Sensitivity Focusing Through Coil Angle Variation in Eddy Current Testing

Eddy current sensors are extensively utilized across various nondestructive testing (NDT) applications. The idea of magnetic field focusing (MFF) is to constrain the magnetic field distribution within a certain area for spatial resolution improvement in imaging. In eddy current testing (ECT), the sensitivity distribution is not only related to the excitation field but also to the receiving field. Therefore, the focusing of both magnetic fields toward the designated area has the potential to focus sensitivity distribution, thus improving spatial resolution in defect imaging. This article defines the 'focus area' of sensitivity and investigates sensitivity focusing in ECT through coil angle variation. Magnetic fields from a single coil of different tilt angles are calculated based on the second order vector potential (SOVP) approach and the 2-D Fourier Transform. The sensitivity to permeability for one pair of coils is investigated and the focus areas of different coil systems are evaluated. Numerical and experimental results prove that higher spatial resolutions on permeability perturbations (sensitivity focusing for permeability variation) are feasible by changing the coil tilt angles.