An evaluation of ultrasonic arrays for the static and dynamic measurement of wheel–rail contact pressure and area

Abstract

The interfacial contact conditions between a railway vehicle wheel and the rail are paramount to the lifespan, safety and smooth operation of any rail network. The wheel–rail interface contact pressure and area conditions have been estimated, calculated and simulated by industry and academia for many years, but a method of accurately measuring dynamic contact conditions has yet to be realised. Methods using pressure-sensitive films and controlled air flow have been employed, but both are limited. Ultrasonic reflectometry is the term given to active ultrasonics in which an ultrasonic transducer is mounted on the outer surface of a component and a sound wave is generated. This ultrasonic wave packet propagates through the host medium and reflects off the contacting interface of interest. The reflected waveform is then detected and contact area and interfacial stiffness information can be extracted from the signal using the quasi-static spring model. Stiffness can be related to contact pressure by performing a simple calibration procedure. Previous contact pressure measurement work has relied on using a focusing transducer and a two-dimensional scanning arrangement which results in a high-resolution image of the wheel–rail contact, but is limited to static loading of a specimen cut from a wheel and rail. The work described in this paper has assessed the feasibility of measuring a dynamic wheel–rail contact patch using an array of 64 ultrasonic elements mounted in the rail. Each element is individually pulsed in sequence to build up a linear cross-sectional pressure profile measurement of the interface. These cross-sectional, line measurements are then processed and collated resulting in a two-dimensional contact pressure profile. Measurements have been taken at different speeds and loads.

Real-time non-invasive measurement and monitoring of wheel–rail contact using ultrasonic reflectometry

“…Dynamic measurements showed good agreement with static test measurements and Hertz theoretical predictions in terms of general contact pressure levels. In contrast to previous work, 17,18 the pitch-catch strategy achieves a truly non-invasive method of monitoring wheel-rail contact conditions, and no modification to the rail structure is required. With the relevant software and hardware upgraded, the new approach is expected to characterise and monitor contacts between the instrumented rail track and all passing train wheels on operating high-speed lines.…”

“…The number of measurements as the rail vehicle passes over the array transducer as a function of pulse repetition frequency. 17 …”

“…Ultrasonic reflectometry, however, has proved useful for machine–element contact measurements 15 and particularly for static wheel–rail contact determination. 16,17 A high-resolution characterisation technique for wheel–rail contacts using a 64-element ultrasonic array has been developed in previous work by Brunskill et al 17 and Zhou et al 18 Yet unlike conventional ultrasonic methods for defect detection, where defects can be indirectly inferred through inspection data or can be reached easily with guided waves, characterisation of contacts needs ultrasound beams striking precisely at the contact interfaces, whereas for the longitudinal elastic body waves normally used in this scenario, structural modifications of contact bodies are usually essential for proper sensor displacement. 18 In most cases, such modifications are strictly forbidden in railway engineering and hence limit the applicability of the technique.…”

“…Yet the complexity of wheel/rail profiles and difficulties in simulating surface roughness, which is dominantly important for contact behaviours, limit the application of FEM to be more as a validation approach. Ultrasonic reflectometry, however, has proved useful for machine-element contact measurements [15] and particularly for static wheel-rail contact determination [16,17]. A high-resolution characterisation technique for wheel-rail contacts using a 64element ultrasonic array has been developed in previous work by the authors [17,18].…”

“…Averaged-peak contact pressure (MPa) comparison (numbers in brackets are the peak pressures before averaging). The number of measurements as the rail vehicle passes over the array transducer as a function of pulse repetition frequency 17. …”