The contact between dissimilar materials occurs in many machine elements where one of the contacting parts is manufactured from low modulus materials such as lip seals, o-rings, and metal on polymer prosthetic hip joints. Contacts of this sort of are often operated in the iso-viscous elastohydrodynamic lubrication (I-EHL) regime. Typically, they have been studied using a numerical approach due to lack of sensor of instrumentation for measuring oil film thickness. By developing the technology of sensors such as electromagnetic radiation and magnetic resistance techniques, the phenomenon of lubrication in I-EHL regime has shown results which are better understood. However, the experimental study that has been conducted to date is only appropriate for laboratory-based measurements.
This thesis deals with the ultrasonic reflection methods to measure an oil film thickness and contact between dissimilar materials where these methods could be applied in-situ. This warrants special attention because there are two drawbacks of measuring of oil film thickness and contact by using bulk longitudinal wave between dissimilar materials (such as rubber and steel) which have mismatched acoustic impedance. One is the attenuation. The ultrasonic signal will be reduced when passed through the rubber. The other is accessibility. The wave must pass normally through the interface and so the transducer must be mounted on the rubber itself.
There are two methods that can be used to measure oil film thickness using ultrasonic reflection: amplitude and phase shift. The amplitude method has been proved successfully for measuring oil film thickness between two similar materials and between two materials with little difference in acoustic impedance, but it fails for contact between two acoustically dissimilar materials. In this case, the phase shift method has the potential to measure oil film thickness. The results show that this method is valid for measuring thin films (h < 40 μm) for contact between Perspex and steel.
The application of ultrasonic reflection techniques to measure the lubricant film thickness in iso-viscous elastohydrodynamic lubrication regime has been investigated. The reflection of ultrasonic pulses from the interface between the nitrile sphere and Perspex disk was recorded for a range of lubricated, dry, static, and dynamic contact conditions. In this way, profiles of oil film thickness were created for various loads and sliding speeds. The phenomenon of a wedge-shaped constriction in lubricant film was observed, especially at low speeds. It was also possible to observe cavitation effects on the signal in the exit region. The measured central film thickness results are compared with published models of the lubrication mechanism and experimental data obtained from optical methods. The result shows that the oil film thickness was measured in the region of 1 to 6 µm. The data was shown to be consistent with previous published experimental work using optical methods but somewhat lower than theoretical solutions.
Ultrasonic surface waves that are commercially used for non-destructive evaluation (NDE) and damage detection have been also developed to measure contact between soft and hard materials. The measurements were made by using variable and fixed wedge transducers. The reflection coefficient of Rayleigh waves at point and line contacts was measured to study the characteristic of compliant contacts in dry and lubricated conditions. The results show that the increased load causes a decreased reflection coefficient. Therefore, the reflection coefficient of Rayleigh wave at interface between soft and hard materials can be developed as a sensor for o-ring and lip seals and this sensor could be positioned remotely from the contact.
