The Investigations On Lamb Wave Testing Technique In Aluminum Plate And Nonlinear Chirplet Transform Time-frequency Analysis Method

The dispersion characteristic of Lamb wave is an important basis for testing. Determining and analyzing the signal accurately are the key problems to solve for promote the application of Lamb wave testing technique. Time-frequency analysis methods are the most effective ways to determine and explain dispersive Lamb waves. Based on the ultrasonic guide wave theory, time-frequency analysis methods are investigated in this work to characterize dispersive Lamb waves.Firstly, the process of solving Rayleigh-Lamb equation has been simplified. The Lamb wave dispersion curves in aluminum plate are plotted by numerical calculation method using Matlab software.Secondly, several traditional time-frequency analysis methods are introduced, such as STFT and CT. The dissertation deeply investigates on the novel parametric time-frequency analysis method——Nonlinear Chirplet Transform(NCT). By replacing the linear chirp kernel in CT by nonlinear polynomial kernel and choosing the proper kernel characteristics parameters, NCT can render a time frequency distribution of excellent concentration for signals whose instantaneous frequency is a nonlinear function of time. Besides, an algorithm is developed to estimate the parameters, and the effectiveness of the algorithm is validated by numerical simulation signals.Finally, time-frequency analysis methods are used to analyze the FEM simulation and experiment broad-bandwidth Lamb wave signals. Simulation and experiment results meet a good agreement with theoretical numerical results respectively. The dissertation gives time frequency distributions of a0 and s0 mode Lamb wave over 1MHz frequency range including STFT, CT and NCT. By using theoretical numerical results and irritation algorithm in which time-frequency analysis regions are manually selected to estimate kernel characteristic parameters, NCT can accurately reflect Lamb waves signals whose instantaneous frequency is a nonlinear function of time. The study demonstrates that NCT is effective in characterizing multi-mode, dispersive Lamb waves.