Sub-surface defect detection with motion induced eddy currents in aluminium

Nondestructive testing in situations where there is a moving media has always been a challenging task. Motion induced eddy current testing is a good solution for testing metallic surfaces, as it does not require contact with the sample. This work presents a method to detect sub-surface defects in non-ferromagnetic material with motion induced eddy currents. A numerical model was used to verify the progression of eddy currents along the depth of a conductive plate and to obtain the magnetic field perturbation in the presence of sub-surface defects. A probe, including a permanent magnet to induce eddy currents and a magnetic sensor was moved in the vicinity of an aluminium plate with sub-surface defects to obtain experimental data and the results compared to those obtained with the numerical model. At higher speeds, the time of diffusion of deeper eddy currents takes, makes it possible to also estimate the depth of a sub-surface defect by measuring how far away the perturbation is from the moving magnet.