Improved Measurements of Samples Simulating Corrosion in Lap-Seams of Aluminum Aircraft

Abstract

AC magnetic bridges offer a technique for continuous electromagnetic scanning of surfaces and lap seams of aging aluminum aircraft to detect material losses due to corrosion. AC magnetic bridges are particularly responsive to changes in conductance because nearly all the electromagnetic field generated is in contact with the sample as compared with conventional eddy-current devices where the high reluctance of that portion of the magnetic field not in contact with the sample has a tendency to mask the small portion of the field in contact with the sample. Further, the eddy-current technique masks the effect of the electromagnetic interaction with the sample through the requirement that the changes produced by changes in the sample be detected in series with the impedance of the eddy-current coil. This latter effect requires the use of relative high frequencies in conventional eddy-current devices. Use of the ac magnetic bridge (the bridge) solves all of these problems while providing the physical convenience of conventional eddy-current scans if such a scans were effective. The frequencies used for bridge operation are usually of the order of 100 Hz to 20 kHz and offer greater sample penetration than conventional eddy-current devices which usually operate well above this range. Further, ac magnetic bridges maintain their sensitivity to material loss at values of lift off of 20 mils or greater, well above the usual values of paint coating thicknesses.