近表面弹性性质连续变化材料中激光声表面波的理论研究

基于金属材料近表面层弹性性质（杨氏模量）的连续变化,建立了一种激光在基底上的梯度材料中激发声表面波的理论模型,并用有限元方法模拟了脉冲激光作用于材料上表面激发出的超声波及其传播过程.讨论了表面层厚度的变化以及表面层弹性性质的变化对热弹条件下产生的声表面波波形特征的影响,并分别计算了两种情况下的声表面波的相速度色散,得到了两种情况下声表面波的变化规律.     

近表面层黏性模量梯度变化的复合平板中激光热弹激发声表面波的传播特性

考虑复合平板近表面层的黏性模量变化特性,建立激光在复合平板中热弹激发声表面波的有限元模型,系统研究复合平板中声表面波的传播特性,并讨论近表面层的黏性模量、厚度与Lamé常数对声表面波衰减特性的影响,在此基础上,结合理论计算的色散与衰减曲线对声表面波传播特性进行验证.本研究为激光超声技术无损评估复合平板近表面层的力学性能提供理论指导,同时也为研究复合平板表面微裂纹问题提供可行性思路.     

由于表面粗糙引起的激光声表面波色散的实验和理论研究

表面粗糙是材料制造过程中必有的副产物, 粗糙表面会引起其中传播的声表面波的速度发生变化. 在利用激光声表面波对材料性质进行评估时, 常用宽带的激光声表面波速度频散特性对材料性质进行反演. 为了研究表面粗糙度是否能作为反演的特征参数之一, 本文建立了激光在表面粗糙样品中激发声表面波、聚偏氟乙烯换能器宽带接收声表面波的实验装置来研究不同粗糙度表面对声表面波速度的影响; 理论上建立了激光在粗糙表面中激发声表面波的计算模型, 利用有限元法得到声表面波的时域特征, 并进一步得到声表面波的速度色散曲线, 理论结果和实验结果能很好地拟合. 这为利用激光声表面波对表面粗糙的评估提供理论和实验依据. 关键词： 表面粗糙 激光声表面波 速度色散 聚偏氟乙烯传感器 有限元法     

激光激发黏弹表面波有限元数值模拟

研究黏弹性材料中激光激发的Rayleigh波的传播特征. 考虑到黏弹性材料的黏性特征,在频域内建立黏弹性材料中激光激发Rayleigh波的有限元数值模型. 在验证有限元频域数值模型正确性的基础上,模拟脉冲激光作用在黏弹性材料上激发出Rayleigh波,进而讨论激光激发的黏弹Rayleigh波的传播特征,并比较黏弹性材料与弹性材料中激光激发的Rayleigh波差异,同时分析了材料的黏性劲度参量变化对Rayleigh波特征的影响. 关键词： 表面波 激光超声 有限元方法 黏弹性     

利用激光超声研究功能梯度材料中声表面波的传播特性

利用有限元的方法建立了脉冲激光在功能材料中激发声表面波的理论模型,根据该理论模型分别分析了脉冲激光在不同空间调制下所激发的声表面波(SAWs)在功能梯度材料表面的传播特性,以及不同力学参量变化对所激发的声表面波传播特性的影响。模拟结果表明:线源激发的宽带声表面波在功能梯度材料表面传播时,声表面波会出现明显的色散效应;当使用光栅调制的脉冲激光在功能梯度材料上激发声表面波时,与均匀材料中所激发的声信号相比,激发频率发生相应的频移现象。该模型为功能梯度材料的非接触表征提供了理论基础。     

激光热弹激发涂层/基底系统中声表面波的有限元数值模拟

激光激发声表面波提供了评价涂层或薄膜的弹性性质的有效手段。考虑激光作用过程中材料热物理参数随温度变化的实际情况,在研究网格大小和时间步长这二个重要参数和求解稳定性的基础上,建立了优化的有限元模型数值模拟两种典型涂层基底系统中声表面波的激光热弹激发和传播过程。数值模拟结果表明:除了掠面纵波外,在快基底慢涂层系统中得到了正常的色散表面波,而在慢基底快涂层系统中得到了反常色散表面波。进一步,采取减小激发激光光斑的办法,使激发出来的声表面波频率高于慢基底快涂层系统中的截止频率时,在慢基底快涂层系统中得到了伪表面波。     

横观各向同性材料中激光超声谱有限元数值模拟

研究了横观各向同性材料中激光超声波的传播特征.基于谱有限元方法,建立了横观各向同性材料中激光超声的数值模型.利用谱有限元方法模拟脉冲激光作用于材料上产生超声波及其传播的过程.讨论了横观各向同性薄板的各向异性及各向同性平面内超声波的传播特征,并分析了材料厚度的变化对产生超声波模态的影响.     

