Abstract
We examined the ability of detecting defects in multilayer aluminum plates by using an HTS SQUID gradiometer in an unshielded environment. The planar HTS SQUID gradiometer with 1mm×1mm pickup loops and 1mm baseline was fabricated by using HTS multilayer and ramp-edge junction technologies, and cooled by thermal conduction from a LN2 cryostat. We tried non-destructive evaluation (NDE) of a structure consisting of 5–10 layers of 2mm thick aluminum plates with a through hole, which has a shape of slit, 30mm in length and 0.5mm in width only in the bottom layer, by changing the frequency of eddy current induced by a double-D type coil. It was found that the observed frequency and depth dependences of the peak gradiometer signal were well fitted by theoretical curves taking account of decay of eddy current and defect-induced magnetic field depending on the distance between the defect and the gradiometer. By employing 200Hz frequency, the slit in the 10-layer structure could be clearly detected, indicating the ability of detecting defects located in depth more than twenty times larger than the gradiometer baseline.
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