Analysis of the Lift-Off Effect in Motion-Induced Eddy Current Testing Based on Semi-Analytical Model

The lift-off effect is a widely occurring phenomenon in electromagnetic nondestructive testing. The change in lift-off distance influences the signal amplitude and the stability of the detection signal. The effect has been relatively well studied in conventional eddy current testing and pulsed eddy current testing, but not in motion-induced eddy current (MIEC) testing. In this article, the influence of magnetizer lift-off on the distribution and intensity of the MIEC has been analyzed based on an analytical model. Then, a semi-analytical model (SAM) is proposed for fast computation of defect signals. Based on the SAM and the finite-element method (FEM), the influence of lift-off on the defect signal has been comprehensively studied. The proposed SAM gives results similar to those obtained by FEM, and, at the same time, it extensively increases the computation efficiency. Finally, MIEC testing experiments were performed with a high-speed rotating aluminum disk. The results show that the defect signal amplitude decreases exponentially with the increase of the probe lift-off, and it is a superimposing effect of the magnet lift-off and sensor lift-off.