声学测微计及其在薄层测厚中的应用

文章描述的声学测微计是以监测伪Sezawa波为基础的。假定在半无限大介质的上表面有一薄的复盖层,当层介质中的横波声速较底下介质的横波声速小时,可有许多表面波模式沿层状介质的表面传播,其中基模为瑞利波,第二个模为Sezawa波。实验所用样品中,基片材料为42-合金,上面薄层材料是金。当一平面超声波通过水耦合斜入射到样品上时,在样品表面层激发的前两个模的相速度色散曲线如图1所示,图中k为波数,d为薄层厚度。Sezawa波有一个截止波数,在这个波数,Sezawa波的相速度等于基片中的     

横观各向同性材料中激光超声谱有限元数值模拟

研究了横观各向同性材料中激光超声波的传播特征．基于谱有限元方法,建立了横观各向同性材料中激光超声的数值模型．利用谱有限元方法模拟脉冲激光作用于材料上产生超声波及其传播的过程．讨论了横观各向同性薄板的各向异性及各向同性平面内超声波的传播特征,并分析了材料厚度的变化对产生超声波模态的影响．     

用于铝板缺陷无损检测的激光超声有限元模拟研究

激光超声技术能够在材料表面形成超声波，是实现材料缺陷无损检测的重要环节。借助Abaqus有限元分析工具，基于激光超声热弹机制建立了轴对称铝板的表面缺陷模型，模拟了激光激发产生的表面波在材料中的传播特性及其与铝板缺陷的相互作用过程。数值模拟实验表明，铝板表面缺陷的分布深度值越大，反射波越强，并且当缺陷深度达到一定程度时，反射波的幅值趋于稳定；但缺陷的分布宽度对于反射波的影响则十分有限。所得结论为基于激光超声的材料缺陷的定量检测及识别提供有效的理论依据。     

Laser-induced ultrasonic waves in steels with gradient changes of elastic property

Taking account of the gradient changes of the near-surface elastic property, a model of multiple gradient layers on a semi-infinite substrate is established. A finite element method (FEM) model is developed to simulate the laser-induced thermo-elastic generation and propagation of acoustic waves in the system. The two-dimensional fast Fourier transform (2-D FFT) method has been used to analyze the ultrasonic signal. The phase velocity dispersion is obtained for a number of waves of different modes obtained and is analyzed in systems with different spatial variations of the material elastic properties and different affected layer depths. The model can be used to extract the elastic parameters of the near-surface regions in an effective way.     

Elastic measurements of layered nanocomposite materials by Brillouin spectroscopy

Surface Brillouin spectroscopy makes it possible to measure surface elastic wave propagation parameters at frequencies up to 20 GHz or more. This enables us to measure the elastic properties of surface layers only a small fraction of a micrometre thick. The wavelength and incident angle of the light determine the wavenumber of surface elastic waves (SAW) that scatter the light inelastically, and their frequency can be found by measuring the change in wavelength of the scattered light. By analysing the elastic wave modes present in the surface, the elastic properties can be deduced. We have used this technique to measure the elastic properties of layered nanocomposite materials, which are widely used in the packaging industry. 12 microns polymer films (PET) were coated with glass oxide layers of thickness as little as 25 nm, to give transparent nanocomposite structures with excellent gas barrier properties. In order to understand and model the behaviour of these films under deformation, it is necessary to determine the elastic properties of the different layers. Evaluation of the elastic properties presents several challenges. First, the oxide layers are much thinner than the wavelengths of the surface phonons in surface Brillouin spectroscopy (and hence the depth probed), which usually lie in the range 250-500 nm. The anisotropic elastic properties of the PET substrate must therefore be measured accurately, and this can be done using bulk Brillouin spectroscopy. Second, a thin layer of metal (usually 10-20 nm) must be deposited on the glass surface so that the surface phonons scatter the light effectively. The elastic properties of the glass layer can then be deduced from surface Brillouin spectroscopy measurements, by simulating the surface wave modes of the metal/glass/polymer composite, and adjusting the parameters to give the best fit. In this way it is possible to observe how the properties of the glass vary as a function of thickness, and in turn to understand how to improve systematically the properties under deformation.     

Investigation of surface acoustic waves in laser shock peened metals

Laser shock peening is a well-known method for extending the fatigue life of metal components by introducing near-surface compressive residual stress. The surface acoustic waves (SAWs) are dispersive when the near-surface properties of materials are changed. So the near-surface properties (such as the thickness of hardened layers, elastic properties, residual stresses, etc.) can be analyzed by the phase velocity dispersion. To study the propagation of SAWs in metal samples after peening, a more reasonable experimental method of broadband excitation and reception is introduced. The ultrasonic signals are excited by laser and received by polyvinylindene fluoride (PVDF) transducer. The SAW signals in aluminum alloy materials with different impact times by laser shock peening are detected. Signal spectrum and phase velocity dispersion curves of SAWs are analyzed. Moreover, reasons for dispersion are discussed.     

