A critical analysis of ultrasonic echoes coming from natural and artificial flaws and its implications in the derivation of probability of detection curves

The structural integrity of safety components during service is strictly related to different factors, such as the performance of the adopted non-destructive method, the crack growth behaviour of the material and the load history during service. The present research is focused on ultrasonic non-destructive testing effectiveness which, during both the manufacturing stage and in-service inspections, is essentially based on the adoption of suitable sample blocks for sensitivity calibration and the reliable knowledge of the probability of detection curve of the adopted non-destructive testing method. Firstly, a comparison between the ultrasonic responses coming from artificial flaws and natural fatigue cracks is carried out, proposing a novel approach for their interpretation and coming to some interesting conclusions regarding the effective design of sample blocks for sensitivity calibration. Eventually, in order to improve their flexibility and the possible extrapolation to similar cases, probability of detection curves were also derived based on the proposed approach, showing interesting advantages with respect to the traditional one.