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Abstract In this study the nonlocal bond-based peridynamic (PD) theory was applied to simulate Lamb wave transmission in 2D bimaterial plates. Plates of unlike materials were jointed end-to-end to make a bimaterial plate. The surface of the plate was then excited tangentially by single-frequency and multi-frequency force signals. The influence of changing material properties on travelling of the symmetric and antisymmetric Lamb waves were studied in detail. The phase and group velocities of travelling wave in bimaterial plates were calculated and it was found that dissimilar material properties of two joint plates can significantly alter the amplitude and arrival time of the wave. Moreover, Fast Fourier Transform (FFT) of the time history of symmetric Lamb wave velocity was calculated and verified. Accuracy and consistency of PD wave model was confirmed entirely using Spectral Finite Element Method (SFEM). The comparison between the results of two applied methods shows a good agreement. [ABSTRACT FROM AUTHOR]

Abstract Scanning laser Doppler vibrometry (SLDV) is a promising method for visualizing defects in plate-like structures. The Lamb wave-based SLDV method using continuous excitations has been recently developed in nondestructive evaluation (NDE). In addition, various signal processing techniques have been developed to enhance damage detection capability for the method. To visualize defects, the A 0 mode is commonly used because of its sensitivity to the thickness variations, i.e. the wave speed changes according to the thickness of a plate. The A 0 mode is easily generated by lead zirconate titanate (PZT) transducers. However, for thick plates, the A 0 mode is found to be less sensitive to thickness variations compared to thin structures. To address this issue, a mode-tuned inter-digital transducer (IDT) is adopted to efficiently generate and utilize the symmetric modes for damage detection and visualization of thick plates. Based on the wavelength of the S 0 mode using the dispersion curve of the plate, the IDT is designed and fabricated for efficient symmetric mode generation. To evaluate the capability of the mode-tuned IDT-based SLDV, a 6-mm-thick plate (40 mm × 40 mm) with defects of 0.3 mm, 0.6 mm, 0.9 mm, and 1.2 mm depths is used. The S 0 mode is selected for its wavenumber sensitivity and the degree of separation between modes. It is demonstrated that the detectability of shallow defects in a thick plate is improved compared to that when using the A 0 mode. [ABSTRACT FROM AUTHOR]

The transmission behavior of Lamb waves and the possible occurrence of resonance at an adhesive butt joint of plates are studied experimentally. To this purpose, two 2.5-mm thick aluminum alloy plates are bonded at their edges using cyanoacrylate-based adhesive. Bonded plate specimens with different joint conditions are prepared by changing the bonding procedure. The measurements are performed for the transmission characteristics of the lowest-order symmetric (S0) and antisymmetric (A0) Lamb modes for the frequency range of 0.4–0.6 MHz below the cut-off frequency of the higher-order modes. The experimental results show that the transmission coefficients of the S0 and A0 modes exhibit different frequency-dependent characteristics depending on the joint condition. Furthermore, for the incidence of the S0 mode at the center frequency of 1 MHz, the transmitted S0 mode in weakly bonded specimens shows a long oscillation tail due to the resonance effect. The experimental results are discussed in the light of the theoretical results based on the spring-type interface model. The interfacial stiffnesses identified from the transmission coefficients are shown to be correlated with the bonding condition of the joint and give reasonable estimates of the resonance frequencies of weakly bonded specimens. [ABSTRACT FROM AUTHOR]

Effective application of the Lamb waves for structural health monitoring and damage identification intensively relies on the accurate damage-related feature extraction in the received signals. Most of existing signal processing methods extract the damage-related features from the time–frequency joint spectrum which requires a quite amount of effort. In this paper, the soft-thresholding process, based on different signal decomposition methods, is introduced to damage identification so that the damage-related signal features can be manifested more distinctively. By applying two popular signal decomposition methods (i.e., the discrete wavelet transform (DWT) and the empirical mode decomposition (EMD)), the signal of interest can be represented by a series of components with different frequencies. Since most noises exist in the high frequency range, it is feasible to alleviate noise by restricting the energy of high-frequency components. Finally, a denoised signal is synthesized using the corresponding reconstruction method. As an application, the soft-thresholding process is performed to detect a small crack on an isotropic aluminum plate under the white Gaussian noise contamination. The results, from both the numerical finite element simulation and experimental test, indicate that the soft-thresholding process is capable of effectively reducing the effect of noise, convincingly improving the sensitivity of damage identification, and discriminating relatively small damage. [ABSTRACT FROM AUTHOR]

Structures with composite materials in daily use are subjected to impact. Determining if the material is damaged is an important aspect of structural health monitoring of composites. In this paper, a method to determine structural damage as a result of impact and localize the position of the damage based on lamb wave mode is presented. First, the paper analyses the characteristics of lamb wave mode generated by force loading on the plate. These results are validated based on the analysis of experimental results. From the results, it can be concluded that S 0 mode lamb waves were generated by damage of the plate structure and it can be used as the characteristic signal of damage. Then, empirical mode decomposition (EMD) is utilized to decompose to the Intrinsic Mode Functions (IMF) of S 0 mode inside the emission signals of complex acoustics to realize if damage occurs and localize the position of impact damage on the composite material plate. [ABSTRACT FROM AUTHOR]