Ultrasonic determination of normal and shear interface stiffness and the effect of Poisson's ratio.

The response of machine elements and structures depends on the stiffness of their joints. These joints are made up of rough surface contacts with both normal and shear stiffness. The stiffness of an interface can be determined using an ultrasonic method. The proportion of the ultrasonic wave amplitude reflected at a rough surface contact can be related to the interface stiffness using a simple quasi-static spring model. The reflection of longitudinal ultrasonic waves gives normal stiffness whilst transverse wave reflection gives shear stiffness. For a given interface, the stiffness is a function of the number of asperity contacts, their size, and distribution. This in turn depends on the contact pressure and the elastic surface properties in the near surface asperity region. However, considerations of the elastic contact of a rough surface, indicate that the ratio of shear to normal stiffness may be a function of the Poisson's ratio of the material only. Normal and shear stiffness measurements have been carried out for different rough surface contacts as a function of nominal contact pressure over a number of loading cycles. The ratio of shear to normal stiffness was found to be largely constant for a particular loading case. Measurements of the stiffness ratio agree well with predictions based on an elastic contact model; although calculation of Poisson's ratio from the results was found to be inaccurate.