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...     

激光在近表面弹性性质梯度变化的材料中激发超声波的数值分析

激光激发超声波为评价材料近表面弹性性质提供了有效的手段. 考虑到由于冲击硬化、表面热处理、表面氧化等引起的金属材料近表面层弹性性质的变化，建立了一种激光在基底上的梯度材料中激发超声波的理论模型. 用有限元方法模拟了热弹条件下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，研究了近表面层离散的层数对超声波的产生和传播的影响，并分别讨论了表面层“变硬”和“变软”两种情况下声表面波的模式变化及用二维傅里叶变换得到各模式的色散曲线. 为进一步研究近表面层的弹性性质建立合理的计算模型及材料性质的反演提供了理论依 关键词： 超声波 有限元法 近表面弹性性质 色散     

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.     

利用PVDF传感器检测激光超声的实验研究

简要介绍了激光超声技术以及声表面波的基本特点、激光超声产生和接收的基本原理及激光超声技术的应用。概述了聚偏二氟乙烯（PVDF）压电薄膜材料的结构、性质和应用，以及薄膜压电性产生的机理。对PVDF换能器的设计思路和实验方法进行了简单讨论。具体实验采用脉冲激光器激发声表面波，利用PVDF传感器接收实验信号，调试实验信号，得出波形，并对实验现象作出初步分析。证实了该实验装置应用于激光超声无损检测的可行性与可靠性。     

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。     

利用光差分技术检测激光激发声表面波定征薄膜材料

利用激光激发声表面波及改进的光差分检测技术,研究沉积在基片表面的薄膜样品的特性和参量。根据实验获得的声色散曲线和基片已知参数,结合层状媒质中声表面波传播理论,拟合得到薄膜材料的弹性模量、密度和厚度等物理量。结合扫描电子显微镜和X光衍射结果分析,并与文献值比较,表明所采用的技术是定征薄膜材料的弹性模量和力学参量的有效方法。     

利用激光超声技术研究表面微裂纹缺陷材料的低通滤波效应

利用激光激发声表面波的理论模型,研究了被激发宽带声表面波在具有表面微裂纹缺陷金属材料上的传播特性.对具有不同形状的表面缺陷模型进行了数值分析.结果表明:表面微裂纹缺陷有明显的低通效应,缺陷深度越大高频截止频率就越低,缺陷深度与低通滤波的截止频率呈近似线性关系;缺陷的宽度增大对表面波透射能量有明显的衰减作用. 关键词： 激光超声 声表面波 有限元方法 低通滤波器     

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.     

退火温度对声表面波检测器电极 表面粗糙度的影响*

该文实验研究了退火温度对声表面波检测器电极表面粗糙度的影响。电极表面的粗糙度随着退火温度不同而变化,实验中分别选择常温(25?C)、200?C和300?C作为退火温度对两种镀膜方式制备的声表面波器件进行退火,最后得到退火温度和电极表面粗糙度的对应关系。从实验结果来看,退火温度为200?C时,得到的电极表面粗糙度最大。该研究为声表面波检测器表面粗糙度优化及灵敏度提升提供了基础。     

激光激发声表面波在缺陷板材中散射过程的有限元分析

利用有限元法模拟了金属板材中激光激发的声表面波经过缺陷位置时发生散射的瞬态过程,采用线状激光源作为超声导波的激发源.针对三种不同深度的表面缺陷以及三种亚表面缺陷的模型进行了对比计算,结果显示缺陷的深度及位置对声表面波的时域特征存在显著的影响.表面缺陷深度越深将产生较大幅度的表面反射回波,亚表面缺陷的影响将取决于缺陷上顶面距离板材上表面的距离.因此,数值模拟结果表明通过分析激光产生的表面波形可以判定近表面缺陷的尺寸和所处的位置.     

