Quantitative Fractography for Improved Probability of Detection (POD) Analysis of Bolt Hole Eddy Current

Fatigue cracks are prone to develop around the fasteners found in multilayer aluminum structures such as ageing aircraft. The determination of their probability of detection (POD) using eddy current techniques is critical for risk assessments used in evaluating serviceability. Normally, a large number of samples of both fatigue cracks and electric discharge machined notches are required for such a study. In this study, the use of quantitative fractography (QF) to efficiently gather POD data for bolt hole eddy current is demonstrated. The technique can be readily automated, significantly reducing man-hour requirements. The method is applied to mid-bore cracks, grown without starter notches, in order to simulate naturally occurring fatigue cracks. POD of the mid-bore cracks is examined using a probit analysis. An a _90/95 is the probability that 90% of the cracks of size a or larger will be detected 95% of the time. An a _90/95 of 0.34 mm was obtained for mid-bore crack depth. This value is consistent with skin depth considerations at the 1.6 MHz inspection frequency. An examination of the POD based on crack length found that it was related to that of depth according to the average length-to-depth aspect ratio of 2 to 1.