Experimental validation of an eddy current probe dedicated to the multi-frequency imaging of bore holes

This paper deals with the experimental characterization of an array probe dedicated to the eddy current imaging of sub-millimetric surface breaking defects appearing in bore holes of metallic parts. The probe is constituted of a large inducer generating an uniformly oriented EC flow within the inspected material, and a sensing array probe featuring bobbin coils to sense the radial component of the magnetic field resulting from the eddy currents/defects interactions within the wall of the bore hole. The probe was designed with accurate defect characterization in view, i.e. to provide multi-frequency and high spatial resolution images with a reduced acquisition time, so as to enhance the informative content of the acquired eddy current data. An experimental set-up was build in order to validate the imaging performances of such a probe. A prototype featuring a large inducer and a single sensing coil which can be accurately positioned in the sensing area, has been developed in order to evaluate the sensing performances as well as to study the influence of the sensing array configuration on the imaging performances. The experimental results demonstrate a good sensing ability of the designed probe in the 10-800 kHz frequency range, with peak-signal-to-noise ratios higher than 36 dB at 10 kHz (and 62 dB at 800 kHz) for defects featuring dimensions as small as 0.4 mm x 0.2 mm x 0.2 mm. Furthermore, a staggered row arrangement of the sensing array was proposed so as to significantly reduce the error due to the sampling step resulting from the pickup coils geometry (from 35% to less than 9% in the worst case). The experimental evaluation of the probe provides promising prospects for the accurate characterization of defects, by means of advanced multifrequency signal processing algorithms. (C) 2012 Elsevier B.V. All rights reserved.