横向各向同性固体中圆柱状散射体对水平偏振剪切波的散射

给出横向各向同性介质中SH波的标量波动方程,并通过简单的坐标变换,将其化为标准的Helmholtz方程.建立了求解散射问题的积分方程,利用边界单元方法数值计算了横向各向同性固体中圆柱状空洞及刚性散射体对SH波的散射场分布。重点分析了各向异性对空洞散射体散射场指向性的影响.     

平面波在横向各向同性均匀介质中的传播

导出均匀各向异性介质当中具有横向各向同性性质的一类介质中平面波的严格解,给出了慢度曲面的三维分布图形,研究了属于这类介质的单向纤维增强复合材料中声波的传播规律。平面波的特征用慢度矢量、波矢量、偏振矢量和群速度等物理量来刻画.由于引入了描述介质各向异性特性的方向矢量,各物理量最终以与座标系无关的形式来表达,因而具有普遍的适用性。     

Transient elastic waves in a transversely isotropic laminate impacted by axisymmetric load

A method of reverberation-ray matrix has been extended to the investigation of the field of wave propagation in a transversely isotropic laminate. By using the decomposition in a local coordinate system, any complicated waves can be separated into a departing part and an arriving part, which are expressed in the local scattering matrix at structural interface. Together with the local phase matrix, we obtain a certain wave transmitting from a layer to the neighboring one. Thus, the wave propagation in the whole laminate can be described when assembling the local information with global phase and global permutation matrices. This method is perfectly suitable for evaluating the transient waves involving a large number of generalized-rays. In this paper, the method is applied to laminate made of transversely isotropic material. Numerical results show the influence of the change of thickness and elastic constants of the layers on the wave propagation in laminate.     

On the material constants measurement method of a fluid-saturated transversely isotropic poroelastic medium

The fluid-saturated transversely isotropic poroelastic medium could be widely found in nature, e.g., the sedimentary rocks underground. To determine the eight independent material constants for the transversely isotropic poroelastic medium, a series of tests are discussed. Two undrained tests and one drained test are suggested as a set of tests of the least amount. For the verification purpose, two additional drained tests are also introduced as an option. The atmospheric dried test is discussed as a replacement of the traditional infiltrated drained test to save the time waiting for an equilibrium state. Some microscopic material constants, i.e.,the unjacketed bulk coefficients, the porosity, and the compressibility of porous fluid, are measurable but unnecessary to determine the independent material constants of a poroelastic medium.     

利用井孔偶极弯曲波反演横向各向同性孔隙地层渗透率的研究

对利用井孔偶极弯曲波衰减特性反演横向各向同性孔隙地层的纵、横向渗透率进行了理论方法和数值研究.对软地层,在方差函数极值行为分析的基础上,用最小二乘反演对多组理论模拟数据进行处理,得到的纵、横向渗透率精度都比较高,这与软地层弯曲波衰减对纵向渗透率的敏感程度明显比硬地层高有关;用添加不同强度的随机噪声综合反映现场资料可能带来的误差,对多道间提取的衰减叠加求平均可明显的抵消噪声的影响;对快速孔隙地层偶极声测井理论模拟数据也尝试性地进行了纵、横向渗透率反演,发现,虽然硬地层弯曲波的衰减对纵向渗透率很不敏感,但纵向渗透率的取值偏离真值却对横向渗透率的反演结果有明显影响.     

Determination of the elastic constants by coherent neutron scattering in polycrystals

It is shown that elastic constants which usually can be determined in single crystals only may be measured in polycrystals by inelastic coherent neutron scattering. Measurements are reported for polycrystalline samples of aluminium, copper and stainless steel. The method is best suited for the determination of the elastic constants connected with transverse phonons.     

Time-resolved study of the spectral characteristics of supercontinuum pulses propagating in scattering media

The spectral and temporal characteristics of supercontinuum pulses propagating through monodisperse scattering media consisting of sub-lambda-sized particles have been measured with a broad-bandwidth cross-correlated frequency-resolved optical gating (XFROG) technique. The results show that the ballistic component of the supercontinuum preserves a phase relationship among its spectral components, which acquire a temporal shift in relation to propagation in a non-scattering medium.     

Surface waves propagating around the periphery of a piezoelectric cylinder of hexagonal symmetry with time dependent elastic stiffness

Acoustic surface waves propagating around the periphery of a piezoelectric cylinder of hexagonal symmetry are investigated. A co-axial structure with a piezoelectric insulator as the inner cylinder and electric conductor as the outer cylinder is considered. The dispersion equation is derived and solved under certain approximations assuming the elastic stiffness to be time dependent as in Voigt's model of viscoelasticity. The final expressions for the mechanical displacement component and electric potential are obtained. Ultimately some important parameters of such waves as group velocity, Poynting vector and power flow components have also been determined.I am grateful to Dr. A. K. Pal for his helpful suggestions in the preparation of the paper.     

Determination of elastic constants of trigonal crystals by the rectangular parallelepiped resonance method

We present a theoretical method to calculate the resonance frequencies of a rectangular parallelepiped of a trigonal (D₃, C_3v, D_3d crystal, and apply it to determine the elastic constants of Al₂O₃ by the rectangular parallelepiped resonance (RPR) method. We demonstrate complete agreement between observed and calculated resonance spectra up to the thirty-ninth mode. The determined elastic constants of Al₂O₃ agree well with previous data. This study offers a new tool for the determination of elastic constants of a trigonal crystal.     

