Eddy Current Imager for the Detection of Buried Flaws in Large Metallic Structures

The authors present a prototype of an eddy current imager dedicated to the high speed inspection of aeronautic riveted lap joints. The system provides high resolution images of the complex magnetic field distribution at the surface of the inspected structure, and allows thorough defect characterization to be achieved. The system was implemented for the detection of buried notches placed next to the rivets of a laboratory made lap joint mock-up. Associated to a dedicated signal processing method, the system allows 6 mm deep EDM notches (0.2mm×3mm×7mm) to be detected and localized within the depth of the inspected structure, and appears very promising for the characterization of buried defects in actual riveted lap joints. 1. Introduction The detection of corroded areas and fatigue cracks is a major preoccupation for the maintenance of ageing aircraft. Indeed aeronautic structures, such as wing or fuselage riveted lap joints, are submitted to strong mechanical and climatic constraints. These constraints may induce the apparition of cracks, specially in the vicinity of the rivets. Cracks can then propagate from one rivet to the others, specially in the areas already weakened by corrosion, and lead to dramatic issues. In order to prevent accidents as to optimize the lifetime of the structures, it is necessary to detect the defects in their early stage with the best possible accuracy and reliability. Among the available non-destructive evaluation techniques, eddy currents (EC) sensors are widely used for the inspection of aluminium based alloy riveted structures, since they are sensitive to defects and easy to implement. However a typical commercial aircraft is constituted of tens of thousands of rivets ; the use of conventional EC sensors featured by a reduced number of sensing elements implies time consuming - and therefore expensive - inspection procedures. In this context, efforts have been made to develop alternative solutions such as the magneto optic / eddy current imager (MOI) designed by Physical Research Instrumentation (PRI) in the early 90's, dedicated to the real time visualization of images relative to the presence of cracks and corrosion in a large inspection area (76mm diameter) (1). The MOI combines the use of an inductor required to induce eddy currents into the inspected structure, and the use of the Faraday rotation occurring in a magneto- optic (MO) sensor operating in polarized light, to locate high values of the normal component of the resultant magnetic field at the surface of the structure. The designer evaluates that the inspection time should be reduced by a factor 1/10 in comparison with conventional EC sensors (1). However, due to the use of a MO indicator featured by a "two-state" magnetization loop, the MOI provides only "two-level" images (1-2). These two levels are resulting from a binary comparison to an adjustable polarization