area of contact is therefore less than the geometric area of contact. In this project we have been using ultrasound to study this phenomenon. Using the following method we can identify a real area of contact and resulting contact pressure, and also investigate the ways in which asperities contact.
The apparatus is simple; a transducer is used to direct ultrasound of high, wide band, frequency. The amount of ultrasound reflected is proportional to the stiffness of the contact. A closely conforming contact will be stiff (i.e. an increase in the nominal contact pressure will cause only a small deflection). The stiffness varies from zero to infinity as the reflection coefficient varies from unity to zero. Thus, reflection, and hence stiffness, can be used to understand the nature of the asperity contacts.
The figure shows the reflection back from an elliptical contact between two curved surfaces. The transducer was scanned across the interface measuring contact stiffness at each point. The lighter the shade, the higher the contact stiffness and hence the greater the conformity. From this scan it can be seen that the resolution of the technique was sufficient to measure variations in contact along the interface. The rough surface profiles mean that the ideal Hertzian contact profile is never reached.
The same method has been used to investigate the repeated contact of rough metal surfaces. In this way we can study elastic, plastic and cyclic loading phenomena.
This graph shows the contact stiffness increasing as the load is increased. The first loading cycle shows largely plastic asperity contact. Successive load/unload cycles result in 'shakedown' to an elastic state.
The results have been compared with several analytical and numerical models of rough surface contact. Contact models are easily adapted to give output in terms of contact stiffness.
This work has developed into several new areas; measurement of oil films, mixed liquid/solid interfaces, and measurement of contact pressure in machine element joints.
- Dwyer-Joyce, R. S., Drinkwater, B. W., and Quinn, A.M., (2001), “The Use of Ultrasound in the Investigation of Rough Surface Interfaces”, ASME Journal of Tribology, Vol. 123, pp. 8-16. (ISSN 0742-4787)
- Drinkwater, B. W., Dwyer-Joyce, R. S., and Cawley, P., (1997), “A Study of the Transmission of Ultrasound Across Solid-Rubber Interfaces”, The Journal of the Acoustical Society of America, Vol. 101, issue 2, pp. 970-981. (ISSN 0001-4966).
- Drinkwater, B. W., Dwyer-Joyce, R. S., and Cawley, P., (1996), “A Study of the Interaction between Ultrasound and a Partially Contacting Solid-Solid Interface”, Proceedings of the Royal Society Series A, Vol. 452, No. 1955, pp. 2613-2628, London. (ISSN 1364 5021).