Improving Probability of Detection of Bolt Hole Eddy Current Inspection

Abstract

Probability of detection (POD) of fatigue cracks and electric discharge machined (EDM) notches in 7075-T6 aluminum was evaluated for a bolt-hole eddy current inspection system. The POD study involved simulated bolt holes in coupons representative of wing areas on CC-130 and CP-140 aircraft. The data were obtained from 24 inspectors who inspected 468 coupons that contained a set of coupons with 45 EDM notches and 72 laboratory-grown fatigue cracks located at the inner surface corner of the bi-layer structures. A single point calibration based upon a 0.76 mm (0.030 inches) deep corner notch set to 20% of screen height (SH) was utilized. Large variability in detection capability between inspectors was observed. A portion of this variability was attributed to significant background noise on the order of 5% screen height (SH). Normalization of each inspector's data to signal amplitudes from notches at twice the depth (1.5 to 1.6 mm (0.060 inches)), close to 40% SH, reduced the relative variations and resulted in a 10–20% improvement in POD. These results indicate that calibration with signal responses from notches larger than that currently used would improve the POD for this inspection system and demonstrate the potential of using POD data to improve inspection systems.