Ultrasonic assessment of wear-induced modifications in engineering contacts

Wear in engineering contacts, which occurs as an unavoidable phenomenon linked to the relative motion between the interacting bodies, may originate significant changes in contact features such as size and shape of the nominal contact area, contact pressure distribution and real contact area (RCA). While in some cases such alterations have little (or no) practical effects, there are situations in which the deviation of contact parameters from the original design is a primary source of malfunctioning and unexpected early failures; thus, the analysis of contact modifications may represent an indirect yet effective method for estimating wear effects. On the basis of these considerations, this study proposes the application of an ultrasonic-based technique to investigate the way wear alters contact conditions on three couples of relevant engineering interest (wheel-rail, tire-ground and gear-rack systems). Basically this method analyzes the reflection of high-frequency ultrasonic waves from the stressed interface, a parameter that is known to be related to contact conditions. The size and shape of the nominal contact area are obtained practically in real time by graphically processing the reflection data while further post-processing procedures allow the assessment of contact pressure distribution. A series of experiments was performed on artificially altered surfaces (to simulate both random and controlled wear patterns) by scanning the contact interface with 10 and 15 MHz frequency focussed transducers and the results, mainly expressed in terms of ‘contact maps’ allow us to state that the ultrasonic method may represent a very effective tool in investigating wear-induced modifications of contact conditions.