Thick-walled steel pipes, which bear high internal pressure, are widely applied in nuclear power and pressure pipelines. If there are defects in the inner wall, they are easy to expand and cause accidents. Therefore, the thick-walled steel pipe must be subject to non destructive-testing after production. For the magnetic flux leakage (MFL) testing method, the detection sensitivity gradually decreases with the increase of wall thickness. To solve this problem, a new structure of MFL probe is proposed in this paper. The influence of the iron core permeability on the MFL signal is analyzed theoretically, and the effect of the core length and diameter on the MFL signal is analyzed by simulation. The variation of the MFL signal with the change of the iron core and coil lift-off is studied respectively. The simulation results are verified by experiments. It is found that the lift-off of the iron-cored coil is determined by the iron core position. Based on this phenomenon, an MFL array probe is designed, which can be used for online detection of thick-walled steel pipes to improve the detection sensitivity of inner wall defects.
Magnetic flux leakage (MFL) testing is widely used in non-destructive testing of ferromagnetic components. In view of the serious attenuation of the leakage magnetic field (LMF) caused by the transmission of LMF in the lift-off layer between the measuring point and the workpiece, this paper introduces an MFL detection method based on the slotted ferromagnetic lift-off layer (SFLL). The conventional non-ferromagnetic lift-off layer is changed to a ferromagnetic lift-off layer with a rectangular slot. The magnetic sensor is fixed above the slot and scans the workpiece together with the lift-off layer. First, the detection mechanism of the new method was studied by an equivalent LMF coil model. The permeability perturbation effect and the magnetization enhancement effect were analyzed in the new method. Based on the detection mechanism, the lift-off tolerance of the new method was investigated. Then, the LMF enhancement and lift-off tolerance of the new method in the steel plate detection model were studied. Finally, experiments were conducted to compare the new method with the conventional method. The simulation and experimental results show that the slotted ferromagnetic lift-off layer enhances the amplitude of the MFL signal and is tolerant to the lift-off value. This method provides a new idea for optimizing the design of the MFL sensor and improving the sensitivity of MFL detection at a large lift-off value.
Detecting inner- and outer-surface discontinuities of drill pipe is of great significance to the evaluation of the quality of the drill pipe. This paper proposes a method based on a magnetized eddy current testing technique to detect inner- and outer-surface discontinuities by analyzing the difference of the imaginary part signal characteristics of the receiving coil. For eddy current testing, the outer-surface discontinuities cause the local conductivity to be zero, while inner-surface discontinuities cause the perturbation of the magnetic permeability on the material surface. In this paper, the effects of conductivity distortion and permeability perturbation on induced eddy currents are analyzed by simulation. The conductivity distortion increases the magnetic field above the discontinuity compared to the magnetic field without the discontinuity, while the permeability perturbation reduces the magnetic field. Next, the difference in coil impedance can be used to distinguish the inner- and outer-surface discontinuities. Finally, the feasibility of the method is verified by experiments, and the results show that the inner- and outer-surface discontinuities can be discriminated.
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