Study On Calculation Methods For Defects Characteristic Signals In Metals Of Electromagnetic Nondestructive Testing

Due to the complex working conditions and harsh working environment,there probably be some defects on the metal components.And the defects in the metal components may lead to terrible accidents.Therefore it is of great significance to the health monitoring of metal components and the detection of the defects.The electromagnetic nondestructive testing method has the characteristics of safety,green,convenient and economical.Forward problems of eddy current testing(ECT)and electromagnetic thermal coupling are studied in this paper.The ECT problem is studied by analytical and numerical simulation method;while electromagnetic thermal coupling problem is studied by numerical simulation and experimental method.For the analytical calculation of forward problem in eddy current nondestructive testing,there are more research on the models without defect,and less research on the models containing defects;for electromagnetic thermal coupling problem,there are more research on the static models,and less research on the dynamic models.Aims to the two existing problems,three aspects are studied in this thesis:(1)The research on the analytical calculation method of cylindrical defect detection signal.A conductive metal plate with a cylindrical defect is modeled,the analytical expression of integral form to the magnetic potential vector is obtained.The analytical expression of series form is obtained by truncated region eigenfunction expansion(TREE)method.Finally,the analytical solution in time domain is calculated via inverse Laplace transform.The analytical method for cylindrical defect model proposed by this thesis has high calculation accuracy but the calculation efficiency is 10 times than the numerical simulation.(2)The research on the analytical calculation method of elliptic cylindrical defect detection signal.An analytical calculation method for elliptical cylindrical defect model is presented on the on the basis of the calculation method of the cylindrical defect model.An empirical formula for the calculation of elliptical cylindrical defect model is constructed.The calculation results are compared with the results of numerical simulation.The application scope of the empirical formulas is also studied.The elliptical cylindrical defect model is approximated to a rectangular shape defect model,and the calculation error of this approximation is studied,too.This study is of greatsignificance for the establishment of a nondestructive eddy current analytical model for metal components with natural defects.(3)The research on induced heating and electromagnetic thermal coupling.A 3D finite element simulation model for static electromagnetic thermal coupling and a 2D for dynamic electromagnetic thermal coupling are constructed.Three experimental platform are set up to study the influence of different material,different exciting coils,different speed,different exciting current between the detection signals.The first experimental platform is for static electromagnetic thermal coupling model;the second one is for the translational model at a low speed;the third one with a metal turntable is for the high-speed model.