Detecting hidden damage at the second layer of lap joint structures in aircraft by using eddy current techniques is difficult because the cracks are usually located beneath the head of the rivets, which cause large signals that mask the relatively smaller signals from the second layer cracks. In addition, using lower frequency excitation for deeper penetration of the eddy currents reduces the signal-to-noise ratio. Several new techniques have been tried, such as using a sheet inducer to increase the penetration depth or using sensors that have a frequency independent response, to improve the signal to noise ratio [1, 2]. To better extract the information due to the cracks beneath the rivet at the second layer, we have developed a phase analysis of the magnetic field produced from the eddy currents induced by the sheet inducer.
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