首页 | 本学科首页   官方微博 | 高级检索  
相似文献
 共查询到20条相似文献,搜索用时 31 毫秒
1.
Analytical expressions for the dispersion of the phase velocity and the inverse attenuation length of Rayleigh waves are derived with allowance made for a thin (as compared to the length of the surface wave) isotropic damaged surface layer that is contiguous with vacuum and located on the surface of a hexagonal crystal with the sixfold axis perpendicular to the surface. It is demonstrated that, in the limit of long wavelengths (as compared to the characteristic inhomogeneity size), which is of greatest interest for experimenters, the change in the dispersion of the phase velocity of Rayleigh waves is proportional to the second power of the frequency, whereas the inverse attenuation length of Rayleigh waves is proportional to the fifth power of the frequency. The inverse attenuation length of the Rayleigh wave is calculated numerically. The calculation method previously proposed by one of the authors (Kosachev, 1998) is generalized to the case of an isotropic damaged layer on an anisotropic (hexagonal) substrate.  相似文献   

2.
The Spectral Analysis of Surface Waves (SASW) is a popular technique in seismics for imaging the ground subsurface. It uses the dispersive properties of Rayleigh waves in a transversely homogeneous, multilayered medium. The SASW approach is being transposed into the civil engineering domain to characterize subsurface damage in concrete structures. Such a damage consists in a few millimeters thick surface layer with porosity slightly higher than in the sound material. It is induced by contact with moisture or chemicals at the surface of the structure and may facilitate penetration of aggressive agents. In this study, two-layered mortar samples are made to mimic concrete cover damage in real structures. The dispersive behavior of Rayleigh waves arises when the wavelength is comparable to the thickness of the first layer. Given the small thickness of this layer, it requires increasing the frequency up to several hundreds of kHz, which means high attenuation and low signal-to-noise ratio. Rayleigh waves with 0.5 MHz central frequency are generated into the samples by the wedge method. Phase velocity dispersion curves are obtained by broadband phase spectroscopy from signals received at various distances from the source. The signal processing is first validated on simulated signals with known dispersion law. Then, the measured dispersion curves are compared with the theoretical curve for a two-layered medium, following Haskell's approach. The measured curve displays the general behavior expected from theory. However, a three-layered, visco-elastic model would be necessary to get a better fit and to estimate more accurately the parameters of each layer.  相似文献   

3.
The Frenkel-Biot theory is used to study the reflection of elastic waves from the boundary of a non-Newtonian (Maxwell) fluid-saturated porous medium. The velocity and attenuation of a Rayleigh surface wave propagating along the boundary of the medium are determined. Two models of a fluid-saturated porous medium are used for calculation: with pore channels of a fixed diameter and with a lognormal distribution of pore channels in size. The results of calculations show that, when the fluid in the porous medium is characterized by a small Deborah number (i.e., exhibits non-Newtonian properties), the velocity of Rayleigh waves exhibits a considerable frequency dispersion. The results also suggest that, in principle, it is possible to estimate the Deborah number from the measured frequency dispersion of the Rayleigh wave velocity.  相似文献   

4.
激光激发黏弹表面波有限元数值模拟   总被引:1,自引:0,他引:1       下载免费PDF全文
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  相似文献   

5.
分层介质半空间瑞利波的时频分析   总被引:6,自引:3,他引:3  
对分层介质半空间瑞利波的频散特性,用一种时频分析方法——重排的平滑伪魏格纳维尔分布(RSPWVD,Reassignment of Smooth Pseudo-Wigner Ville Distribution)进行了分析和研究。对均匀半空间和两层介质半空间的理论和实验研究表明,由于层状介质中瑞利波的频散曲线存在多个模式,所获得的群速度频散曲线在不同的频段显示出来的模式是对应表面位移幅度占主导作用的模式。频散曲线的这种模式判定对利用层状半空间的瑞利波反演介质参数是必须预先了解的。  相似文献   

