225 Inversion of 3-D Surface Cracks From Leakage Magnetic Field Data

Abstract

A method is proposed for estimating the sizes of surface cracks in magnetic materials. The method is based on applying magnetic field, determining leakage magnetic field, in the vicinity of a crack, by moving Hall element on the surface of the material along one or two scanning lines crossing the crack, and measuring the corresponding Hall voltage distribution. A dipole model of a crack is utilized, in which a surface crack is considered as being full with magnetic dipoles aligned parallel to the applied field, and whose density varies linearly along the depth of the crack. The crack sizes and the parameters of the distribution of magnetic dipoles along the crack depth are computed by crack inversion, which represents a regression for the Hall voltage distribution. Hall voltage distributions are measured on ferromagnetic steel samples containing one artificial surface crack. Some crack inversions are performed for estimating the maximum crack depth and the crack width of cracks with rectangular, and isosceles triangular cross-sections along the long crack axis. The fast and accurate estimation of the maximum crack depth, and the crack width by such crack inversions could be important for pipeline inspection. Another crack inversions are performed for determining the cross-section along the long axis of the investigated cracks, and the results are satisfactorily.