Estimation of defect depth in non destructive evaluation utilizing superconducting quantum interference device picovoltmeter

Eddy current testing utilizing a cooled normal pickup coil and a high-T_c superconducting quantum interference device (SQUID) picovoltmeter was performed both experimentally and analytically. In the experiment, we successfully detected a small crack on the back surface of the Cu plate by moving the coil in unshielded environment. First, we showed a method to avoid a drift of the detected signal that was caused by the variance of lift-off. Next, we clarified the dependencies of the detection signal on the excitation frequency and thickness of the Cu plate. It was shown that an optimum frequency that maximizes the detected signal exists. Since this frequency changed with the thickness of the Cu plate, the frequency dependence could be used to estimate the depth of the crack from the surface of the Cu plate. The experimental results were analyzed by taking account of the phase and amplitude of the signal field caused by the crack. Good agreement was obtained between experiment and analysis.