DIFFERENTIATION OF SURFACE AND INTERNAL DEFECTS BY USING TAIL FIELD LEAKAGE DATA AT HIGH-SPEED MFL TESTING

Abstract

In the practice of magnetic flux leakage non-destructive testing the problem of distinguishing signals generated by dangerous internal defects from signals generated by surface flaws is very acute. In this paper on the basis of three-dimensional computer simulation it was shown that at significant inspection speed the area of the maximum magnetization of the most remote from the surface metal layers under the influence of emerging eddy currents is shifted behind the rear pole of the magnetizing system. Studies of magnetic induction distributions in the testing object in the form of a rail from speed, from the distance of the section under consideration relative to the rear pole and from the depth of the layer are carried out. Also, the distributions of the field in the presence of two types of defects – surface and internal – are studied. It was found that the magnetic field distribution behind the rear pole differs significantly from the distribution in the interpole space. Two contrary magnetic fluxes are formed behind the rear pole: in the bulk of the object the magnetic flux moves in one direction and near the surface magnetic flux flows in the opposite one. Since the flux directions are opposite, the signals from surface and internal defects have different polarity, which can be used to differentiate them.