Studies to optimize the probe response for velocity induced eddy current testing in aluminium

• Motion induced eddy currents were used in defect detection.
• Rare earth permanent magnets were used as source of magnetic field.
• A Hall sensor was used as magnetic field sensor.
• Finite element modelling was used to optimize the probe sensitivity using multiple magnets.
• Several defect orientations were tested.

Detection and localization of surface and near surface defects in metallic objects using faster and simpler methods is always a matter of great interest in non-destructive testing (NDT). Early defect detection is of utmost importance to maintain safety of the structure and to reduce the maintenance costs. This work proposes a NDT method based on velocity induced eddy currents to detect the surface defects in electrically conductive metal. The approach used is original as it is the resultant magnetic field generated by the eddy currents of the test material induced by the permanent magnet motion, that is measured in order to detect defects. For this purpose new kind of moving magnetic probes were designed and fabricated. Each probe consists of permanent magnets which, due to the movement, induces eddy currents in the sample and a Hall effect sensor able to measure the resultant magnetic field. The total magnetic field has the information of the perturbation of the induced currents produced by the defect. Commercial simulation software was used for the optimization and design of the probe. In order to test the performance and feasibility of the proposed method several experiments were performed on an aluminium plate specimen having linear defects machined with different orientation and depths. The results were obtained by scanning the probe on test specimen at a constant speed. Experimental results confirm that the proposed method with the proposed sensing solution can be an NDT tool to detect the defects in the electrically conductive materials where motion is involved, for example in the inspection of railroads.