Determination of Surface Crack Orientation Based on Thin-Skin Regime Using Triple-Coil Drive–Pickup Eddy-Current Sensor

Electromagnetic sensors have been used for inspecting small surface defects of metals. Based on the eddy-current (EC) thin-skin regime, a revised algorithm is proposed for a triple-coil drive–pickup EC sensor scanning over long surface crack slots (10 mm) with different rotary angles. The method is validated by the voltage measurement of the designed EC sensor scanning over a benchmark (ferromagnetic) steel with surface defects of different depths and rotary angles. With an additional sensing coil for the designed EC sensor, the defect angle (or orientation) can be measured without spatially and coaxially rotating the excitation coil. By referring to the voltage change (due to the defect) diagram (voltage sum versus voltage different) of two sensing pairs, the rotary angle of the surface crack is retrieved with a maximum residual deviation of 3.5%.