Research On Nonlinear Ultrasonic Guided Wave Full Path Imaging Method For Anisotropic CFRP Structural Damage

Anisotropic carbon fiber reinforced polymer(CFRP)is widely used in engineering structures because of its excellent weight reduction function while ensuring material performance.Due to the different properties,damage is easy to occur inside,which seriously damages the integrity of the structure.The propagation and accumulation of damage occupy most of the time from damage appearance to structural failure.Early detection of damage is of great significance to the safe operation of structures and the maximization of resource utilization.Nonlinear ultrasonic guided wave is very sensitive to the micro damage or state change in materials through the interaction of sound wave and micro damage,which is conducive to early structural damage detection.However,the nonlinear component is very weak,and the omni-directional velocity of anisotropic CFRP structure is different in all directions,which brings challenges to the imaging detection of anisotropic CFRP structure damage.Probabilistic imaging method can obtain high-quality damage imaging effect through a sparse sensor networks,which provides a good opportunity for the damage imaging detection of nonlinear ultrasonic guided wave.However,the weak nonlinear component leads to large fluctuation of damage features,and the robustness of imaging with partial paths is poor.Therefore,aiming at the problem of accurate detection and location imaging of the early damage in anisotropic CFRP structure,this thesis focuses on the full path non-elliptical probability imaging(FPNEPI)method of nonlinear ultrasonic guided wave.The main research work of this thesis is as follows:(1)In order to solve the problem that it is difficult to obtain the omni-directional velocity of anisotropic CFRP structure due to the different omni-directional velocity,a method of multi-angle fitting extracting omni-directional velocity by combine the theoretical dispersion curve and numerical simulation is proposed.The transformation mechanism of the stiffness coefficient matrix of anisotropic CFRP structure is derived,and the omni-directional modal propagation characteristics are further studied.The omni-directional safe solution method of anisotropic CFRP structure is deduced,and the dispersion curve is drawn.By combining the dispersion curve with the simulation of multi angle anisotropic CFRP structure,the omni-directional guided wave velocity of anisotropic CFRP structure is obtained by one-time fitting,which effectively reveals the azimuth-velocity characteristics of CFRP structure,and lays the foundation for the subsequent nonlinear ultrasonic guided wave imaging detection of CFRP structure.(2)Aiming at the problem that the nonlinear components of guided wave signal are very weak,which makes it difficult to extract nonlinear features,a nonlinear WP-SPWVD damage quantitative characterization method is proposed.Firstly,a new method is proposed to determine the starting point of the signal according to the intersection of the data envelop fitting lines,which improves and enriches the acquisition method of the start and end time of the guided wave signal.Then,the reconstructed nonlinear component is obtained by reconstructing the fundamental frequency component and nonlinear harmonic component in the guided wave signal.For crack damage,the reconstructed nonlinear components are obtained by wavelet packet decomposition,and the nonlinear scattering damage index(NSDI)is defined to evaluate the degree of damage to different paths.For the delamination damage,the nonlinear WP-SPWVD method combining the reconstructed nonlinear component based on wavelet packet with SPWVD is proposed,and the nonlinear damage index(NDI)is defined.The quantitative characterization of anisotropic CFRP structure damage is realized.(3)Aiming at the problem that there is no analytical solution for the imaging process of anisotropic CFRP structure and it is difficult to achieve accurate damage location imaging,a full path non-elliptic probability imaging method based on nonlinear ultrasonic guided wave is proposed.By introducing the reference point,defining the time coefficient and comparing the actual damage time difference with the reference point,the analytical solution process of damage location is avoided.The nonlinear damage index is combined to estimate the damage probability of the discrete reference point.The scale control coefficient is embedded in the probability distribution function to enhance the robustness of the algorithm.Finally,the accurate location and imaging of the anisotropic CFRP structuren are realized.The validity and accuracy of the FPNEPI method are verified by delamination and crack damage simulation.The results show that the proposed full path non-elliptic probability imaging can intuitively and accurately display the damage information of anisotropic CFRP structures.(4)Aiming at the validity and application of nonlinear ultrasonic guided wave FPNEPI method,a series detection and application experiments of anisotropic CFRP structural damage were carried out.The experimental results show that NSDI can effectively detect crack damage.Compared with LSDI,the damage probability energy E_r of threshold imaging using NSDI is increased by 4.06%.NDI based on nonlinear WP-SPWVD method has a good detection effect on layered damage.The delamination damage is in the area with maximum damage probability,and the threshold imaging E_r reaches 75.94%,which is 70.64%higher than that of EPI(Elliptical Probability Imaging).Compared with FFT(fast Fourier transform),STFT(short time Fourier transform)and ST(S-transformation),the threshold imaging E_r of the proposed nonlinear WP-SPWVD method in improved by at least 32.42%,which indicates the reliability of the nonlinear WP-SPWVD method.The application of nonlinear WP-SPWVD method in delamination damage detection of anisotropic CFRP structure of marine rim-driven thruster shows the effectiveness and universality of FPNEPI method for anisotropic CFRP structure damage detection.