6.
Bonello B  Charles C  Ganot F 《Ultrasonics》2006,44(Z1):e1259-e1263
We have studied the propagation of a surface acoustic waves (SAW), in a structure constituted by a 2D phononic film (a few micrometers thick and having lattice constants of a few hundreds of micrometers in the two directions of the propagation plane) deposited onto a homogeneous semi-infinite substrate. First, we have calculated the dispersion relations of the acoustic modes by using a plane waves expansion method. We found that the surface branch exhibits both the folding effect and a band gap for the propagation along some particular directions. This is a very interesting result which demonstrates that the effects related to the existence of the band gap (sound velocity dispersion, diffraction, refraction, ultrasound tunneling, etc.) can all appear, even if the thickness of the phononic film is much less than the penetration depth of the SAW. Then, we used an all-optical technique to monitor the spectral content of the SAW propagating along the GammaX direction in the reduced Brillouin zone. We show that a wave with frequency in the stop band, is destructively diffracted after it propagates through less than ten periods. Finally, we report on measurements of the Rayleigh wave phase velocity and we show that the transit time is independent of the distance traveled inside the phononic crystal, suggesting that tunneling trough the sample is involved.  相似文献   

7.
An inhomogeneous layer element method is presented to analyze the dispersion of waves and characteristic wave surfaces in plates of functionally graded piezoelectric material (FGPM). In this method, the FGPM plate is divided into a number of layered elements. The elemental elastic and electric properties are assumed as linear functions of the thickness to adopt the variety of the material property of FGPM. The Hamilton principle is applied to determine the governing equations. The phase velocity surface, phase slowness surface, phase wave surface, group velocity surface, group slowness surface, and group wave surface for FGPM plate are formulated using Rayleigh quotient and the orthogonality condition of the eigenvectors. These six surfaces are then used to illustrate the characteristics of waves in FGPM plates. Numerical examples are presented using the present formulations to analyze dispersions and characteristics of waves in FGPM plates.  相似文献   

8.
The excitation and propagation of the guided waves in a stratified half-space and a Rayleigh wave exploration method in shallow engineering seismic exploration are studied in this paper. All the modes of the guided waves are calculated by the bisection method in the case where the low velocity layers are contained in a stratified half-space. Cases when the formation shear wave velocity gradually decreases from the top to the bottom layers are also studied. The dispersion curves obtained in actual Rayleigh wave exploration are usually noncontinual zigzag curves, but the dispersion curves given by the elastic theory for given modes of the guided waves are smooth and continual curves. In this paper, the mechanism of zigzag dispersion curves in Rayleigh wave exploration is investigated and analyzed thoroughly. The zigzag dispersion curves can give not only the possible positions of the low-velocity layers but also the other information on the formation structure (fractures, oil, gas, etc.). It is found that the zigzag dispersion curves of the Rayleigh wave are the result of the leap of the modes and the existence of low velocity layers in a stratified half-space. The effects of the compressional wave velocity, shear wave velocity, and density of each layer on zigzag dispersion curves and the relationship of the low velocity layers to zigzag dispersion curves are also investigated in detail. Finally, the exploration depth of the Rayleigh wave is discussed. The exploration depth of the Rayleigh wave is equal to the wavelength multiplied by a coefficient that is variable and usually given by the work experience and the formation properties of the local work area.  相似文献   

9.
Cho YS 《Ultrasonics》2002,40(1-8):227-230
This study presents the results of the non-destructive testing using spectral analysis of surface waves (SASW) based on high-strength concrete materials. This SASW method was used to evaluate the compressive strength of single-layer high-strength concrete slabs through a correlation with the surface wave velocities. This paper also presents the relationship between the theoretical and experimental compact dispersion curves when the SASW test is applied to multi-layer thin high-strength concrete slab systems with a finite thickness. The test results show that the surface wave velocity profile obtained from the theoretical dispersion curve has lower values than the profile obtained from the experimental compact dispersion curve under the condition of a finite thickness due to different boundary conditions and reflections from the boundaries. Based on the measured response, an experimental study was conducted to examine if the dispersive characteristics of Rayleigh wave exist in the multi-layer high-strength concrete slab systems. This study can be utilized in examining structural elements of high-strength concrete structures and can also be applied in the integrity analysis of high-strength concrete structures with a finite thickness.  相似文献   

10.
 三折螺旋波纹波导使TE11模和TE21模相互耦合,其色散曲线能够在宽频带内与电子回旋共振,因此需要对色散方程进行研究。提出了适合工程实用的行波模式和返波模式的色散方程,并对方程中的耦合系数进行了简化,误差在1%左右。利用CST软件的VBA语言对螺旋波纹波导进行建模和计算,根据模拟得到的传输特性曲线的特点,提出一种模拟方法,将模拟与理论计算得到的色散曲线作对比,误差在5%左右。  相似文献   

