On Non-Maximizable Ultrasonic Responses and POD Curves

An effective ultrasonic inspection practice requires the maximization of echo responses due to indications, before their evaluation in terms of amplitude and size. This is achieved pointing the acoustic axis of the sound beam to the reflecting area of the indication, in a way to get back the maximum possible sound energy. However, considering some operative cases, such a response maximization is not feasible, mainly due to geometrical constraints impeding the inspection of the whole control area applying a constant sensitivity. The traditional end inspection of solid railway axles by a rotating probe mounting conventional sensors falls back into this kind of inspections. In particular, inspection angles are fixed and the probe holder cannot move longitudinally allowing for response maximization of in-service damages located, for example, along the body. It follows some control areas cannot be inspected using the maximum sound pressure. The present research shows how the derivation of POD curves for non-maximizable ultrasonic responses cannot be carried out by the traditional statistical approach and a novel one is proposed, based on experiments and numerical simulations.