Lift-off effect reduction based on the dynamic trajectories of the received-signal fast Fourier transform in pulsed eddy current testing

Reduction of interferences such as the lift-off effect in pulsed eddy current (PEC) testing has been difficult. A new approach based on the dynamic trajectories of the fast Fourier transform (FFT) of the received signals is presented in this paper for reducing the lift-off effect in PEC. The PEC probe scans the surface of the test sample and a set of received signals for a corresponding set of scanning points is stored. FFT is performed on all these stored signals, and the real and imaginary parts for a particular FFT harmonic of all the stored scanning points are plotted in a complex plane to form a dynamic trajectory. The defect trajectory can be recognized from the lift-off trajectory by observing the angle between them, as in conventional eddy current testing. By moving the defect trajectory in the complex plane for each harmonic to a zero-lift-off point and performing an inverse fast Fourier transform (IFFT) based on the new trajectories for all the harmonics, the traditional PEC time domain differential signals for a defect can be reconstructed. Then, the acquired results are amplified with a coefficient related to the inspection settings and final differential signals closely approximating the original defect differential signals without lift-off are obtained. Experimental results demonstrate that the proposed method is highly effective, particularly for the simultaneous inspection of internal and surface defects, although the location of the defect in the depth direction is unknown before inspection.