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 proporti on of an incident wave; this proportion is known as the reflection coeffici ent. 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 cons idering the interface as a spring. The proportion of the incident wave refl ected (reflection coefficient) is then a function of the stiffness of the i nterface and the frequency of the ultrasonic wave. The sensitivity of the u ltrasonic technique has been quantified using a simple model, from which th e stiffness of individual gaps and contacts are calculated and their effect on the ultrasonically measured stiffness predicted. The reflection of ultr asound at a static interface between a rough, nominally flat aluminum plate and a rough, nominally flat hardened steel punch has been investigated. Pl astic flow on first loading was evident, while repeated loading was largely elastic. However, subsequent cycles indicate a small amount of further pla sticity and contact irreversibility. The effect of surface roughness on the resultant contact has also been investigated. A simple plastic contact mod el 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 c ontracts remains constant over most of the loading whereas the number of co ntacts increases almost linearly. The contact stiffness has also been model ed with two well known elastic rough surface contact models. These models p redicted a lower interface stiffness than was observed in the experiments. However they provide a useful way of interpreting the ultrasonically measur ed interface stiffness data.