An electron in the field of two classical plane waves propagating in slightly different directions

Solutions of the Dirac and Klein-Gordon equations have been obtained for an electron moving in the field of two plane waves propagating in slightly different directions. The solutions were obtained as an expansion in terms of the small angle between the two waves. Terms linear in this angle were considered.     

Determination of the elastic constants of copper on copper-whiskers by the flexural vibration method

Copper-whiskers with different orientations were excited in flexural vibrational modes. From the resonance frequencies and the cubic compressibility, the elastic constants c_ik of copper were determined. The results are in good agreement with values obtained by other authors using other methods with macroscopic single crystals of copper. No enhancement of Young's modulus, reported recently in thin metallic films, was seen in Cu-whiskers.     

各向同性媒质中球体声散射的共振模式缺损

以有机玻璃中铁球的共振散射为例，研究了各向同性媒质中球状散射体的共振模式缺损问题 . 通过对纵波和横波散射系数的数值分析，指出在纵波入射时的散射纵波（简称p→p散射） 中零阶分波会出现共振模式缺损，导致共振模式缺损的数值原因是纵波散射系数分式中分子 和分母零点位置几乎相同. 关键词： 共振散射 模式缺损     

Response of thermal source in a transversely isotropic thermoelastic half-space with mass diffusion by finite element method

The two-dimensional problem of generalized thermoelastic diffusion material with thermal and diffusion relaxation times is investigated in the context of Lord-Shulman theory. As an application of the problem, a particular type of thermal source is considered and the problem is solved numerically by using a finite element method. The components of displacement, stress, temperature distribution, chemical potential, and mass concentration are obtained. The resulting quantities are depicted graphically for a special model. Appreciable effect of relaxation times is observed on various resulting quantities.     

Modeling the quasicompressional wave field of a rectangular transducer in a transversely isotropic solid

A fast model for simulating the transient quasicompressional wave field of a rectangular ultrasonic transducer directly coupled to a transversely isotropic elastic half-space of general orientation is developed. The so-called two-tier asymptotic approach and the uniform stationary phase method are used to derive the high-frequency asymptotics of time-harmonic displacements. Then, transient fields are modeled by means of harmonic synthesis. In geometrical regions, the formulas involve elementary and inside boundary layers, well-known special functions (Fresnel integral and generalized Fresnel integral), and are applicable in the radiating near field. The asymptotics elucidate the physics in terms of various arrivals and give explicit dependence of the radiated waves upon model parameters. The asymptotic code is tested against a direct numerical solution. It is at least a thousand times faster but describes accurately both arrival times and amplitudes of various pulses radiated by the transducer.     

Response of thermal source in a transversely isotropic thermoelastic half-space with mass diffusion by using a finite element method

The two-dimensional problem of generalized thermoelastic diffusion material with thermal and diffusion relaxation times is investigated in the context of the Lord-Shulman theory.As an application of the problem,a particular type of thermal source is considered and the problem is solved numerically by using a finite element method.The components of displacement,stress,temperature distribution,chemical potential,and mass concentration are obtained.The resulting quantities are depicted graphically for a special model.An appreciable effect of relaxation times is observed on various resulting quantities.     

An increase in the wavelength of light pulses propagating through a fiber

An increase in the wavelength of an optical pulse propagating through a conventional fiber is considered. It is shown that the light wavelength increases gradually owing to a transfer of part of the pulse energy to acoustic waves.     

Determination of third-order nonlinearity in ZnTe by use of femtosecond laser pulses

Third-order nonlinearity induced by femtosecond pulses in ZnTe is investigated experimentally. We employ the transmission z-scan technique, using femtosecond pulses at 810 nm, to determine Kerr-type nonlinearity in ZnTe. The implications of our results to the terahertz (THz) radiation generation are discussed.     

Determination of the elastic constants of a composite plate using wavelet transforms and neural networks

An inverse method based on a combination of the wavelet transform and artificial neural networks is presented. The method is used to recover the elastic constants of a fiber-reinforced composite plate from experimental measurements of ultrasonic Lamb waves generated and detected with lasers. In this method, the elastic constants are not recovered from the dispersion curves but rather directly from the measured waveforms. Transient waveforms obtained by numerical simulations for different elastic constants are used as input to train the neural network. The wavelet transform is used to extract the eigenvectors from the Lamb wave signals to simplify the structure of the neutral network. The eigenvectors are then introduced into a multilayer internally recurrent neural network with a back-propagation algorithm. Finally, experimental waveforms recoded on a titanium-graphite composite plate are used as input to recover the elastic constants of the material.     

A numerical investigation of the acoustic mode waves in a deviated borehole penetrating a transversely isotropic formation

A 2.5-dimensional method in frequency wave-number domain is developed to investigate the mode waves in a deviated borehole penetrating a transversely isotropic formation. The phase velocity dispersion characteristics of the fast and slow flexural mode waves excited by a dipole source are computed accurately at various deviation angles for both hard and soft formations. The sensitivities of the flexural mode waves to all elastic constants in a transversely isotropic formation are calculated. Numerical results show that, for a soft formation, the fast flexural mode wave is dominated by c66 at high deviation angles and low frequencies, while the slow flexural mode wave is dominated by c44 at the same conditions. Waveforms in time domain are also presented to support the conclusions.