质量偏心Timoshenko梁的振动波特性研究

在研究船舶、潜艇等工程结构的低频振动时,通常可以将其简化为质量在截面内分布非均匀的梁结构,此质量偏心会引起弯-纵耦合。针对弯-纵耦合的质量偏心Timoshenko梁,推导了其截止频率的解析表达式;探讨了质量偏心对其纵振波、传播弯曲波及衰减弯曲波波数的影响规律;研究了三组波数下纵向/弯曲位移比随频率及质量偏心的变化。分析结果表明,质量偏心会降低梁的截止频率,偏心率越大,降低越明显;弯曲衰减波会在截止频率处转变为弯曲传播波;质量偏心使得非频散的纵向振动波转变为频散波;纵向振动与弯曲振动的耦合在质量偏心率或频率增大时,会进一步加强。     

层间光滑接触的双层厚壁圆筒中周向导波

通过对双层厚壁圆筒周向导波的频散现象的研究,以及与单层圆筒周向导波特性的比较,发现周向导波在双层厚壁圆筒中传播时,会发生明显的频散现象和模态干涉现象,其第1阶模态在高频时接近于无频散Rayleigh面波。在其高阶模态上,层间效应被弱化,因此,应将层间界面裂纹无损检测的频率激发范围,集中于低阶模态频率上。研究发现,第2阶模态,具有在层间界面上集中能量的特点,因此,可用于层间界面裂纹的周向导波无损检测技术中。     

黏弹性正交各向异性空心圆柱中纵向导波的传播

基于线性三维弹性理论和Kelvin-Voigt模型,采用勒让德正交多项式展开法推导了黏弹性正交各向异性空心圆柱中纵向导波的波动方程,数值求解了波动方程并阐述了相关方程的含义。首先计算了大径厚比下黏弹性管的相速度频散曲线和衰减曲线,并与已发表文献的结果进行了对比,验证了程序的正确性,并进一步计算了低阶纵向导波的位移分布和应力分布曲线,验证了方法的可靠性。然后利用方程的解耦特性,分别求解了不同径厚比、不同黏性常数下纵向模态和扭转模态的频散和衰减曲线,研究了径厚比和黏性常数效应对两种模态的影响。最后针对扭转模态导波,研究了材料相关黏弹性常数对其频散特性和衰减特性的影响。      

波纹管壳结构中振动波的传播

研究了波纹管壳结构中的振动波。运用摄动方法与结构振动的波动方法得到了波纹管结构波动解的形式;在此基础上考察了振动波的频散特性。结果表明：摄动波是以传播波和衰减驻波形式存在;研究了振动波通过圆柱壳－波纹壳不连续截面时的传播特点,从频散特性角度对弯曲波传播进行了解释。本文为工程实际中波纹管接头的降噪预报提供了一种理论方法。     

粘性液体对管道中扭转导波传播特性的影响研究

对带有液体的圆柱/管中的波传播进行了研究。在流动为层流的假设下,研究了有粘性液体管道中扭转导波的位移模式和传播特性。分析了波的频散和衰减曲线,得到了粘性、密度等和第一阶扭转模态衰减的关系,确定了粘性液体对扭转导波的影响,为扭转导波在损伤检测中应用做了一些理论工作。数值计算结果与某文献中实验结果符合较好,一定程度上证明了液体中的层流假设,说明了第一阶扭转模态在损伤探测中的优势。     

周向导波在不同连续性条件多层圆筒中的传播

通过对多层厚壁圆筒中周向导波频散曲线研究,发现周向导波在多层圆筒中传播时,会发生明显的频散现象和模态干涉现象,第1阶模态在高频时接近于无频散的Rayleigh面波。通过对不同连续性条件的多层筒与单层筒的频散曲线比较,以及筒的层数对频散曲线的影响,都充分说明了检测层间界面缺陷的周向导波应该集中在低阶模态上。最后对不同连续性条件多层筒的位移研究,最终推断出第2阶模态具有在层间界面附近集中能量的特征,可应用于层间界面缺陷的周向导波无损检测技术中。     

正交各向异性空心圆柱体中的纵向导波

基于三维线弹性理论,采用勒让德正交多项式展开法,推导了正交各向异性材料中纵向导波的耦合波动方程,并对耦合波动方程进行了数值求解。首先,为确定方法的适用性和准确性,利用Disperse软件求解各向同性空心圆柱体中纵向导波的频散曲线,并将其与勒让德正交多项式展开法求解结果相对比,二者结果完全一致。然后,讨论了勒让德正交多项式截止值对轴对称导波频散曲线收敛性的影响,并从数值计算的角度分析了产生影响的原因。最后,针对碳纤维缠绕的复合材料空心圆柱体,分别求解纵向、扭转和弯曲三种不同模态纵向导波的相速度频散曲线。计算了不同径厚比下的弯曲模态相速度频散曲线,分析径厚比的变化对频散曲线的影响。     

