Optimal Frequency of Low-Frequency Eddy-Current Testing for Detecting Defects on the Backside of Thick Steel Plates

Defect detection on the backside of thick steel plates is essential for the safety maintenance of most infrastructures in which they are used. Eddy-current testing (ECT) is a promising method to detect the defects in a steel plate, and inspections at low frequencies are necessary to suppress the skin effect. However, if the frequency is reduced further, the signal from the detection coil also becomes smaller. Therefore, we assessed the optimal frequency for the detection of slit defects on the backside of a 10 mm thick steel plate based on experiments and electromagnetic simulations using the finite-element method. The results suggest that the optimal frequency to detect the defects is approximately 8-10 Hz when the height of the slit exceeds 6 mm. Similarly, in the simulation, the signal was maximized when the frequency was 8-10 Hz, which validates the experimental results. Furthermore, we propose a method to emphasize the deeper defects using multi-frequency data as a fundamental study. The results show that the backside defects are depicted more clearly.