复合材料无损检测中Lamb波的优化

如何解决Lamb波检测机理的复杂性并将之运用于复合材料损伤检测是个重要课题.以玻璃纤维增强的复合材料层合板为研究对象,搭建试验平台,利用集成在复合材料层合板上的PZT压电陶瓷片作为驱动元件和传感元件,对接收的Lamb波信号进行分析与研究,从信号频率范围、信号中心频率、波振幅值、波形个数和信号激励形式等五个方面完成Lam...     

基于概率损伤算法的复合材料板空气耦合Lamb波扫描成像

采用非接触空耦传感器在准各向同性复合材料板中激励出单一的Lamb波模态,用于分层缺陷的扫描检测。扫描时,激励和接收传感器置于复合材料板同侧并相对倾斜布置,传感器沿2个正交方向同步线性扫描,得到不同位置的检测信号。对不同扫描路径下的检测信号进行连续小波变换,提取激励频率下的小波系数包络信号,对分层缺陷进行成像。在此基础上,利用概率损伤算法定义损伤指数,结合不同方向的损伤指数实现分层缺陷成像。采用全加法和全乘法对2个正交扫描方向得到的成像结果进行数据融合,实现了分层缺陷的定位和重构。并在成像算法中引入阈值,进一步提高了分层缺陷的定位精度以及重构质量。     

《基于谱元法的梁结构中Lamb波传播特性研究》

把谱元法应用于结构中弹性波的传播的模拟,建立基于二维谱元法的梁结构模型,提出模拟结构裂纹损伤的节点分离法;研究无损伤和含有损伤的铝合金梁结构中Lamb波的传播规律,通过和理论解对比,验证谱元法模拟结构中Lamb波传播的有效性;研究不同程度损伤对响应信号的影响,探讨了梁结构损伤识别方法。结果对基于Lamb波的结构损伤识别方法的研究具有借鉴意义。     

基于谱元法的复合材料裂纹梁Lamb波传播特性研究

基于谱元法提出了一种弹簧元来模拟复合材料梁由于横向裂纹导致的轴弯耦合效应,分析复合材料裂纹梁中Lamb波的传播特性。由断裂力学的相关知识求得弹簧元的刚度,建立复合材料裂纹梁的损伤谱元模型。通过模拟复合材料裂纹梁内Lamb波传播,并和传统的有限元结果进行比较,验证了所提出模型的可行性和有效性。推导了频域内Lamb波各模态的能量计算公式,裂纹处的能量守恒证明了所提出模型的正确性,同时计算表明复合材料梁中裂纹处反射与透射的Lamb波各模态能量随着裂纹深度的变化规律具有单调性,结论可以为定量识别复合材料梁裂纹提供实用依据。     

Lamb波传播特性研究

应用三维弹性理论对Lamb波频散曲线进行理论建模.在Lamb波主动监测系统中,应用Gabor小波变换理论分析Lamb波信号的时延及相位角,获得了板中Lamb波的相群速度频散曲线.比较理论曲线与实验数据,证明了三维弹性理论建模方法的有效性.     

基于压缩感知的复合材料板Lamb波场重构及损伤成像

为了解决全波场数据获取耗时的问题,引入压缩感知算法对波场进行稀疏表示.然而受限于导波在复合材料中复杂的传播特性,压缩感知在复合材料板中的应用成为难题.为此通过考虑导波在复合材料板中传播的各向异性波数特性,构建计算频带内不同角度下的波数库用于波场重构.同时在损伤分析阶段,提出了一种无需基准信号的复合材料损伤散射波场分离技...     

基于Lamb波和Hilbert变换的复合材料T型加筋损伤监测

为了监测整体成型复合材料结构的损伤,提出了一种基于Lamb波和Hilbert变换的能量损伤指数。首先,通过应用Hilbert变换提取Lamb波信号的波形包络;然后选取具有最大峰值的波包,将此波包在结构出现损伤后的能量变化值与损伤前的能量之比作为损伤指示;该方法不需要选择特定的Lamb波模式,克服了Lamb波在复合材料结构中存在的频散、多模式及模式转换给信号分析带来的困难;最后在复合材料T型加筋的损伤演化试验中,对该能量损伤指数进行了应用验证研究。研究结果表明：该能量损伤指数可以用于复合材料T型加筋的损伤监测,当能量损伤指数（EDI）值达到0.62时,所研究的复合材料T型加筋确定有损伤产生。     

考虑材料各向异性的纤维增强聚合物基复合材料板损伤Lamb波检测和定位

超声Lamb波法是检测板状结构损伤的常用方法,然而纤维增强聚合物基复合材料（Fiber reinforced plastics,FRP）本身的各向异性会对Lamb波损伤定位的精度造成较大的影响。为了解决此问题,在传统椭圆法的基础上,提出了考虑材料各向异性的时间概率密度法的损伤定位方法。该方法通过考虑不同传播方向的A₀模态波的群速度变化,计算FRP板上任意一点存在损伤的反射波的走时,得到声波传播时间图。创建声波传播时间与实际损伤反射波走时的映射关系,可得到能表征损伤存在概率的时间概率密度图,通过相应的数值分析和实验研究,发现本方法误差比传统椭圆法误差的减小率可达到70%以上,论证本方法对各向异性FRP板损伤定位的可行性和准确性。      

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.     

