层状板中Lamb波的频散特性研究

利用矩阵递推方法，建立了层状板中Ｌａｍｂ波的特征方程以及相应的位移和应力分布计算公式，由此分析了双层板、软夹层板和硬夹层板中Ｌａｍｂ波的频散特性，特别指出软夹层的存在对Ｌａｍｂ波的频散特性有显著影响。该结论对工程测试分析有一定指导意义。     

Influences of Elastic and Viscous Moduli on Laser-Generated Lamb Waves in Viscoelastic Plates

The study of the propagation of laser-generated Lamb waves in viscoelastic plates, after taking into account the viscoelastic and thermophysical properties, is reported. Based on the thermoviscoelastic theory, a finite-element model for the laser-generated Lamb waves is developed in the frequency domain, and the temporal temperature and displacement waveforms are obtained by using inverse fast Fourier transforms. The influences of each elastic and viscous modulus $(C_{11}^{\prime },\; C_{22}^{\prime },\; C_{12}^{\prime },\; C_{66}^{\prime },\; C_{11}^{{\prime }{\prime }},\; C_{22}^{{\prime }{\prime }},\; C_{12}^{{\prime }{\prime }},$ and $C_{66}^{{\prime }{\prime }})$ on the propagation of the laser-generated Lamb waves (mainly for the $S_{0}$ and $A_{0}$ modes) are investigated in detail. The numerical results show that the attenuation of the $S_{0}$ mode is mainly determined by $C_{22}^{\prime },\; C_{12}^{\prime },\; C_{11}^{{\prime }{\prime }}$ , and $C_{12}^{{\prime }{\prime }}$ , and is independent of $C_{66}^{\prime }$ and $C_{66}^{{\prime }{\prime }}$ . In addition, the attenuation of the $A_{0}$ mode is more influenced by $C_{11}^{\prime },\; C_{22}^{\prime },\; C_{11}^{{\prime }{\prime }},$ and $C_{12}^{{\prime }{\prime }}$ . The finite-element method in the frequency domain provides a useful tool to investigate the mechanical properties of the viscoelastic plates.     

Laser-Generated Lamb Waves Propagation in Multilayered Plates Composed of Viscoelastic Fiber-reinforced Composite Materials

The propagation characteristics of laser-generated Lamb waves in multilayered fiber-reinforced composite plates with different fiber orientations and number of layers have been investigated quantitatively. Considering the viscoelasticity of the composite materials, we have set up finite element models for simulating the laser-generated Lamb waves in two types of the multilayered composite plates. In the first type, different fiber orientations are adopted. In the second one, different number of layers are considered. The results illustrate the occurrence of attenuation and dispersion, which is induced by the viscoelasticity and multilayer structure, respectively.     

基于三维弹性理论的碳纤维板中Lamb波建模

基于主动Lamb波的结构健康监测是目前复合材料结构损伤监测技术研究的热点之一,了解Lamb波的传播特性对进行可靠的损伤监测非常重要.本文结合经典三维弹性理论与Lamb波的运动位移方程,对碳纤维复合材料板中传播的Lamb波传播特性进行了建模研究,在此基础上推导了碳纤维板的相速度频散曲线,并讨论了Lamb波传播方向与坐标轴之间的夹角及碳纤维铺层方向对频散曲线的影响,建模结果证明了这种建模方法的正确性.     

Lamb Waves for Damage Localisation in Panels

Abstract: In this study, Lamb wave propagation phenomena have been used to localise discontinuities in aluminium and composite panels. For that purpose, piezoelectric transducers have been used to excite and register Lamb waves in the panels. Excited waves that propagate and reflect from the panel edges and discontinuities were registered by piezoelectric sensors as voltage changes in time. Three different experiments have been conducted to investigate the wave propagation phenomena. In the first experiment, the attenuation and dispersion of Lamb waves in the aluminium panel has been studied. The second experiment has been oriented to localise an additional mass attached to the same panel. A dedicated signal‐processing algorithm has been developed to aid the localisation procedure that allowed to extract relevant features of the discontinuity. The algorithm has been tested on signals from the panel with and without the additional mass. The results obtained have been compared to find the position of the discontinuity. The third experiment concerned a more realistic damage scenario when piezoelectric transducers have been arranged as a clock‐like array to find a crack in a composite panel.     

