Magnetic leakage flux testing with probes: physical principles and restrictions for application

Abstract This article reviews various factors which affect magnetic flux leakage testing with probes. The authors' results for quasi-stationary magnetization are compared with results obtained by other authors for the DC magnetization. In particular, lift-off effects of the probe in combination with the influence of the defect width are discussed. For lift-off values much larger than the crack widths, which is the case in practice, the leakage field amplitude increases with the crack width for a fixed crack depth. This effect is responsible for uncertainties in defect size estimation and demands better criteria than amplitude criteria alone.