有粘弹薄层负载时薄板中的Lamb波传播

从弹性波理论出发,结合边界条件,导出了一面有粘弹薄层负载时薄板中Lamb波的色散方程,通过数值计算,研究了薄板中Lamb波传播色散、衰减等特性及负载层参数对Lamb波传播的影响。最后提出了一种结合Lamb波和SH波微传感实现粘弹薄膜材料复Lame参数λ*和μ*的定征方法。     

复合结构螺旋超材料对光波前的高效调控

近年来,超材料和超表面因为一些不同于传统材料的新奇性质一直被广泛研究,而基于超材料或者是超表面的波前控制也是其中的一个热门研究领域.迄今为止,已经提出了很多不同的结构来对反射光和透射光的波前进行调控,在已知的结构中,反射光的波前调控效率已经可以达到较高数值,但是很少有报道能够使用超材料简单高效地调制透射光的波前.本文提出了一种由相同几何结构的左旋和右旋结构复合而成的螺旋超材料.通过使用时域有限差分方法进行仿真,发现这种螺旋结构将会在入射光和透射光之间引入一个可控的相位变化,从而可直接对透射光波前进行调控.仿真结果还表明,该复合结构螺旋超材料在较宽的波长范围内可以达到近64%的透射率.最后通过将该螺旋材料沿着X轴排布成有着连续相位变化的阵列,可以在近红外区域(1.0—1.4μm)观察到反常折射现象,仿真结果与理论计算得出的反常折射角十分符合.     

基于透射型几何相位编码超表面的太赫兹波束自由操控

为了实现太赫兹波束的自由操控,基于Pancharatnam-Berry几何相位理论,提出一种多层C型单元结构,实现在目标频率下0到2π的太赫兹波透射传输相位调控。基于广义斯涅耳定律,构建2-bit和3-bit编码超表面,在圆偏振入射条件下,实现透射型太赫兹波束的散射角度调控。采用数字信号处理理论中的傅里叶卷积运算,对不同周期序列的编码超表面进行四位制编码卷积运算,获得新的编码序列,实现对透射太赫兹波散射角度的自由调控。利用多层C型编码超表面单元结构对其进行旋转编码,实现1阶和2阶涡旋相位板,产生不同阶数太赫兹波涡旋光束。     

一种光可调的多功能太赫兹几何相位超表面北大核心CSCD

基于金属-光敏硅组合圆环提出了一种光可调太赫兹超表面,可用于实现多功能的波前操控。该超表面单元由正反两面的结构层和中间介质层组成,其中结构层是开口方向相反的金属环,开口处由光敏硅进行填充。在光照强度较低时,超表面可以将入射的圆极化波转换为交叉极化的透射波;随着光照强度升高,透射波将逐渐被完全抑制。根据几何相位原理,通过旋转金属-光敏硅组合圆环,透射的交叉极化波会携带额外的相位因子,并可实现完全的2π范围相位覆盖。通过合理排列超表面单元结构,可以对透射波的波前实现任意操控。利用提出的光可调超表面,在较低光照条件下实现了高效的异常折射、透镜以及轨道角动量产生器;在较高光照条件下,抑制了其透射效率,可有条件地选择应用功能,表现出较好的灵活性。提出的光可调超表面在太赫兹成像、通信、雷达等方面具有较大的潜在应用价值。     

A0模态Lamb波聚焦透镜的结构设计及实验研究

基于折射率的弯曲波透镜多在板结构上打孔或挖槽,这种设计破坏了原有的结构,降低了板的刚度和稳定性。鉴于此,通过在板表面镶嵌棱柱的方式设计了A0模态Lamb波的聚焦透镜。首先,详细讨论了棱柱的结构参数对Lamb波带隙的影响机理,获取了波速与结构尺寸的定量关系,并实现了透镜的结构设计;其次,有限元仿真了该聚焦透镜的工作性能,包括聚焦位置、焦点处能量分布、聚焦尺寸、工作带宽等;最后,通过实验验证了该透镜设计的正确性。研究结果表明,设计的透镜能够使弯曲波聚焦在预先设定位置,且在不改变结构参数的情况下具有一定的工作频率带宽。该透镜设计方法具有聚焦性能优越、刚度强、易于加工等优点,为声聚焦透镜在无损检测、能量收集等领域的实际应用提供了参考。     

宽带透射吸收极化无关超材料吸波体

提出了一种新的基于磁性吸波体材料的具有低频透射和高频宽带吸收特性的超材料吸波体.该超材料吸波体在1 GHz的透射系数为-0.5 dB,具有较好的低频透射特性,可以实现对低频信号的相互通信;在频率大于8.4 GHz的频段,吸收率均大于80%,基本覆盖整个X波段和Ku波段,实现高频宽带吸收.此外,由于该超材料吸波体的单元金属周期结构具有较好的四重旋转对称性,因而是极化无关的.该透射吸收超材料吸波体设计简单,实用性强,具有较强的潜在应用价值.     