环形定子的激光致表面波机理及可视化探测研究

开展了用于新型激光驱动马达的环形定子的激光致表面波机理及实验研究. 提出一种带有凹槽阵列结构的环形定子新设计, 建立了激光在环形定子表面激发表面波的物理模型, 揭示了影响表面波幅值的关键因素; 采用一种新颖的激光致表面波可视化探测方法, 在波长1053 nm, 脉宽30 ns, 单脉冲能量1 mJ的激光激发下, 对表面波在铜质环形定子表面的传播特性进行了可视化探测实验. 理论与实验研究表明: 当激发光斑的位置紧邻凹槽阵列时, 沿着圆环向凹槽方向传播的表面波会被齿状凹槽阵列迅速衰减和吸收, 而沿着圆环向远离凹槽方向传播的表面波能够持续传递, 从而首次实现了激光致表面波在环形定子上的单向传播; 而对没有凹槽阵列结构的圆环进行的对比实验表明, 激光致表面波在圆环表面双向传播, 最终因相互混叠和串扰等而处于混乱状态. 由激光在该种环形定子表面激发出的单向表面波, 可望在光致表面波马达及驱动机构中获得应用. 关键词： 激光致表面波 环形定子 表面波可视化 激光驱动     

A comparison of laser ultrasound measurements and finite element simulations for evaluating the elastic properties of tissue mimicking phantoms

Advances in the field of laser ultrasonics have opened up new possibilities in medical applications. This paper presents a finite element modelling technique, which studies laser generated surface acoustic waves in different concentration of soft tissue mimicking agar-agar phantoms. In addition, we propose a novel approach that utilises a low coherence interferometer to detect the laser-induced surface acoustic waves from the tissue mimicking phantoms. A Nd:YAG focused laser line-source is applied to the agar-agar phantoms, which as the same with the FE simulation. The generated SAW signals are detected by a time domain low coherence interferometry system. SAW phase velocity dispersion curves from both of the FE simulation and experiment are calculated. By comparison, we show that the experimental results agree well with those of the FE simulation and theoretical expectations. This study is the first report that a laser-generated SAW phase velocity dispersion technique is applied to soft materials. This technique may open a way for laser ultrasonics to detect the mechanical properties of soft tissues, such as skin.     

Frequency dependence of laser ultrasonic SAW phase velocities measurements

Advances in the field of laser ultrasonics have opened up new possibilities in applications in many areas. This paper verifies the relationship between phase velocities of different materials, including hard solid and soft solid, and the frequency range of SAW signal. We propose a novel approach that utilizes a low coherence interferometer to detect the laser-induced surface acoustic waves (SAWs). A Nd:YAG focused laser line-source is applied to steel, iron, plastic plates and a 3.5% agar–agar phantom. The generated SAW signals are detected by a time domain low coherence interferometry system. SAW phase velocity dispersion curves were calculated, from which the elasticity of the specimens was evaluated. The relationship between frequency content and phase velocities was analyzed. We show that the experimental results agreed well with those of the theoretical expectations.     

Laser-ultrasonic surface wave dispersion measurements on surface-treated metals

Surface acoustic wave (SAW) velocity spectroscopy has been long considered to be one of the leading candidates for nondestructive characterization of surface-treated metals because of its ability to probe the material properties at different penetration depths depending on the inspection frequency. We developed a high-precision laser-ultrasonic technique to study the feasibility of SAW dispersion spectroscopy for residual stress assessment on shot-peened metals. This technique is capable of measuring SAW dispersion with a relative error of 0.1% over a frequency range from 2 to 15 MHz. Our experimental results obtained from shot-peened aluminum 2024-T351 samples indicate that the dispersion of the surface wave is a superposition of different effects of surface treatment in the material, including surface roughness, compressive residual stress, and cold work. Although the surface roughness induced component is often the dominating part of the overall dispersion, the experimental results also indicate that it is feasible to observe a perceivable change in the dispersion of the SAW when the specimen is heat-treated at different temperatures, which has no perceivable effect on the surface roughness. The part of the dispersion, which changes during annealing via thermal relaxation, is due to near-surface residual stresses and the decay of texture, although at high frequencies nonuniform grain coarsening could also play a significant role.     

