基于压电晶体传感器的椭圆损伤定位算法

压电晶体传感器作为激励部件可以在结构中产生高频弹性应力波,其同裂纹等局部损伤发生相互作用将产生波动的能量耗散、波形反射以及波形干涉等现象.利用附在金属板上的压电激励器在结构产生的高频弹性应力波,并通过嵌入或附着在结构中的压电传感器进行接收,提出了一种基于TDOA模型的椭圆损伤定位算法.利用该算法可以获得损伤部位坐标的显式表达式,并能够对损伤区域的大小做出估计.仿真计算结果和实验证明该算法能够提供实时的"主动"损伤检测手段,同时可以准确快速地对结构中可能存在的损伤进行定位.     

Identification of solitary-wave solutions as an inverse problem: Application to shapes with oscillatory tails

The propagation of stationary solitary waves on an infinite elastic rod on elastic foundation equation is considered. The asymptotic boundary conditions admit the trivial solution along with the solution of type of solitary wave, which is a bifurcation problem.     

High Order Accurate Finite Difference Modeling of Seismo-Acoustic Wave Propagation in a Moving Atmosphere and a Heterogeneous Earth Model Coupled Across a Realistic Topography

We develop a numerical method for simultaneously simulating acoustic waves in a realistic moving atmosphere and seismic waves in a heterogeneous earth model, where the motions are coupled across a realistic topography. We model acoustic wave propagation by solving the linearized Euler equations of compressible fluid mechanics. The seismic waves are modeled by the elastic wave equation in a heterogeneous anisotropic material. The motion is coupled by imposing continuity of normal velocity and normal stresses across the topographic interface. Realistic topography is resolved on a curvilinear grid that follows the interface. The governing equations are discretized using high order accurate finite difference methods that satisfy the principle of summation by parts. We apply the energy method to derive the discrete interface conditions and to show that the coupled discretization is stable. The implementation is verified by numerical experiments, and we demonstrate a simulation of coupled wave propagation in a windy atmosphere and a realistic earth model with non-planar topography.     

基于稀疏导波的裂纹定位和尺寸评估

材料在使用过程中,受各种因素的影响,会不可避免地产生诸多损伤,破坏材料结构完整性的同时,也具有一定的安全隐患;在无损检测中,尤其是导波检测领域,由于缺乏高效方法对材料结构中存在的损伤进行尺寸检测,因此使用导波方法对材料进行微裂缝等损伤的检测的过程中仍然存在很多困难;对一种基于稀疏导波的损伤检测方法进行了研究,可以在进行结构损伤定位的同时对损伤尺寸进行测量;研究表明,使用导波进行损伤检测时,由损伤而产生的复杂反射波中包括很多与损伤形状和尺寸相关的信息,但是这些信息并不明显,且信号重叠无法区分;因此,提出一种稀疏lamb波方法来分解由材料损伤产生的反射信号的各个分量;在此基础上,通过确定各个反射信号分量相应的传播时间来计算材料损伤尺寸;最后,在具有人工损伤的铝板上进行实验验证,结果表明,基于稀疏导波损伤检测的方法是可行的。     

Design of phononic materials/structures for surface wave devices using topology optimization

We develop a topology optimization approach to design two- and three-dimensional phononic (elastic) materials, focusing primarily on surface wave filters and waveguides. These utilize propagation modes that transmit elastic waves where the energy is contained near a free surface of a material. The design of surface wave devices is particularly attractive given recent advances in nano- and micromanufacturing processes, such as thin-film deposition, etching, and lithography, which make it possible to precisely place thin film materials on a substrate with submicron feature resolution. We apply our topology optimization approach to a series of three problems where the layout of two materials (silicon and aluminum) is sought to achieve a prescribed objective: (1) a grating to filter bulk waves of a prescribed frequency in two and three dimensions, (2) a surface wave device that uses a patterned thin film to filter waves of a single or range of frequencies, and (3) a fully three-dimensional structure to guide a wave generated by a harmonic input on a free surface to a specified output port on the surface. From the first to the third example, the resulting topologies increase in sophistication. The results demonstrate the power and promise of our computational framework to design sophisticated surface wave devices.     

Damage detection of structures using spectral finite element method

This paper brings together the principles, equations, and applications of damage modelling and elastic waves propagation, both traditional and state-of-the-art in a review form. It begins with the relevant fundamentals of damage modelling, derives the basic equations of fracture mechanics and elastic wave propagations, and covers advanced topics and applications of Lamb waves that are at the forefront of today’s research. The results obtained indicate that the current approach is capable of detecting cracks and delaminations of very small size, even in the presence of considerable measurement errors. The sections are filled with case studies, worked examples and exercises that make this paper an outstanding resource.     

