Analytical Approaches To Eddy Current Nondestructive Evaluation For Stratified Conductive Structures

Eddy current nondestructive testing, as one kind of the five main nondestructive testing methods, has the properties of fast detection, non-contact and low cost, which has been widely used in aerospace, nuclear industries and high speed train vehicles manufacture. Supported by the sub-project---pulsed eddy current nondestructive evaluation for inter-erosion of stratified conductive structures, this paper mainly studied on analytical modeling and experiments for harmonic eddy current and pulsed eddy current, and developed analytical model of differential eddy current pick-up sensors for stratified conductive structures. The main researches included:1. Analytical solutions for coils above stratified conductors were provided. The Extended Truncated Region Eigenfunction Expansion method (ETREE) is applied to stimulate the eddy current response to stratified conductive structures and compared with Cheng-Dodd-Deeds'integral form solutions. Fan followed an original method to study the eddy current field for multilayered conductive structures and developed the model based on the reflection and transmission theory of the electromagnetic waves. We did the experiment and compared the experimental result with the Fan's model, which agreed well.2. Analytical modeling of pulsed eddy current was developed based on analytical modeling of harmonic eddy current for stratified conductive structures. And the experimental setup was developed to confirm the simulating result of analytical model.3. In this paper, analytical modeling for differential pick-up sensors was established based on the reflection and transmission theory of electromagnetic wave for stratified conductive structures. New kind of eddy current sensors for detecting multilayered conductive structures were found through the simulation optimization, which can eliminate the magnetic field produced by the isolated coil. This kind sensors only detect the magnetic field change caused by the layered conductive structures, so the signal of the magnetic field change caused by the layered conductive structure avoid being concealed by the signal of the magnetic field produced by the isolated coil. 4. Low carbon steel thickness changes were detected by using pulsed eddy current experimental equipment. And through experimental results, we can get the response characteristics of pulsed eddy current signal for low carbon steel.