Effects of surface roughness on surface acoustic wave propagation in semiconductor materials

The effect of surface roughness on adhesion and tribological properties of films and interfaces is of key importance. Therefore, it is of utmost importance to be able to measure this quantity and to predict the effects that different roughness levels may cause. Roughness affects the propagation of surface acoustic waves on a material but there is little useful quantitative data on the topic. This work investigates the dispersive effect of roughness on surface acoustic wavepackets (30-200 MHz frequency range) for different degrees of nanometer roughness on silicon (0 0 1) and (1 1 1) surfaces, we show that the roughness-induced frequency dispersion effect is significant, and that although available theories agree qualitatively with the results, the theory is not adequate to predict the real SAW dispersion. These experimental results have considerable implications for design of SAW devices, for accuracy of Brillouin spectroscopy measurements, and for possible applications to non-destructive testing of materials. Previously unknown dispersive effects on anisotropic crystal surfaces are also demonstrated.     

MOCVD法生长SAWF用ZnO/Diamond/Si多层结构

使用等离子体辅助MOCVD系统在金刚石,硅衬底上成功地制备了氧化锌多层薄膜材料,通过两步生长法对薄膜质量进行了优化。XRD测试显示优化后的样品具有c轴的择优取向生长,PL谱测试表明样品经优化后不仅深能级发射峰消失,同时紫外发射峰增强。对优化后的样品的表面测试显示出较低的表面粗糙度。比较氧化锌多层薄膜结构的声表面波频散曲线,ZnO薄膜声表面滤波器受膜厚和衬底材料的影响较大。当ZnO薄膜较薄时,在它上面的传播速度将与衬底上的传播速度接近,与其他衬底上生长的薄膜相比,以金刚石这种快声速材料为衬底的ZnO多层薄膜结构,声表面波滤波器的中心频率将提高1倍左右。     

Photoacoustic diagnosis of human teeth using interferometric detection scheme

We have investigated the mechanical and acoustic properties of human teeth using the laser generation of surface acoustic wave (SAW) technique. The materials investigated included normal and decayed teeth, which have similar grain sizes and different thicknesses. The tissue responds to the laser-induced stress by thermoelastic expansion. The informative features of this method allow one to determine sample thermal, optical, and acoustical properties that depend on the peculiarities of the sample compositional structure. An interferometric detection experimental scheme is applied for detection generated SAW pulses. The surface displacement curves shape of normal and decayed human teeth are shown. The dispersion curves for SAW pulses were determined by Fourier analysis. The result is an almost linear dependence of SAW velocity on frequency for a normal tooth, the magnitude of the thermoelastic expansion of the normal tooth reaches its peak at 0.344 μs, a SAW phase velocity of 2500 ms^?1 between 0.0008 and 5 MHz was determined. For abnormal teeth, the magnitude of thermoelastic expansion of the normal tooth reaches its peak at 1.3 μs, the measured velocity was 3225 ms^?1. Due to the inhomogeneity of abnormal teeth perpendicular to the propagation direction, strong differences in their dispersion curves were obtained. The detection of acoustic waves is the basis of photoacoustic methods, which can be used for diagnostic purposes.     

Investigation of titanium nitride coating by broadband laser ultrasonic spectroscopy

We present a laser ultrasonic method to investigate a titanium nitride(TiN) coating specimen.The technique is based on the principle of surface acoustic wave (SAW) dispersion during acoustic propagation on a half-space with the presence of a thin layer.Due to the high efficiency of laser line-source excitation,we have been able to generate and detect a SAW with an excellent signal-to-noise ratio in a wide frequency band.An inverse fitting algorithm was employed to extract simultaneously the thickness and the elastic parameters of the TiN coating from the experimental SAW velocity dispersion curve.     

Energy accumulation effect and parameter optimization for fabricating of high-uniform and large-area period surface structures induced by femtosecond pulsed laser

In this paper, theoretical and experimental investigations are performed to understand the effect of laser energy accumulation on morphologies of femtosecond laser-induced periodic surface structures (LIPSSs). Based on energy accumulation effect we put forward, scanning velocity, distance of two adjacent scan lines and pulsed laser energy density are optimized to fabricate high-uniform and large-area period surface structures for the first time. Our theoretical results can offer clues to choose experiment parameters in order to obtain high-quality LIPSSs which have extensively emerging and potential applications in optoelectronics, phononics and so on.     

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.