Analysis of the Liftoff Effect in Motion-induced Eddy Current Testing

Liftoff effect is a widely exist phenomenon in electromagnetic nondestructive testing. The change of liftoff distance influences the signal amplitude and the stability of detection signal. It has been relatively well studied in conventional eddy current testing and pulsed eddy current testing, but not in the motion-induced eddy current testing. In this paper, the influence of magnetizer liftoff on the distribution and intensity of the motion-induced eddy current has be analyzed based on a theoretical model. Then, the change of defect signal with the magnet liftoff is analyzed by finite element simulation. Finally, motion-induced eddy current testing experiments are carried out with a high-speed rotating aluminum disk. The results show that both the intensity of eddy current and the signal amplitude decreases exponentially with the increase of liftoff distance.