| The application of steel pipe,rail and other ferromagnetic structures is extensive in “the Belt and Road” strategy.The non-destructive testing of them is very important.Deep internal micro defects have always been a technical difficulty in magnetic flux leakage(MFL)testing.This paper focuses on the local magnetic permeability perturbation caused by internal defects of ferromagnetic materials and its measurement,and proposes a new principle and method of electromagnetic non-destructive testing,which realizes high precision detection of internal deep defects.Based on the nonlinear magnetic properties of ferromagnetic materials under DC magnetization,the mechanism of internal magnetic field disturbance caused by defects is analyzed.Theoretical analysis reveals that the magnetic permeability perturbation(MPP)caused by the defect corresponds to different characteristics of each stage of the ?-H curve.The analytical model and the finite element model are used to obtain the spatial distribution,characterization method,transfer rules and influencing factors of the magnetic permeability perturbation of different buried depth defects.The source-conversion mechanism of "defect-permeability perturbation" in ferromagnetic materials is discussed.A magnetic permeability perturbation testing(MPPT)method is proposed for the first time.Based on the Dodd-Deeds model,the analytical model of the placed coil integral under magnetic permeability perturbation is derived.The threedimensional finite element simulation model is established by the equivalent permeability method.The simulation results show that the MPP affects the eddy current distribution and affects the sensing signal of the probe under the AC excitation.In order to improve the sensitivity,the differential sensing coil was used to detect the permeability perturbation.The influencing factors such as differential spacing,defect size,lift-off and excitation frequency are analyzed.For 25 mm thick steel plates,a crack of 0.5 mm deep,25 mm long,and 0.5 mm wide on the back surface can be detected.The MPPT method breaks through the detection limit of traditional MFL testing.The multi-source effects in MPPT are further explored.The defect itself,the magnetic permeability perturbation around the defect and the MFL generated by the defect are the signal sources.The dominant role of the three signal source under different internal and external conditions is clarified.The MPP changes with the DC magnetization current,which has different effects on the MPPT signal.As the magnetizing current increases,the signal amplitudes of internal and external defects exhibit strong non-monotonic changes.For external defects,the MFL plays a dominant role in the MPPT signal under large liftoff.However,for internal defects,the MPP plays a dominant role in the MPPT signal of deep defects.Furthermore,a large lift-off detection method based on MPPT probe is proposed.Experiment results show that the method can detect surface defects of 0.5mm depth,25 mm length and 0.5mm wide when the lift-off is greater than 10 mm,which exceeds the detection capability of the conventional MFL testing method.It is difficult to detect thick-walled small-diameter steel pipes for MFL testing and ultrasonic testing.By comparing and analyzing the MPPT method and traditional MFL testing and eddy current testing,the advantages of the MPPT method is clarified.The influence of the radius of curvature on the MFL and surface magnetic permeability of the steel tube surface was discussed.For a 56 mm diameter steel pipe with a thickness of 12 mm,a detection signal of a crack of 0.6 mm deep,25 mm long,and 0.5mm wide on the inner wall was obtained experimentally. |
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