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

Abstract

Electromagnetic sensors have been used for inspecting small surface defects of metals. Based on the eddy-current thin-skin regime, a revised algorithm is proposed for a triple-coil drive-pickup eddy-current 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 %.

Measuring the Magnetic Polarizability Tensor Using an Axial Multi-Coil Geometry

“…Recent progress in mathematical theory has demonstrated that the MPT is the coefficient array in the first term in an asymptotic expansion of the perturbed field [6]- [9]. Although the motivation of this paper is toward humanitarian demining, the MPT and Electromagnetic Induction Spectroscopy have also been successfully applied widely in MD, for example to the detection of unexploded ordnance [10]- [12], walk-through metal detectors [13], metal recycling [14], workpiece recognition [15], non-destructive testing [16]- [18] and buried object detection [19]. MPT and broadband inductive sensing have also been previously proposed for landmine detection [20]- [25].…”