Mechanism Of Magnetic Memory Testing And Simulation Of Leakage Magnetic Testing For Metal Defects

Structural steel and reinforced concrete structures are widely used in civil engineering.As a non-contact testing method,magnetic nondestructive testing technology can test defects by measuring the magnetic field changes on the surface of structures.In this paper,defect testing in steel beams and reinforced concrete is taken as the research object.Firstly,the magneto-mechanical coupling microscopic mechanism of magnetic memory testing technology for metal component defects is studied,and the magneto-mechanical coupling law in the magnetic memory testing signal from defects of steel beam under three-point bending is discussed experimentally.For the magnetic leakage testing technology of metal defects,based on the magnetic constitutive test results,the nonlinear magnetic hysteresis magnetization constitutive of the tested metal material is established,and the defect leakage detection simulation considering the real constitutive behavior of the material is realized.The simulation of magnetic flux leakage(MFL)testing of defects and the analysis and evaluation of corrosion defects of reinforced concrete structures based on MFL testing signals are discussed through numerical simulation.The main contents of this article are as follows:(1)The magneto-mechanical coupling microscopic atomic mechanism of defect magnetic memory testing technology was investigated by first-principles calculations.Using the density functional theory plane-wave pseudopotential method,the GGA method was used to calculate the magneto-mechanical coupling effects of α-Fe and its doping systems with different elements from the atomic level.The influence of tension,compression,and shear on the magnetic properties of the systems was analyzed.The results show that there are differences in the energy and magnetic moment changes of the doped system under the same strain,and the doped elements have an impact on the straininduced magnetic moment enhancement.Different types of strain have different effects on the variations of magnetic moment and energy,with shear strain having the most significant impact,followed by tension,and compression having the little impact.(2)Based on the metal magnetic memory testing method,a steel beam defect detection test was carried out,and the magneto-mechanical law in the metal magnetic memory detection signal of steel beam defects under three-point bending was discussed.The results show that the gradient value curve of the magnetic memory signal around different types of defects exhibited a "peak-valley" phenomenon.Combined with the distance between the gradient curve and extremum,the position of transverse and longitudinal defects of different sizes can be initially determined.However,due to the asymmetry of oblique defects on the scanning line,the size of oblique defects cannot be accurately identified using the gradient curve alone.Other methods need to be used to assist in identifying oblique defects in practical engineering.(3)Based on the magnetic constitutive test results,a defect leakage magnetic testing simulation was conducted considering the material’s real constitutive behavior.For the MFL testing technology of metal defects,the Epstein frame equipment was used for constitutive testing,and the nonlinear magnetic hysteresis magnetization constitutive parameters of the tested metal material were established.Based on the obtained nonlinear magnetic hysteresis magnetization constitutive,a MFL testing simulation of surface defects on the magnetic material was carried out.The results show that within the required range of test parameters,the constitutive curves obtained for specimens of different sizes are consistent under the excitation magnetic field.By comparing the field-magnetization curves,it can be seen that compared with the relative permeability model,the results obtained from the real material constitutive model show that the magnetic signal has a significant nonlinearity with the increase of the applied excitation field.(4)The analysis and evaluation of corrosion defects in reinforced concrete structures based on MFL signals were discussed through numerical simulation.By comparing the MFL signals from the Gaussian theory model and the simplified model commonly used in the research of MFL testing at the present stage under different degrees of corrosion,a quantitative evaluation of the corrosion degree was carried out based on the characteristic parameters of the corrosion signal.The results show that the variation law of the peak-to-valley difference of the Gaussian theoretical model is in good agreement with the experimental results obtained from actual corrosion tests.The relative error of the Gaussian theoretical model for the inversion results of the average corrosion degree is significantly lower than that of the simplified model.This confirms the applicability of the Gaussian theory model for analyzing MFL signals of corrosion defects in reinforced concrete,enabling a quantitative evaluation of corrosion parameters.