Methods for assessment of probability of detection for nondestructive inspections

© 2008 Dr. Catherine Ann HardingAssessment of the probability of detection (POD) of defects with non-destructive s inspection (NDI) is essential to integrity management of safety-critical structure, particularly in the aerospace industry. Traditionally, probability of detection is measured empirically by a POD trial, in which a number of inspections are performed under field (or simulated field) conditions on a set of specimens containing representative defects. These POD trials are very expensive and for many applications are not feasible. This thesis develops improved techniques for cost-effective measurement of POD. Methods for statistical analysis of hit/miss POD data are reviewed and the behaviour of selected methods is explored using simulations. Superior methods for applying confidence limits on POD from hit/miss data are identified, which are valid for much smaller sample sizes than traditional methods, such as that found in US military handbook, MIL-HDBK-1823 (1999). Model-assisted probability of detection assessment uses physics-based or empirical models to predict POD, which has the potential to reduce the requirement for conventional POD trials, increase the portability of POD information across related applications, and provide a basis for reliability assessment for applications where trials are not feasible. In this thesis, model-assisted POD is applied to a specific non-destructive inspection procedure developed for detection of cracks at fastener holes in the lower wing skin of Royal Australian Air Force F-111 aircraft. This application of POD modelling uses data obtained from field trials and laboratory experiments for angle-beam shear-wave ultrasonic detection of fatigue cracks at fastener holes, including human factors effects. A fully representative POD trial was not feasible for this inspevtion procedure, and so this application provides important real-world context for the development of model-assisted POD