金属薄板中导波的模态识别和波速测定

按照Lamb波理论,板内导波由多阶对称波和反对称波组成。当波长远大于板厚时,利用经典平板理论对Lamb方程进行简化,此时板内主要有零阶对称波S0和反对称波A0两种模态。认识薄板内声波模态,不仅有助于分析声源性质,提取有效信号,而且可以提高声发射检测中时差定位的准确性。利用PAC公司的Fieldcal标准信号发生器和Nielsen Hsu断铅法作为信号源在0.7 mm薄钢板上进行声波传播实验。通过比较实测波速和理论波速甄别板内的声波模态。实验发现对称波S0实测波速和理论波速比较吻合,而反对称波A0的波速由于频散而较难精确测量。综合两种模态波的波速情况,声发射检测中利用对称波速度进行声源定位计算更加接近薄壁结构声波的传播状况。     

二阶波浪力和聚焦位置对畸形波作用下张力腿平台动力响应的影响研究

通过波浪聚焦和波列叠加模型模拟生成了满足畸形波定义的畸形波。求解畸形波作用下张力腿平台动力响应,并分析二阶波浪力和畸形波聚焦位置对张力腿平台动力响应的影响。结果表明:畸形波会引起张力腿平台纵荡、垂荡和纵摇的大幅值运动;二阶差频波浪力会引起纵荡方向的大幅振荡,纵摇运动主要由二阶和频波浪力贡献,垂荡运动则由二阶和频波浪力引起的垂向高频振荡和二阶纵荡差频力引起的大幅值下沉运动组成;因不同频率波浪力之间的相位差,使得张力腿平台纵荡、垂荡响应极值与纵摇响应极值分别发生在畸形波作用在后立柱和平台中心时。     

间隙波在功能梯度压电板和压电半空间结构中的传播

研究了间隙波在功能梯度压电板和压电半空间结构中的传播性质.功能梯度压电板的材料性能沿x2方向呈指数变化,首先推导了间隙波传播时的解析解,利用界面条件得到了间隙波的频散方程,基于推导的频散方程,结合数值算例分析了功能梯度压电材料的梯度、压电层厚度以及材料性能对间隙波相速度的影响,研究结果对功能梯度压电材料的覆层结构在声波器件中的应用具有重要的参考价值.     

畸形波作用下二阶波浪载荷对张力腿平台动力响应的影响

畸形波易对海上的建筑物造成极大的危害。为研究畸形波作用下张力腿平台的动力响应特性,考虑张力腿平台的六自由度运动与张力腿非线性恢复刚度,建立非线性耦合运动方程。结合随机频率相位角调制法生成的畸形波波面时历,计算在畸形波条件下平台所受的一阶及二阶和、差频波浪载荷,并采用数值方法求解平台六自由度的运动。结果表明,在畸形波作用下,一阶、二阶波浪载荷均受到畸形波的影响,但各波浪载荷成分的增幅在不同自由度上并不相同。以纵荡为代表的低频运动主要受二阶差频波浪载荷影响,以垂荡、纵摇为代表的高频运动受二阶和频波浪载荷的影响。而由平台水平面内运动引发的下沉运动在二阶差频波浪载荷的作用下显著增大,从而诱发了垂荡运动产生了显著增加,因此在畸形波作用下垂荡运动同时受到二阶和频及差频波浪载荷的影响。此外,由于畸形波具有冲击载荷的特性,不同自由度运动幅值出现时刻并不相同。     

H—S波前传感器时域采样频率对探测误差的影响

波前传感器是自适应光学系统的重要组成部分。讨论了Ｈａｒｔｍａｎｎ－０Ｓｈａｃｋ－（Ｈ－Ｓ）波前传感顺的采样频率对探测误差的影响，理论分析结果说明，时域采样频率引入的探测误差Ｈ－Ｓ波前传感器的重要误差源，其大小随时域采样频率的增大而下降。     

有限声束的二阶积累声场

有限声束的二阶积累声场邓明晰（重庆后勤工程学院物理教研室重庆·６３００４２）吕霞付蔡绍皙（重庆大学生物工程研究院重庆·６３００４４）１引言在实际应用中，用于激发声波的换能器辐射面总是有限宽的，因此，研究有限声束在介质中的声传播性质就具有重要的实际意义...     

