The Use of Ultrasound in the Investigation of Rough Surface Interfaces

The measurement of ultrasonic reflection has been used to study the contact between rough surfaces. An incomplete interface will reflect some proportion of an incident wave; this proportion is known as the reflection coefficient. If the wavelength is large compared with the width of the gaps in the plane of the interface then the reflection mechanism can be modeled by considering the interface as a spring. The proportion of the incident wave reflected (reflection coefficient) is then a function of the stiffness of the interface and the frequency of the ultrasonic wave. The sensitivity of the ultrasonic technique has been quantified using a simple model, from which the stiffness of individual gaps and contacts are calculated and their effect on the ultrasonically measured stiffness predicted. The reflection of ultrasound at a static interface between a rough, nominally flat aluminum plate and a rough, nominally flat hardened steel punch has been investigated. Plastic flow on first loading was evident, while repeated loading was largely elastic. However, subsequent cycles indicate a small amount of further plasticity and contact irreversibility. The effect of surface roughness on the resultant contact has also been investigated. A simple plastic contact model is described which allows prediction of the average size of the asperity contacts and their number. This model shows that the average size of the contacts remains constant over most of the loading whereas the number of contacts increases almost linearly. The contact stiffness has also been modeled with two well known elastic rough surface contact models. These models predicted a lower interface stiffness than was observed in the experiments. However they provide a useful way of interpreting the ultrasonically measured interface stiffness data.