Eigenvalue-Based Approach for Enhancement of Eddy Current Images of Shallow Defects

An eigenvalue-based approach is proposed for enhancement of eddy current images. The images are obtained by raster scanning an absolute eddy current probe over defects in a ferromagnetic carbon steel plate and by measuring the changes in probe impedance. During eddy current testing of ferromagnetic materials, variations in local magnetic permeability produce incoherent noise as compared to systematic impedance changes due to defects. In the proposed approach, eigen pairs of image covariance matrix are determined, and only the significant eigen pairs that have maximum information of the impedance changes due to defects are utilized to reconstruct images free from noise attributable to permeability variations. The capability of the proposed approach is assessed using eddy current images of a hole and a machined notch in a carbon steel plate.