In situ evaluation of contact stiffness in a slip interface with different roughness conditions using ultrasound reflectometry

Abstract

Understanding the dynamic condition of the interface between a railway wheel and rail is important to reduce the risks and consider the effectiveness of countermeasures for tribological problems. Traditionally the difficulty in obtaining accurate non-destructive interfacial measurements has hindered systematic experimental investigations. Recently, an ultrasound reflectometry technique has been developed as a direct observation method of a rolling–sliding interface; however, the topography dependence under the high contact pressures in a wheel–rail contact has not been clarified. For this reason, a novel in situ measurement of the contact stiffness using ultrasound reflectometry was carried out for three different levels of roughness. A contact pressure equivalent to that in a wheel–rail interface was achieved by using a high-pressure torsion test approach. The dynamic change of contact stiffness with slip was measured using ultrasound and the influence of roughness was investigated. The measured changes were validated using a newly developed numerical simulation, and mechanisms to explain the observed behaviour were proposed in terms of fracture and plastic deformation of the asperity bonds. These findings could help in understanding the traction characteristics for different roughness conditions and also assist in understanding damage mechanisms better, such as wear and rolling contact fatigue.

Transition of the friction behaviour and contact stiffness due to repeated high-pressure contact and slip

“…Furthermore, in this experiment, the shear stiffness in the circumferential direction tended to be higher than that in the radial direction for most of the test cycles, especially for smoother surfaces. These directiondependent properties were not clear in the previously reported single-cycle results [34]. Figures 11(b) and 11(d) show that this tendency appears in the later cycles.…”

“…The results showed that there was a correlation between the change in friction coefficient and contact stiffness with repeated rollingsliding, and a near linear relationship was obtained. Additionally, the authors also applied the ultrasonic measurement technique to the high-pressure torsion (HPT) test and investigated the dynamic change of contact stiffness with slip [34]. As a result, it was found that the deformation of surface asperities and frictional behaviour due to slip differed depending on the surface topography.…”

“…Since the details of the high-pressure torsion test equipment and ultrasonic measurement are described in previous works [34,35] only the basic structure is explained here. Figure 2 shows the appearance of the HPT testing equipment.…”

“…With the application of tangential load, the asperities plastically deform rather than break bonds owing to the interlocks of the tips by the penetration and the greater heights of asperities. Once Coulomb limit passed and macro-slip occurs, plastic deformation and rounding of surface peaks accelerates [34].…”