Detection and Imaging of Internal Cracks by Tangential Magnetic Field Component Analysis using Low-Frequency Eddy Current Testing

Abstract. Eddy current testing using a pickup coil is usually applied to the surface detection of cracks because of its high frequency. However, for metal infrastructure, detection of not only the surface but also internal cracks is desired. Therefore, we have developed a low-frequency eddy current testing system to inspect interior structures using a magneto-resistive (MR) sensor with a wide measuring frequency range, ranging from DC to high frequencies. For imaging of the crack shape, especially the front edge of the crack, a tangential component detection and analysis method that can directly detect the eddy current distribution was developed. This distribution reflects the shape of the crack because the current distribution is changed by the existence of the crack and the current is concentrated at the front edge of the crack. Eddy current testing is difficult to apply to steel constructions because of magnetic noise because of changes in permeability and magnetization of ferromagnetism. In order to investigate the influence of magnetism, we compared two different types of materials: aluminum plates as a non-magnetic material and steel plates as a magnetic material. We found that, regardless of the material, internal cracks were imaged by the highly sensitive MR sensor at a low frequency and with tangential magnetic field measurements.