One of the trends in eddy-current (EC) NDE is to probe deeper by working at lower frequencies. In aircraft NDE, frequencies as low as 300 hertz have been used to inspect lap joints involving several layers of aluminum plate [1–5]. Remote-field EC works at a few tens of hertz to penetrate the walls of ferromagnetic tubes such as oil-well casings or heat-exchanger tubes in power plants [6,7].
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