New Frontiers in Guided Wave Excitation and Manipulation

An axisymmetric guided wave mode is excited independently within a circular tube structure to reduce the complexity of signal interpretation through the prevention of unwanted wave polarisations and reflections. However, it is difficult to use the axisymmetric guided wave to determine the circumferential position and coverage of a defect within the circular tube structure. Non-axisymmetric guided waves can be used to mitigate the limitation of the axisymmetric guided wave through the adoption of a partially covered transducer design and analysis of the propagation characteristics. The partial excitation of non-axisymmetric guided waves can facilitate the arrangement of a transducer during defect detection. This paper reviews state-of-the-art research on non-axisymmetric guided waves for determining the axial positions, circumferential positions, and circumferential lengths of defects. First, the fundamental analysis of a specific non-axisymmetric guided wave mode based on the normal mode expansion method and beam directivity analysis method reveals that the propagation characteristics of the wave mode are closely related to the working principle and configuration of the corresponding transducer. Then, the advantages and disadvantages of the different types of transducers and transducer arrays for the excitation of non-axisymmetric guided waves are introduced and discussed. Finally, the current defect detection methods based on non-axisymmetric guided waves are discussed and summarised. This review can promote the application of non-axisymmetric guided waves in defect detection.

Guided waves, elastic waves propagating through bounded structures, play a pivotal role in various applications, including ultrasonic non-destructive testing and structural health monitoring. Recently, elastic metamaterials artificially engineered to exhibit physical properties not typically seen in nature have emerged as a ground-breaking approach, heralding a new era in guided wave-based technologies. These metamaterials offer innovative solutions to overcome the inherent constraints of traditional guided wave-based technology. This paper comprehensively reviews elastic metamaterials from their fundamental principles to diverse applications, focusing on their transformative impact in guided wave manipulation.

We report a numerical study of a reconfigurable topological waveguide based on honeycomb-lattice elastic phononic crystals (EPCs) which consist of two kinds of cavities filled with water. We can realize the EPCs with different symmetries by adjusting the water depth of the cavities, and obtain a Dirac cone for the EPCs composed of the cavities with the same water depth, in which the Dirac frequency can be modulated by adjusting the water depth. When the water depths of the cavities are different, the inversion symmetry of the EPC is broken, destroying the two-fold degeneracy of the Dirac point, and opening an omnidirectional bandgap. Based on EPC-I and EPC-II with opposite valley Hall phases, we design a valley topological waveguide of elastic wave, and obtain valley edge states in the domain wall (DW). Importantly, by adjusting the water depths, we can achieve the conversion between EPC-I and EPC-II, and realize arbitrary DWs for the propagations of elastic waves in the topological waveguide. Finally, we discuss an interesting application of a path-selective waveguide based on a linear interference mechanism. The designed reconfigurable topological waveguide provides an effective method to manipulate valley topological transports of elastic waves, and a theoretical basis for designing advanced topological devices.

Phased arrays have been a cornerstone of non-destructive evaluation, sonar communications, and medical imaging for years. Conventional arrays work by imparting a static phase gradient across a set of transducers to steer a self-created wavefront in a desired direction. Most recently, space-time-periodic (STP) phased arrays have been explored in the context of multi-harmonic wave beaming. Owing to the STP phase profile, multiple scattered harmonics of a single-frequency input are generated which propagate simultaneously in different directional lanes. Each of these lanes is characterized by a principal angle and a distinct frequency signature that can be computationally predicted. However, owing to the Hermitian (real) nature of the spatiotemporal phase gradient, waves emergent from the array are still bound to propagate simultaneously along up- and down-converted directions with a perfectly symmetric energy distribution. Seeking to push this boundary, this paper presents a class of non-Hermitian STP phased arrays which exercise a degree of unprecedented control over the transmitted waves through an interplay between gain, loss, and coupling between its individual components. A complex phase profile under two special symmetries, parity-time (PT) and anti-PT, is introduced that enables the modulation of the amplitude of various harmonics and decouples up- and down-converted harmonics of the same order. We show that these arrays provide on-demand suppression of either up- or down-converted harmonics at an exceptional point—a degeneracy in the parameter space where the system's eigenvalues and eigenvectors coalesce. An experimental prototype of the non-Hermitian array is constructed to illustrate the selective directional suppression via time-transient measurements of the out-of-plane displacements of an elastic substrate via laser vibrometry. The theory of non-Hermitian phased arrays and their experimental realization unlock rich opportunities in precise elastoacoustic wave manipulation that can be tailored for a diverse range of engineering applications.

