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Nondestructive Evaluation

Using Electric Current Perturbation for Second-Layer Inspection of Fastener Holes on Aircraft Structure


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image of SwRI-developed eddy current probe array


This SwRI-developed eddy current probe array was developed to provide improved inspection performance, speed, and coverage for turbine disks. The probe has 16 coils, which are interfaced to the eddy current instrument using an SwRI-developed multiplexer unit.


image showing flaw sensitivity to 0.050-inch flaw


Flaw sensitivity to 0.050-inch flaw

Electric current perturbation (ECP) was used by Southwest Research Institute (SwRI) engineers as a technique to detect cracks in fastener holes after the holes were repaired on the A-10 aircraft at wing-station 23 (WS 23). This repair required enlarging the hole and installing an Inconel bushing to fit the original fastener. The nominal thickness of the bushing is 0.050 inch.


Engineers designed and tested a probe on an Air Force Research Laboratory standard representing the complex stack-up of WS 23 complete with 0.050- and 0.100-inch flaws in an aluminum layer representing the wing skin. The goal was to detect the smallest flaw closest to the 4340 steel attachment fitting on the wing standard.


Laboratory tests were performed at multiple frequencies to find the best penetration depth of induced currents through the second layer representing a bushed hole. Both absolute and differential receiver coils were used to compare the sensitivity to flaws from each approach.


Using the original stack-up at the WS 23 attachment fitting area, the 0.050-inch flaw was detected using 10 kHz as the drive frequency. The data plot shows flaw sensitivity to the 0.050-inch flaw.


For more information about our nondestructive evaluation (NDE) capabilities, electric current perturbation technology, or how you can contract with SwRI, please contact Clinton J. Thwing, at or (210) 522-3989.


Contact Information

Clinton J. Thwing


(210) 522-3989

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 9 technical divisions.