Estimation of elastic stress in the near-surface layers of solid objects caused by thermoelastic deformations under absorption of nonstationary laser radiation

Analytical expressions for the calculation of radial and tangential components of the elastic stress tensor that are generated by the nonstationary laser radiation in the near-surface layers of solid objects are derived in the framework of a 3D model and quasi-static approximation. The components of the stress tensor are analyzed in the systems with strong and weak focusing of laser radiation on the surface of the object.     

Love-type waves in functionally graded piezoelectric material (FGPM) sandwiched between initially stressed layer and elastic substrate

This paper investigates the propagation behavior of Love-type surface waves in three-layered composite structure with initial stress. The composite structure has been taken in such a way that a functionally graded piezoelectric material (FGPM) layer is bonded between initially stressed piezoelectric upper layer and an elastic substrate. Using the method of separation of variables, frequency equation for the considered wave has been established in the form of determinant for electrical open and short cases on free surface. The bisection method iteration technique has been used to find the roots of the dispersion relations which give the modes for electrical open and short cases. The effects of gradient variation of material constant and initial stress on the phase velocity of surface waves are discussed. Dependence of thickness on each parameter of the study has been shown explicitly. Study has been also done to show the existence of cut-off frequency. Graphical representation has been done to exhibit the findings. The obtained results are significant for the investigation and characterization of Love-type waves in FGPM-layered media.     

Acoustoseismic wave fields generated by surface seismic vibrators

Results of experimental and theoretical studies of acoustoseismic wave fields generated by surface seismic vibrators are presented. In experiments with high-power seismic vibrators operating in a frequency range of 5–10 Hz, acoustic waves were recorded at distances up to 50 km from the source. The long-range sound propagation from seismic vibration sources was observed in a near-surface waveguide arising due to temperature inversion. The effect of the acoustoseismic induction, i.e., excitation of surface seismic waves by the acoustic wave arriving from the vibrator, was also detected. The results of mathematical modeling of the acoustoseismic field generation by an operating seismic vibrator are presented. They include the modeling of the radiation of a harmonic acoustic wave’s by the vibrator, its trapping by the near-surface waveguide, the long-range low-frequency acoustic wave propagation in the presence of the waveguide, and the induction of a surface seismic wave by the arriving harmonic acoustic wave. It is shown that a seismoacoustic wave propagating at the boundary between the elastic earth and the atmosphere is an analog of the Stonely wave that appears in the presence of a near-surface low-temperature layer in the atmosphere.     

Observation of topological valley transport of elastic waves in bilayer phononic crystal slabs

In this Letter, we propose a unique bilayer design of phononic crystal slabs that are constructed by two layers of snowflake phononic crystal plates connected by a honeycomb array of cylinders. By tuning orientations of snowflake-shaped holes in both layers, we achieve two kinds of valley-projected topological elastic insulators distinguished by conventional and layer-polarized topological valley Hall phases. Then, between different conventional and layer-polarized topological valley Hall phases, two kinds of edge modes, layer-mixed and layer-polarized edge modes, are obtained and explored. In finite-size samples, the interesting topological transport properties, which the elastic wave can propagate alternatively between both layers and only in a single layer, are realized by exciting layer-mixed and layer-polarized edge states. In addition, we design an interlayer converter to realize conversion of the elastic wave propagation between both layers. Our research promotes the development of topological elastic insulators and provides a route for various practical applications.     

Numerical Analysis of Multilayer Waveguides Using Effective Refractive Index Method

With the help of the effective refractive index method we have numerically analyzed a multilayer planar waveguide structure and calculated the propagation constants, confinement factors, and transverse electric (TE) modes. A five-layer waveguide model has been provided to analyze the electro-magne tic wave propagation process. The analysis method has been applied to the 980 nm laser with active layer of GaInAs/GaInAsP strained quantum wells, GaInAsP confinement layers and GaInP cap layers. By changing the thickness of confinement layers, we obtained confinement factor as high as 95% with higher TE modes TE1 and TE2. The results are in good agreement with the experiment by A. Al-Muhanna et al. and give the new idea to enhance output power of semiconductor lasers. The analysis method can also be extended to any other slab multilayer waveguide structures, and the results are useful to the fabrication of optic-electronic devices.     

Application of narrow band laser ultrasonics to the nondestructive evaluation of thin bonding layers

In this paper, a modified laser induced grating technique (LIG) has been utilized to generate narrow band surface waves in an epoxy-bonded copper-aluminum layered structure. A high performance optical interferometer system was utilized to detect the laser-generated surface waves. The dispersion of surface wave in an epoxy-bonded copper-aluminum specimen was measured and compared with the theoretical solution. An inverse algorithm based on the simplex method was then introduced to determine the bonding thickness as well as the elastic properties of the bonding layer. The inversion results demonstrated that the thickness in the microm range or the elastic properties of the bonding layer could be successfully determined.