正交各向异性空心圆柱体的周向SH波

基于线性三维弹性理论,得到了正交各向异性管道中位移表示的周向波控制方程.波动方程中的轴向位移独立于其它两个方向.即波的传播可以分解为SH波和类Lamb波.通过贝塞尔函数求得了这个周向SH波频散方程.方程中决定周向SH波特性的弹性常数有两个,一个决定了波的频率,另一个决定了波速.最后分别讨论了两个弹性常数和波数变化对周向SH波位移模式的影响.     

圆柱壳中结构振动波的传播特性

计算了圆柱壳中低阶和高阶周向模态结构振动波的频散结果，得到了相应的频散曲线。着重分析了近场波和衰减驻波的性质及其随频率变化的规律，结果表明：纵向伸缩近场波和扭转近场波都不具有通常意义上的纵向伸缩和扭转的性质，弯曲近场波则具有通常意义上弯曲性质；衰减驻波大体上是弯曲性质的驻波。文中把低阶和高阶周向模态情况下波的频散规律也作了比较。     

温度对水饱和孔隙介质压缩波速度的影响

温度作用使孔隙介质密度、弹性模量等参数发生改变,从而影响声波传播性质。文章基于变形体能量守恒定律,考虑热机耦合和两相介质热耦合,推导水饱和孔隙介质的热传导方程;考虑孔隙水和固相孔隙介质存在的三种耦合性,基于热作用下应力应变关系分析,建立了热机耦合声波传播模型;设计温度变化沉积物样品声学测量实验,当温度由2℃增加到27℃时,压缩波速度平均增大88m·s^-1运用热机耦合声波传播模型解释了实验测量结果的上升趋势,获得理论计算值与测量值相对偏差范围为-3．03％～1．19％。     

狭缝空腔间粘弹性体中弹性波传播特性的研究

利用笛卡尔坐标下的Kelvin-Voigt线性粘弹性模型,研究了无限长粘弹性狭缝通道中波的传播和衰减。利用自适应绕数求根方法,针对具有狭缝形空腔周期性分布覆盖层的吸声问题,求得频散方程的根,并且得到无限长狭缝空腔中相速度频散曲线和衰减曲线。分析了各阶模式对无限长粘弹性狭缝结构中波传播的相速度和衰减的影响。通过引入复粘弹性模量,粘弹性波动方程具有弹性波动方程相对应的形式,计算中可用复值粘弹性模量代替相应的弹性模量。粘弹性狭缝结构中所有波传播模式均存在衰减,高阶波在某个频率以下衰减非常大,而在该频率以上逐渐减小;最低阶模式的相速度与材料无损耗的情况非常接近。粘弹性狭缝结构中波传播的衰减特性不仅与狭缝的结构参数有关,还与粘弹性材料本身的复弹性模量有关。     

Micromachined thin film plate acoustic resonators utilizing the lowest order symmetric lamb wave mode

Thin film integrated circuits compatible resonant structures using the lowest order symmetric Lamb wave propagating in thin aluminum nitride (AlN) film membranes have been studied. The 2-mum thick, highly c-oriented AlN piezoelectric films have been grown on silicon by pulsed, direct-current magnetron reactive sputter deposition. The films were deposited at room temperature and had typical full-width, half-maximum value of the rocking curve of about 2 degrees. Thin film plate acoustic resonators were designed and micromachined using low resolution photolithography and deep silicon etching. Plate waves, having a 12-mum wavelength, were excited by means of both interdigital (IDT) and longitudinal wave transducers using lateral field excitation (LW-LFE), and reflected by periodical aluminum-strip gratings deposited on top of the membrane. The existence of a frequency stopband and strong grating reflectivity have been theoretically predicted and experimentally observed. One-port resonator designs having varying cavity lengths and transducer topology were fabricated and characterized. A quality factor exceeding 3000 has been demonstrated at frequencies of about 885 MHz. The IDT based film plate acoustic resonators (FPAR) technology proved to be preferable when lower costs and higher Qs are pursued. The LW-LFE-based FPAR technology offers higher excitation efficiency at costs comparable to that of the thin film bulk acoustic wave resonator (FBAR) technology     

