Linear magneto-optic imager for non-destructive evaluation

Magneto-optic (MO)/eddy current imagers appear to be good alternatives to conventional eddy currents sensors for defect detection in large metallic structures. Indeed, they allow short time inspection of large structures such as airplanes fuselage or wings, thanks to the visualization of large images relative to the integrity of the inspected structure. However, the existing MO imagers only allow “two-level” images to be produced, which are possibly highly degraded by perturbation patterns due to the magnetic domains of the MO sensor. These drawbacks severely limit the defect detection possibilities and forbid thorough defect characterization such as 3D image inversion to be carried out. In this paper, the authors present a patented linear MO imager (LMOI) dedicated to thorough defect detection and characterization. The device provides complex images obtained with high signal to noise ratio, which are the true measurement of the normal magnetic field at the surface of the inspected structure. The basic principle of the LMOI consists in the combination of a dedicated MO sensor featuring a linear and hysteresis-free magnetization loop, used with an original image acquisition system based on a stroboscopic approach, and a specific high sensitivity eddy current inductor. A first LMOI integrated prototype is described. Complex images obtained for the inspection of riveted lap joints featuring surface or buried flaws placed close to the rivets are presented and discussed. Finally further enhancements of the device are proposed.