用非饱和三相孔弹模型研究浅层土壤中地震波的传播特性

研究浅层土壤中声波耦合的地震波的传播特性, 用于声波探雷技术的机理分析. 根据浅层土壤具有孔隙度和可压缩性的特点, 利用非饱和三相孔隙介质中的地震波模型, 研究了土壤孔隙度、含水饱和度等参数对地震波传播特性的影响. 计算结果显示: 在给定的参数条件下, 地震波的传播速度和衰减系数均随频率的增加而增加; 纵波的传播速度随孔隙度的增加而减小, 横波的传播速度随孔隙度的增加而增加; 地震波的传播特性随含水饱和度的增加变化比较复杂. 通过对计算结果与已发表实验结果的比较分析, 讨论了解析方法的可行性, 为声-地震耦合机理及其在声波探雷研究中的应用提供了一定的理论基础. 关键词： 声-地震耦合 地震波 孔隙度 声波探雷     

大气风场和温度对无线电声波探测系统探测高度影响的数值研究

从声波扰动介质中的电波波动方程出发, 使用时域有限差分(FDTD)方法, 结合声波传播的FDTD 模型, 构建了描述声波和电波相互作用的数值模型, 并运用该模型分析风场和温度对无线电声波探测系统的探测高度的影响. 数值模拟结果表明: 温度与风场剖面的存在改变声波和电波散射回波的传播轨迹; 温度梯度剖面主要影响声波的传播速度, 风场剖面导致作为电波散射体的声波波阵面的偏移, 降低电波散射回波的强度并改变回波路径, 使得接收数据减少, 限制无线电声波探测系统的探测高度; 在强风背景下, 若降低声波散射体高度, 电波散射回波“聚束点”的偏移会有较大的改善, 但同时意味着探测高度的降低. 为了改善风场背景下无线电声波探测系统的探测高度, 可以使用双基地雷达或者增大接收天线面积等方法来实现.     

Experimental and numerical study on damage detection in an L-joint using guided wave propagation

Longitudinal and flexural wave propagation in a steel L-joint is considered in this paper. Particular attention is paid to damage detection aspects. Experimental investigations were conducted on an intact L-joint as well as on an L-joint with a notch. Velocity time histories of elastic waves propagation have been applied to find the location of damage. To model longitudinal as well as flexural wave propagation including lateral and shear deformations, the special frame spectral element in the time domain, based on the Mindlin-Herrmann rod and Timoshenko beam theories, was formulated. As a result this paper discusses in detail the possibility of detection of damage in an L-joint and it compares the usefulness of the application of axial and flexural waves in non-destructive damage detection for this typical structural component.     

The dynamic analysis of a cracked Timoshenko beam by the spectral element method

The aim of this paper is to introduce a new finite spectral element of a cracked Timoshenko beam for modal and elastic wave propagation analysis. The proposed approach deals with the spectral element method. This method is suitable for analyzing wave propagation problems as well as for calculating modal parameters of the structure. In the paper, the results of the change in modal parameters due to crack appearance are presented. The influence of the crack parameters, especially of the changing location of the crack, on the wave propagation is examined. Responses obtained at different points of the beam are presented. Proper analysis of these responses allows one to indicate the crack location in a very precise way. This fact is very promising for the future work in the damage detection field.     

管道弯头对低频纵向导波传播特性影响分析

管道弯头显著改变了导波传播特性,影响了对检测信号的解读,研究弯头对导波传播特性的影响是实现复杂管道系统导波检测的基础。采用半解析有限元法计算弯管导波频散曲线,分析了弯管导波频散曲线所呈现的不同特征,并基于弯管导波频散曲线,以低频L(0,1)模态导波为研究对象,实验研究了低频L(0,1)模态导波通过管道弯头时的模态变换特征。研究结果发现,当L(0,1)模态导波通过管道弯头时,不仅会发生L(0,1)到F(1,1)的模态变换,还会模态变换出反向L(0,1)模态导波,即弯头反射现象,且随着激励频率的降低和弯头弯曲半径的减小,弯头反射现象愈发明显。研究结果将深化对弯管导波传播特性的认识,推动导波检测技术在复杂管道系统检测中的应用。      

A large ultrasonic bounded acoustic pulse transducer for acoustic transmission goniometry: modeling and calibration