Excitation of plate waves in thickness measurements of layers deposited on thin plates

Layer thickness measurements with an acoustic micrometer using pseudo-Sezawa waves in which ultrasonic waves are obliquely applied to a layered surface of a specimen have been proposed. A case in which the plate thickness of the specimen is so thin that it cannot be regarded as a half space is studied. A number of modes of plate waves are then excited in addition to pseudo-Sezawa waves. The plate waves, giving rise to the appearance of extra dips in the power spectrum of reflected waves, cause difficulties in the measurements. To prevent the excitation of plate waves, it is proposed that a mask of a sound-insulating material with a slit aperture should be placed on the layered surface of the specimen. Experiments and theoretical calculations, using lead frames of LSI chips as typical test specimens with thin substrates, were performed to demonstrate the effectiveness of the present method in preventing the excitation of plate waves.     

多极声源在裸眼井外的纵横波辐射特性

声波远探测方法已逐渐成为石油测井的一门重要应用技术,声源向井外地层中的辐射特性是远探测声场的主控因数。推导了单极、偶极和四极子声源的远场位移渐近解与辐射指向性公式,模拟分析了低频和常规测井频率下多极子声源在井外地层中的辐射图样,优选出不同声源辐射到地层中的优势波类型,并对比了不同频率下优势波指向性的变化规律。结果表明:单极辐射P波和偶极辐射SH横波在各自的辐射声场中占据了主导地位,它们对应的最佳激发频段分别在10 k Hz和4 k Hz左右,而在四极子声源的辐射声场中,P波和SH横波的指向性相近,均优于SV横波,且两者在7 k Hz左右达到最佳,为多极子声波远探测技术的应用提供了理论支持。     

Probing thermomechanics at the nanoscale: impulsively excited pseudosurface acoustic waves in hypersonic phononic crystals

High-frequency surface acoustic waves can be generated by ultrafast laser excitation of nanoscale patterned surfaces. Here we study this phenomenon in the hypersonic frequency limit. By modeling the thermomechanics from first-principles, we calculate the system's initial heat-driven impulsive response and follow its time evolution. A scheme is introduced to quantitatively access frequencies and lifetimes of the composite system's excited eigenmodes. A spectral decomposition of the calculated response on the eigemodes of the system reveals asymmetric resonances that result from the coupling between surface and bulk acoustic modes. This finding allows evaluation of impulsively excited pseudosurface acoustic wave frequencies and lifetimes and expands our understanding of the scattering of surface waves in mesoscale metamaterials. The model is successfully benchmarked against time-resolved optical diffraction measurements performed on one-dimensional and two-dimensional surface phononic crystals, probed using light at extreme ultraviolet and near-infrared wavelengths.     

Phononic plate waves

In the past two decades, phononic crystals (PCs) which consist of periodically arranged media have attracted considerable interest because of the existence of complete frequency band gaps and maneuverable band structures. Recently, Lamb waves in thin plates with PC structures have started to receive increasing attention for their potential applications in filters, resonators, and waveguides. This paper presents a review of recent works related to phononic plate waves which have recently been published by the authors and coworkers. Theoretical and experimental studies of Lamb waves in 2-D PC plate structures are covered. On the theoretical side, analyses of Lamb waves in 2-D PC plates using the plane wave expansion (PWE) method, finite-difference time-domain (FDTD) method, and finite-element (FE) method are addressed. These methods were applied to study the complete band gaps of Lamb waves, characteristics of the propagating and localized wave modes, and behavior of anomalous refraction, called negative refraction, in the PC plates. The theoretical analyses demonstrated the effects of PC-based negative refraction, lens, waveguides, and resonant cavities. We also discuss the influences of geometrical parameters on the guiding and resonance efficiency and on the frequencies of waveguide and cavity modes. On the experimental side, the design and fabrication of a silicon-based Lamb wave resonator which utilizes PC plates as reflective gratings to form the resonant cavity are discussed. The measured results showed significant improvement of the insertion losses and quality factors of the resonators when the PCs were applied.     

