Coupling Impedance Analysis And Structure Design Of Coil Probe Used For CFRP Eddy Current Testing

With the continuous expansion of the application scope and scale of carbon fiber reinforced polymer(CFRP),the demand for nondestructive testing technology becomes more and more urgent.According to the characteristics of weak conductivity and heterogeneous multiphase structure,eddy current method has a good application prospect in the detection of damage in CFRP.At present,the eddy current method has low accuracy and fuzzy image for the defect detection of CFRP,which is difficult to meet the requirements of practical engineering application.Coil probe is the core component of eddy current testing system,whose structural parameters directly determine its impedance performance and eddy current testing accuracy.Therefore,based on the analysis of the characteristics of the coupling impedance between the coil probe and the CFRP material,a high-resolution eddy current coil probe is developed.At the same time,combined with the signal processing technology of space-frequency domain,the eddy current imaging of the micro fiber distribution and the defects such as fiber missing,fracture and crack delamination in the carbon fiber composite plate is realized.The main contents of this thesis are as follows:Firstly,the mathematical calculation method of the coil impedance and the coupling impedance model between the coil probe and the conductor material are established,and the reliability of the model is verified by simulation analysis and experiment.The influence of CFRP defect and input current frequency on the coil impedance is further studied.Firstly,the RLC equivalent circuit model of coil and eddy current detection system is established,and the calculation method of theoretical value of resistance,inductance and capacitance of spiral coil is proposed.Then,the influence of crack defect and input current frequency on the impedance characteristics of the coil is calculated by COMSOL.Finally,the effects of conductivity and crack size on the impedance characteristics of the coil are studied by sweep frequency test.Secondly,in order to enhance the anti-interference ability of the coil probe and further improve the sensitivity of eddy current detection,the coupling mutual inductance circuit model of the separated eddy current probe is established.From the perspective of coil mutual inductance,the coupling mutual inductance and applicability of different space structure separate type probes are studied.At the same time,a kind of flat cut probe with self-adjusting zero characteristic is proposed,and its zero mutual inductance characteristic is verified and analyzed by finite element simulation.Based on the coupling impedance circuit model between the separated probe and the conductor,the coupling mutual inductance of the absolute value probe,T-R probe and the flat cut probe are compared.The zero mutual inductance characteristic of the flat cut probe is proved by the simulation,and the superiority of the flat cut probe to the eddy current detection of the weak conductive CFRP material is further illustrated.Thirdly,the C-scan imaging performance of the probe is studied based on the characteristics of the eddy field point diffusion function.Because the waveform characteristics of the detection signal are determined by the spatial structure and size of the probe coil,in order to obtain the high-resolution eddy current imaging of the CFRP plate,the concept of the point spread function is introduced firstly,and the point spread function of the separated probe with different spatial structures is studied,which verifies the superiority of the eddy current detection ability of the flat cut probe.Then,the two-dimensional plane scanning of multi-layer CFRP laminates is carried out by using eddy current probe,which verifies the high resolution of the flat cut probe to detect the CFRP laminates.At the same time,the finite element simulation model is used to study the layer selection characteristics of the flat cut probe,and the rotation detection of the orthogonal and four direction CFRP plates is carried out,which verifies the directivity detection ability of the flat cut probe to the CFRP laminated plates.Finally,based on the eddy current image and two-dimensional Fourier transform,a method for separating the fiber striations in unidirectional layers of multi-directional CFRP laminates is proposed,which lays a foundation for the quantitative analysis and evaluation of defects in CFRP materials in the future.