A large, flat ultrasonic transmitter and a small receiver are developed for studies of material properties in acoustic transmission goniometry. While the character of the wave field produced by the transmitter can be considered as a plane wave as observed by the receiver, diffraction effects are noticeable near critical angles and result in the appearance of weak but detectable arrivals. Transmitted ultrasonic waveforms are acquired in one elastic silicate glass and two visco-elastic acrylic glass sample plates as a function of the angle of incidence. Phase velocities are determined from modeling of the shape of curves of the observed arrival times versus angle of incidence. The waveform observations are modeled using a phase propagation technique that incorporates full wave behavior including attenuation. Subtle diffraction effects are captured in addition to the main bounded pulse propagation. The full propagation modeling allows for various arrivals to be unambiguously interpreted. The results of the plane wave solution are close to the full wave propagation modeling without any corrections to the observed wave field. This is an advantage as it places confidence that later analyses can use simpler plane wave solutions without the need for additional diffraction corrections. A further advantage is that the uniform bounded acoustic pulse allows for the detection of weak arrivals such as a low energy edge diffraction observed in our experiments.     

Finite element modelling of ultrasound, with reference to transducers and AE waves

Finite element (FE) modelling has a role to play in simulating elastic wave propagation associated with structural vibrations, acoustic phenomena and ultrasound problems. In this work we have used the PAFEC software [PAFEC finite element software PACSYS, Strelley Hall, Nottingham, NG8 6PE, UK]. With the advent of increased computer power and greater availability of software these simulations have become more readily available and will provide improved insight into wave propagation problems. Simulations have been undertaken of transient wave propagation in steel plates with an attached simple resonant transducer. This simulates acoustic emission (AE) propagation in plate like structures relevant to many industrial applications. Simulations for short propagation distances suggest the resonant transducer voltage signal carries information on the plate-waves propagating in the structure, overlaid with the piezoelectric resonance and some information might be extracted from the transducer signal. Looking at the wave propagation information alone, a great deal of variability is seen in the displacement profile for different source types, orientations and locations. Although users have expressed a need for calibration of the AE detection process, this idea remains problematic since the complete generation and detection system has the features of a chaotic system. Using FE modelling a method of "point calibration" might be available, for some specific AE applications such as crack growth along known paths.     

裂缝性地层黏弹性地震多波波动方程

裂缝检测是目前国内外石油勘探界研究的一个热点问题，如何确定裂缝方位等参数是石油公司面临的难题，而解决该难题就要确定裂缝方位等参数与地震波场传播之间的定量关系.但是目前所采用的裂缝性地层介质模型不能完全定量地反映裂缝的方位特征和衰减特征.针对该问题，建立了具有任意裂缝方位的裂缝性地层介质模型；并构造了时间增量的方法，将非线性的卷积积分采用近似的方法实现，建立了以位移场表示的具有任意方位角的黏弹性方位各向异性介质的波动方程.该波动方程定量地给出了黏弹性波场特征与裂缝走向的关系，描述了黏弹性地震波在这种介质中的 关键词： 裂缝 各向异性 黏弹性 波动方程     

磁纳米粒子介导的靶向剪切波弹性成像*

组织力学特性与其生理病理变化过程密切相关。因此,对组织力学特性的分析有望为疾病诊断提供重要依据。超声弹性成像可以定量分析组织的剪切模量,但在检测的特异性和灵敏度等方面仍存在局限性。针对这一问题,该文发展一种磁纳米粒子介导的靶向剪切波弹性成像新方法。该方法是基于磁纳米粒子在脉冲磁场作用下产生磁致振动,从而导致周围组织的剪切波传播,通过超声探测粒子振动及剪切波传播即可获得磁纳米粒子的分布及周围组织的弹性信息。基于该方法,该文在仿体水平实现对直径约3 mm的磁纳米粒子标记区域的振动检测及其周围组织的弹性信息获取。该研究有望为疾病的精准诊断提供新的思路。     

Laser ultrasonic anomalous wave propagation imaging method with adjacent wave subtraction: Application to actual damages in composite wing

Laser ultrasonic wave propagation imaging methods have great potential for integrated structural health management and non-destructive evaluation. However, application of these techniques to complex structures in the field is difficult because they give rise to complicated wave propagation patterns. We developed an anomalous wave propagation imaging method with adjacent wave subtraction using laser ultrasonic scanning to solve this problem. The proposed method is suitable for non-destructive evaluation of complex structures because it highlights the propagation of anomalous waves related to structural discontinuities, and suppresses complex incident waves without the need of pre-stored reference data. In this study, the method was applied to a real composite wing subjected to bending and impact tests. The method enhanced the visibility of the anomalous waves related to damages such as stringer tip debonding, skin-spar debonding, and invisible impact damage. Based on these anomalous waves, variable time window amplitude mapping was performed to show the damage location, size, and shape resemble to the actual damage. Comparisons showed that the methods performed better than the ultrasonic A-scan in terms of damage detection and sizing accuracy. The presence of structural elements such as spars, stringers, ribs, and surface-mounted PZT elements did not adversely affect the inspection. The proposed wing test setup with a built-in ultrasonic propagation imaging system for automatic NDE could be easily expanded throughout a hanger for maintenance inspection.     

