EDDY CURRENTS: Investigation of directional eddy current complex measurements for defect mapping

This paper reports an experimental study investigating the interaction of the real and imaginary components of eddy currents with defects and the resulting change in magnetic field under rectangular (directional) coil and circular (non-directional) coil excitation, for the purposes of evaluating surface defects in conductive materials. Eddy currents are induced when an excitation coil is placed near the surface of a conductive material. Discontinuities such as cracks disturb the flow of the eddy currents and thus affect the induced current and magnetic field. These changes can be detected using a magnetic field sensor (Hall sensor). Measurements using an eddy current probe moved over a defect with an X-Y scanner were obtained experimentally. The results illustrate that the imaginary component of the magnetic field responses under directional magnetic field excitation using a rectangular-shaped excitation coil can provide further defect sizing information alongside the real component of the magnetic field, in comparison with a circular-shaped excitation coil.