11.
左手系材料界面上的非线性TE电磁波   总被引:4,自引:4,他引:0  
针对电磁波在非线性左手系材料中的传播性质, 分别研究了左、右手系材料界面以及两左手系材料界面上非线性TE表面波的传播行为讨论了导波的频率特性、色散关系以及群速度随频率的变化规律分析表明,两种界面上的非线性TE表面波均存在频率通带和禁带,且带宽是传播功率的函数揭示了在一定条件下,左、右手系材料界面上既可以支持正向传播的非线性TE表面波,也可以支持反向传播的非线性TE表面波;两左手系材料界面上表面波的传播性质因材料参量的变化差异较大,一定参量条件下,该界面上仅支持反向传播的非线性TE表面波.  相似文献   

12.
The effect of a weak surface, near-surface and interfacial inhomogeneity on the frequency dependence of the surface wave velocity and of the SH (shear horizontal) wave reflectivity in isotropic elastic media is studied analytically and numerically. The inhomogeneity is modeled as an infinite planar layer with continuously varying properties. Weak inhomogeneity may markedly affect the dispersion of the Rayleigh velocity and especially of the reflectivity. It is demonstrated how this effect, particularly pronounced at high frequency, depends on the extent of inhomogeneity. The material data for damaged and ideal concrete and several simple examples of inhomogeneity profiles are utilized for the numerical calculations based on the Peano expansion. The use of explicit low- and high-frequency approximations is also exemplified. Among these, simple WKB asymptotics are shown to be particularly helpful for the Rayleigh velocity in the case of a prominent inhomogeneity attached to the surface and for the reflection on weak interfaces.  相似文献   

13.
含有孔隙的层状材料中声表面波传播特性的理论研究   总被引:2,自引:0,他引:2  
洪轲  袁玲  沈中华  倪晓武 《声学学报》2011,36(2):150-155
研究含有孔隙的层状材料中Rayleigh波的传播特性。采用本征函数展开法,并利用孔隙率与材料的弹性常数和密度之间的关系,模拟了不同孔隙率情况下铁基氧化铝层状材料中Rayleigh波的相速度色散曲线,分析了孔隙率对铁基氧化铝层状材料中Rayleigh波相速度的影响。Rayleigh波色散曲线的变化规律能同时反映层状材料中弹性模量、泊松比、密度和孔隙率的信息,为含有孔隙的层状材料参数的反演提供了理论依据。  相似文献   

14.
A flat transducer was constructed, using a 9-microm-thick PVDF (polyvinylidene fluoride) film for generation and detection of high-frequency ultrasonic waves, and used for measurements of the phase velocity of longitudinal waves traveling along the thickness direction in a very thin material. The transducer has a useful wideband frequency characteristic extending from 10 MHz to over 150 MHz. Measurements of the phase velocity of the longitudinal waves are carried out using a 0.212-mm-thick glass slide and a 0.102-mm-thick stainless-steel shim, using water as a coupling medium. The thickness limit for this measurement appears to be approximately 20 microm. The phase velocity of the longitudinal mode is obtained as a function of frequency in the frequency domain by using a modified sampled continuous wave (cw) technique. It can also be measured in the time domain by using a broadband pulse of short duration.  相似文献   

15.
分层固体板中导波的激发与频散特性   总被引:2,自引:0,他引:2       下载免费PDF全文
针对无限大弹性分层固体板,研究了结构中导波的频散和激发特性。首先使用传递矩阵法推导分层板模型中导波的频散方程,然后用二分法求取导波各模式的频散曲线,进而分析结构中导波的频散特性。结果表明:在速度递增或递减的分层板中,基阶模式和高阶模式的高频极限分别等于低速层的瑞利波速和横波波速。对于含低速夹层的分层板,所有模式的高频极限都等于低速层的横波速度。在导波激发特性方面,研究了在具有一定宽度的法向力源作用下的分层板中导波各模式在结构中的法向位移谱。发现在速度递增的分层板结构中基阶模式是主导模式,而对于速度递减和含低速夹层模型,主导模式在不同的频段范围内对应不同的导波模式。  相似文献   

