Study On Some Key Problems In Eddy Current Testing Of Multi-layered Conductive Structures For Thickness Measurement And Flaw Detection

It's widely recognized that thickness measurement and defect inspection of multi-layered conductive structures are complex and urgent problems to tackle in a number of industries ranging from aerospace industry to atomic engineering. Supported by a National Defence Project, several key problems in the thickness measurement and defect non-destructive testing for multi-layered structures using Eddy Current Testing (ECT) method are studied starting with a general survey of the state of the art of ECT techniques.In chapter 1, the applications of non-destructive testing and evaluation (NDT&E) in various fields are briefly introduced and a review of ECT theory and technology is presented. The current research situation at home and abroad of thickness measurement and defect inspection of multi-layered structures using ECT method is discussed and the contents of this dissertation and new ideas involved are introduced.In chapter 2, the forward ECT problem in multi-layered thickness measurement is investigated. Based on the theory of the axially symmetric and time harmonic electric and magnetic fields the impedance model of a cylindrical, air-cored eddy-current probe coil placed over an N-layer conductive structure is derived in the form of integrals of first-order Bessel functions after introducing the vector potential. The mathematical model can be calculated by a matrix method, which greatly simplifies the procedure.In chapter 3, the computing techniques including the solving method of the Bessel function integrals for the probe coil impedance mathematical model are firstly investigated and then an ECT simulator for multi-layered structure thickness measurement is developed. The coil impedance responses due to the variations of parameters of the structure and probe coil and excited frequency are simulated. The simulated and experimental results are compared and the agreement is well.In chapter 4, the principle of multi-thickness measurement using multi-frequency ECT method is first introduced and three types of inversion models to inverse the detected thickness from the probe coil impedance /voltage measurements are put forward. They are the numerical optimization inversion model using the least square method, the feed forward BP Neural Networks (NNs) model and the Radial Base Function NNs model. Inversions are carried out using the experimental data and results are compared and discussed.In chapter 5, ECT techniques for scanning inspection of defects in multi-layered structures are studied. A finite element analysis (FEA) forward model is proposed and simulation for different types of flaws that have different shape, locations and dimensions in the configuration is carried out based on the ANSYS software. The simulation program is written by the ANSYS Parametric Design Language (APDL). Methods to determine the locations and dimensions of the defect in light of the probe coil impedance and phasecurve are put forward. Results of simulation and experiments are compared and the validation of the forward model, simulation work and instrument developed is proved.In chapter 6, signal-processing techniques for noise elimination including the discrete wavelet transform and wavelet packet analysis methods are studied. Their results are compared and the best de-noising method is proposed.In chapter 7, an experimental ECT system for multi-layered thickness measurement and defect inspection is developed successfully and the bottleneck software and hardware system are discussed.In chapter 8, important results and conclusion of the dissertation are summarized and the future research of multi-layered structure thickness measurement and defect inspection using eddy current testing is prospected.