The influence of non-uniform wall thickness on MFL testing for a steel pipe

A non-uniform wall thickness inevitably forms during the production and use of a steel pipe, leading to a sensitivity difference in the magnetic flux leakage (MFL) during non-destructive testing of the steel pipe. To promote the MFL non-destructive testing technology, the magnetic characteristics of pipe walls with non-uniform thicknesses were analysed by applying magnetic refraction theory Magnetic field lines in the thin wall leak into the air; however, magnetic field lines in the air are absorbed into the thick wall, producing background magnetic fields with different distributions. The induced flux intensity of the thin wall is larger than that of the thick wall. As a result, different magnetic fields are produced by identically-sized defects during testing, but in pipe walls with different thickness features their relative intensities descend from large to small in the thin to the thick walls, respectively, causing sensitivity differences. These results were verified by relevant MFL experiments using drill pipes.