Long Range Detection of Defects in Composite Plates Using Lamb Waves Generated and Detected by Ultrasonic Phased Array Probes

This article presents a technique for the generation and detection of Lamb waves guided along large plate-like structures made from various types of materials (metal, polymer, fibre-reinforced composite, etc.). A multi-element matrix ultrasonic probe is driven using the well-known phased array principle, for launching and detecting pure Lamb modes in/from specific directions along the plate, which are arbitrary for isotropic materials and limited to specific directions for anisotropic materials, e.g. principal directions or directions for which both phase and group velocities are collinear. The probe is gel-coupled to the tested specimen and allows quick inspection of large area from its fixed position, even of zones with limited access. The technique, which takes into account the frequency dispersive effects, is different than SHM-like (Structural Health Monitoring) inspection, since all transmitting or receiving elements are grouped together in a localized area defined by the active surface of the probe, and not permanently attached to the tested structure. The use of a multi-element probe, for long range Lamb waves-based inspection, is also distinctive from that usually performed, which consists of very local inspection of a material by steering the ultrasonic beam below and nearby the probe. A prototype is presented, as well as measurements of its performances in terms of modal selectivity and directivity. Finally the detection and localisation of a through-thickness hole in a large aluminium plate, of a delamination-like defect in a carbon epoxy composite plate and of an impact damage on a stiffened composite curved plate are shown.     

Frequency Dependence of Second-Harmonic Generation in Lamb Waves

The frequency dependence of the second-harmonic generation in Lamb waves is studied theoretically and numerically in order to examine the role of phase matching for sensitive evaluation of material nonlinearity. Nonlinear Lamb wave propagation in an isotropic plate is analyzed using the perturbation technique and the modal decomposition in the neighborhood of a typical frequency satisfying the phase matching. The results show that the ratio of the amplitude of second-harmonic Lamb mode to the squared amplitude of fundamental Lamb mode grows cumulatively in a certain range of fundamental frequency for a finite propagation distance. It is also shown that the frequency for which this ratio reaches maximum is close but not equal to the phase-matching frequency when the propagation distance is finite. This feature is confirmed numerically using the finite-difference time-domain method incorporating material and geometrical nonlinearities. The fact that the amplitude of second-harmonic mode becomes high in a finite range of fundamental frequency proves robustness of the material evaluation method using second harmonics in Lamb waves.     

杂质吸收对一维掺杂声子晶体缺陷模的影响

为了研究杂质吸收对一维掺杂声子晶体缺陷模的影响,引入复波数,推导出一维掺杂声子晶体的转移矩阵,计算了一维掺杂声子晶体的透射系数和反射系数随衰减系数的变化特征.得出杂质的衰减系数对一维掺杂声子晶体透射波中和反射波中的缺陷模都有显著的影响.在透射波中,随着衰减系数的增加缺陷模峰高迅速降低,κ在0～0.002k范围内缺陷模比较明显.当衰减系数增加到0.005k时,缺陷模几乎消失.反射波中的缺陷模随着衰减系数的变化也有相同的特征.     

多模式Lamb波在层合板检测中的应用研究

以层合板复合材料为实验对象,采用超声Lamb波为手段,通过检测信号的时频分析,有效确定缺陷类型对Lamb波在复合材料中传播的影响.     

基于遗传算法的声子晶体梁振动传输特性优化设计

以工程中常用的梁类结构为背景,研究了利用声子晶体带隙来抑制梁中振动传播的设计方法,在考虑附加质量、振幅等约束条件下,用遗传算法来优化设计声子晶体梁的力学参数,从而实现"按需设计".基于传递矩阵法建立了局域共振声子晶体梁振动传输特性的计算方法,以某频段内的插入损失最大为优化目标,通过建立有效的惩罚函数并选择合适的遗传运算策略,建立了考虑质量、振幅约束条件下局域振子参数的优化设计方法,实现了局域共振声子晶体梁振子参数的优化设计,并分析了优化结果的正确性.所提出的优化设计方法对于将声子晶体应用于抑制结构振动传播的工程实践具有重要的实用价值.     

Simulation and Experiments of Airborne Zero-Group-Velocity Lamb Waves in Concrete Plate

The impact-echo (IE) test method has been widely used to determine the thickness of a plate and to locate delaminations in reinforced-concrete structures. It was later realized that the IE mode is related to the Zero-Group-Velocity S1 Lamb mode (S₁ZGV). An efficient inspection of a structure using rapid, noncontact, air-coupled sensing technology is of great interest. The objective of this research is to investigate the effectiveness of air-coupled sensing to detect the ZGV Lamb mode signals. Numerical simulation and experimental verification were performed to study the in-air ZGV vibration responses induced by a point impact applied on a concrete plate. Two concrete plates, one without delamination and one with delamination, were tested and simulated. For each plate, the simulation results agreed well with the experimental results, and both methods estimated the plate thickness precisely. The numerical simulations also visualized the pressure field in the air and are helpful for interpretation of the experimental results. The numerical and experimental results suggest that the air-coupled sensing technology can serve as an effective and rapid inspection tool for large concrete structures.     