To experimentally measure the complicated vibration and wave characteristics of a shell, a 3D scanning laser Doppler vibrometer is a competent but costly instrument due to the requirement of exactly aligning each point when scanning the shell. Here we propose a simplified measuring method just by utilizing a single-point laser vibrometer fixed on a motorized positioning system. The clamp can be rotated to adjust the incident angles and translated to capture the whole tested region. During each test in a specific incident angle, the signals are interpolated to generate a continuous wave field in both the time domain and the frequency domain, eliminating the need for alignment. The in-plane and out-of-plane wave fields are obtained from the measured 3D signal using the projection relationship, and then verified both experimentally and numerically. Furthermore, we show that the present method can be used to test complex wave fields, such as the scattering field by obstacles on a cylindrical shell. The present work may stimulate systematically experimental studies on the wave propagation and vibration on shells.

The influence of size effects on one-dimensional defective phononic crystal (PnC) sensors based on simplified strain gradient elasticity theory (SSGET) is studied in this paper. PnCs have been widely used in high-sensitivity gas and liquid sensors by introducing defects to disrupt the perfect PnC modes. In comparison with classical elasticity theory, the SSGET includes two microstructure-related material parameters that can accurately reflect the size effects of the structure. In this paper, the stiffness matrix method was used to calculate the transmission coefficients of the proposed model, avoiding the numerical instability of the transfer matrix method. The results show that the size effects at the microscale affect the perfect PnC bandgap's frequency range, and the microstructure constants impress the resonant frequency while detecting liquids. Consequently, the accuracy of the sensor is reduced. These findings provide a theoretical basis for designing microscale PnC sensors.

We propose a reconfigurable phononic crystal (PnC) for detecting the concentration of solutes in liquids. The designed PnC consists of liquid-filled hollow pillars and connecting bars. The finite element method is used to calculate the transmission spectra and band structures of PnC filled with various liquids. We fabricate 3D printed samples and conduct corresponding experiments. The results show that sound velocity is the key parameter affecting the frequency of the passing band. As the sound velocity increases, the resonance frequency shifts down. For both NaCl solution and ethanol solution, good linear relationships between the resonance frequency and liquid concentration are established. Experimental results show good agreement with simulations, and stable detection capabilities are maintained in the presence of interference. The impact of fabrication tolerances on sensor performance has also been discussed, with a greater impact on sensitivity and a smaller impact on Q-factor. The reconfigurability also shows the potential of the design of multi-liquid PnC sensors.

This investigation addresses the issue of damage detection and localization in wind turbine blade laminates. This paper proposes a novel approach that integrates the elliptical trajectory and probabilistic imaging method using the Bayesian framework. This method employs multiple damage-sensitive features to enhance the reliability and robustness of sensor arrays. The algorithm is optimized by analyzing the propagation characteristics of Lamb waves in composite blade laminates. A numerical simulation is conducted on a 1.5 MW wind turbine blade laminate model, incorporating the scattered wave signal, wave arrival time, and correlation coefficient as damage characteristic signals. Markov Chain Monte Carlo sampling method is adopted to obtain the posterior distribution of the damage location and achieve accurate localization of blade delamination damage. The experimental results indicate that the damage localization algorithm, which utilizes the Bayesian approach, achieves an accuracy of approximately 97.04% in localizing delamination damage in blade laminates.