Nonlinear elasticity in structurally inhomogeneous materials: An experimental study

Experimental results on the propagation of longitudinal and torsional elastic waves in polycrystalline copper under elastoplastic strain are reported. The strain in the interval of 0–0.06 was created by applying heavy tensile loads to the samples. The Young and shear moduli, Poisson ratio, and nonlinear acoustic parameters of longitudinal and torsional elastic waves were measured as functions of the static strain. The nonlinear acoustic parameters were found to behave anomalously in loading-unloading cycles in the plastic strain region. The experimental results are discussed. The nonlinear properties of structurally inhomogeneous materials are explained in terms of the clapping nonlinearity and hysteretic nonlinearity mechanisms.     

Visual characteristics of inhomogeneous acoustic waves

Experimentally obtained visualizations of propagating inhomogeneous acoustic waves driven by zero-order antisymmetric Lamb waves (flexural waves) in water are presented. The inhomogeneous waves are visualized by optical holographic interferometry. A series of photographs show the evolution in time of instantaneous acoustic pressure distributions associated with propagating inhomogeneous waves. The photographs reveal characteristic features of flexurally driven inhomogeneous waves such as transversely attenuated wavefronts oriented perpendicularly to the plate boundary and a phase propagation velocity along the boundary approximately equal to the plate wave velocity (250 meters/second). Effects due to the dispersive nature of the flexural plate waves are also noted in the photographic series. Features distinguishing these subsonic, inhomogeneous surface waves (also called trapped or evanescent waves) from the leaky, lateral or head wave and also from incompressible fluid motions associated with low frequency vibrations of fluid loaded plates are identified. The relevance of inhomogeneous acoustic waves driven by subsonic flexural waves to practical sound-structure interaction problems is discussed.     

Acoustic phenomena in filled porous glasses by time-resolved spectroscopy

We studied the propagation of ultrasonic acoustic waves in two porous glasses characterized by different pore diameters (4 and 200 nm) filled with carbon tetrachloride. Using a time-resolved optical spectroscopic technique (Transient Grating) we can measure the acoustic dynamics of these systems. The sound velocities, measured in both the samples, show high values compared with the sound velocity of bulk carbon tetrachloride; they cannot be predicted by the simple effective medium model, but are in good agreement with the estimate obtained from the Biot theory.     

The damage of the optical components induced by the stimulated Brillouin scattering

A theory of excitation of ultrasonic waves in the stimulated Brillouin scattering (SBS) process is presented in this paper. By using several reasonable approximations, a numerical calculation of the transient longitudinal SBS shows that large amplitude of acoustic waves can be built up by the nanosecond pulse of high-power laser, which may result in the damage of optical glasses. The maximal density change and the maximal acoustic wave intensity in optical glasses of 5\,cm in thickness are calculated by using different parameters of the high-energy laser, such as the intensity, the pulse width, and the wave length. The damage threshold of the optical glasses is about 80 GW/cm^{2} when using a 1064 nm laser. The dynamic mechanism of SBS is the electrostriction effect of the components coupling with the high-power laser.     

非均匀流体介质内部散射声场重建的逐层计算方法*

入射声波激励下非均匀流体介质内部散射声场的重建方法对超声层析成像具有重要意义。以往采用矩量法求解,但该方法全域离散形成的复数满秩矩阵规模随着分辨率与计算精度的提高而急剧增大,对算力具有很高的要求,一定程度上限制了其在实际中的应用。为克服上述缺陷,本文以逐层离散、逐层计算为核心思想,以声散射基本公式与近场声全息理论为基础,推导出逐层计算非均匀流体介质内部散射声场的理论公式并给出对应的几何离散模型。为验证该方法的可行性,以矩量法为参照,对同样的介质模型进行介质内部声场重构仿真。结果表明,逐层算法不仅可以有效地重建非均匀流体介质内部散射声场,且大幅度减小了求解规模。     

Chain Degradation under Low-Intensity Sonication of Polymer Solutions in the Presence of Filler: Mechanism of Ultrasonic Degradation of Flexible Chain Macromolecules

Routine dispersion of fillers in polymer solutions in a usual ultrasonic cleaning bath has been shown to lead to chain degradation. It is the filler presence, only, that has been demonstrated to provoke chain degradation under low-intensity sonication. A critical analysis of the literature concerned with the effects arising from propagation of acoustic waves in a liquid and an experimental study of ultrasonic degradation of polymers in solution were carried out. Based on these results, the mechanisms of chain degradation were discussed. Our previously proposed universal mechanism of chain degradation in inhomogeneous hydrodynamic fields has been shown to explain the basic facts repeatedly confirmed over the years of studying the ultrasonic degradation: (i) the existence of a limiting molecular weight such that macromolecules with lower molecular weights are not subject to degradation and (ii) the dependences of degradation rate on polymer molecular weight, polymer concentration, and temperature.     

