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.     

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     

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     

激光超声中首达纵波脉冲的一种新解释

当脉冲激光辐射到样品物体上时,会产生脉冲超声波(包括体波和表面波)。利用脉冲激光作为激励源,可避免与样品的机械接触,可对样品实行远距离的非接触式无损检测。 传统的光声理论利用了经典热弹耦合方程(CTE)该理论假设热扩散的速度为无限大,它的热传导方程是一个抛物型方程。人们利用这个方程在解释实验上     

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.     

研究激光激发的声表面波与材料近表面缺陷的振荡效应

根据激光激发声表面波的热弹运动方程及热传导方程, 采取有限元技术对方程进行求解, 得到声表面波传播波形图. 当声表面波经过近表面缺陷时, 声表面波与近表面缺陷之间产生一种振荡效应, 通过近表面缺陷的振荡波形幅值存在一个逐渐增加后又逐渐减小的过程. 当声表面波经过不同深度的近表面缺陷时, 振荡信号中心频率存在一定的变化规律. 数值仿真结果表明: 当近表面缺陷深度从0.1 mm到0.5 mm变化时, 振荡效应产生的振荡信号中心频率从0.4 MHz到0.76 MHz变化, 振荡信号中心频率与近表面缺陷深度呈近似线性关系, 这为近表面缺陷的定量检测提供了一种理论基础.     

声表面波介质表面受力条件下的波速变化研究

为了计算力负载直接作用在声表面波传播表面时波速的变化特性,通过有效材料系数将负载引入,并用广义Green函数计算声速,得到了数值计算的结果,并用声表面波谐振器进行了实验.实验结果表明,将力负载直接加载在声表面波的传播表面时,波速变化量和力负载大小近似为线性关系,该加载方式下,实验中声表面波谐振器的谐振频率对外加质量负载有较高的灵敏度,可达1900 Hz/g.     

短路金属栅对正入射瑞利波的散射

由压电晶体动力学方程等出发,用电表面边界微扰技术得到了入射声表面波在栅区激发的微扰电场及电荷分布;将这些电荷作为表面激发源,用积分变换得到了包括散射声表面波和体波的一般积分解。数值计算了YZ-LiNbO_3,表面上短路金属栅对正入射瑞利波散射的声表面波,给出了其反射系数,前向散射系数等与栅条宽度的关系;进而导出了声表面波经过金属栅时的相速变化。理论计算与实验测量结果符合较好。     

超声波波型转换与表面波的检测

分析了超声波纵波、横波、表面波3种波型的产生条件和声速关系,检测了超声波在固体表面的波型转换,并测量了声表面波的速度,讨论了声表面波在超声延迟线方面的应用.     

声表面波渡越时间法检测黏弹性介质表面凹痕深度的研究

研究声表面波渡越时间法检测黏弹性介质表面凹痕的深度.基于频域动力学平衡方程,采用有限元方法分别在有表面凹痕的弹性和黏弹性介质中,建立激光激发声表面波的数值模型.研究材料的黏性引起的声表面波波速和频散的变化,对表面凹痕检测的影响,在此基础上,根据反射和透射声表面波的传播路径及到达时间,利用渡越时间法检测黏弹性介质中的表面...     

基于单孔洞近似的高纯铝部分层裂实验的数值模拟研究

基于单孔洞近似,对不同撞击速度下高纯铝的部分层裂实验进行了数值模拟研究,讨论了微孔洞长大对波传播的影响及其在自由面速度波剖面上的表现. 通过分析微孔洞周围的应力场变化,认识到实测自由面速度波剖面出现"回跳"特征并不能说明材料发生完全层裂,其直接原因是样品内部微孔洞长大所引起的局部卸载效应. 将计算得到的自由面速度波剖面和微孔洞相对体积与实验结果进行了对比分析,发现两者均符合很好,表明采用单孔洞增长来近似描述部分层裂样品中随机损伤发展及其对波传播的影响是可行的. 关键词： 层裂 孔洞增长 自由面速度波剖面 微孔洞相对体积     

Soda lime玻璃材料中失效波形成和传播研究

 为了研究冲击载荷作用下Soda lime玻璃材料中失效波的形成和传播,通过轻气炮加载平板撞击实验,采用双螺旋锰铜压阻传感器,在一发实验中同时测量4种不同厚度试件背面与有机玻璃背板间界面处的纵向应力时程曲线,根据测量结果得到试件中失效波的传播轨迹。通过改变碰撞速度,对不同加载条件下的失效波形成和传播规律进行了研究,结果表明,Soda lime玻璃材料在冲击作用下产生失效波所需的延迟时间随冲击载荷的增加而减小,失效波传播速度随冲击载荷的增加而增加。最后采用弹性微裂纹统计模型描述冲击载荷作用下Soda lime玻璃的破坏机制,并将模型嵌入LS-DYNA有限元程序中,模拟试件在不同加载条件下的平板碰撞,所得横向应力和自由面粒子速度曲线均可用于表征失效波破坏现象。根据数值模拟结果分析失效波的传播轨迹,与实验测量结果符合较好。     

管间界面特性对周向超声导波传播特性的影响

采用界面弹簧模型对圆管结构的管间界面特性进行描述, 推导出含弱界面的圆管结构中声波沿周向传播时的位移场及应力场的数学表达式. 在此基础上采用导波的模式展开分析方法, 给出了与管间界面特性及激励源密切相关的周向超声导波模式展开系数的解析表达式. 数值分析了管间界面特性的变化对周向超声导波的频散和声场产生的影响. 理论与数值分析结果表明, 通过选择适当的驱动频率及周向导波模式, 可使周向超声导波的相速度及圆管外表面的位移场随管间界面特性的变化表现出非常敏感且单调的性质. 这一结果有助于采用周向超声导波方法准确定征圆管结构的管间界面特性.     

