Simultaneous subsurface defect detection and contact parameter assessment in a wheel–rail system

In the last few years, a non-conventional ultrasonic approach has been proposed by the authors to experimentally assess the main contact parameters in wheel–rail systems. This technique, which is based on the analysis of the reflection of high frequency ultrasonic waves from the wheel–rail interface, makes it possible to obtain graphic maps from which we can observe the shape of the contact area, measure its size, and also collect qualitative information about contact pressure distribution. Furthermore, ultrasonic waves are quite well known as one of the primary non-destructive testing techniques useful in checking the presence of cracks in wheels, rails and axles. Thus, it was thought to combine these two approaches to obtain a single tool capable of supplying at the same time information about wheel–rail contact and the possible presence of subsurface cracks, which are typically located a few millimeters below the surface. This study reports the results of a series of experiments, performed on realistic wheel–rail couplings, in which the wheel was characterized by the presence of subsurface defects of known geometry. For each case tested, the whole ultrasonic waveform (which contains information about both the contact interface and a certain thickness of the wheel to investigate) was acquired and analyzed with dedicated software. The original data, expressed in terms of amplitude of reflected peaks, have been processed to obtain a number of graphic maps which, layer by layer, starting from the surface, give an idea first of all of contact conditions and, at the same time, of what kind of anomalies are present below it. These data are potentially useful in supporting numerical studies on the propagation of rolling contact fatigue (RCF) subsurface cracks by providing both actual contact pressure distribution and geometry of the cracks.