复振型导数计算的可变移频值方法

实际结构系统由于存在多种不同性质的阻尼其动态特性很复杂,振型导数的计算也比较困难。采用模态加速和移频的思想发展了一种基于模态叠加的复振型导数计算方法。首先对控制方程进行移频处理,利用广义幂级数展开式获得模态迭代公式,并利用迭代结果与各阶振型表示复振型导数;然后把系统的广义动柔度矩阵表示为已知的低阶模态与截断的高阶模态之和,高阶模态部分采用多个矩阵多项式与一个广义幂级数的乘积表示,并利用系统的低阶模态和系统矩阵进行计算;各阶移频值表示为相应的移频系数与复特征值的乘积,它们仅与最低阶模态移频值的模和本阶模态的单位复特征值有关,而最低阶模态的移频系数通过精度分析获得。给出了合适的模态加速迭代次数。该方法仅需进行一次系统矩阵的分解就可获得高精度的多个复振型导数。算例表明方法正确、高效。     

温度对大鼠肝脏中剪切波与频散特性影响研究

剪切波相速度和衰减的频散特性的准确测量具有重要的意义。温度变化可能会对生物软组织的力学特性产生影响,为了研究温度的变化对剪切波相速度和衰减频散特性测量的影响,以大鼠肝脏为研究对象,基于声辐射力的剪切波成像的方法,通过离体实验来获取大鼠肝脏内部不同温度下的剪切波相速度和衰减频散特性曲线。实验结果发现,随着温度的升高,剪切波相速度逐渐减少,衰减频散逐渐增大,特别是在较高的频段(270~380 Hz)的变化更为显著。结果表明温度是测量大鼠肝脏中剪切波相速度和衰减频散特性中不可忽略的因素,这对今后的动物实验研究以及疾病评估提供了实验参考,具有重要的意义。     

成渝高速铁路某高架桥段地面三向振动特性分析

地面振动是高速铁路运营期的主要环境问题之一，为了研究高速铁路高架桥段地面振动的传播和衰减规律，选择成渝高速铁路某桥梁段进行了现场地面三向振动测试。在时域和频域内分析了地面三向振动的时程特征和频谱特征，以及垂向振动、水平向振动随距离的传播特性。结果表明，在同一距离处，横向、纵向振动加速度幅值及有效值均大于垂向；三向振动随距离的增加，加速度幅值及有效值均呈衰减趋势；垂向和纵向振动中，高频成分衰减迅速，低频衰减速度较缓，而横向则是低频和高频均衰减迅速，但远场仍是以低频为主；三向振动远场优势频率均在10Hz左右；计权垂向振级高于水平向振级，未计权的水平向振级均大于垂向振级。     

Analysis of cladded acoustic fibers of hexagonal crystal symmetry

A cladded acoustic fiber consisting of a core and an infinite thick cladding both having a hexagonal crystal symmetry is analyzed. The crystalline Z axes of both core and cladding are parallel to the fiber axis. Dispersion equations and field distributions for all modes are derived, and previously reported results for the isotropic case are retrieved. Numerical results for a few lowest-order pure guided torsional, radial-axial and flexural modes are shown. Material considerations are discussed.     

Elastic stress transmission in cellular systems—Analysis of wave propagation

A closely packed array of thin-walled rings constitutes an idealisation of a cellular structure. Elastic waves propagating through such structures must do so via the ring (cell) walls. A theoretical investigation into the propagation of elastic stresses in thin-walled circular rings is undertaken to examine the nature of wave transmission. Three modes of motion, corresponding to shear, extensional and flexural waves, are established and their respective velocities defined by a cubic characteristic equation. The results show that all three waves are dispersive. By neglecting extension of the centroidal axis and rotary inertia, explicit approximate solutions can be obtained for flexural waves. Employment of Love's approach for extensional waves [Love AEH. A treatise on the mathematical theory of elasticity, 4th ed. New York: Dover Publications; 1944. p. 452–3] enables approximate solutions for shear waves to be derived. The three resulting approximate solutions exhibit good agreement with the exact solutions of the characteristic equation over a wide range of wavelengths. The effects of material property, ring wall thickness and ring diameter on the three wave modes are discussed, and the results point to flexural waves as the dominant means of elastic energy transmission in such cellular structures. Wave velocities corresponding to different frequency components determined from experimental results are compared with theoretical predictions of group velocity for flexural waves and good correlation between experimental data and theory affirms this conclusion.     

