Upper Limit Frequency Analysis of Eddy Current Testing for High Conductive Plate

Eddy current testing (ECT) based on electromagnetic induction can detect surface and near surface defects on conductive materials. Frequency is an important parameter in ECT detecting non-ferromagnetic plate materials due to the skin effect of electromagnetic wave. Using magnetic induction tomography (MIT) detection of biological tissue for reference, the specific relationships between the amplitude response and phase response and frequency are investigated based on the equivalent circuit and the weak skin effect assumption which is feasible below an upper limit frequency. The effects of the tested plate thickness and size on the upper limit frequency are verified by finite element simulation. The results show that the amplitude response is proportional to the square of the frequency and the phase response is proportional to the frequency. The effect of plate thickness on the upper limit frequency is much larger than the effect of plate size in terms of variation per millimeter. And the ratio of skin depth to thickness (STR) decreases with the thickness increases. This study can offer a guidance of skin depth according to the thickness of the tested plate object, and then provide a reference frequency range of ECT detecting non-ferromagnetic plate materials.