纹影法在超声场成像中的应用

纹影法成像系统具有分辨率高、成像速度快等特点,可以实现稳态声场分布以及声波传播的瞬态过程快速成像。将纹影法应用于瞬态声传播特性以及声子晶体声场的成像研究。在瞬态成像时,记录了单脉冲辐照固体表面产生的泄漏瑞利波和泄漏纵波的传播过程,并且与仿真结果进行比较,两者的泄漏波辐射角较一致。在声子晶体声场的成像中,通过改变入射频率,观察到了声子晶体在其工作频率时背表面的稳态声场,为其俘获颗粒能力的研究与分析提供了直观的实验依据。     

利用有限长FEM/BEM提取COM参量

0引言声表面波(SAW)器件作为移动通信系统的核心元器件获得了广泛应用。耦合模式(COM)模型是目前低损耗射频SAW器件设计的主流工具,COM参量的精确与否直接决定了器件模拟的精度。传统的COM参量是常数,能够很好地模拟瑞利波器件;但对于漏波器件,声波传播中不断向基片体内泄漏的能量随频率变化,而且漏波的传播特性也是色散     

基于高分辨率Radon变换的瑞利波反演在役路基动模量

为了探索在役路基动模量的一种动态反演与成像方法,该研究通过瑞利波在弹性层状体系中的理论频散方程,揭示了瑞利波在基准路基路面结构中的理论频散特征,基于高分辨率线性Radon变换建立了路基路面结构中瑞利波频散曲线的提取方法。在结合高分辨线性Radon变换提取的频散曲线与路基工作区细化分层的基础上,建立了在役路基工作区动模量的反演成像方法。通过在足尺道路试槽内点对点依次进行瑞利波、落锤式弯沉仪(fallingweightdeflectmeter, FWD)、承载板和弯沉测试,建立了各测试指标之间的关系。结果显示,瑞利波反演的路基动模量与其他测试方法相应结果的相关系数均大于0.80,且反演成像结果能够正确、清晰地反映路基承载能力与压实工况。     

瑞利方程根的研究

瑞利方程的三个根中有两个都被认为是增根,而现在随着计算技术和检测技术的发展,瑞利方程又重新有了研究价值[1].本文列出了推出瑞利波方程的各项方程,用数值方法计算在泊松比为0.44的情况下的弹性波位移场和能流.     

超薄覆层低频超声类瑞利波定征方法

本文提出低频超声类瑞利波定征方法来估计覆层/基层层状复合结构超薄覆层的纵波声速、横波声速、厚度和密度等参数。这里“超薄”的概念是指覆层厚度h远小于覆层中的声波波长λ,导致在时域上覆层前后界面的各次回波信号相互混迭。文章采用水楔耦合方式,利用超声漏类瑞利波频域分析方法获取层状复合结构超声类瑞利波色散曲线,提出以色散特性为基础的低频超声类瑞利波覆层定征方法和在最小二乘意义下以色散曲线为基础的反向算法对覆层进行了参数估计。文中分析了影响估计准确性的各种因素,研究了低频超声类瑞利波定征方法对覆层各参数的灵敏度及其在误差传递中的意义。实验结果证实,对于银箔/玻璃层状复合结构,该方法能够准确估计厚度小到26μm银箔覆层的声学参数。     

超薄覆层低频超声类瑞利波定征方法

本文提出低频超声类瑞利波定征方法来估计覆层／基层层状复合结构超薄覆层的纵波声速,横波声速,厚度和密度等参数。这里“超薄”的概念是指覆层厚度ｈ远小于覆层中的声波波长λ,导致在时域上覆层前后界面的各次回波信号相互混迭。文章采用水楔耦合方式,利用超声漏类瑞利波频域分析方法获取层状复合结构超声类瑞利波色散曲线,提出以色散特性为基础的低频超声类瑞利波覆层定征方法和在最小二乘意义下以色散曲线为基础的反向算法对     

光弹法研究固体圆柱横截面上的脉冲声场

为了研究超声脉冲波在钢、铁、铝等金属棒内的传播规律。检验已有的声学理论,采用和金属声速接近的光学玻璃为样品,当超声垂直于样品侧壁入射时,利用研制的动态光弹成像系统研究了玻璃园棒内超声脉冲纵波、横波、瑞利波声场的传播特性。自动记录了0—99．9μs时间内各种超声波声场在玻璃园棒横截面上的传播图像（时间间隔0．1μs）,由此来模拟超声脉冲波在金属棒横截面中的传播行为。     

匀质软夹层地基瑞利波弥散特性

本文利用有限单元法和解析法推导了求上软下硬地基，夹层地基及道路结构瑞利波弥散曲线及位移分布的计算公式，得到了兼有有限单元法计算简单及解析法精度高之优点的计算方法，且编制了相应的计算机程序，并着重讨论了匀质软夹层地基瑞利波的弥散特性。     