基于自重影响的连续轴扭/弯耦合振动的研究

考虑轴的自重的影响,研究轴的质量中心分布线与旋转中心线不重合时的扭/弯耦合振动情况。研究中,基于轴的基本理论建立了轴的扭/弯耦合振动的方程组,并根据振型叠加法以及模态的正交性进行了耦合方程组的求解。对简支轴仅考虑自重情况下轴的扭/弯耦合振动进行了自由振动和强迫振动的研究,对与理论研究条件相同的实际简支轴进行了耦合振动响应的实测研究。研究结果表明:第一、三、五…阶弯曲振动是跟第二、四、六…阶扭转振动相耦合的,而第二、四、六…阶弯曲振动则是跟第一、三、五…阶扭转振动相耦合。     

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。     

Acoustic wave propagation along cladded fibers of cubic crystalsymmetry

An approximate method is proposed to analyze the acoustic wave propagation characteristics in nonpiezoelectric cladded fibers of cubic crystal symmetry. The fiber axis coincides with the crystalline Z axis. This approximate method utilizes coupled mode equations and the exact solutions for the corresponding cladded isotropic fiber are used as the approximation base. We concentrate our analysis on cubic fibers with weak anisotropy where the first order approximation obtained from the coupled mode equations can provide accurate results. Dispersion curves of a few lower order flexural, torsional and radial-axial modes in such a fiber with infinite cladding are presented for the first time. Higher order approximations are also discussed. We show the accuracy of the perturbation formula, reveal that the torsional and radial-axial modes are not pure in cubic fibers, and inspect the modal birefringence phenomenon     

Guided Wave Tuning Principles for Defect Detection in Tubing

Dispersion diagrams for longitudinal modes and several flexural modes are given. A partial loading oblique incidence technique was introduced for non-axisymmetric guided wave generation. Acoustic fields for partially loaded generation of guided waves were obtained along a sample Inconel steam generator tube surface. The axial field and the circumferential fields were non uniform. Even though the acoustic field was much more complicated than in the case of axisymmetric modes, the study of non-axisymmetric mode cannot be avoided; excitation of a single symmetric mode is often difficult due to limited access and transducer efficiency and there is also mode conversion after scattering from defects. For 100% inspection coverage of tubing and piping, three dimensional tuning (distance, frequency, and incident angle tuning), was employed. A single combination of incident angle, position, and frequency may miss defects in blind spots. However, complete inspection coverage of a whole cross sectional area over a certain distance of tubing was successfully demonstrated through a multi crack detection experiment by using the three dimensional tuning concepts. Also, the use of non-axisymmetric guided waves for a large distance inspection capability was successfully demonstrated.     

Analysis of weakly guiding cladded acoustic fibers of hexagonal crystal symmetry

An approximate analysis of the weakly guiding cladded acoustic fiber consisting of a core and infinite thick cladding both with hexagonal crystal symmetry is presented. The crystalline Z axis is parallel to the fiber axis. Weak guidance conditions require that the stiffness constants of the core are slightly less than those of the cladding, and the density of the core and cladding are almost the same. Approximate dispersion and cutoff equations are derived for all pure guided modes. Dispersion curves of several lower-order guided torsional, radial-axial and flexural modes are evaluated using both exact and approximate formulas and they are in good agreement. A simple approximate dispersion equation for leaky (longitudinal-type) modes is also obtained.     

采用波动法研究有限尺寸加肋L型板结构的振动特性

本文采用波动方法研究有限尺寸加肋L型板结构的振动及其抑制问题。将有限尺寸方钢平板处理成平板结构弯曲和面内运动与方钢结构的弯扭运动和轴向运动的耦合连续模型，利用波动方法分析有限尺寸加肋L型板结构任意位置的动力学响应。数值结果表明，与有限元方法相比，波动法不仅可以准确地计算有限尺寸方钢平板结构的低频振动响应，而且能有效地计算其中高频振动响应。方钢布设在两板连接处，提高其抗扭刚度可提高减振效果；方钢布设于源板沿长度方向的三分之二位置处或靠近连接处的接受板上，具有较优的减振效果。     

Coronal magnetohydrodynamic waves and oscillations: observations and quests

Coronal seismology, a new field of solar physics that emerged over the last 5 years, provides unique information on basic physical properties of the solar corona. The inhomogeneous coronal plasma supports a variety of magnetohydrodynamics (MHD) wave modes, which manifest themselves as standing waves (MHD oscillations) and propagating waves. Here, we briefly review the physical properties of observed MHD oscillations and waves, including fast kink modes, fast sausage modes, slow (acoustic) modes, torsional modes, their diagnostics of the coronal magnetic field, and their physical damping mechanisms. We discuss the excitation mechanisms of coronal MHD oscillations and waves: the origin of the exciter, exciter propagation, and excitation in magnetic reconnection outflow regions. Finally, we consider the role of coronal MHD oscillations and waves for coronal heating, the detectability of various MHD wave types, and we estimate the energies carried in the observed MHD waves and oscillations: Alfvénic MHD waves could potentially provide sufficient energy to sustain coronal heating, while acoustic MHD waves fall far short of the required coronal heating rates.     

基于有限元法的管形振子声场特性研究

利用有限元分析软件ANSYS,对管形振子的振动模态及在清洗槽中的辐射声场进行数值计算分析,并用铝箔腐蚀的方法记录了管形振子的声场分布情况。结果表明基于线性理论的ANSYS软件能够较好地预测低强度的超声场;管形振子在清洗槽中的辐射声场在径向方向出现驻波,驻波沿管形振子的轴向间断出现,驻波数等于振子的半波长数,此外作耦合振动的管形振子和非耦合振动的振子相比其辐射的声场更均匀,表明这种新型辐射器有很好的应用前景。