Strain energy density of a circular cavity buried in semi-infinite functionally graded materials subjected to shear waves

In this paper, based on the theory of multiple scattering of elastic waves, employing wave functions expansion method, multiple scattering and strain energy density in semi-infinite functional graded materials with a circular cavity are investigated, the analytical solution of the problem is derived, and the numerical solution of the strain energy density factors around the cavity is also presented. The effects of the distance between the cavity and the edge of the materials, the wave number and the non-homogeneous parameter of materials on strain energy density factors are analyzed. From analysis, it can be seen that when the non-homogeneous parameter of materials is less than zero, it has less influence on the maximum strain energy density factor around the cavity; however, it has greater influence on the distribution of strain energy density factors around the cavity. When the non-homogeneous parameter of materials is greater than zero, it has greater influence on both the maximum strain energy density factor and the distribution of strain energy density factor around the cavity, especially in the case that the distance between the cavity and the edge is comparatively little.     

MULTIPLE SCATTERING AND DYNAMIC STRESS ANALYSIS OF ELASTIC WAVES IN A FIBER—REINFORCED COMPOSITE WITH INTERFACES

Based on the theory of elastic dynamics, multiple scattering of elastic waves and dynamic stress concentrations in fiber-reinforced composite are studied. The analytical expressions of elastic waves in different regions are presented. The mode coefficients of elastic waves are determined in accordance with the continuous conditions of displacement and stress on the boundary of the multi-interfaces. By using the addition theorem of Hankel functions, the formula of scattered wave fields in different local coordinates are transformed into those in one local coordinate to determine the unknown coefficients and dynamic stress concentration factors (DSCFs). The influences of the distance between two inclusions, material properties and structural size on the DSCFs near the interfaces are analyzed. As examples, the numerical results of DSCFs near the interfaces for two kinds of fiber-reinforced composites are presented and discussed. The project supported by the National Natural Science Foundation of China (19972018)     

光纤陀螺随机游走系数的分析研究

分析了产生随机游走误差的机理,分别采用有关标准和Allan方差进行了随机游走系数的分析和计算。结果表明:随机游走系数是衡量光纤陀螺噪声水平的重要指标,其值的大小体现了陀螺的极限精度。采用Allan方法表述光纤陀螺的静态指标,物理概念清晰,方法简便。     

单侧摩擦约束夹杂物的瞬态弹性波散射的时域边界元分析--反平面情况

应用时域边界元法研究了瞬态SH波对单侧摩擦约束夹杂物的散射问题，假设摩擦遵守库仑定量，当入射SH波足够强时界面会出现局部滑移，发现了一种有效的迭代技术以确定边界上的未知区域（滑移和粘着区）。算例计算了无限域中圆柱埋置夹杂物对瞬态SH波的散射问题。     

浅埋的圆柱形孔洞对SH波的散射与地震动

研究了浅埋的圆柱形孔洞对以任意方向入射的平面SH波的散射与地震动问题。利用复变函数和多极坐标方法构造了问题的位移解。当入射波的波长与圆孔的半径相比较小时,地震动将受到较大的影响。影响地震动有三个主要参数:(1)SH波的入射角0;(2)入射波波数,即圆柱形孔洞的半径与入射波半波长之比;(3)h/R,即圆柱形孔洞至表面的距离与圆孔半径之比。当较大时,地震动幅值变化激烈,位移幅值可出现跳动和放大的现象。当h/R增大至10～12时,位移幅值变化恢复至半空间的情况,表明圆柱形孔洞的影响可被忽略。     

Off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid sphere

In this paper, the off-axial acoustic scattering of a high-order Bessel vortex beam by a rigid immovable (fixed) sphere is investigated. It is shown here that shifting the sphere off the axis of wave propagation induces a dependence of the scattering on the azimuthal angle. Theoretical expressions for the incident and scattered field from a rigid immovable sphere are derived. The near- and far-field acoustic scattering fields are expressed using partial wave series involving the spherical harmonics, the scattering coefficients of the sphere, the half-conical angle of the wave number components of the beam, its order and the beam-shape coefficients. The scattering coefficients of the sphere and the 3D scattering directivity plots in the near- and far-field regions are evaluated using a numerical integration procedure. The calculations indicate that the scattering directivity patterns near the sphere and in the far-field are strongly dependent upon the position of the sphere facing the incident high-order Bessel vortex beam.     