The analogy between acoustic waves and water surface waves with refraction

The similarity between acoustic waves in an inhomogeneous medium and water surface waves with variable water depth is established. The two systems are analogous if they both are characterized by the same index of refraction field. Based on this analogy, acoustic refraction problems are treated by utilizing a surface wave simulation achieved with a water wave tank. Experimental results obtained from the wave tank for various situations are used to deduce the behavior of the corresponding acoustic systems. It is concluded that there are two major advantages in employing a water wave tank. The first is that the entire wavefront configuration can visually be observed; the second is that water surface waves can be investigated in the laboratory much more readily and more economically than acoustic waves.     

Sound wave propagation in a vertically inhomogeneous ocean

Propagation of sound waves generated by a time dependent acoustic source in a vertically inhomogeneous ocean is considered. The effect of the solid bottom is included so that both the longitudinal and shear waves can be excited inside the bottom. The possibility of exciting a lateral shear wave by the acoustic source is also discussed. Although the results presented here are formal and general, physical interpretations have been offered whenever possible.     

Influence of the porosity on the dispersion of the phase velocity of longitudinal acoustic waves in isotropic metal-matrix composites

The influence of the volumetric porosity of isotropic metal-matrix composite materials, which are reinforced with ceramic microparticles, on the dispersion of the phase velocity of longitudinal acoustic waves is investigated. For this purpose, the method of broadband acoustic spectroscopy with a laser source of ultrasound and piezoelectric detection of nanosecond ultrasonic pulses is used. Composite samples based on a silumin matrix with added silicon carbide (SiC) microparticles in different mass concentrations (3.8–15.5%) were investigated. As the concentration of SiC particles in a sample increases, its porosity that is determined using the hydrostatic-weighing method also increases. The simultaneous increase in the filler concentration and porosity leads to the appearance of a dispersion of the phase velocity of longitudinal acoustic waves in the sample within the frequency range of 3–25 MHz. The obtained empirical relationship between the relative change in the phase velocity and the sample porosity can be used to obtain a proximate quantitative estimate of the bulk porosity of the isotropic metal-matrix composite materials.     

Effect of anomalous transparency of a liquid-gas interface for sound waves

Problems of acoustic wave propagation near the interface between media are discussed. The processes that occur at a liquid-gas interface are considered, and their characteristic features related to the effect of inhomogeneous waves are pointed out. The transmission factors of spherical and cylindrical waves incident on the aforementioned interface are calculated with allowance for the contribution of the inhomogeneous component. Experimental dependences of the transmission factor on frequency and source depth measured with respect to the interface are presented. It is shown that the lower the frequency generated by the source and the closer the source to the water-air interface, the higher the acoustic transparency of the interface.     

Low frequency electrostatic nonlinear structures in an inhomogeneous magnetized non-Maxwellian electron–positron–ion plasma

The properties of low frequency (coupled acoustic and drift wave) nonlinear structures including solitary waves and double layers in an inhomogeneous magnetized electron–positron–ion (EPI) nonthermal plasma with density and temperature inhomogeneities are studied in a simplified way. The nonlinear differential equation derived here for the study of double layers in the inhomogeneous EPI plasma resembles with the modified KdV equation in the stationary frame. But the method used for the derivation of nonlinear differential equation is simple and consistent to give both the stationary solitary waves and double layers. Further, the illustrations show that superthermality κ, drift velocity and temperature inhomogeneity have significant effects on the amplitude, width, and existence range of the structures.     

弯折波导杆的超声斜入射方法*

波导杆技术是一种有效的超声辅助检测方法,在提升高温关键承压设备安全运行方面具有重要作用。现有的波导换能器因直入式发射声波,常被用来在线监测壁厚的腐蚀进程,难以对指定方向的缺陷进行有效检测。为了解决这个难题,基于斯涅耳定理以及导波的频散特性提出了一种弯折结构的波导杆来实现超声斜入射。首先分析了弯折波导杆中水平剪切波的传播特性,探究了波导杆的厚度、弯折角度对杆中声波传播的影响规律;然后,进一步研究了弯折波导杆在半无限空间内的声束指向性;最后通过一对弯折波导杆在线检测结构内部缺陷的试验证明了波导超声斜入射的应用价值。研究结果为基于斜入射技术的波导超声在线监测提供了坚实的理论基础。     

非均匀波导中的声聚焦

理论研究了声波在非均匀波导中的空间聚焦问题,利用多模态导纳法构建波导内任意位置处声压与入射声压在模态域的映射关系,计算使声波聚焦于空间某位置时的最佳入射波,并画出了相应的聚焦声场.研究了三种非均匀情况:水平变截面波导、含散射体波导以及声速垂直变化波导.结果表明,当输入最佳入射波时,在非均匀波导中可以产生良好的单点或多点声聚焦效果,声波的聚焦过程充分地利用了波导结构及介质非均匀性对声波的散射作用.     