16.
The properties of harmonic surface waves in an elastic cylinder filled with a liquid are studied. The case of elastic material for which the shear wave velocity is higher than the sound velocity in a liquid is considered. The wave motion is described based on the complete system of equations of the dynamic theory of elasticity and the equation of motion of an ideal compressible liquid. The asymptotic analysis of the dispersion equation in the region of large wave numbers and qualitative analysis of the dispersion spectrum showed that in such a waveguiding system there exist two surface waves, the Stoneley and the Rayleigh waves. The lowest normal wave forms the Stoneley wave on the internal surface of the cylinder. In this waveguide phase, velocities of all normal waves, except for the lowest one, have the velocity of sound in the liquid as their limit. Therefore, the Rayleigh wave on the external surface of the cylinder is formed by all normal waves in the range of frequencies and wave numbers in which phase velocities of normal waves of the composite waveguide and the lowest normal wave of the elastic hollow cylinder coincide.  相似文献   

17.
该文针对我国高速铁路轨道板缺陷的非接触动态检测问题,研究了空气耦合超声兰姆波在轨道板中的传播规律。首先,给出了轨道板中超声兰姆波的相速度和群速度频散曲线,结果表明:随着频厚积的增加,频散现象越明显,并且A0相速度收敛于Rayleigh波的波速。然后,建立轨道板中波传播的有限元模型,计算得到兰姆波传播的群速度为2220 m/s,且二维傅里叶变换系数的较大值沿Rayleigh波的频散曲线分布。最后,在沪杭高铁嘉兴南站进行了现场测试,以8.8°倾斜角向轨道板激励产生超声兰姆波,激发产生的兰姆波模态群速度为2325 m/s,且二维傅里叶变换分析其系数的较大值沿Rayleigh波的频散曲线分布。有限元计算结果和实验结果均与理论计算结果一致。该研究为后续轨道板缺陷的非接触动态检测提供了理论依据和实验方法。  相似文献   

18.
A new method is proposed for calculating the dynamic elastic constants of an isotropic plate from measurements of the impact-echo resonance and Rayleigh wave velocity. Poisson's ratio is shown to be a single-valued function of the ratio between thickness frequency and Rayleigh wave velocity. This dependence is derived theoretically from the condition of resonance at the minimum frequency of the first-order symmetric Lamb mode. A finite element model is developed to determine how this frequency varies with Poisson's ratio. The results obtained by modal analysis and the power-spectral density technique are in good agreement with those calculated as the solution of the S1 Lamb mode equation. The method is verified by impact-echo tests on concrete and methacrylate plates. A laser interferometer is used to detect the vibration. Thickness frequencies are accurately identified by applying the multicross-spectral density to the signals detected at several points close to the impact point. In a separate experiment, Rayleigh waves are generated by the mediator technique. The wave velocities are determined from the arrival times of the surface wave at several points. Finally, the main sources of uncertainty are evaluated.  相似文献   

19.
In this paper, Lamb wave propagation in a homogeneous and isotropic non-classical micro/nano-plates is investigated. To consider the effect of material microstructure on the wave propagation, three size-dependent models namely indeterminate-, modified- and consistent couple stress theories are used to extract the dispersion equations. In the mentioned theories, a parameter called ‘characteristic length’ is used to consider the size of material microstructure in the governing equations. To generalize the parametric studies and examine the effect of thickness, propagation wavelength, and characteristic length on the behavior of miniature plate structures, the governing equations are nondimensionalized by defining appropriate dimensionless parameters. Then the dispersion curves for phase and group velocities are plotted in terms of a wide frequency-thickness range to study the lamb waves propagation considering microstructure effects in very high frequencies. According to the illustrated results, it was observed that the couple stress theories in the Cosserat type material predict more rigidity than the classical theory; so that in a plate with constant thickness, by increasing the thickness to characteristic length ratio, the results approach to the classical theory, and by reducing this ratio, wave propagation speed in the plate is significantly increased. In addition, it is demonstrated that for high-frequency Lamb waves, it converges to dispersive Rayleigh wave velocity.  相似文献   

20.
窄频带Lamb波频散特性研究   总被引:1,自引:0,他引:1       下载免费PDF全文
李富才  孟光 《物理学报》2008,57(7):4265-4272
利用连续小波变换对从铝合金板结构中俘获的Lamb波信号进行分析,获得波信号在时间-尺度域的等高线和等高线脊线.根据Lamb波的频散特征、时间-尺度域等高线脊线的斜率和波在不同尺度下的到达时间,识别了Lamb波信号中各信息包的模式,并匹配出基础阶模式窄频带Lamb波在铝合金板结构中传播的实际频散曲线.对试验的Lamb波信号分析的结果表明该方法对于研究和应用窄频带Lamb波的频散特性是有效的. 关键词: 连续小波变换 Lamb波 频散  相似文献   

设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号