超声Lamb波混频激励效率与模式选择

研究了超声Lamb波混频的激励效率与模式选择。在非零能量流和相匹配条件的基础上,结合Lamb波在结构中的状态分布,提出了以激励效率参量作为超声Lamb波混频模式选择的依据。根据混频条件筛选混频模式对,计算相应的激励效率参量,并通过超声Lamb波混频的仿真模拟与实验测量进行验证。通过对比仿真与实验中得到的非线性参量与激励效率参量的关系,证明了激励效率参量作为模式选择依据的有效性。研究表明,激励效率参量可以有效地表征超声Lamb波混频的激发效率,从而筛选非线性效应明显的模式对,更好地实现损伤的检测与表征。     

窄频带Lamb波频散特性研究

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     

有粒滞液层负载时薄板中类Lamb波的传播

本文从弹性波传播理论出发，结合边界条件，导出了薄板一面有粘滞液层负载时板中类Lamb波传播的色散方程；研究了液层粘滞引起的类Lamb波衰减．数值计算表明，板和液层的驻波共振振动引起极中类Lamb波对称和反对称模式的模式转换以及衰减系数的变化。文中对ktd《1时，Ao模式类Lamb波的微质量和粘滞传感特性也作了讨论。     

铝板中Lamb波检测的实验研究

Lamb波在传播过程中具有频散及多模态特性,若相关参数选择不当,会导致在实际应用中信号相互叠加而无法识别。本文基于Lamb波的频散曲线是其频散方程实数解分布的特点,采用二分法绘制了铝板中Lamb波的频散曲线、波结构曲线和入射角曲线。根据曲线选择S0模态的Lamb波对1mm厚铝板中不同类型的缺陷进行检测。实验结果表明,S0模态的Lamb波对裂纹型缺陷和贯穿型缺陷十分敏感,但对于裂纹型缺陷,其幅值变化并不与缺陷大小成线性关系,并且S0模态Lamb波的声场指向性十分集中,在偏离声束轴线时无法检测到缺陷。     

Ultimately thin metasurface wave plates

Optical properties of a metasurface which can be considered a monolayer of uniaxial metamaterials ‐ parallel‐plate and nanorod arrays – are investigated. It is shown that such metasurface acts as an ultimately thin sub‐100 nm wave plate. This is achieved via an interplay of epsilon‐near‐zero and epsilon‐near‐pole behavior along different axes in the plane of the metasurface allowing for extremely rapid phase difference accumulation in very thin metasurface layers. These effects are shown to not be disrupted by non‐locality and can be applied to the design of ultrathin wave plates, Pancharatnam‐Berry phase optical elements and plasmon‐carrying optical torque wrench devices.

     

Simulation of Lamb wave's interactions with transverse internal defects in an elastic plate

Lamb wave’s interactions with transverse internal defects in an elastic plate are investigated in this paper to help practical inspection work with guided wave. A 2D frequency domain hybrid boundary element method approach previously mainly used to study Lamb wave’s interactions with external defects in elastic plates is adopted in this work and extended to study the cases of internal defects. Simulation examples are presented to illustrate the reflection and transmission coefficients’ variations with various parameters including defect’s height, width, testing fd for internal symmetrical and non-symmetrical cracks, with symmetry defined with respect to the middle plane of the plate. This simulation could be a valuable tool for the research of Lamb wave’s applications in nondestructive testing (NDT) field, as the problem of internal defects could be difficult to study experimentally because of the inconvenience in machining internal defects in a real plate.     

Time-domain analysis of second-harmonic generation of primary Lamb wave propagation in an elastic plate

Within the second-order perturbation approximation,this paper investigates the physical process of generation of the time-domain second harmonic by a primary Lamb wave waveform in an elastic plate.The present work is performed based on the preconditions that the phase velocity matching is satisfied and that the transfer of energy from the primary Lamb wave to the double frequency Lamb wave is not zero.It investigates the influences of the difference between the group velocities of the primary Lamb wave and the double frequency Lamb wave,the propagation distance and the duration of the primary Lamb wave waveform on the envelope shape of the time-domain second harmonic.It finds that the maximum magnitude of the envelope of the second-harmonic waveform can grow within some propagation distance even if the condition of group velocity matching is not satisfied.Our analyses also indicate that the maximum magnitude of the envelope of the second-harmonic waveform is kept constant beyond a specific propagation distance.Furthermore,it concludes that the integration amplitude of the time-domain second-harmonic waveform always grows with propagation distance within the second-order perturbation.The present research yields new physical insight not previously available into the effect of generation of the time-domain second harmonic by propagation of a primary Lamb wave waveform.     