Propagation of Bleustein-Gulyaev waves in a prestressed layered piezoelectric structure

Based on the theories of nonlinear continuum mechanics, piezoelectricity and elastic waves in solids, theoretical analysis of Bleustein-Gulyaev surface acoustic wave propagation in a prestressed layered piezoelectric structure are described. Numerical calculations are performed for the case that the layer and the substrate are identical LiNbO(3) except that they are polarized in opposite directions. It is found that an almost linear behavior of the relative change in phase velocity versus the initial stress is obtained for both surface electrically free and shorted cases. Potential applications in the design of acoustic wave devices are suggested.     

Stable gravity wave of arbitrary amplitude in finite depth

A combination and modification of two existing methods, which involves balancing static and dynamic pressure differences between points along the surface and conserving mass through cross sections below the surface in the reference frame moving with the phase velocity, is applied to surface gravity waves of arbitrary amplitude in water of finite depth. For a given still water depth and wave height the method determines in closed form the phase velocity, wavelength, and wave profile of the stable wave. The main assumption is that the horizontal component of the fluid velocity be independent of depth. The motion is not assumed to be irrotational. The wavelength of the stable wave is found to be about 3.6 times the still water depth for infinitesimal amplitude, and at finite amplitude the wavelength decreases as the amplitude increases. Therefore, shallow water waves are concluded to be unstable even at infinitesimal amplitude, for which the assumption is accurate. Previously it has been argued that only at finite amplitude will shallow water waves change form as they propagate. The wave profile is found to be sinusoidal for infinitesimal amplitude and to be asymmetric at finite amplitude, the crests being higher and narrower and the troughs shallower and broader. These results are consistent with well-known theoretical work and laboratory measurements.     

Coating thickness affects surface stress measurement of brush electro-plating nickel coating using Rayleigh wave approach

A surface ultrasonic wave approach was presented for measuring surface stress of brush electro-plating nickel coating specimen, and the influence of coating thickness on surface stress measurement was discussed. In this research, two Rayleigh wave transducers with 5 MHz frequency were employed to collect Rayleigh wave signals of coating specimen with different static tensile stresses and different coating thickness. The difference in time of flight between two Rayleigh wave signals was determined based on normalized cross correlation function. The influence of stress on propagation velocity of Rayleigh wave and the relationship between the difference in time of flight and tensile stress that corresponded to different coating thickness were discussed. Results indicate that inhomogeneous deformation of coating affects the relationship between the difference in time of flight and tensile stress, velocity of Rayleigh wave propagating in coating specimen increases with coating thickness increasing, and the variation rate reduces of difference in time of flight with tensile stress increasing as coating thickness increases.     

Measurement of velocity variations along a wave path in the through-thickness direction in a plate

In this paper there is given a method to predict ultrasonic wave velocity variations along a wave path in the through-thickness direction in a plate from thickness resonance spectra. Thickness resonance spectra are numerically calculated and two simple rules used to predict the entire ultrasonic wave velocity variation are derived. In the calculation, the wave path is assumed to be straight along the thickness direction and the velocity variation is assumed to be either as a parabolic curve dependence or a linear dependence with respect to the distance from the surface and to be symmetric with respect to the plate center. To see if the numerical calculation method is reliable, thickness resonance frequencies of a sample with three-layers were measured by EMAT (electromagnetic acoustic transducer) with a good agreement between the measured and the calculated frequencies. This method can be applied to the ultrasonic measurement of material characteristics, internal stress or various other properties of plate materials.     

Local surface skimming longitudinal wave velocity and residual stress mapping

Local variation in surface skimming longitudinal wave (SSLW) velocity has been measured using a scanning acoustic microscope. A very narrow width electrical impulse has been used to excite the transducer of the acoustic lens. This permits the separation of the SSLW signal from the direct reflected signal in the time domain. A simple method of measuring the time delay between the directly reflected signal and the SSLW signal at two defocuses has been utilized for the local measurement of SSLW velocity. The variation in the SSLW velocity measured over an area of the sample is scaled and presented as an image. The method has been implemented to image the variation of the SSLW velocity around a crack tip in a sample of Ti-6Al-4V. Since the SSLW velocity is known to change linearly with the stress, the SSLW velocity image is considered as a representation of the image of stress around the crack tip. Local stress variation in the same region of the crack tip is directly measured using x-ray diffraction. The SSLW velocity image is compared with the x-ray diffraction stress image. The contrast in the two images, spatial resolution, and the penetration depth into the sample of acoustic waves and x rays are discussed.     

Depression of the ferroelastic phase transition in LiCsSO<Subscript>4</Subscript> by uniaxial stress

In the lithium-caesium sulphate crystal in the temperature range of ferroelastic phase transition, the uniaxial stress σ _X induced changes in velocity and attenuation of the longitudinal ultrasonic wave propagating in the direction [010] are studied. The phase transition is close to the three-critical point and the critical exponent is κ = 0.27 ± 0.02. The stress applied drastically decreases the stepwise change in the wave velocity at T _C up to its disappearance at 2 MPa. In the temperature range between T _C and T _C − 6 K, the stress leads to an increase in the wave velocity and a decrease in its attenuation. This range was interpreted as that of co-existence of ferroelastic and incommensurate ordering, in which the stress influences the density of solitons leading to stiffening of the crystal lattice.     

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.