Analysis of microwave propagation effects using two-dimensional electrooptic field mapping techniques

In high-speed electronic devices and monolithic microwave and millimetrewave integrated circuits the propagation of electromagnetic waves plays an important role. In so-called travelling wave devices and circuits these propagation effects are applied to design and realize functions not available in lumped elements. In order to exploit fully the potential of wave propagation effects, experimental investigations have to be carried out which, however, require a measurement technique to allow spatially resolved detection of microwave potential or field distributions inside elements and circuits and along the electrical interconnects. In this paper, it is shown by several examples, that the two-dimensional electrooptic field mapping technique is an excellent tool to study wave propagation effects up to millimetrewave frequencies, with submicrometre spatial resolution and without electromagnetic interference.     

轨道板中超声兰姆波传播的有限元计算和实验*

该文针对我国高速铁路轨道板缺陷的非接触动态检测问题,研究了空气耦合超声兰姆波在轨道板中的传播规律。首先,给出了轨道板中超声兰姆波的相速度和群速度频散曲线,结果表明:随着频厚积的增加,频散现象越明显,并且A0相速度收敛于Rayleigh波的波速。然后,建立轨道板中波传播的有限元模型,计算得到兰姆波传播的群速度为2220 m/s,且二维傅里叶变换系数的较大值沿Rayleigh波的频散曲线分布。最后,在沪杭高铁嘉兴南站进行了现场测试,以8.8°倾斜角向轨道板激励产生超声兰姆波,激发产生的兰姆波模态群速度为2325 m/s,且二维傅里叶变换分析其系数的较大值沿Rayleigh波的频散曲线分布。有限元计算结果和实验结果均与理论计算结果一致。该研究为后续轨道板缺陷的非接触动态检测提供了理论依据和实验方法。     

Nonlinear surface acoustic waves: realization of solitary pulses and fracture

A laser-based technique for the contact-free generation and detection of strongly nonlinear surface acoustic wave (SAW) pulses with amplitudes limited by the materials strength has been developed. The effects of nonlinear propagation of short elastic surface pulses with finite strength in isotropic solids, such as fused quartz, anisotropic solids, such as silicon, and dispersive media were investigated. Solitary surface wave propagation was observed in layered structures for normal and anomalous dispersion. In addition, a SAW-based method for evaluating the critical fracture stress of anisotropic brittle solids, such as single crystal silicon, is introduced.     

熔石英光学材料紫外皮秒激光损伤的瞬态特性研究

光学元件激光损伤是限制高功率激光装置输出能力的关键因素,为了理解光学元件激光损伤过程,提高光学元件抗激光损伤性能,利用偏振阴影显微镜成像技术和光电探测技术研究了紫外皮秒激光诱使熔石英光学元件损伤的时间分辨动力学过程。结果显示了紫外皮秒激光作用过程中冲击应力波的传输特性、瞬态吸收的演变过程以及裂缝的发展过程。结果表明,冲击应力波的传输速度约为6.9μm/ns;532nm波长的激光瞬态吸收在激光作用之后2.5μs时激光吸收达到最大值,之后缓慢下降,整个持续时间可达50μs以上;损伤裂纹在7.5ns时刻就基本停止增长。研究结果对理解皮秒激光的损伤机制有重要意义。     

High-frequency guided ultrasonic waves for hidden defect detection in multi-layered aircraft structures

Aerospace structures often contain multi-layered metallic components where hidden defects such as fatigue cracks and localized disbonds can develop, necessitating non-destructive testing. Employing standard wedge transducers, high frequency guided ultrasonic waves that penetrate through the complete thickness were generated in a model structure consisting of two adhesively bonded aluminium plates. Interference occurs between the wave modes during propagation along the structure, resulting in a frequency dependent variation of the energy through the thickness with distance. The wave propagation along the specimen was measured experimentally using a laser interferometer. Good agreement with theoretical predictions and two-dimensional finite element simulations was found. Significant propagation distance with a strong, non-dispersive main wave pulse was achieved. The interaction of the high frequency guided ultrasonic waves with small notches in the aluminium layer facing the sealant and on the bottom surface of the multilayer structure was investigated. Standard pulse-echo measurements were conducted to verify the detection sensitivity and the influence of the stand-off distance predicted from the finite element simulations. The results demonstrated the potential of high frequency guided waves for hidden defect detection at critical and difficult to access locations in aerospace structures from a stand-off distance.     