层状场地脉动卓越频率对应的理论计算深度研究*

地脉动观测是一种简便有效的场地土层动力特性测试方法。本文依据场地脉动能量中瑞利波为主的特性,基于层状场地瑞利波弥散特征曲线,研究了不同场地条件下脉动卓越频率对应的瑞利波波长与场地自振频率理论计算深度之间的关系。计算结果表明：场地一定深度范围内土层剪切波速差异特性是其主要影响因素。如果土层波速差别较小或者某一波速占主导地位,则理论计算深度与瑞利波波长比值约为1/4a(a为瑞利波波速与剪切波速之比),与均匀场地结果接近;反之,土层波速差异越大,两者比值与1/4a偏离越远,且1/4a为比值上限。     

流体—固体成层介质中瑞利波特性的进一步讨论

利用解析法建立了两流体固体介质中斯通利波的特征方程及其位移计算公式，并进一步分析讨论了流体固体介质中瑞利波的弥散特性和传播规律。两半无限流体固体介质中斯通利波波速度是非频散的，波速比ｃ／ Ｖｓ 比匀质半空间中瑞利波波速比小，且ｃ／ Ｖｓ 受泊松比μ影响较小，一般情况模量比ｑ 对ｃ／ Ｖｓ 影响很小。流体固体成层介质中瑞利波具有弥散性，波长短时，相速度ｃ 趋于最上层固体的斯通利波波速，波长长时则趋于最下层固体的瑞利波波速     

Ultrasonic imaging by leaky Rayleigh waves

Leaky Rayleigh waves are used for the ultrasonic imaging of surface-breaking or near-surface defects. The phenomena which take place when a leaky Rayleigh wave propagates and interacts with a flaw are described in order to explain both how the ultrasonic image is built using a single transducer and what the main characteristics of these images are. In particular, the influence of the ultrasonic frequency is investigated in detail.     

Finite element simulation and visualization of leaky Rayleigh wavesfor ultrasonic NDE

The generation and propagation properties of transient leaky Rayleigh waves are characterized by a two-dimensional finite element model. The displacement vector is used as the primary variable for the solid medium and a potential scalar, which is a replacement for the pressure, is taken as the fundamental variable for the fluid medium. The coupled system of finite element equations are solved in the time domain by direct integration through the central difference scheme. Three configurations are considered: the conversion of a Rayleigh surface wave into a leaky Rayleigh wave, a focused beam probing a fluid/solid interface at the Rayleigh angle, and the interaction of a defocused wave with the interface. The wave velocity in the fluid (water) is lower than the Rayleigh wave velocity in the solid (aluminum). The wave propagation profile in each case is predicted by the model. The finite element model proves to be an effective tool for surface acoustic device design and ultrasonic NDE     

Ray representation of longitudinal lateral waves in acoustic microscopy

For object materials having a large enough Rayleigh velocity, the V(z) (where V is the output voltage and z is the defocus distance) variation is mainly due to interference between the fields of the geometrically reflected wave and the leaky Rayleigh wave. However, for materials, such as organic compounds, having a low Rayleigh velocity, the leaky Rayleigh wave is not excited. For this case, the lateral wave resulting from propagation along the surface of the longitudinal wave plays a significant role in determining the V(z) dependence. The effect of the lateral wave contribution on V(z) is studied. Ray optics is to derive an expression giving the influence of the longitudinal lateral wave. Good agreement is found between the theory and measurements for z not near zero. Because of the ease with which the longitudinal wave velocity can be obtained from V(z), one can conveniently determine the elastic constant c(11 ) of isotropic materials using the acoustic microscope.     

Ultrasonic leaky interface waves for composite-metal adhesive bond characterization

Highly stressed or damaged regions in aircraft structures are increasingly being reinforced by adhesively bonding an overlay of a unidirectional boron fiber-epoxy composite to the aircraft metallic substrate. While conventional C-scan ultrasonics can be used to detect disbonding of the overlay, nondestructive methods are also required to detect weak adhesive bonding. This paper describes a preliminary experimental investigation of the existence of ultrasonic leaky interface waves between overlays and 2024-T351 aluminum alloy and D6ac steel substrates, and their possible use for detection of weak bonds. The approach used was to excite these leaky waves using Rayleigh waves generated by a laser line-source on the substrate. The existence of the leaky waves, traveling normal to the composite fibers, between boron-epoxy overlays and either an aluminum or steel substrate, was confirmed by observations of the quasishear pulses leaked to the top of the overlay, and of the Rayleigh wave transmitted to the free surface beyond the overlay. When quantitative measurements were possible, measured values of the complex interface wave speed agreed satisfactorily with predicted values. This paper also demonstrates one case for which leaky interface waves are sensitive to weak bonding caused by the presence of a contaminant on the substrate surface prior to bonding: application of a low-concentration aqueous solution of hydraulic oil to the aluminum substrate prior to bonding of the overlay caused a marked increase, over that for the well-bonded case, in the observed amplitude of the quasi-shear pulses leaked to the top of the overlay by decay of the interface wave. More experiments are needed to establish whether leaky interface waves are sensitive to bond condition for various other combinations of substrate, overlay and contaminant.     

