Fast Fourier Transform Based NDT Approach for Depth Detection of Hidden Defects Using HTS rf-SQUID

We present a new approach for depth detection of hidden defects based on the analysis of the frequency spectrum of the output waveform in a nondestructive testing (NDT) system. In our eddy current Superconducting quantum interference device (SQUID) based NDT system, we apply multiple frequencies to its single excitation coil in a magnetically unshielded environment. The excitation coil is a planar D-shaped printed circuit board coil, and a high-T
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gradiometer YBCO rf-SQUID is used as the electromagnetic sensor in this system. An automated two-dimensional nonmagnetic scanning robot is used to test samples with intentional defects at different depths. In this approach, a diagram labeled “FFT relative changes” is assigned to each test sample. The optimum points of the FFT relative changes diagram occur at the optimum frequencies directly related to the depth of the corresponding defects. The relation between the optimum frequency and the corresponding depth in our system is obtained both experimentally and by simulation. An excellent agreement between the practical and the simulation results confirmed the accuracy of the proposed method.