| In this study, the improved active Lamb wave propagation-based damage identification methods are developed for metallic and composite laminated plates. Existing studies are first reviewed followed by a comprehensive study.;Second, a damping boundary is developed in both the numerical finite element modeling and experimental implementation to absorb the Lamb wave reflection at plate edges. Based on the purified wave field, the Lamb wave propagation can be accurately characterized and easily interpreted, through different time frequency spectral analysis methods.;Third, a signal denoising method based on the soft-thresholding of decomposed Lamb wave components is presented. By applying two representative signal decomposition methods, the signal of interest is decomposed into a series of components with different frequencies, and the noise is alleviated by restricting the energy of high-frequency components. The results show that the denoised signals present damage related wave features more distinctively and improve the damage identification accuracy.;Fourth, an enhanced single transmitter and multi-receiver (ESTMR) array is proposed. Structural responses collected by various sensing points in ESTMR are shifted and summed to generate a time-velocity spectrum where the forward and backward waves can be extracted. Since the first backward wave indicates the reflection from damage, the damage can be located by multiplying the corresponding Lamb wave group velocity with the time-of-flight. Three baseline-free quantification parameters using the pixel values of the reconstructed damage image manifest a convincing improvement of ESTMR over the classic single transmitter and multi-receiver array.;Finally, a new damage identification method considering hybrid wave components is proposed to monitor small-sized non-penetrating damages, and a damage index is developed by windowing the Hilbert-Huang spectrum. The numerical and experimental results demonstrate that the hybrid wave components-based damage identification method has higher damage sensitivity and noise immunity than the one based on the single wave component.;In summary, different improved methods of damping boundary, signal denoising, enhanced sensor array, and hybrid wave components are presented for active Lamb wave propagation-based damage identification of plate-like structures, and they contribute to better detecting damage in structures and promoting the wave propagation-based methods in real time structural health monitoring. |
