As a promising non-destructive testing (NDT) method, magnetic flux leakage (MFL) testing has been widely used for steel structure inspection. However, MFL testing still faces a great challenge to detect inner defects. Existing MFL course researches mainly focus on surface-breaking defects while that of inner defects is overlooked. In the paper, MFL course of inner defects is investigated by building magnetic circuit models, performing numerical simulations, and conducting MFL experiments. It is found that the near-surface wall has an enhancing effect on the MFL course due to higher permeability of steel than that of air. Further, a high-sensitivity MFL testing method consisting of Helmholtz coil magnetization and induction coil with a high permeability core is proposed to increase the detectable depth of inner defects. Experimental results show that inner defects with buried depth up to 80.0 mm can be detected, suggesting that the proposed MFL method has the potential to detect deeply-buried defects and has a promising future in the field of NDT.

Eddy current array testing (ECAT) is a useful tool in evaluating defect sizes. However, it is difficult for ECAT to evaluate inner defects in ferromagnetic materials due to thin skin effect. In this paper, DC magnetization-based ECAT (DCMECAT) is proposed to quantify buried defects existing in the inner surface of steel plates. Relying on permeability disturbance in the defect vicinity, DCMECAT can build links between the testing signals and defect sizes. Results show that the performance of evaluating defect sizes is better under the saturated magnetization state. Further, three features, i.e., the signal width, the peak voltage, and the contour orientation, are proposed to evaluate the width, the depth and the orientation of defects, respectively. Relationships between features and defect sizes are investigated by using least-squares regression. Further, a testing example is shown in this paper, and the evaluation error is 14%.

In order to detect tiny cracks in bearing rings, a high-sensitivity magnetic flux leakage (MFL) method based on magnetic induction head is proposed and investigated in this paper. First, the MFL sensing method based on magnetic induction head is theoretically illustrated. A magnetic core with an open gap is applied to guide leaked magnetic flux into a detection magnetic circuit, where an induction coil is placed to detect the change of the magnetic flux. To obtain an optimal gap width for higher sensitivity, its influence on MFL sensitivity is analyzed by building magnetic circuits. Then, to realize automatic inspection for bearing rings, a U-shape magnetizer is proposed and 3-D numerical simulations are conducted to investigate the signal characteristics of the proposed method and the sensitivity affected by gap width. Furthermore, MFL experiments for bearing rings were performed and it was found that the proposed method has high sensitivity for tiny cracks and when the gap width is approximately equal to the width of the crack, the greatest sensitivity will be obtained. Finally, the automatic MFL testing system for bearing rings is designed and employed in the production line. In-line tests show that the novel MFL system can fulfill the inspection requirement for bearing rings.

As an efficient non-destructive testing method, magnetic flux leakage (MFL) testing has been widely used for damage detection in ferromagnetic materials. The magnetic dipole theory is a traditional method to calculate the magnetic leakage field. In this paper, the magnetic leakage field functions of a slot in a 2-D infinity plate are obtained using the magnetic dipole theory. Equations of the magnetic leakage field are converted into the convolution of two other functions. Spatial spectral equations of the MFL are developed according to the convolution theorem using the Fourier transform (FT) method. The results are analytical functions different from other numerical methods, such as the discrete FT and fast FT methods, and the calculation speed is fast. The results show that the y-component of the magnetic leakage field has 90° phase shift with the x-component. This result has clear physical significance compared with the traditional MFL equations using the magnetic dipole theory. Different spatial spectral curves for different defects are presented in this paper. Since there is a sinc function for the width value in the equation, the width value can be deduced from the first zero-crossing point in the spatial spectrum curves. A new method is proposed to determine the width value using the relationship w =1/fs, where fs is the first zero cross point. Two specimens with different cracks are investigated, and the experimental results show this method can be used as an inverse MFL data interpretation technique.

Magnetic flux leakage (MFL) testing has been widely used as a non-destructive testing method for various materials. However, it is difficult to separate the influences of the defect geometrical parameters such as depth, width, and length on the received leakage signals. In this paper, a “near-field” MFL method is proposed to quantify defect widths. Both the finite element modelling (FEM) and experimental studies are carried out to investigate the performance of the proposed method. It is found that that the distance between two peaks of the “near-field” MFL is strongly related to the defect width and lift-off value, whereas it is slightly affected by the defect depth. Based on this phenomenon, a defect width assessment relying on the “near-field” MFL method is proposed. Results show that relative judging errors are less than 5%. In addition, the analytical expression of the “near-field” MFL is also developed.