Simultaneous measurements of metal plate thickness and defect depth using low frequency sweeping eddy current testing

In this paper, a new low frequency (50 to 300 Hz) sweeping eddy current testing (ECT) method is developed to simultaneously detect defect and measure thickness for metallic plates. The proposed method has been derived through analytical solution (AS) and verified by finite-element method (FEM) and experimental data. New signal characteristics from both AS and FEM demonstrate that the maximum value of ΔZabs (difference between |Zsample| and |Zair|) and its corresponding frequency (lower than 300 Hz) can be utilized to distinguish the defect depth in metallic plate and thickness of metallic plate, respectively. It finds that the frequency corresponding to the maximum value of ΔZabs decreases as conductivity δ, thickness d and lift-off distance L increases. At 1 mm lift-off distance, the accuracy of the estimated thickness of the aluminum plate is within 2 %, and the accuracy of the estimated defect depth is within 17.5 %.