Motion-induced eddy current testing of composite materials

Modern composite materials are gaining more and more importance in mechanical engineering. Due to the complex structure of most of these materials, traditional NDT methods do not satisfy the measurement requirements. In this paper we address the capabilities and limitations of the non-destructive testing method of motion-induced eddy currents for (non-ferromagnetic) composite materials. The specimen moves with constant velocity through a magnetic field, which is created by a fixed permanent magnet. The interaction of induced eddy currents and the primary magnetic field re-sults in the Lorentz force acting on the specimen. Due to the third Newton law, the reaction force acts on the magnet system itself and is measured in all three spatial dimensions. Every force component has a characteristic profile for a certain defect-free specimen. Anomalies in the specimen affect the eddy currents due to variations of local conductivity. These deviations influence the measured force profiles from which the location, size and type of the defect in the specimen may be determined. Two types of magnet systems have been applied: a cylindrical magnet and a radial Halbach array with a ferromagnetic disc. The cylindrical magnet produces a dipole-like field, whereas the Halbach array with the additional disc creates a field concentrated right below the magnet system. Experiments show, that the Halbach array is very well suited for thin speci-mens. The defect response signal is higher due to the stronger eddy currents caused by the focused magnetic field. Two different types of composite materials have been experimentally tested: Carbon fiber reinforced plastic (CFRP) and glass laminate aluminum reinforced epoxy (GLARE). For CFRP four samples were fabricated, whereas one was tested. For GLARE two samples were used with defects in different depth.