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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In order to test the fretting fatigue cracks with high sensitivity, which occurred on the wheel seat of hollow axles of Shinkansen vehicles, the design of a focused ultrasonic transducer was proposed by a new method. The method was achieved by designing a suitable surface shape of the ultrasonic transducer through a reverse analysis of the wave propagation in the axle so that all radiated waves from the transducer were focused into a point region of the wheel seat with a same phase. A composite piezoelectric material was applied to the focused ultrasonic transducers in order to test with a shorter ultrasonic pulse. As the result of evaluating the performance of the developed focused ultrasound transducer, comparing with the previous plate-like ultrasonic transducer made using PZT, a great improvement was demonstrated in respect of sensitivity and directivity. Furthermore, the method inclining the ultrasonic beam slightly to the circumference direction and the method making the frequency characteristic of the transducer high were devised in order to reduce the influence of echoes by press fit. Consequently, it was demonstrated that a focused incline ultrasonic transducer of 10 MHz was possible to evaluate up to an artificial crack of about 0.17 mm depth with the SN ratio of 9 dB.

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