Research On Key Technologies Of Nondestructive Testing Of Ferromagnetic Materials Defects Based On Multi-frequency Electromagnetics

The pipeline transportation industry has become the lifeline of the country's development.At the same time,the detection technology for pipeline defects has become a hot research point in recent years,especially the research on non-destructive testing technology is progressing.Electromagnetic non-destructive testing technology has gradually gained the attention of everyone in recent years.Compared with the traditional alternating magnetic flux leakage detection technology,multi-frequency electromagnetic non-destructive testing technology has its own unique advantages.First,multi-frequency electromagnetic detection technology inherits all the advantages of AC electromagnetic detection technology,with accurate mathematical model,can be applied Non-contact measurement;in addition,multi-frequency electromagnetic detection technology has high detection efficiency,and can be used for both ferromagnetic materials and non-ferromagnetic materials,and has wider applicability;and,multi-frequency electromagnetic detection technology combined different excitation frequencies The detection characteristics can simultaneously reflect different parameters of the detection defect.This article will study from the following aspects.First,analysis of the principle of alternating electromagnetic detection technology.The basic theory of electromagnetic field is introduced.On this basis,the theoretical analysis of AC electromagnetic detection technology is carried out,and the realization principle of multifrequency electromagnetic detection technology is expounded.The calculation method of magnetic field distribution is studied,including analytical method and numerical method.The equivalent point dipole in the magnetic dipole model is used to simulate metal surface crack defects,and the trend of magnetic field level and vertical component on the detection path is analyzed..For the electromagnetic non-destructive testing technology,several key factors affecting the detection effect are briefly described,including the characteristics of the defect,the excitation frequency,the metal material,and the lifting height.Secondly,construction and defect feature analysis of single frequency excitation model.The finite element analysis method is briefly introduced.Based on the ANSYS finite element simulation software,a spatially induced magnetic field distribution model for simulating metal surface crack defects is established for the inherent properties of the plate to be tested.In the case of the influence of the parameters of the defect(including length,width,depth and course)on the spatial magnetic field distribution,including the horizontal component amplitude of the magnetic signal,the horizontal and vertical horizontal spacing,the vertical component peak-tovalley height difference,and the peak domain.Magnetic field distribution characteristics such as the length of the tangential direction.Thirdly,design of multi-frequency electromagnetic detection implementation.The realization process of multi-frequency electromagnetic detection technology is designed.The sensitivity of the defect shape at different frequencies is analyzed,and the most sensitive frequency bands corresponding to length,depth,width and angle are obtained.The multifrequency excitation signal is generated by superposing the signal of the appropriate frequency,and the magnetic field distribution under the multi-frequency excitation is compared with the magnetic field signal under the single-frequency excitation to obtain the magnetic field distribution characteristics of the multi-frequency electromagnetic detection technology.It is concluded that the spatial magnetic field signal generated by multi-frequency electromagnetic excitation cannot be directly applied to the existing defect quantization algorithm.Fourthly,defect iterative inversion based on multi-frequency finite element model.The defect inversion iterative algorithm based on multi-frequency finite element model is studied,and the parameters of defects are reconstructed.The EMD method is used to decompose the target multi-frequency signal and the predicted multi-frequency signal into magnetic signals at three different frequencies,and three objective functions introducing the Pearson coefficient are set as the decision conditions of the iterative inversion algorithm.Three objective functions approximating the predicted defect contour parameters from three degrees of freedom,enhancing the convergence of the algorithm and having higher prediction accuracy.