Effect of impact loading in the β phase of a Cu-Al-Ni single-crystal alloy on its elastic and inelastic characteristics measured at a frequency of 5 MHz

The internal friction and velocity of ultrasonic waves in the β phase of Cu-Al-Ni single crystals are measured at a frequency of 5 MHz in the temperature range 190-300 K, including the range of thermoelastic martensite transformation. The effect of impact loading (5 GPa) on the elastic and dissipative characteristics of the samples is investigated. The results obtained are discussed within the framework of existing theoretical concepts on the martensite transformation mechanisms responsible for the acoustic characteristics of a material.     

On the feasibility of remote palpation using acoustic radiation force

A method of acoustic remote palpation, capable of imaging local variations in the mechanical properties of tissue, is under investigation. In this method, focused ultrasound is used to apply localized (on the order of 2 mm3) radiation force within tissue. and the resulting tissue displacements are mapped using ultrasonic correlation based methods. The tissue displacements are inversely proportional to the stiffness of the tissue, and thus a stiffer region of tissue exhibits smaller displacements than a more compliant region. In this paper, the feasibility of remote palpation is demonstrated experimentally using breast tissue phantoms with spherical lesion inclusions, and in vitro liver samples. A single diagnostic transducer and modified ultrasonic imaging system are used to perform remote palpation. The displacement images are directly correlated to local variations in tissue stiffness with higher contrast than the corresponding B-mode images. Relationships between acoustic beam parameters, lesion characteristics and radiation force induced tissue displacement patterns are investigated and discussed. The results show promise for the clinical implementation of remote palpation.     

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。     

基于蜷曲空间结构的近零折射率声聚焦透镜

研究基于蜷曲空间结构的近零折射率声聚焦透镜.根据近零折射率材料的声波方向选择机理,采用蜷曲空间结构为基本单元进行排列,设计具有特定入射与出射界面的几何结构,对透射声波的出射方向进行调控,实现了平面声波与柱面声波的聚焦效应,并深入讨论了透镜内部刚性散射体对声聚焦性能的影响.在此基础上,改变近零折射率透镜的出射界面,可以精确调控声波阵面的形状与方向.该类型透镜具有单一的单元结构、高聚焦性能及高鲁棒性等优点.研究结果为设计新型近零折射率声聚焦透镜提供了理论指导与实验参考,同时也为研究声波阵面的调控提供了新思路.     

Review on second-harmonic generation of ultrasonic guided waves in solid media(Ⅰ): Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation(SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.     

Review on second-harmonic generation of ultrasonic guided waves in solid media (I):Theoretical analyses

Considering the high sensitivity of the nonlinear ultrasonic measurement technique and great advantages of the guided wave testing method, the use of nonlinear ultrasonic guided waves provides a promising means for evaluating and characterizing the hidden and/or inaccessible damage/degradation in solid media. Increasing attention on the development of the testing method based on nonlinear ultrasonic guided waves is largely attributed to the theoretical advances of nonlinear guided waves propagation in solid media. One of the typical acoustic nonlinear responses is the generation of second harmonics that can be used to effectively evaluate damage/degradation in materials/structures. In this paper, the theoretical progress of second-harmonic generation (SHG) of ultrasonic guided wave propagation in solid media is reviewed. The advances and developments of theoretical investigations on the effect of SHG of ultrasonic guided wave propagation in different structures are addressed. Some obscure understandings and the ideas in dispute are also discussed.     

Acoustic properties of proton exchanged LiNbO3 investigated by Brillouin scattering

The propagation of surface acoustic waves at microwave frequencies (10¹⁰ Hz) was studied on proton exchanged LiNbO₃ crystals by means of Brillouin scattering. The proton exchange causes a large velocity reduction for surface acoustic waves propagating in the x–y plane of ay-cut crystal as well as for longitudinal bulk acoustic waves travelling in the proton exchanged sub-surface region. The velocity reduction amounts to about 20% for both types of waves. The corresponding elastic constants are reduced even by about 40% since the density remains almost constant. This softening seems to involve both the shear and compressional elastic constants, but in an anisotropic way.Thus by proton exchange it is possible to build acoustic waveguides adjacent to the surface, similar to the construction of optical waveguides. By a lateral control of the proton exchange rate optical elements for ultrasonic waves, for example, acoustic lenses can be produced without deformation of the flat surface.The absorption of surface acoustic waves on proton exchanged surfaces is stronger than on pure LiNbO₃ indicating a novel absorption mechanism becoming active in the proton exchanged material.