基于时间反转加权分布的复合材料Lamb波损伤成像

超声Lamb波是检测板状结构损伤的常用方法,然而碳纤维增强聚合物基复合材料（Carbon Fiber ReinforcedPlastics,CFRP）本身的各向异性会对Lamb波的损伤成像和定位造成很大的影响。且大多数检测方法均采用健康结构的检测信号作为参考信号,用差信号的方法来实现损伤成像,该过程容易受到待测结构和实验环境变化等外界因素的影响。针对该问题,采用时间反转和加权分布成像相结合的方法,将其应用在复合材料板状结构的Lamb波损伤检测和成像中。仿真结果表明,该方法能够有效地实现板中单源脱层损伤和多源脱层损伤的二维成像与定位,且具有较高的精度和准确性。     

Parallel implementation of spectral element method for Lamb wave propagation modeling

The proposed spectral element method implementation is based on sparse matrix storage of local shape function derivatives calculated at Gauss–Lobatto–Legendre points. The algorithm utilizes two basic operations: multiplication of sparse matrix by vector and element‐by‐element vectors multiplication. Compute‐intensive operations are performed for a part of equation of motion derived at the degree of freedom level of 3D isoparametric spectral elements. The assembly is performed at the force vector in such a way that atomic operations are minimized. This is achieved by a new mesh coloring technique The proposed parallel implementation of spectral element method on GPU is applied for the first time for Lamb wave simulations. It has been found that computation on multicore GPU is up to 14 times faster than on single CPU. Copyright © 2015 John Wiley & Sons, Ltd.     

Some aspects of numerical simulation for Lamb wave propagation in composite laminates

Some aspects of numerical simulation of Lamb wave propagation in composite laminates using the finite element models with explicit dynamic analysis are addressed in this study. To correctly and efficiently describe the guided-wave excited/received by piezoelectric actuators/sensors, effective models of surface-bounded flat PZT disks based on effective force, moment and displacement are developed. Different finite element models for Lamb wave excitation, collection and propagation in isotropic plate and quasi-isotropic laminated composite are evaluated using continuum elements (3-D solid element) and structural elements (3-D shell element), to elaborate the validity and versatility of the proposed actuator/sensor models.     

Control of wave propagation response using quasi crystals: A formulation based on spectral finite element

This paper presents wave propagation studies in quasi crystal structures and quasi crystal reinforced aluminium structures. The analysis is performed using frequency domain spectral finite element formulation. The analysis considers different 2-D decagonal and 3-D icosahedral quasi crystals. First, wave propagation analysis of quasi crystal structure alone is performed and the propagation of phonon and phason modes for different quasi crystals are studied. The study includes the propagation of axial and transverse wave responses in these quasi crystals. The study has found that the amplitude of the phason modes is very small compared to the phonon modes and the increase of the phason mode content (through increase in R) increases the phason mode amplitude, without affecting the phonon mode amplitudes. It is shown that the dominant axial phonon mode is non-dispersive and the dominant flexural phonon mode is dispersive. In the next study, the aluminium beam structure is reinforced with different quasi crystals in different configurations and the wave propagation of axial and transverse responses are studied. For all the combinations of quasi crystal aluminium beam combination, there is substantial suppression of responses both for the axial and the bending responses. Unsymmetrical configuration produces substantial non-dominant phonon modes which propagate dispersively. It is found that for a symmetric bi-morph configuration, the response is reduced significantly, about 68% and 75% for axial loading and 80% and 78% for flexural loading, respectively, for the 2-D decagonal quasi crystal and the 3-D icosahedral quasi crystal.     