Ultrasonic Lamb wave method for sizing of spot welds

Two spot-welded thin sheets can be treated as two elastic waveguides coupled by a small common welded region. It is proposed to use Lamb waves to evaluate the quality of the weld. The feasibility of selecting the working frequency and mode number of the Lamb wave so that no resonance of the elastic vibrations appears in the weld region is considered analytically. This condition ensures a one-to-one correspondence between the mean diameter of the spot-weld region and the coefficient of transmission of the Lamb waves through it.A simple technique is suggested for determining the transmission coefficient and the scattering patterns of Lamb waves at the weld region. Experiments indicate that, to a satisfactory approximation, the wave transmission coefficient is proportional to the cube of the weld diameter. This allows a unique determination of the dimension of the welded region on the basis of the amplitude of the transmitted signal.     

Low-frequency band gaps in one-dimensional thin phononic crystal plate with periodic stubbed surface

Using supercell plane wave expansion method, the Lamb wave band structure of one-dimensional thin plate with periodic stubs is investigated. The numerical results show that flat bands will appear and band gap can exist in a low-frequency domain. The position of the flat bands and width of the low-frequency Lamb wave band gap can be tuned by the stub height, plate thickness and filling fraction. The band gap is obtained by opening the folding points of the same plate modes not the crossing point of different plate mode when the stub height is small.     

Nondestructive testing in human teeth using a leaky Lamb wave device

A Lamb wave interdigital transducer mounted on a layered substrate composed of two plates, a thin piezoelectric ceramic plate and an acrylic plate, operating at a liquid-solid boundary, is investigated for ultrasonic nondestructive testing of the layer thickness in human teeth. A higher-order mode having a phase velocity higher than the longitudinal wave velocity in the human teeth can be used for nondestructive testing. In the combination of the two layers, the fourth mode of leaky Lamb wave is most favorable for a frequency-controllable radiation angle of an ultrasound beam into a water layer as an acoustic coupler. In the configuration of an acoustic delay line, the layer-thickness measurement in vivo, evaluated from the time interval between two reflected ultrasound echoes, is successfully realized under a thin water layer as the acoustic coupler.     

基于层状模型的功能梯度板中兰姆波传播研究

功能梯度板中的兰姆波的传播在实际工程中有着非常广泛的应用。采用层状模型研究了兰姆波在材料特性沿厚度方向连续变化的功能梯度板中的传播特性。通过数值计算获得了层状板中兰姆波的色散关系,并与已有结果进行了比较,获得了材料属性沿厚度方向呈指数变化和多项式变化时功能梯度板中兰姆波的波速和位移解。当材料属性连续变化时,兰姆波各阶模态的波速与位移都将发生变化。相比于兰姆波的高阶模态,低阶模态的波速变化更加明显。本文的研究可为功能梯度板的设计提供参考。     

Lamb wave sensors array for nonviscous liquid sensing

The interdigital transducer (IDT) can excite Lamb wave in a piezoelectric plate loading with a liquid layer, and the phase velocity of Lamb wave is associated with the properties of the liquid layer. In this paper, the concept of effective permittivity is introduced to study the Lamb wave’s potential application in liquid sensing. Considering the measuring of ideal nonviscous liquid, the sensors array is designed to sense the density and the dielectric constant of the liquid layer simultaneously. Using LiNbO₃ as piezoelectric material, in order to improve the sensors array sensitivity and the electro-mechanical coupling coefficient, the optimized results including plate thicknesses and cut orientations are presented by numerical simulation. These studies show that the Lamb wave sensors array can be potential in liquid sensing.     

Lamb wave band gaps in locally resonant phononic crystal strip waveguides

Using finite element method, we have made a theoretically study of the band structure of Lamb wave in a locally resonant phononic crystal strip waveguide with periodic soft rubber attached on the two sides of epoxy main plate. The numerical results show that the Lamb wave band gap based on local resonant mechanism can be opened up in the stub strip waveguides, and the width of the local resonant band gap is narrower than that based on the Bragg scattering mechanism. The results also show that the stub shape and width have influence on the frequency and width of the Lamb wave band gap.     

非线性兰姆波在阶梯板中传播的有限元仿真

在非线性兰姆波检测中,累积二次谐波是重要的检测信号.该文利用有限元法仿真了非线性兰姆波在阶梯板中的产生和传播过程,通过二维离散傅里叶变换分析了兰姆波的传播模态和各个模态通过波导横截面的能流大小,给出了各个模态在阶梯板中的反射、透射系数,总结了其随阶梯板厚度的变化规律.研究发现,随着台阶增高,散射效应使得兰姆波更难入射波...