Previous research [1–4] has demonstrated that transient eddy-current (EC) techniques have significant advantages for the nondestructive evaluation (NDE) of hidden corrosion in multilayer aircraft structures when compared with conventional EC NDE. By sampling the time-domain response to a step or pulsed excitation using a high-speed digitiser, the probe response is effectively captured over a wide range of frequencies within a single measurement of only a few milliseconds duration, giving a very significant performance advantage over swept-frequency methods. In addition, direct measurement of the transient magnetic field (eg. using a Hall effect device), rather than the voltage induced in a pickup coil, allows transient EC NDE to probe more deeply into a multilayer structure compared to conventional EC techniques, giving improved sensitivity to deeply buried defects including hidden corrosion [1, 2, 5].
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