Some defects in metallic plates present complex forms that difficult their geometric characterization. This is the case of defects with ramifications where the eddy currents cannot penetrate some concavity areas. The illumination of the material under test with a coherent excitation field presents some advantages, such as the possibility of applying the excitation into different directions, thus obtaining different patterns that may be correlated to increase the definition of the acquired signals. The generation of the coherent excitation is obtained by using a planar coil that produces a uniform excitation field inside a given area.
It is not easy to correlate the magnetic field patterns, obtained by using single component giant magneto-resistor (GMR) sensors, with the real geometry of the defects. The interpretation of the measured data is much easier if the data are inversed to obtain the geometry of the eddy current lines inside the conductor.
The inversion process was performed using the the discrete Fourier transform of the field data and of the elementary dipole current kernel. The inversion was followed by Tikhonov regularization and automated determination of the regularization parameter.
Results obtained for a defect with three linear segments in a star configuration are depicted in Fig.1.
Figure