Versatile Instrument for Non-Destructive Inspection of Subsurface Flaws around Fasteners

One of the most important problems in the nondestructive inspection of aircraft is the detection of cracks and corrosion under fasteners, in the lower layers of lapped structures. Existing methods for inspecting such structures require either that the fastener be removed, or that the instrument be redsigned for each new inspection problem.I propose to remove these limitations with a new eddy-current sensing system which varies that applied-field distribution and sensor configuration to fit the gemometry of the inspection problem, thus maximizing the flaw signature and minimizing the background signals due to the fastener head and nearby structures. This new approach is possible in part because of new magnetic sensors which provide very low noise over a wide frequency range.The new instrument will detect cracks and corrosion around ferrous and nonferrous fasteners of varying size, in structures made of aluminum, titanium, or ferrous metals, at depths ranging from 1 mm or less to 10 mm or more. Its high sensitivity at low frequencies will enable the inspection of very thick structures, and its low noise will signicantly reduce inspection times.