Analysis of the lift-off effect of a U-shaped ACFM system

Non-contact inspection is the main advantage of the alternating current field measurement (ACFM). The lift-off effect lowers the mutual-inductance between the ACFM probe and the test sample. The lift-off value has a significant influence on electromagnetic signal and the measurement accuracy of a U-shaped ACFM system. To optimize the U-shaped ACFM system, the lift-off value is determined through simulation analysis and experimental study in this paper. The distribution and intensity of the alternating magnetic field signals above the crack at different lift-off values for the U-shaped ACFM probe are simulated in ANSYS. The relationship between the lift-off value and the amplitudes of the electromagnetic signals is studied using a linear least squares fitting method. By analyzing the simulation results, 4 mm is selected to be the optimal lift-off value for the U-shaped ACFM probe. To verify the analysis of lift-off effect, real crack inspection experiments are conducted using the U-shaped probe of an ACFM prototype in laboratory. The results show that a 4 mm lift-off value is acceptable for this U-shaped ACFM probe considering both inspection sensitivity and stability.