Stimulated Raman scattering in natural crystals of fluorapatite,Ca5(PO4)3F

Hexagonal Ca₅(PO₄)₃F, known as natural crystal fluorapatite and oldest host‐crystal for Ln³⁺‐lasant ions, is presented as a Raman‐active material. High‐order Raman‐induced χ⁽³⁾‐nonlinear processes are discovered in natural crystals of fluorapatite under picosecond pumping at 1.064 μm and 0.532 μm wavelength. A multitude of Stokes and anti‐Stokes components is generated in the ultraviolet, visible and near‐infrared spectral region by stimulated Raman scattering (SRS) and Raman four‐wave mixing (FWHM), resulting in a frequency comb with a width of 520 THz. The spectral lines are identified and attributed to the ν₁(A_g) vibration mode of the tetrahedral [PO₄] units which is related to a Raman shift of ω_SRS ≈ 965 cm⁻¹. The first Stokes steady‐state Raman gain coefficient in the near‐infrared spectral range is estimated to be >0.38 cm·GW⁻¹. Finally, a short review of SRS‐promoting vibration modes and observed χ⁽³⁾‐ nonlinear interactions in all known SRS‐active natural crystals (minerals) is given.

     

Recent advances in wavefront shaping techniques for biomedical applications

Due to the highly inhomogeneous distributions of refractive indexes, light propagation in complex media such as biological tissue experiences multiple light scattering events. The suppression and control of multiple light scattering events are investigated because they offer the possibility of optical focusing and imaging through biological tissues, and they may open new avenues for diagnosis and treatment of several human diseases. In order to provide insight into how new optical techniques can address the issues of multiple light scattering in biomedical applications, the recent progress in optical wavefront-shaping techniques is summarized.     

Effect of nanocrystallinity on lattice dynamics in Bi2Te3 based thermoelectrics

The lattice dynamics in as‐cast and nanocrystalline thermoelectric Bi₂Te₃ based p‐type and n‐type material were investigated using inelastic neutron scattering. Generalized densities of phonon states show substantial agreement between the lattice dynamics in as‐cast samples and previous studies. The lattice dynamics in the nanocrystalline materials differ significantly from its as‐cast counterparts in the acoustic phonon regime. In nanocrystalline p‐type and n‐type compounds, the average acoustic phonon group velocity was found to be reduced to 80(5)% and 95(2)% of the value in as‐cast material. It is argued that point‐defect and strain contrast scattering may play an important role for the understanding of lattice thermal conductivity in (nanocrystalline) Bi₂Te₃ based thermoelectrics beside the observed decrease of sound velocity. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Charge correlations in cobaltates La2–xSrx CoO4

We report on an elastic neutron scattering study of the charge correlations in La_2–xSr_x CoO₄ with x = 1/3, 0.4 and 0.5. We found that the checkerboard charge ordering correlations present in the x = 0.5 sample persist in the x = 0.4 and 1/3 materials. These checkerboard charge ordering correlations are robust and explain the occurrence of nano‐phase separation in layered cobaltates for Sr‐concentrations away from half‐doping. The half‐integer reflections then arise from the nanometer‐sized hole‐rich regions (blue areas in title figure) instead of the undoped ones (red areas in title figure). The appearance of nano‐phase separation is an important ingredient for understanding the formation of hour‐glass shaped magnetic excitation spectra in La_2–xSr_x CoO₄.

Nano‐phase separation in La_2–xSr_x CoO₄ (schematically). Red areas: undoped La₂CoO₄ islands, blue areas: checkerboard charge ordered regions; black, green and blue balls represent nonmagnetic Co³⁺ ions, magnetic Co²⁺ ions and oxygen ions, respectively; green arrows indicate Co²⁺ spins [1, 2].