Quantitative magneto-optic imager for non-destructive evaluation

Magneto-Optical Imagers (MOI) appear to be good alternatives to conventional eddy current 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 "real time" images relative to the presence of defects [1]. The basic principle of the MOI is to combine a magnetic inductor, used to induce the circulation of eddy currents into the structure under test, with an optical set-up used to image the resultant magnetic field, thanks to the Faraday effect occurring in a magneto-optical garnet. The MOI designed by G. L. Fitzpatrick and Physical Research Instrumentation provides two-level images relative to the presence of defects, with an adjustable detection threshold. These so-called qualitative images, although highly contrasted, are rather poor and limited in terms of defect characterization possibilities. In, this paper, the authors present a new kind of MOI, called Quantitative Magneto-Optical Imager (Q-MOI), based on the use of a dedicated "linear" magneto-optical garnet associated with a specific instrumentation. ne Q-MOI should considerably reduce the inspection time and allow to fully characterize the encountered defects. First images obtained with a demonstration prototype are shown for surface and buried flaws and further enhancements of the device are proposed.