Effect of periodic surface roughness on V(z) curves for the line-focus acoustic microscope

A specimen with a periodic surface profile is considered to estimate the effect of surface roughness on the V(z) curve for the line-focus acoustic microscope. The Fourier optics approach is used to obtain the response of the lens and the Rayleigh-Fourier method is used to obtain the reflection coefficients for plane wave incidence from the fluid side on the periodic surface. An integral expression is obtained to calculate V(z) curves for periodic surface profiles. The V(z) curves are used to calculate the leaky Rayleigh wave velocities by applying the fast Fourier technique. Numerical results are presented to display the effect of sinusoidal surface roughness on the V(z) curves and the corresponding leaky Rayleigh wave velocities.     

Analysis of leaky-surface-wave propagating under periodic metal grating

A detailed field analysis is presented for a leaky surface wave propagating under a periodic metal grating, using a theory that neglects the effect of mass loading due to the grating. The approach is based on Floquet's theorem and the coupled equations of wave motion with unperturbed mechanical and perturbed (or periodic) electrical boundary conditions, yielding a general field solution applicable to any material and to arbitrary connections to the grating. As a key step, the periodic boundary equations are solved by combining them into a set of infinite homogeneous equations through algebraic treatment and performing orthogonal integration with respect to space harmonics. The advantage in using this method results from there being no need to use assumptions or complicated expressions anticipating an accurate solution if sufficient space harmonics are considered. It is shown that the theory proposed here can be directly extended to solve simpler SAW problems. An analysis is carried out for LiNbO(3) for both the leaky wave and Rayleigh wave, taking into account dispersion relations, propagation attenuation of the leaky wave, and other field distributions. Theoretical and experimental results for the width of the first stopband are discussed.     

Fast, all-optical Rayleigh wave microscope: imaging on isotropic and anisotropic materials

A fast, non-contact Rayleigh wave scanning microscope is demonstrated, which is capable of scan rates of up to a maximum of 1000 measurements/s with typical speeds of up to 250 measurements/s on real samples. The system uses a mode-locked, Q-switched Nd:YAG laser operating at a mode-locked frequency of 82 MHz and a Q-switch frequency of 1 kHz. The Q-switch frequency determines the upper limit of the scanning rate. The generating laser illumination is delivered and controlled by a computer-generated hologram (CGH). The generating laser produces around 30 pulses at 82 MHz and additional harmonics at 164 and 246 MHz and above. The microscope can operate at these harmonics provided the spatial bandwidth of the optics and the temporal bandwidth of the electronics are suitable. The ultrasound is detected with a specialized knife-edge detector. The microscope has been developed for imaging on isotropic materials. Despite this, the system can be used on anisotropic materials, but imaging and interpreting images can be difficult. The anisotropy and grain structure of the material can distort the Rayleigh wavefront, leading to signal loss. A model has been developed to simulate polycrystalline-anisotropic materials; this is discussed along with possible solutions that would overcome the problems associated with anisotropy. Rayleigh wave amplitude images are demonstrated on silicon nitride at 82 and 164 MHz and on polycrystalline aluminium at 82 MHz.     

Periodic Green's functions analysis of SAW and leaky SAWpropagation in a periodic system of electrodes on a piezoelectriccrystal

A method of periodic Green's functions with a propagation factor exp(iβx), unknown in advance, is used to calculate dispersion curves and attenuation coefficients for Rayleigh- and leaky- waves propagating in a periodic system of thin electrodes on a piezoelectric surface. To describe the charge distribution on the electrodes both a step approximation and Chebyshev polynomials are used, the last being more adequate in most cases. Numerically determined values of the Green's function are used and interpolated either linearly or using a modified variant of Ingebrigtsen's formula. Such basic parameters as stopband width, stopband center frequency, wave velocity and attenuation in the stopband are found. These parameters can be used in the coupling-of-modes (COM) analysis and design of SAW devices. The analysis includes bulk wave radiation and scattering. The dependence of the corresponding attenuation coefficient on frequency is determined. Results obtained allow the determination directly and properly of the COM parameters and the design of SAW devices having large number of electrodes most precisely and rapidly. Numerical results for Rayleigh waves on YZ-LiNbO₃ and leaky waves on 36°YX-LiTaO₃ substrates are presented     

Propagation properties of longitudinal leaky surface waves on lithium tetraborate

Theoretical and experimental results of longitudinal leaky surface waves with a higher phase velocity than that of ordinary leaky surface waves and a low propagation loss on lithium tetraborate (LBO) are investigated in detail. They propagate along the surface with a phase velocity close to that of longitudinal bulk wave, slightly radiating two kinds of shear bulk waves (or one shear bulk wave in the case that one of two shear wave terms is uncoupled) into the solid. Most surface components of the mode consist of a longitudinal wave term and an electromagnetic wave term. The detailed propagation properties of the longitudinal leaky surface waves on LBO with the Euler angles (phi, theta, 90 degrees ) are investigated theoretically and experimentally. The (011) cut of LBO was found to be desirable for higher frequency SAW devices. One of the reasons why that mode on LBO has a low propagation loss is also discussed.