Magneto-optic/eddy current imaging of aging aircraft: a new NDI technique

Abstract

An eddy current instrument called a magneto-optic/eddy current imager (MOI) has been developed for the purpose of nondestructive inspection (NDI) of aging aircraft. The MOI produces realistic, real-time, eddy current images of both surface-breaking fatigue cracks and hidden multilayer cracking or corrosion. While it also works on steel and titanium, the MOI was designed to yield optimum results on aluminum aircraft skins and structural components. At eddy current frequencies of 25.6--102.4 kHz, most surface-breaking fatigue cracks in aluminum are easily imaged; while at lower frequencies hidden multilayer cracks, corrosion, and aircraft substructure may be imaged. The advantages of the MOI are its speed (5 to 10 times faster than conventional eddy current NDI), its easily interpreted image information, the near elimination of false calls, the elimination of the need for paint or decal removal, the reduction of operator fatigue, and the easy documentation of results via video tape or 35 mm film. MOI images for both fatigue cracks and corrosion are illustrated. Recent improvements in the technology, including rotating or multidirectional (by nonmechanical means) eddy current excitation are illustrated. Finally, the application of the MOI to specially tagged composite materials is illustrated.