Comparison of hit/miss and ‘â versus a’ POD calculations for short surface cracks using inductive thermography

Abstract

Inductive thermography is an inspection technique that consists of the heating of a sample or a component by short induced eddy current pulses (duration of 50-100 ms) while an infrared camera records the evolution of the surface temperature. Surface cracks in metals can be excellently detected by this inspection technique. Both the eddy currents and the heat diffusion are disturbed by cracks, therefore becoming visible in the infrared images. The recorded image sequence is processed using the Fourier transform, and the resulting phase image is analysed directly by technicians (human-based detection) or developed defect detection algorithms (computer-based), to localize the defects. For industrial applications, the reliability of an inspection technique is very important. This is why it is necessary to calculate the probability of detection (POD) of the technology considered for each case study. The goal of this investigation is to compare the two standard parametric POD calculation techniques, ‘hit/miss’ and ‘â versus a’, for detecting surface cracks with inductive thermography on the nickel-based austenitic superalloy Inconel 718. First, artificial defects are considered for an ‘â vs a’ POD analysis and results are compared to finite element simulations. Additionally, real cracks on TIG welds, created with a Varestraint test machine, have also been considered and corresponding ‘hit/miss’ POD calculations have been performed. However, it is important to note, that the deeper a crack, the larger obstacle it creates for the eddy current and for the heat flow. Hence not only the defect length, but also its depth affects the signal around it. This implies that the calculated POD depends on both the crack length and depth and that in some cases ‘hit/miss’ POD analysis will not be enough.