基于振动响应分析的盆式绝缘子损伤检测系统

目前盆式绝缘子损伤检测方法为被动式,且存在漏检现象。提出了一种基于Lamb波的主动激励式盆式绝缘子损伤检测系统的设计方案。该系统通过粘贴在盆式绝缘子表面的压电元件激发Lamb波,利用Lamb波在盆式绝缘子内部传播过程中遇到损伤时传播特性会发生改变的特征,通过差值法和阈值判断法对接收端的Lamb波分析处理判断出盆式绝缘子内部损伤状况。试验结果表明所设计的系统能够准确的检测出微小损伤并确定损伤区域,具有良好的应用前景。     

Nonlinear acceleration waves in porous media

We review three theories explaining why waves in a diffusion problem can travel with a finite speed. We then briefly look at a class of equivalent fluid theories for sound propagation in porous media. Finally, we derive a Cattaneo model for an elastic material containing a distribution of voids. Nonlinear acceleration wave motion in such a class of materials is also considered.     

Axial magnetic field effect on wave propagation in bi-layer FG graphene platelet-reinforced nanobeams

This paper aims to investigate the size-dependent wave propagation in functionally graded (FG) graphene platelet (GPL)-reinforced composite bi-layer nanobeams embedded in Pasternak elastic foundation and exposed to in-plane compressive mechanical load and in-plane magnetic field. The small-scale effects are taken into account by employing the nonlocal strain gradient theory that contains two different length scale parameters. The present two nanobeams are made of multi-composite layers. Each layer is composed of a polymer matrix reinforced by uniformly distributed and randomly oriented GPLs. The GPLs weight fraction is graded from layer to other according to a new piece-wise rule and then four distribution types will be established. Our technique depends on applying the four-variable shear and normal deformations theory to model the wave propagation problem. The equations of motion are obtained using Hamilton principle. These equations are then analytically solved to obtain the wave frequencies and phase velocities of the waves. The calculated results are compared with those published in the literature. The impacts of the length scale parameters, foundation stiffness, in-plane magnetic field, weight fraction of graphene, graphene platelets distribution type and beam geometry on the propagating waves in the FG GPLs nanobeams are discussed in details. It is found that the strength of the composite beams may be enhanced with increasing in the GPLs weight fraction and magnetic field leading to an increment in the phase velocity and wave frequency of the present system.

     

Influence of natural disasters on ground facilities

The purpose of this article is to study the problem of the propagation of waves that result in earthquakes in different geological media: homogeneous, multilayer, gradient, with fractured layer, and karst cavern. The authors pose the problem of analyzing the impact of waves on ground structures: buildings and dams. Numerical solutions of problems of wave propagation in heterogeneous media are obtained. On the basis of the analysis of wave patterns, the types of waves propagated from the focus of the earthquake are qualified. The comparison of the impact of elastic waves on the day surface for the cases of different geological media is done. Synthetic seismograms for these media are obtained. The influence of elastic waves on the stability of ground structures is qualitatively examined. The grid-characteristic method for triangle meshes with the formulation of boundary conditions on interfaces of rock-crack, building-rock, rock-water, and dam-water, as well as free surfaces in an explicit form, is used in this paper.     

One dimensional elasto-plastic wave interaction and boundary reflections

Elasto-plastic wave propagation in a one dimensional rod is treated by utilizing finite element analysis. The time dependence is handled by means of a variable time step numerical integration method. Elasto-plastic reflections from a fixed boundary and interaction of plastic waves are discussed.

Theoretical expressions are presented for determining the stress response for a material exhibiting bilinear hardening behavior during elasto-plastic wave interaction and boundary reflection. These expressions are employed to evaluate predictions made by the numerical procedure.

In the computations, steep wave fronts are approximated by ramps. The steepness of these ramps may be increased by refining the finite element grid. Excellent agreement is obtained between numerical and theoretical elastic and inelastic stress response levels.     

Design of phononic crystals for self-collimation of elastic waves using topology optimization method

Self-collimating phononic crystals (PCs) are periodic structures that enable self-collimation of waves. While various design parameters such as material property, period, lattice symmetry, and material distribution in a unit cell affect wave scattering inside a PC, this work aims to find an optimal material distribution in a unit cell that exhibits the desired self-collimation properties. While earlier studies were mainly focused on the arrangement of self-collimating PCs or shape changes of inclusions in a unit cell having a specific topological layout, we present a topology optimization formulation to find a desired material distribution. Specifically, a finite element based formulation is set up to find the matrix and inclusion material distribution that can make elastic shear-horizontal bulk waves propagate along a desired target direction. The proposed topology optimization formulation newly employs the geometric properties of equi-frequency contours (EFCs) in the wave vector space as essential elements in forming objective and constraint functions. The sensitivities of these functions with respect to design variables are explicitly derived to utilize a gradient-based optimizer. To show the effectiveness of the formulation, several case studies are considered.     

