31 March 1998 Detection and imaging of corrosion around wing skin fasteners using the dripless bubbler ultrasonic scanner
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The galvanic action between steel fasteners and aluminum wing skins of aircraft often leads to hidden exfoliation corrosion around the countersink surface of the fastener heads. To detect and evaluate the severity of such corrosion defects, the Dripless Bubbler ultrasonic scanner was applied. This technique uses a focused beam of high frequency ultrasound in a closed-cycle, water-coupled scan of wing skin test panels containing corroded and uncorroded fasteners. With full waveform acquisition, not only the lateral extent but also the depth profile of the corrosions around the fastener heads were mapped out, subject to shadowing of defects at different depth. The technique is capable of providing quantitative assessment of the severity of the corrosion. In tests conducted to evaluate different techniques, the Dripless Bubbler has shown high probability of detection and low false call rate. The presence of paint on the surface did not degrade the performance of the technique. In addition, the Dripless Bubbler was also used on wing skin panels containing repair 'blend-out' regions that had 0.020' to 0.100' of metal removed from the surface by grinding. Corrosions around fasteners in the blend-out regions were also detected.
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David K. Hsu and Daniel J. Barnard "Detection and imaging of corrosion around wing skin fasteners using the dripless bubbler ultrasonic scanner", Proc. SPIE 3397, Nondestructive Evaluation of Aging Aircraft, Airports, and Aerospace Hardware II, (31 March 1998); doi: 10.1117/12.305063; https://doi.org/10.1117/12.305063

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