太赫兹波探测光子晶体涂层覆盖目标的可行性

覆盖于高温目标表面的光子晶体红外涂层可实现对目标红外辐射的抑制,而太赫兹波所具有的强穿透特性使其对该类目标的探测成为可能。以相关文献中设计的光子晶体涂层为例,采用特征矩阵理论对0.3~3 THz频率范围内的太赫兹波在该类涂层中的传输特性进行理论计算和分析,重点研究了不同入射角度的太赫兹波在该类涂层中的传输特性。研究发现,上述太赫兹波段处于光子晶体的带隙之外,0.3~0.5 THz频率范围内的太赫兹波对该类红外涂层具有较强的穿透特性,其光谱透过率大于90%;而在2.4~3 THz范围内,其在涂层上具有较强的反射,且整个波段内的吸收率小于0.2%。当入射角小于60°时,其对太赫兹波的传输特性影响较小;进一步增大入射角,其透过率逐渐降低,而反射率逐渐增大。研究结果证明了利用太赫兹波进行涂层覆盖目标探测的可行性,有望利用太赫兹雷达探测弥补红外探测系统的不足。     

Difference between a photon's momentum and an atom's recoil

When an atom absorbs a photon from a laser beam that is not an infinite plane wave, the atom's recoil is less than variant Planck's k in the propagation direction. We show that the recoils in the transverse directions produce a lensing of the atomic wave functions, which leads to a frequency shift that is not discrete but varies linearly with the field amplitude and strongly depends on the atomic state detection. The same lensing effect is also important for microwave atomic clocks. The frequency shifts are of the order of the naive recoil shift for the transverse wave vector of the photons.     

黏弹介质包裹的充液管道中低频轴对称波传播特性

研究埋地充液管道中低频轴对称波传播特性。将土壤考虑为黏弹介质,结合Kennard薄壳方程和Kelvin-Voigt线性黏弹性模型,引入土壤载荷矩阵,推导出土-管滑移情形下流体主导波和管壁压缩波的相速度表达式。通过数值模拟计算得到流体主导波和管壁压缩波的频散和衰减曲线并进行可靠性验证,分析两种波引起的管壁径向位移之比,讨论厚径比和品质因子对流体主导波传播的影响。结果表明,黏弹介质对流体主导波和管壁压缩波的相速度影响较小,但对衰减影响较大;流体主导波对管壁径向位移有较大的影响,是泄露噪声传播的主要载体;厚径比越大,流体主导波的相速度越大,衰减越小;而品质因子越大,流体主导波的频散和衰减都越小。研究结果可为埋地充液管道的泄漏检测提供一定的理论参考。      

New Method for Determination of Excited State Lifetime

The use of optical methods for the detection of plasma waves requires the consideration of propagation properties of waves as well as the excited state lifetimes. In case of known propagation properties of waves the excited state lifetime can be determined from the frequency-dependent decrease of the wave amplitude. The proposed technique represents a new independent method for the determination of excited state lifetimes. This paper deals with first studies on excited state lifetime based on the frequency response of the amplitude of free streaming electron waves in He-dc-discharges. The results demonstrate the correctness of the idea about the decrease of wave signal in consequence of excited state lifetime.     

Piezoelectric transducer parameter selection for exciting a single mode from multiple modes of Lamb waves

Excitation and propagation of Lamb waves by using rectangular and circular piezoelectric transducers surface-bonded to an isotropic plate are investigated in this work. Analytical stain wave solutions are derived for the two transducer shapes, giving the responses of these transducers in Lamb wave fields. The analytical study is supported by a numerical simulation using the finite element method. Symmetric and antisymmetric components in the wave propagation responses are inspected in detail with respect to test parameters such as the transducer geometry, the length and the excitation frequency. By placing only one piezoelectric transducer on the top or the bottom surface of the plate and weakening the strength of one mode while enhancing the strength of the other modes to find the centre frequency, with which the peak wave amplitude ratio between the S0 and A0 modes is maximum, a single mode excitation from the multiple modes of the Lamb waves can be achieved approximately. Experimental data are presented to show the validity of the analyses. The results are used to optimize the Lamb wave detection system.