Fourier-transformed eddy current technique to visualize cross-sections of conductive materials

A pulsed eddy current NDE system is useful for detecting deep flaws in conductive materials. We have developed a pulsed eddy current NDE system using a magneto-resistive sensor. The induction coil generates a pulsed magnetic field which has a frequency component between 0 and 3 kHz, and a maximum amplitude of about 4μT. Aluminum plates with flaws at depths of 1 and 2 mm were measured, and the intrinsic frequency responses of the plates were obtained using a proposed Fourier transformation method. Cross-sectional images of the samples could then be directly constructed from the intrinsic responses. We also demonstrated estimation of quantitative values for the depths of flaws. The estimated depth was well defined for a sample with a flaw at a depth of 2 mm. The estimated value was slightly different for a sample with a flaw at a depth of 1 mm due to lack of frequency components above 3 kHz. These results indicate that the proposed system is a very promising tool for NDE tomography.