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Research On Analytical Model Of Eddy Current Magnetic Field And Electromagnetic Characteristics Caused By Defects

Posted on:2020-12-27Degree:DoctorType:Dissertation
Country:ChinaCandidate:F JiangFull Text:PDF
GTID:1361330605472810Subject:Mechanical engineering
Abstract/Summary:PDF Full Text Request
Based on the difficulty in the quantitative evaluation of defects in traditional eddy current testing at present,the fundamental reasons are that the internal mechanism is inadequate and the response of the spatial magnetic field is not clear.The key to accurate quantification of defect parameters is to obtain the size and distribution characteristics of the magnetic field around the defect as comprehensively and accurately as possible.However,the traditional method of calculating and analyzing coil impedance is not suitable for the development of modern non-destructive testing.Therefore,the theoretical model of eddy current magnetic field and the research of electromagnetic characteristics caused by defects are proposed in this paper.In terms of theoretical modeling,aiming at obtaining the analytical solution of the problem,the solution in the form of series of the magnetic field is systematically studied in order to realize the fast,convenient and accurate calculation of space magnetic field.The changes in magnetic permeability and conductivity at the defect are comprehensively considered in the theoretical model,and corresponding technical measures are taken for the structural characteristics of different tested objects,thus providing theoretical support for the study of defects in various types of structures.In terms of defect-magnetic field response,by analyzing the interactions between the electromagnetic response characteristics and the geometrical dimensions,the hidden depth,the shape as well as other parameters of the defect,the feature parameters representing each physical quantity are extracted to realize the quantitative research of the defects.Specifically,this paper has carried out the four aspects of research as follows:(1)Based on Maxwell's equations,the magnetic vector potential equations of crack and non-crack regions excited by a long straight coil with rectangular cross-section are established by applying the method of separating variables and setting effective region.Then,the analytical solution of eddy current magnetic field for a rectangular surface crack is obtained by treating the long and thin crack with physical boundary and applying the continuous condition at each interface.According to the changes of electromagnetic characteristics between the surface crack and the excitation coil,the influences of the crack depth and width on the magnetic induction intensities and phases are studied.The quantitative characteristics of geometric parameters are extracted.In addition,a feasibility study has been carried out to detect cracks with different hidden depths and shapes using information contained in the magnetic field.First,the surface crack model is extended to a general model with arbitrary hidden depth.Thus,an analytical solution for the magnetic field distribution around the surface of a conducting plate that contains a long hidden crack is formulated.Next,based on the proposed modelling,the influences of the different hidden depth cracks on the surface magnetic field are analyzed.The results show that the horizontal and vertical components of the magnetic field near the crack are becoming weaker,and the second peak of the magnetic field distribution and the phase information of the magnetic field can quantitatively and qualitatively determine the hidden depth of the crack.Considering the difficulty of crack shape recognition,the mathematical model is further optimized to establish the relationship between signal characteristics and crack shape,so as to improve the ability of crack section shape identification.(2)In order to improve the ability of traditional eddy current technology to detect and evaluate corrosion defects,the series expression of magnetic induction intensity above the conductive plate containing corrosion are established,and the mechanism of corrosion hole on electromagnetic field has been revealed.Based on the proposed model,the influences of size and hidden depth of cylindrical corrosion on the surface magnetic field are analyzed.The two characteristics of transverse influence node and longitudinal depression depth are extracted from a butterfly-shaped graph to quantitatively analyze and evaluate the hidden corrosion.The influences of excitation current on the evaluation of a cylindrical corrosion in a plate are discussed,and the results show that this method still has high sensitivity under low frequency excitation.It is found that the radial magnetic induction intensity increases as the excitation current frequency increases,whereas the axial magnetic induction intensity decreases.As a result,the influence node for detecting the radius and the depression depth for detecting the hidden depth are obviously dependent on the excitation current frequency in terms of the butterfly-shaped graph.The excitation current amplitude has the same effect on different corrosion holes and does not affect the phase of magnetic induction intensity.(3)By setting up the axial solution region and establishing different differential equations,an analytical model of a cylindrical coil encircling metal rod is established.The mechanism of the electrical conductivity of the metal rod and the rod radius on the surrounding magnetic field is analyzed.Moreover,butterfly graph is proposed to comprehensively evaluate the effect of conductivity and rod radius on the radial and axial magnetic induction intensities.This method expands the application of eddy current testing in the field of material performance evaluation and material sorting.Besides,an analytical model for a metal rod with a coating layer is proposed to evaluate circumferential crack from the signals of surface magnetic field.The calculation results can show the three-dimensional distribution of axial and radial magnetic induction intensities on the surface of a metal rod clearly.The analysis of interaction mechanism demonstrates that induced eddy currents on the surface of the metal rod with cracks appears to be propelled toward the inner layer of the metal rod and the presence of a circumferential crack directly causes a decrease in the induced eddy current on the inner surface of the rod.The results calculated from the analytical model indicate that the model is capable of providing an accurate variation in the magnetic field due to circumferential cracks at different depths.(4)In order to perfect the theoretical system of eddy current magnetic field testing,the mathematical models of metal tube under two different configurations of the bobbin probe and the encircling probe are established.Through the calculation of the radial and axial magnetic induction intensities,the detection effect of the inner and outer circumferential cracks on the metal tube is studied.The intrinsic principle of the difference in detection sensitivity between the two probes is explained.According to the theoretical model of eddy current magnetic field response,the relationship between the distribution of the magnetic field in the inner region of metal tube and the geometrical size of cracks is analyzed.The results show that the analytical model can effectively explain and analyze the variation of the surrounding magnetic field caused by the crack.The crack characteristic parameters extracted from the magnetic field have certain quantitative recognition ability.In addition,the characteristics of magnetic field changes caused by the wall thickness of metal tube are studied,and the influences on crack detection are also discussed.The establishment of theoretical model and the study of defect-magnetic field response are helpful to understand the eddy current testing mechanism in various structures including flat plate,metal rod and tube,and expand the application of eddy current non-destructive testing and evaluation.Meanwhile,the analysis results have important theoretical and practical significance for optimizing the parameters of the experimental system,quantitative evaluation of defects,and establishing the inversion model.
Keywords/Search Tags:eddy current testing, analytical model, quantitative evaluation, magnetic field, crack, corrosion, interaction mechanism
PDF Full Text Request
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