Ultrasonic inspection of solid railway axles by a phased array rotating probe applied to blind holes manufactured at their ends

The traditional approach to ultrasonic inspection of solid railway axles, applied in Italy since the ‘70s of the last century, is based on the so called “rotating probe”, a device composed by mono-crystal piezoelectric transducers generating longitudinal ultrasonic waves differently tilted in order to inspect all of the critical sections of the axle from its ends. Even if such approach has proven to be more effective with respect to the conventional one based on single ultrasonic probes applied at the sides of the axle, the known limits of this technique are mainly related to the geometry of axle ends. The present research proposes a novel technical solution able to improve furtherly the performance of the rotating probe, while mitigating, at the same time, its known limits. In particular, the phased array ultrasonic technique is implemented and the application of the rotating probe is no more at the ends of the axles, but at suitable blind holes manufactured on the same ends. The validation of the developed inspection technique is carried out by a dedicated experimental campaign and its reliability is, eventually, derived by a Model-Assisted Probability of Detection (MAPOD) approach, which required a generalization of the traditional statistical procedure, introduced by Berens, for determining the Probability of Detection curve from inspection data