A spectral finite element for axial-flexural-shear coupled wave propagation analysis in lengthwise graded beam

A new spectral element (SE) is formulated to analyse wave propagation in anisotropic inhomogeneous beam. The inhomogeneity is considered in the longitudinal direction. Due to this particular pattern of inhomogeneity, the governing partial differential equations (PDEs) have variable coefficients and an exact solution for arbitrary variation of material properties, even in frequency domain, is not possible to obtain. However, it is shown in this work that for exponential variation of material properties, the equations can be solved exactly in frequency domain, when the same parameter governs the variation of elastic moduli and density. The SE is formed using this exact solution as interpolating polynomial. As a result a single element can replace hundreds of finite elements (FEs), which are essential for all wave propagation analysis and also for accurate representation of the inhomogeneity. The developed element is used for eliciting several advantages of the gradation, including mode selection, mode blockage and smoothening of stress waves.     

Numerical simulation of the guided Lamb wave propagation in particle reinforced composites

This paper deals with the investigation of the Lamb wave propagation in particle reinforced composites excited by piezoelectric patch actuators. A three-dimensional finite element method (FEM) modeling approach is set up to perform parameter studies in order to better understand how the Lamb wave propagation in particle reinforced composite plates is affected by change of central frequency of excitation signal, volume fraction of particles, size of particles and stiffness to density ratio of particles. Furthermore, the influence of different arrangements is investigated. Finally, the results of simplified models using material data obtained from numerical homogenization are compared to the results of models with heterogeneous build-up. The results show that the Lamb wave propagation properties are mainly affected by the volume fraction and ratio of stiffness to density of particles, whereas the particle size does not affect the Lamb wave propagation in the considered range. As the contribution of the stiffer material increases, the group velocity and the wave length also increase while the energy transmission reduces. Simplified models based on homogenization technique enabled a tremendous drop in computational costs and show reasonable agreement in terms of group velocity and wave length.     

Simulation of three-dimensional elastoacoustic wave propagation based on a Discontinuous Galerkin spectral element method

In this article, we present a numerical discretization of the coupled elastoacoustic wave propagation problem based on a discontinuous Galerkin spectral element approach in a three-dimensional setting. The unknowns of the coupled problem are the displacement field and the velocity potential, in the elastic and the acoustic domains, respectively, thereby resulting in a symmetric formulation. After stating the main theoretical results, we assess the performance of the method by convergence tests carried out on both matching and nonmatching grids, and we simulate realistic scenarios where elastoacoustic coupling occurs. In particular, we consider the case of Scholte waves, the scattering of elastic waves by an underground acoustic cavity, and a problem of marine seismic exploration. Numerical simulations are carried out by means of the code SPEED , available at http://speed.mox.polimi.it .     

Lagrangian finite element method for solitary wave propagation

A finite element method for a study of the propagation of solitary waves has been presented using the Lagrangian formulation. In this method, the computation of the convection terms can be avoided in the basic iteration procedure and the determination of the free surface position is automatically performed. To obtain the velocity and pressure, the velocity correction method is effectively used based on the correction potential. To overcome the distortion of the configuration of finite elements, the rezoning technique has been effectively used. The wave propagation analysis has been carried out and the validity of the present method has been demonstrated.     

Post-buckling finite element analysis of composite cylindrical panels in axial compression

The post-buckling behaviour of composite cylindrical panels in axial compression is investigated using the nonlinear finite element theory. A branch switching algorithm with an ‘eigenmode injection’ is discussed. The effect of the reinforcement angle on the buckling load, as well as the post-buckling behaviour of the panels, are analysed. Examples are considered for separated and closely spaced bifurcation points.

By varying the reinforcement angle of the panels from 0 to 90°, two kinds of buckling modes are discovered for asymmetric and unstable symmetric lowest points of bifurcation. Local minima for the secondary branches of geometrically perfect panels are no less than 15% of their buckling loads, while values of the buckling loads alter more than twice in this region.

A 16-node degenerated shell element with full integration scheme is used.     

Modeling of ultrasonic wave propagation in teeth using PSpice: a comparison with finite element models

Ultrasound is used extensively in the medical field for the detection and characterization of a variety of features in the human body. Finite element models used to understand ultrasonic wave propagation in teeth have been developed so that ultrasound techniques could be realized in dentistry. This paper presents a hypothesis that underlies one possible design of an ultrasonic tool that can be used in a clinical environment, as well as several models that describe acoustic field simulation, propagation, and interaction with the layers of several tooth structures. A complete PSpice model of a single-element transducer based on Redwood's version of Mason's equivalent circuit, a focusing lens, and a multi-layer tooth structure is used to illustrate the validity of this hypothesis. Transmission line theory is employed as a basis for the models of the piezoceramic, the lens, and the different tooth layers. Results clearly depict the transmission and reflection of the ultrasonic waves as they travel through the layers within the tooth structure and point out the noticeable similarity to longitudinal L-wave signatures produced by axisymmetric finite element models presented in earlier studies.