Research On Defect And Stress Of Aerospace Aluminum Alloy Using Pulsed Eddy Current Non-destructive Testing Technology

Pulsed eddy current (PEC) is a new nondestructive testing (NDT) technique developed in recent years. More conductive material information including defects and the stress can be gotten from detection of complex aluminum-alloy structure of aerospace by using wideband exciting, PEC technique has become a hot research area in aviation NDT. On the basis of analyzing the state of the art of the research on PEC technique both inland and overseas, the defects and the stress of aerospace aluminum-alloy are studied in this paper. The brief structure of the research and novel approaches are as follows:1. Optimization design of PEC circular probeBased on the geometric characteristics and boundary conditions of PEC probe, a 2D finite element model of PEC probe is built. The peak value of the difference signals of PEC is adopted to evaluate electricity conductivity by Finite Element Method. The results show that the peak value of the difference signal is a linear relation with the change of the electrical conductivity, and can be adopted to estimate on the electrical conductivity distribution of the conductivity material. This paper reports an investigation into the sensitivity for a pancake PEC probe to variations in the material conductivity of specimens. Three experimental coil parameters are modeled: (a) coil inner radius; (b) coil width; (c) coil height. The optimization design of PEC circular probe is proposed. This paper also investigates relationship between other parameters including excited frequency, lift-off, the ferrite cores and aerospace aluminum-alloy. It will provide the theory guide of PEC sensor design.2. Estimation on defect of the aerospace aluminum-alloy using PEC techniqueTo estimate on the defects of aerospace aluminum-alloy, experiments including the surface defects, the subsurface defects and the thickness changes are carried out using PEC technology. Based on the extraction features in time domain and frequency domain, the research results show that the defect depth sensitivity of aerospace aluminum-alloy is higher than the aluminum-alloy defect width. The quantitative model of defect depth is established for the different defects. The peak value of the spectrum is a new feature in frequency domain. All features extracted are selected and combined to identify the three kinds of defects, the research results show that the combination of the peak value of the spectrum in frequency domain and the peak time in time domain can be more efficient to identify three kinds of the defects.3. Estimation on stress of aerospace aluminum-alloy using PEC techniqueIn order to estimate on the residual stress of aerospace aluminum-alloy, this paper introduces the occuring theory, testing and estimation of the residual stress. Based on the correlation between applied stress and the electrical conductivity, the PEC technique is proposed to estimate on the stress. The brief structure of the research and novel approaches are as follows:(1)In order to investigate the relationship between the peak value of the difference signal and the elastic stress, the elastic stress of AL5083 is carried out. The experimental results show that the peak value of the difference signal is a linear with the applied elastic stress. On the other hand, the peak value of the difference signal by COMSOL FEM software simulation is a linear with the electrical conductivity changes. Therefore, the peak value can be adopted to estimate the electrical conductivity distribution and the applied stress distribution.(2)In order to estimate the residual stress, flat bar samples made of various aluminum-alloys have been tested with different PEC probes such as circular and rectangular coils. The research results show that the elastic dependencies exhibit non-linearity in many cases. These verify that there are the elastic stress and plastic deformation on aluminum-alloys. The plastic deformation sample on aluminum-alloys can be tested using PEC probe through"equivalent elastic coefficient"extracted."Equivalent elastic coefficient"of AL2024 is decreased with the plastic deformation increased using the circular probe and rectangular transverse probe. Overall the PEC responses obtained on the tested materials have demonstrated sufficient sensitivities to tensile stress and plastic deformation.(3)In order to estimate on the different depth stress, PEC response of variable pulse width excitation has been calibrated with respect to known values of electrical conductivity standards. The obtained calibration dependence in principle component analysis (PCA) space has been used to evaluate electrical conductivities of other test materials. The experimental results show that the electrical conductivity PCA space is very efficient to estimate on the stress of the multi-layer metal materials. To verify the validity of the calibration dependence in PCA space, the experiment of bending AL2024 is also carried out. The experimental results verify our stress profile inspection methods using PCA based PCA. The agreement between theory and experiment shows that the present method can be used to estimate on the stress profile of aerospace aluminum-alloys.