Precise Inversion for the Reconstruction of Arbitrary Defect Profiles Considering Velocity Effect in Magnetic Flux Leakage Testing

In magnetic flux leakage type nondestructive testing (NDT), there exists velocity effect, which may cause the distortion of the defect signals and reduce the estimated accuracy of defect profile. In this paper, the distortions are analyzed by using finite-element method (FEM), and the influence of velocity effect on reconstruction of defect profiles is discussed under the condition of the simulation and experiment. This paper proposes an effective method for reconstructing arbitrary defect profiles in different velocity conditions. In the proposed method, the FEM considering velocity effect is employed as the forward model. A weighting conjugate gradient algorithm is applied to update the defect profile iteratively in two gradient directions. The algorithms effectiveness is tested on a series of artificial defects under various velocity conditions. The results demonstrate that the proposed model can achieve the better reconstruction accuracy than the ignoring velocity effect models. The properties of the presented method are so stable and robust that they make our approach a promising technique for the practical application of profile reconstruction for NDT.