Study On Detection And Characterization Methods Of Internal States In Multi-layer Conductive Structures Using Pulsed Eddy Current

Nondestructive testing and evaluation of conductive structures is widely applied in many fields ranging from aerospace,nuclear power,to industrial production process.The thesis is mainly focused on the detection and characterization methods of internal states in multi-layer conductive structures,as part of research work of projects under the support of the National Natural Science Foundation of China and practical engineering applications.On the basis of researching pulsed eddy current detection mechanism,the mapping relationship between the response signal of pulsed eddy current and internal defect in conductive structures has been studied,and also the mapping relationship between the response signal of pulsed eddy current and the testing parameter;The signal feature extraction and analysis techniques in pulsed eddy current testing have been investigated.And methods for qualitative detection,location,and depth quantitative evaluation of internal defects in multi-layer structures are presented.The prototype system of pulsed eddy current testing has been developed and the application example in thermostat production line has been introduced.The main work and innovations are as follows:(1)The physical model and detection mechanism of pulsed eddy current testing has been studied.Pulsed eddy current typical testing signals and response characteristics of eddy current testing signals with varying internal defect parameters and probe lift-off are researched and analyzed,which provide theoretical forward guidance for pulsed eddy current testing and signal analysis.(2)The detection of defects is carried out by using a threshold method based on pulsed eddy current peak values.On the issue of defect localization,under the interference of lift-off fluctuations,Fisher linear discriminant analysis is used to extract the features from testing signals.Then Support Vector Machine(SVM)method based on genetic algorithm parameter optimization is presented to deal with the localization of defects.Multi-layer plates have been used for experimental verification.(3)Concerning the problem of quantitative evaluation of defect depth,the mapping relation and relativity between defect depth and features from classical time domain analysis and principal component analysis have been studied.Combined features are proposed to evaluate the depth of defect.Partial Least Square and Support Vector Machine are used in defect depth regression analysis,and results comparison is analyzed.(4)A prototype system of pulsed eddy current testing is developed.The design and development of architecture,hardware and software of the testing system are put forward.The internal states testing of thermostat is taken as an application case,and the actual application of pulsed eddy current analysis methods and the testing system is introduced.In a word,the research work in this thesis will provide technical support for the testing and evaluation of internal states in conductive structures in many fields.