| Magnetic flux leakage testing (MFL) is a classic industrial nondestructive detection method, which has been widely applied in fields, like safety inspection of magnetic equipment in power system, safety assessment of industrial oil pipeline in service, etc. The detection accuracy of MFL equipment will be directly related to the safety production and operation in application areas. Therefore it is particularly important to do researches and make improvements of MFL technology. Recently with the rapid development of industrial detection and automation technology, and the putting forward of industry 4.0 strategy, MFL technology, as the traditional classical nondestructive testing method, will also face the challenge of technology updating and iterative. Under the background, this paper has studied MFL distribution characteristics and its forward problems, respectively from the circumferential multi-channel, the radial multi-lift-off and the material permeability changes.Firstly, this paper introduces the principle, the implementation method and the mathematical solution of steel tube MFL testing. With the finite element simulation platform, the finite element model of fine steel pipe leakage detection, the model of the Homer magnetizer, hole defects, crack defects, crack model and the corresponding fine air layer have been established. Moreover the model optimum magnetization factor has been analyzed and determined.Secondly, based on the coordinate description method of the three dimensional space domain, this paper has mathematically descried the forward problem of MFL distribution. With the engineering factors being simply processed during the implementation of MFL testing, it sets sup an ideal distribution environment of MFL. By category it described defect parameters, magnetic permeability parameters and MFL information characteristic quantities. Based on the above, a description method of the spatial distribution of has been established by combining with the magnetic permeability change factors, including the expression of MFL in a spatial single point, in a single channel and single liftoff, in the circumferential multichannel and single liftoff, as well as those in a radial multi-liftoff and single channel.Thirdly, it has analyzed the circumferential characteristics of MFL distribution for single hole defect from the circumferential multi-channel angle. Multi-channel correlation method is used to calculate the circumferential distribution characteristics and regularity of MFL, including based on interval channel MFL correlation analysis, correlation analysis, and based on adjacent channel MFL. Correlation coefficient has been established based on correlation analysis of waveform characteristics and relationship between defect location parameter, solved the MFL testing on the problem of weak defect detection rate is low, and based on multi-channel characteristics of MFL distribution of circumferential double hole defects are analyzed.Then, it has analyzed the spatial domain MFL distribution problems of standard circumferential crack defects from the radial multi-lift-off angle, including the axial component, radial component MFL waveform and their derivative waveform characteristic changes in the relationship. Based on this, a compensation algorithm based on the relationship between the peak to valley spacing value of the first derivative of axial component MFL wave and the multi-lift-off, and the correction surface between has been put forward.Finally, from the material permeability changes and steel tube wall partition magnetization state view, it has described the optimum magnetization theory and MFL effect mechanism; Analysis of the MFL distribution characteristics of the wide groove defect and its difference with crack defects MFL has been done; Based on this, it has put forward the "steps" defects which is the subunits defect of crack defects and its standard model and parameter definition. The step defects size parameters and its MFL waveform relations has been carried on the comprehensive analysis of the characteristics, based on the above analysis, a crack defect overlay analysis method was proposed; Finally it has analyzed the pipe inner surface magnetic field distribution characteristics, formation reasons and its difference with the MFL based on the material permeability and explain the forward problems of the characteristics of MFL distribution and formation reasons. |
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