Monitoring the state of the moving train by use of high performance systems and modern computation methods

The objective of this work has been to study the propagation of elastic waves in rails. It presents the comparison of calculations obtained by the grid-characteristic and discontinuous Galerkin methods. The propagation of elastic waves in the presence and absence of the karst inclusion in the ground under the embankment, diagnosed in these cases from the rails, are compared. The wave pictures and diagnosed signals for four types of defects of a fractured character: vertical and horizontal head layering, cross fracture in the head and cracks in the rail web are given. The grid-characteristic method on the curvilinear structural meshes and the discontinuous Galerkin method on the nonstructured triangular meshes make it possible to solve efficiently the tasks on monitoring the state of the moving train and rail, including a great number of integrity violations, dynamic interactions in the train-rail system, and obtain the full wave picture.     

内波极性转变过程序列SAR图像仿真研究

基于Lynett和Liu的水平二维内孤立波传播模型,对内波的极性转变过程进行了序列SAR成像仿真研究。通过分析下降型内孤立波转变为上升型的序列SAR仿真图像,验证了内波极性转变时在SAR图像上出现信号减弱和特征消失的现象。对比于实地星载SAR图像,仿真结果也验证了内波在传播过程中受地形变化影响而出现的传播方向平行于等深线、分裂等现象。     

Ripple phenomenon of displays with nematic liquid crystal

Liquid crystal displays will show ripple if the display surface or display bracket is subjected to tactile forces. In this paper, the ripple of liquid crystal displays is investigated by dealing with elastic wave propagation in a liquid crystal layer. The model proposed for a visco‐elastic medium like liquid crystals (LCs) is generalized by combining the properties of a crystalline solid and an anisotropic fluid. The governing equation is derived by using visco‐elastic and wave equations. In the experiments, a linear motor is used to touch the display panel for producing ripple. Displays of three different amounts of LCs are compared. Experimental results also show that each display panel has its own wave propagation velocity that is not changed by different motor touch speeds. In addition, both theoretical analysis and experimental results depict that displays with a larger amount of LCs lead to slower ripple speed.     

旋转圆环平面外振动的行波法研究

以高速球轴承保持架为应用对象,针对旋转薄壁圆环的平面外振动问题,开展弹性波传播的基本特性研究,分析平面外波动的色散方程、波数、截止频率、相速度以及位移耦合系数.基于行波动力学方法,根据波动正向传递的判据,将正负行波的波数进行分离.结合相位封闭原理和波传递矩阵,建立旋转圆环的频率特征方程.文中算例给出了旋转薄壁圆环平面外振动的固有频率,对其计算结果与文献结果作了比较,验证了计算方法的准确性.     

Simulating the movement of bed and suspended loads in the coastal environment

A dynamic computer simulation model is implemented to simulate wave-induced transport of bed and suspended material, and the associated development of nearshore profiles. The model uses the Airy, Stokes', cnoidal and solitary wave theories to compute the height, length, celerity, steepness, breaker indices, and horizontal, veritical, mass drift and orbital velocities of random waves propagating along a flat slope. With shoreward wave propagation both bed and suspended loads are computed at discrete grid points.The FORTRAN '77 simulation program is executed for 3000 iterations, with each iteration corresponding to one twelfth of a tidal cycle. The simulated results demonstrate that sediment movement is controlled by the magnitude of the drift, orbital and horizontal wave velocity components, with wave breaking having a direct influence on the deposition of transported bed load. The shallow water regions of cnoidal and solitary waves are characterized by the presence of high concentrations of suspended material. Over the runlength of the simulation, transport of bed and suspended material cause the initialized profile state to change through a sequence of ill-defined transient states to a final morphological state. Examination of the sequence of profile states indicate weak resemblances of barred and nonbarred profile configurations.     

Cutoff wave numbers for energy-orthogonal eight-node quadrilateral plane-strain elements

This paper studies the propagation of plane harmonic waves in plane-strain solids discretized by the standard eight-node quadrilateral finite element. This element is formulated in energy-orthogonal form. It means that the stiffness matrix is split into basic and higher order components which are obtained from the mean and deviatoric strain fields, respectively. The major subject is to obtain reference values for the wave number that can be used as optimum cutoff wave number to properly capture a wave field. The procedure is based on the properties of the higher order elastic energy.     

Cutoff wave numbers for energy-orthogonal six-node triangular plane-strain elements

This paper studies the propagation of plane harmonic waves in unbounded media discretized by the standard six-node triangular finite element. The element stiffness matrix is split into basic and higher order components which are obtained from mean and deviatoric strain fields, respectively. This decomposition is applied to the elastic energy. Based on the properties of the higher order energy, two values of the wave number are selected. Depending on the desired precision one of those values can be used as optimum cutoff wave number to properly capture a wave field.