The Measurement And Quantitative Characterization Of Nonlinear Lamb Wave

This thesis aims at the low sensitivity problem in the early micro damage of linear Lamb wave monitoring structure, by conducting nonlinear Lamb wave basic propagation theory, time-frequency signal processing technology and nonlinear Lamb wave test system and so on several aspects of the study of the problem. The nonlinear Lamb wave method and small piezoelectric incentive and sensor model is applied to the structure of the aluminum plate in the early micro cracks and fatigue damage,which provides a new train of thought and method to Lamb wave Structural Health Monitoring technology. The practicality of this technology is expected to its promoting process. The full text of the main research contents are as follows:First of all, it studies the nonlinear Lamb wave basic propagation theory, by solving wave equation to prove nonlinear second harmonic source and the dispersive equation of Lamb wave. It chooses a cumulative effect(S1, S2) symmetry model to detect the nonlinear properties, and material nonlinearity characteristic parameters are presented.Secondly, the nonlinear Lamb wave testing system is designed.The main work content is the charge amplifier design, the bandwidth reaches 3 MHz, magnification reachs 300 times, and it meets nonlinear signal amplification processing.Thirdly, it studies the nonlinear Lamb wave signal extracting. The filter processing technology, short time Fourier transform(STFT) and wavelet transform time-frequency processing methods were studied.The experimental results proved the effectiveness of this method which were applied to extract the fundamental frequency and the double frequency amplitude.Finally, the experiment research was performed to the early cracks and fatigue damage of the structure respectively, the results show nonlinear Lamb wave signals and characteristic parameters are sensitive to structural material nonlinearity change, and validated the feasibility and effectiveness of the nonlinear Lamb wave method to early structure fatigue damage monitoring.The results show that the nonlinear Lamb wave is sensitive to the change of material nonlinearity, while the sensitivity of existing Lamb wave monitoring technology based on linear signal is very low.This research provides a feasible method to the structure fatigue, micro cracks and other early micro damage of online monitoring.