SH导波在钢板缺陷检测中的传播特性

水平剪切(Shear Horizontal,SH)导波在传播的过程中有对称模式和反对称模式,其相速度和群速度主要取决于试件的厚度和频率之积(频厚积)。利用COMSOL有限元分析软件,建立了钢板缺陷的3-D模型,仿真分析了SH0导波在钢板中的传播特性。仿真结果表明SH波在传播过程中很少发生波束方向的改变,无频散和模式转换,信噪比高。利用电磁超声方法激励SH波,对钢板中的裂纹和焊缝缺陷进行了实验,验证了仿真结果的正确性及可行性,为超声导波在板材缺陷检测中的应用提供基础。     

The power flow angle of acoustic waves in thin piezoelectric plates

The curves of slowness and power flow angle (PFA) of quasi-antisymmetric (A0) and quasi-symmetric (S0) Lamb waves as well as quasi-shear-horizontal (SH0) acoustic waves in thin plates of lithium niobate and potassium niobate of X-,Y-, and Z-cuts for various propagation directions and the influence of electrical shorting of one plate surface on these curves and PFA have been theoretically investigated. It has been found that the group velocity of such waves does not coincide with the phase velocity for the most directions of propagation. It has been also shown that S0 and SH0 wave are characterized by record high values of PFA and its change due to electrical shorting of the plate surface in comparison with surface and bulk acoustic waves in the same material. The most interesting results have been verified by experiment. As a whole, the results obtained may be useful for development of various devices for signal processing, for example, electrically controlled acoustic switchers.     

Thermoelastic Lamb waves in a transversely isotropic plate bordered with layers of inviscid liquid

Analysis for the propagation of thermoelastic waves in a homogeneous, transversely isotropic, thermally conducting plate bordered with layers of inviscid liquid or half space of inviscid liquid on both sides, is investigated in the context of coupled theory of thermoelasticity. Secular equations for homogeneous transversely isotropic plate in closed form and isolated mathematical conditions for symmetric and anti-symmetric wave modes in completely separate terms are derived. The results for isotropic materials and uncoupled theories of thermoelasticity have been obtained as particular cases. It is shown that the purely transverse motion (SH mode), which is not affected by thermal variations, gets decoupled from rest of the motion of wave propagation and occurs along an in-plane axis of symmetry. The special cases, such as short wavelength waves and thin plate waves of the secular equations are also discussed. The secular equations for leaky Lamb waves are also obtained and deduced. The amplitudes of displacement components and temperature change have also been computed and studied. Finally, the numerical solution is carried out for transversely isotropic plate of zinc material bordered with water. The dispersion curves for symmetric and anti-symmetric wave modes, attenuation coefficient and amplitudes of displacement and temperature change in case of fundamental symmetric (S₀) and skew symmetric (A₀) modes are presented in order to illustrate and compare the theoretical results. The theory and numerical computations are found to be in close agreement.     

Calculations of plate frequencies from complementary energy formulation

The complementary variational principle has been used to derive the differential equations and the associated boundary conditions of the vibrating plate in terms of bending moments. It is shown that in this formulation, the plate possesses an infinite number of zero frequency modes in which the plate remains in a state of constant strain under a set of self-equilibrating bending moments. In applying the Rayleigh Ritz procedure for the non-zero frequency modes of the plate, it is shown that it the assumed functions are orthogonal to only a finite number of zero frequency modes, then one may obtain frequencies which are lower than the true frequencies of the plate. An iliustrative example is given in the paper.     

Characterization of a Polycrystalline Material with Laser-Excited Nonlinear Surface Acoustic Wave Pulses

High-amplitude nonlinear surface acoustic waves (SAWs) generated by laser pulses were observed in polycrystalline material (stainless steel), and the nonlinear acoustic parameters were evaluated. The velocities of bulk waves and the elastic moduli of the second order were determined by detecting the surface perturbations produced by longitudinal and shear bulk waves (precursors). Consequently, a consistent set of elastic and acoustic constants was obtained by performing all necessary measurements with the same sample using laser excitation and detection techniques.