Ultrasonic evaluation of fatigue cracks at the wheel seat of a miniature wheelset

Ultrasonic testing of axles is an important issue with regard to 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 were inspected using angle probes and grazing shear-horizontal-wave (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 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.