A STUDY OF THE INTERACTION BETWEEN ULTRASOUND AND A PARTIALLY CONTACTING SOLID-SOLID INTERFACE

The measurement of the reflection of ultrasonic waves from a partially contacting solid-solid interface can be used to study the contact con ditions at that interface. This paper describes measurements and predi ctions of the reflection of ultrasonic waves from partially contacting aluminium-aluminium interfaces, performed in the low frequency regime where the wavelength of the ultrasound is large compared to the size of the gaps. The proportion of the incident wave which is reflected at the interface (the reflection coefficient) was measured as a function of frequency with a single wideband ultrasonic transducer. When load was applied across the interface three regions of contact can be seen; no contact, partial contact and perfect contact. In the no contact re gion the measured reflection coefficient was unity at all frequencies. In the partial contact region the measured reflection coefficient inc reased with frequency. No measurements were taken in the perfect conta ct region in which the reflection coefficient is known to be zero at a ll frequencies as this state is the same as a continuous piece of alum inium. The reflection coefficient variation with frequency was modelle d using a spring model, good agreement between experiments and predict ions being achieved. Reflection coefficient measurements were then use d to study the contact between two aluminium surfaces under repeated l oading and unloading cycles. Plastic flow on first loading was evident while subsequent loading cycles revealed largely elastic behaviour. B oth elastic and plastic statistical contact models, as well as a numer ical contact model, were used to predict the variation of interfacial stiffness with pressure. These models agreed qualitatively with the ex perimentally determined stiffness variations and the predicted stiffne ss was within an order of magnitude of the measured value in all cases .