Research On Eddy Current Distribution And Defect Detection Of CFRP

Carbon fiber reinforced polymer(CFRP)has been widely used because of its excellent properties.Nondestructive testing of CFRP components is necessary for ensuring their integrity and reliability.Carbon fiber is conductive,which makes it possible to test CFRP by the eddy current testing(ECT).Finding out the distribution of eddy current(EC)in CFRP and revealing the underlying mechanism are the basis for better studying the ECT of CFRP.Based on the previous studies,the laws of EC distribution in isotropic and anisotropic materials under the excitation of the pancake coil are further ascertained:with the increase of the conductivity of the conductor,the EC in the conductor is more and more concentrated below the winding of the excitation coil;the in-plane EC distribution in unidirectional CFRP is sparse in the fiber direction;for larger conductor conductivity,the rate of EC attenuation under the coil excitation is closer to that under the uniform-field excitation.the EC attenuation in the anisotropic conductor is faster than that in isotropic ones.The underlying mechanisms of the above laws and the previous findings in our group(the attenuation of EC in anisotropic material is faster than that in isotropic one;the EC density in orthogonal CFRP increases around the interface)are studied.A three-coils model is proposed to reveal the mechanism of the in-plane EC distribution.The EC diffusion coefficient and skin depth correction coefficient are proposed,and the EC diffusion-cancellation theory is used to reveal the mechanism of EC attenuation in unidirectional CFRP.By analyzing the influence of conductivity on the electric field,the mechanism of EC attenuation in orthogonal CFRP is revealed.In this dissertation,the laws about EC attenuation in isotropic and anisotropic plates under the excitation of the rectangular coil are found:the attenuation of EC under the excitation of the rectangular coil is faster than that under the excitation of the uniform field;the attenuation of EC in anisotropic material is faster than that in isotropic one;the EC density in orthogonal CFRP increases around the interface.The underlying mechanisms of the above laws are revealed by using the EC diffusion-cancellation theory and analyzing the influence of conductivity on the electric field.The influences of the geometric parameters of the rectangular coil on EC attenuation and the underlying mechanism are studied.Fiber waviness and error of ply angle are two kinds of defects easily produced in the manufacturing process,which will greatly weaken the compressive strength of the CFRP.This dissertation proposes to use the phase of the output voltage of the orthogonal coils to characterize fiber angle,and the study of detecting fiber waviness in the prepreg,unidirectional CFRP,and orthogonal CFRP are performed by simulations and experiments.The results show that the proposed method can be used to characterize fiber waviness,and the probe is able to distinguish fiber directions as small as 0.5°.The dissertation also uses the phase of the output voltage of the orthogonal coils to detect error of ply angle in CFRP.The results show that error of ply angle can be identified by the peak-to-peak value of the curve of the phase signal obtained via rotating the probe.