Improving Probability of Detection for bolt hole eddy current inspection

Probability of Detection (POD) studies are performed in order to obtain an indication of the detection capability of a particular Non Destructive Testing (NDT) system. POD studies are used by the Canadian Forces (CF) in risk assessments and to determine inspection intervals for different aircraft fleets. Bolt-hole eddy current is an NDT technique used to find discontinuities in bolt holes on wing areas of CC-130 and CP-140 aircraft. POD data from 24 separate inspectors at different sites across Canada using the same eddy current equipment was analyzed. The subset of discontinuities studied consisted of 45 Electric Discharge Machined (EDM) notches and 72 laboratory grown fatigue cracks on the inner surface corners of bolt holes. This thesis work involved a study of the physical variables affecting this eddy current system and resulted in a POD analysis approach based on the relation of signal amplitude response to the length of corner discontinuities. This analysis approach allowed a comparison of EDM notch and fatigue crack data as well as an analysis using a combined data set. Due to large variability between inspectors, different normalization approaches were attempted in order to improve the POD findings. A 20% improvement in POD was obtained using a two-point normalization of the EDM notch data. The improvement was attributed to a reduction in the variability of signal amplitude response between inspectors after normalization. This result suggests that normalization, incorporated in the form of an additional calibration measurement, would improve system detection capability.