Research On Debonding Damage Identification Method Of Composite Plates Based On Piezoelectric Nonlinear High Order Harmonics

Composite plate is a very important structural component.Engineering accidents and economic losses might occur due to damages of composite plates.The most common damage form of composite plates is the debonding-induced delamination.The main reason for the delamination of composite plates is the formation and development of interlayered microcracks.Therefore,it is of great significance to accurately detect the interlayered delamination of composite plates.In this dissertation,the comprehensive method of theoretical analysis,finite element simulation and verification test is used to detect the interlayered delamination of composite plates based on the piezoelectric nonlinear ultrasonic Lamb waves,aiming at clarifying the mechanism of the second harmonic generation induced by the microdelamination of composite plates,and proposing a delamination identification method based on the nonlinear coefficient of sensing signals.The main contents of this work are as follows:(1)The theoretical method is used to study the source of the second harmonic in thin plates.Firstly,through one-dimensional wave equations and a nonlinear material constitutive relation of thin plate structure,the second order nonlinear coefficient expression is deduced,and the relationship between the nonlinear coefficients and sensing signals is established;Secondly,the physical meaning of the nonlinear coefficient is explained,and the corresponding relationship among the micro-delamination and nonlinear coefficients as well as the sensing signals is analyzed.Finally,from the microscopic point of view,the micro-delamination can be described by the nonlinear coefficients,and then the mechanism of producing the nonlinear contact sound effect is explained.(2)Based on the comprehensive method of theory and assignment calculation,the propagation behavior and law of Lamb waves in composite plates is studied.The dispersion equation of isotropic single-layer plate is derived,and the theoretical dispersion curve is drawn after assigning parameters.Similarly,theoretical dispersion curves of Lamb wave propagating in composite plates are drawn through the theoretical research,and the effectiveness of the proposed dispersion curves and finite element model(FEM)is verified by the compare of theoretical and numerical results.(3)The mechanism of the second harmonics induced by geometric characteristics of micro-delamination is studied.The effects of parameters,including the width and length of micro-delamination,excitation amplitudes,product of frequency and thickness,on nonlinear ultrasonic coefficients are analyzed through finite element numerical simulation.The results show that the micro-delamination induced second harmonic mechanism is regarded as delaminating interfacial collisions,meanwhile,interactions between Lamb waves and delamination boundary on a micro level.Because of mode transformations,the second harmonic component increases,and the corresponding nonlinear coefficient β also increases.In addition,the relationship between the maximum oscillation area of the delamination and the nonlinear coefficient β reflects a certain regularity from a macro view,that is,the larger the maximum oscillation area,the higher the value of the corresponding nonlinear coefficient β.It also increases accordingly.The micro-delamination identification method based on nonlinear coefficients of ultrasonic Lamb waves is feasible.(4)A second harmonic based micro-delamination identification method for composite plates is proposed and the relevant operation process is established.Based on the second harmonic generation mechanism,a kind of micro-delamination identification method based on nonlinear coefficients of sensing signals is proposed.The relevant basic operation process includes,firstly,scanning the damaged plate and selecting the appropriate excitation frequency;then,according to dimensions of the tested objective,selecting excitation amplitudes to ensure the micro-delamination interfacial effective oscillation and sensor’s effective reception;finally,evaluating the micro-delamination of composite plates through the analysis in frequency domain and the damage index.(5)The efficiency of the proposed micro-delamination identification method for composite plates is validated through the test.The micro-delamination identification test of composite plates based on the second harmonic method is performed,and the experimental results are compared with the simulation ones.The results show that the proposed damage identification method based on the finite element analysis is effective.The main innovations of this thesis include the following:(1)The mechanism of second harmonic induced by micro-delamination of composite plates is clarified;(2)The oscillation nonlinearity of the interfacial micro-delamination of composite plates is the main reason for generating the second harmonic;(3)The second-order nonlinear coefficient β of the sensing signals can effectively characterize the relationship between nonlinear stress waves and geometric characteristics of micro-delamination;(4)An interfacial micro-delamination identification method based on nonlinear coefficients is proposed.