MAGNETOOPTIC EDDY-CURRENT IMAGING OF AGING AIRCRAFT - A NEW NDI TECHNIQUE

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 c urrent images of both surface-breaking fatigue cracks and hidden multi layer cracking or corrosion. While it also works on steel and titanium , the MOI was designed to yield optimum results on aluminum aircraft s kins and structural components. At eddy current frequencies of 25.6-10 2.4 kHz, most surface-breaking fatigue cracks in aluminum are easily i maged, while at lower frequencies (eg., 6.4 kHz) hidden multilayer cra cks, corrosion, and aircraft substructure may be imaged. The advantage s of the MOI are its speed (5 to 10 times faster than conventional edd y current NDI), its easily interpreted image information, the near eli mination of false calls, the elimination of the need for paint or deca l removal, the reduction of operator fatigue, and the easy documentati on of results via video tape or 35 mm film. MOI images for both fatigu e cracks and corrosion are illustrated. Recent improvements in the tec hnology, including rotating or multidirectional (by nonmechanical mean s) eddy current excitation are also illustrated. Finally, the applicat ion of the MOI to specially ''tagged'' composite materials is illustra ted.