In this paper we use eddy currents to determine the depth of linear cracks in aluminum plates. A constant field probe is used to generate the spatially uniform excitation field and a single axis giant magneto-resistor (GMR) sensor is used to measure the eddy currents magnetic field.
To settle a standard measurement process, linear cracks with different depths were machined in one aluminum plate with 4 mm of thickness. By scanning those cracks the magnetic field components parallel and perpendicular to the crack’s line were measured when the eddy currents were launched perpendicularly to the crack’s line. It was verified that better results were obtained using the data acquired with the sensor parallel to the crack. These results were used to extract features correlated to the crack’s dimensions. The measurements were repeated with the probe on both sides of the plate, corresponding to the characterization of surface and subsurface cracks. All the selected features (maximum amplitude, signal energy), extracted from the perturbation signals were measured as a function of the excitation frequency.
Finally, to characterize one crack in a plate of the same thickness and material, the experimental procedure was defined. The plate surface is scanned to detect and locate one crack. The acquired data enables the determination of the crack’s length and orientation. A second scanning is performed with the excitation current perpendicular to the crack and the GMR sensing axis parallel to the crack’s line. The features extracted from the acquired data were compared with those obtained from the standard measurements and a depth value was determined.