Experimental investigation of low-frequency and pulsed eddy current testing in thickness measurement

This paper experimentally compares low-frequency and pulsed eddy current testing techniques in measuring plate thickness. The experimental setups for both methods were built using general commercial devices. A single polarity rectangular wave with a 10 Hz pulse repetition frequency and 50% duty was utilised as the excitation wave in pulsed eddy current testing. Ferromagnetic steel and nonferromagnetic aluminium alloy plates of 1, 3, 5, and 10 mm thicknesses were tested at various lift-off distances. Using peak amplitude, valley amplitude, time to attenuate, logarithmic slope, and reciprocal square root of the logarithmic slope as features, pulsed eddy current testing could effectively evaluate the steel plates thicker than 3 mm and all the aluminium alloy plates. In contrast, low-frequency eddy current testing using a 10 Hz excitation frequency could evaluate both steel and aluminium alloy plates using signal phase. This indicates that redundant frequency components may impair the effectiveness of pulsed eddy current testing. Moreover, low signal-to-noise ratio and simple signal processing methods struggle to detect minor signal variations. Additionally, the linearity of phase versus thickness for steel is better than for aluminium alloy, potentially indicating that ferromagnetic materials enhance the coupling between the coil and the plate.