Analytical Solution Of Pulsed Eddy Current Field And Thickness Measurement For Layered Planar Conductive Structures

Metal components with layered structure are widely used in various industrial fields.Pulse eddy current technology can be used to realize non-destructive testing of layered thickness,which is of great significance to product quality inspection,structure analysis and safety testing of in-service equipment.In this paper,two methods,Fourier series method and Laplace transform method,are used to obtain the analytical solution of the pulsed eddy current field of the planar layered conductive structure,and the technique of the layered thickness pulsed eddy current detection of the metal component is discussed.The main research work is as follows:Fourier series method is used to study the analytical solution of pulsed eddy current field generated by a placed coil in a multilayer conductive structure.Firstly,the single frequency excited conventional harmonic eddy current field is discussed.By using Cheng matrix,the analytical solution of the conventional harmonic eddy current field with multilayer conductive structure and the analytical expression for detecting the induced voltage of the coil are obtained.On this basis,the pulse excitation is decomposed into several harmonics of different frequencies by Fourier decomposition,and the induced voltage signal of multi-layer conductive structure is solved by using the superposition principle of the field.The method is verified by finite element examples.By using Laplace transform,the pulsed eddy current field of the coil placed above the multilayer plate conductive structure is solved in the frequency domain,and the time domain analytical expression of the induced voltage signal of the pulsed eddy current detection of the monolayer plate is solved by the residue theorem.The AWG numerical inversion method is used to solve the induced voltage signal of pulsed eddy current detection of multi-layer plate.The numerical results are compared with the results of finite element method,and the agreement between the numerical results and the results obtained by the finite element method is quite good.Through the comparison of the upper bound of different summation series in the analytic formula,it is pointed out that there exists the precision limit of the summation series.Compared with each other,a suitable method for detecting the induced voltage of multi-layer conductive structure pulse eddy current testing is selected.The penetration process of the eddy current in the conductive structure is studied,and the corresponding relation between the induced voltage curve and the layer thickness is established,which provides the theoretical basis for the inversion of the detection signal.The influence of the excitation coil on the detection signal is studied,and the optimization direction of the excitation coil is put forward.The inversion characteristics of exponential function by AWG numerical inversion method are used to analyze the analytic expression of induced voltage in time domain for single-layer plate conductive structure under pulse excitation,and the confidence interval of the inversion result of AWG method is determined.Based on vortex ring theory,the inversion confidence interval of multi-layered plate structure is also studied.Finally,a numerical example of finite element method is used to verify the inversion method.Through the error analysis of the inversion result,the applicable object of the method is explained.