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Ultrasonic testing of axles is an important issue regarding the integrity assessment of railway vehicles.In this study, several aspects of the ultrasonic evaluation of fatigue cracks in an axle are examined experimentally. Namely, the effects of (i) the bending load applied to the axle, (ii) the presence of a wheel fitted to the axle, and (iii) the nominal frequency and the type of an ultrasonic probe, on the measured ultrasonic echo height of a crack are examined. To this aim, two fatigue cracks of different depths were developed at the wheel seat of a miniature wheelset test piece by rotating bending, and inspected using angle probes and grazing SH-wave probes under bending loads. The echo height of the cracks varied remarkably with the bending stress due to its crack opening/closing effect, accompanying some hysteresis.A possible reason for this is discussed qualitatively based on the ultrasonic wave transmission across the crack surfaces as well as the axle-wheel interface. The performances of different probes in detecting echo signals as well as estimating crack depths are compared and discussed. As a result, it is found that (i) the use of grazing SH-wave probes may be useful for the crack detection, and (ii) the application of bending load to the wheelset will be advantageous in highlighting the echo signal. The observed effect of the nominal frequency of the probe on the echo-height level is examined in a qualitative manner. Finally, the applicability of the present results to full-sized axles is discussed.

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For the ultrasonic testing at the wheel seat of railway axles, quantitative investigation of the reflection and transmission phenomena at the axle-wheel interface is important. This paper describes the influence of the axle-wheel interface on the ultrasonic testing of a fatigue crack in a wheelset by applying the spring interface model. The normal and tangential stiffnesses were identified experimentally for an as-manufactured wheelset at the normal incidence, and the reflection coefficient for the shear-wave oblique incidence was calculated. A parametric study was performed to clarify the influence of these interfacial stiffnesses on the incident-angle dependence of the reflection coefficient. The calculated reflection coefficient at the incident angle of 45° qualitatively explained the relative echo-height decrease due to the presence of a wheel observed experimentally for a wheelset in fatigue loading by rotating bending. The quantitative difference between the experimental and calculated results was considered to be due to the reduction of the effective interference of shrink fit by the wear at the axle-wheel interface during the fatigue loading as well as by the applied bending moment. For the estimated relative echo-height decrease to agree with the experimental results, the interfacial stiffnesses were found to be smaller than the values identified for the as-manufactured wheelset by a factor of 0.5-0.7.