Some Experimental Observations on the Detection of Composite Damage using Lamb Waves

Abstract: This paper deals with the detection of damage in composite plates using Lamb wave scattering from faults and novelty detection. Although previous studies have established the feasibility of the approach, this paper addresses the problem of quantifying the detectable level of damage. In order to accomplish this, two test programmes were conducted. The first tests dealt with the detection of through holes in a composite laminate, while the second series was concerned with detecting internal damage generated by low‐velocity impacts. In both cases, the damages were detected successfully; however, the tests raise some issues concerning the stability of the environment of the test and the associated instrumentation, and these issues were investigated further.     

Superconducting Point Defect in a Two-Dimensional Photonic Crystal

This paper presents the tunability of the high temperature superconducting (SC) point defect modes in a two-dimensional photonic crystal composed of circular cylinders on a square lattice. By some modifications on the plane wave expansion method, we have shown that the properties of the SC point defect modes strongly depend on the temperature and external magnetic fields. The superconductor type and the nearest-neighbor of the defect effects are further investigated. Comparison between SC and conventional defects show that the SC one has only a monopole profile for different values of the defect size, and has very low sensitivity on the disorders, which is an important feature from a manufacturing point of view.     

固-液结构平板掺杂声子晶体中弹性波的缺陷模

利用一维固-液结构平板掺杂声子晶体中弹性波横向受限的条件推导出弹性波在一维固-液结构平板掺杂声子晶体中各个模式满足的关系式,并利用关系式研究了各个模式弹性波的特性。利用转移矩阵计算出弹性波各个模式的缺陷模随模式量子数以及平板厚度的变化特征,得出了一些一维固-液结构平板掺杂声子晶体缺陷模的新特征。     

Three-Dimensional Magnetostatic Waves Caused by Electromagnetic Fields in Metallized Ferrite Plates

Materials Science - By using a well-known technique for finding the characteristics of magnetic fields based on the method of decomposition of unknown quantities in a series in small parameter...     

Lamb Wave Phase Velocity Imaging of Concrete Plates with 2D Arrays

In the nondestructive evaluation of concrete structures, ultrasonic techniques are considered to be more capable than low-frequency techniques such as the impact-echo method. This is especially true with the recent development of ultrasonic transducers, synthetic apertures, and results in an image form, and because low-frequency techniques are usually limited in their evaluation to the frequency of one single resonant mode. With the aim of reducing this gap in capabilities, we present a 2D array and wide-frequency bandwidth technique for Lamb wave phase velocity imaging. The presentation involves a measurement on a newly cast concrete plate using a hammer and an accelerometer as an example. The key concept of the technique is the use of 2D arrays that record a full wave field response over a limited surface subdomain within the complete measurement domain. Through a discrete Fourier transform, a spectral estimate is obtained for the 2D array in the frequency-phase velocity domain. The variation of the phase velocity is then mapped using a stepwise movement of the 2D array within the complete measurement domain. With two different types of 2D arrays, the variation of the phase velocity for the A0 Lamb mode is mapped and displayed in a polar and image plot, and low variation is observed for both cases. This result verifies the expected condition of a homogenous material and plate thickness and, more importantly, highlights the potential of wide-frequency bandwidth techniques based on full wave field data.     

Computational Characterization of Adhesive Layer Properties Using Guided Waves in Bonded Plates

This research develops a guided waves technique to nondestructively characterize the stiffness properties of bonded engineering components. This study first quantifies the influence of the relevant adhesive layer properties—Young’s modulus, Poisson’s ratio and bond thickness—on the dispersion curves of a two-layer bonded system, an aluminum plate with an adhesive tape layer bonded to its lower surface. Both a commercial finite element (FE) code (ABAQUS/Explicit) and the global matrix method (GMM) are used to determine the dispersion relationships of this bonded plate system in the form of frequency-wavenumber and slowness-frequency relations. These dispersion curves are then used to determine a set of adhesive tape parameter sensitive points, whose frequency coordinates represent the solution criteria for a proposed inversion procedure. This inversion is based on the GMM and assumes the three adhesive tape properties are unknown. The performance of this inversion procedure depends on the number of input time-domain signals; it is possible to solve the inverse problem for all three of the unknown adhesive tape properties if multiple input signals are known.     

基于主动Lamb波和小波变换的二维结构损伤定位研究

基于主动监测技术，将结构健康和损伤状态下的Lamb波激励响应信号进行对比，两者的差信号实际上是激励信号通过损伤折射后被接收到的类似声发射源产生的一种信号。该信号与激励响应信号比较将有时间延迟，通过Gabor小波变换确定时间延迟，从而获取损伤位置信息。实验证实这种位置信息是可靠的。