无限介质包围的充液管道中导波的频散特性

利用波传播方法从理论上分析了无限介质包围的充液管道中导波的频散特性.基于经典的薄壳振动理论,求出了不同传输路径中的波数表达式,并利用数值方法对振动方程进行解析研究,分析管道参数和无限介质对导波频散特性的影响.结果表明,管道厚径比、管壁厚度、无限介质和管壁材料与导波频散密切相关.其中管道厚径比和管壁材料对流体主导波(s1...     

基于振动模态分析的钢轨中超声导波传播特性数值计算方法

针对钢轨等复杂异型波导结构中超声导波频散特性难于求解的问题,本文通过振动模态分析提取出钢轨中超声导波传播特性。基于振动特征频率和波动解互相转化的原理,将振动模态分析获得的特征频率解,转换成波动解,进而提取出钢轨的频散特性。同时,还可以由结构振动分析中获得的变形信息,计算出超声导波的波结构。通过对计算出的钢轨中低阶超声导波模态频散特性和波结构分析,选择出适合钢轨无损检测的超声导波模态类型和频率范围,为后续专用超声导波传感器设计和检测实验提供理论支持。     

一维声子晶体中弹性波的全反射贯穿效应

为了研究一维声子晶体中弹性波的全反射贯穿效应,利用转移矩阵法计算了弹性波在大于全反射角入射一维声子晶体的透射率。在透射波中发现了两条透射峰带,即出现了全反射贯穿效应。得出了贯穿效应随入射角的变化规律、贯穿效应随介质厚度的变化规律以及贯穿效应随周期数的变化规律。并利用波的量子理论和渐逝波的理论对一维声子晶体的贯穿效应作出了定性的理论解释     

非周期型理想钻柱系统声传播特性研究

基于一维声传播理论推导出了非周期型理想钻柱系统的频散方程和透射波声衰减方程,数值计算了非周期型钻柱系统几何参量与其频散特性、声衰减特性之间的关系,并利用有限元ANSYS对非周期型理想钻柱系统的声传播特性的结果进行了验证。研究结果表明：非周期型理想钻柱系统的频散特性与多个周期性结构系统频散特性相“与”的结果一致,具有通带较窄而阻带较宽,且频率在阻带范围内的声波沿钻柱传播时声衰减较大的特性。     

基于声辐射模态的声场重建中的测点优化方法

少测点条件下,利用声辐射模态理论重建声场时,测点布置方式是决定声场重建精度的一个关键因素。为求得最佳测点布置方式,提出了一种基于声辐射模态的最优测点选择方法,即基于声辐射模态矩阵的奇异值分解,采用循环迭代的方式,逐次去除对其最小奇异值最敏感的测点,从而得到了一组使重建方程条件数最小的测点。实验结果表明,利用文中提出的最优测点选择方法布置测点,能够对声场进行有效的重建,重建效果优于均匀布置方式,显著提高了声场重建精度。     

基于PolyMAX的声固耦合模态试验研究

白车身的结构模态频率和模态振型反映了汽车车身结构的固有特性,对车内噪声有重要影响。车内空腔跟车身结构一样,同样拥有模态频率和模态振型。采用LMS数据采集系统对某国产SUV进行车内空腔声学模态试验。首先基于传声器阵列的方法获取响应点的信号,然后利用PolyMAX方法提取声学模态频率及振型。将声学模态频率与白车身结构模态频率进行对比分析,结果表明：车内空腔的第一、二阶声学模态分别跟白车身的第四、十阶结构模态有很强的耦合。最后通过实车测试验证了声固耦合共振时低频轰鸣的存在。可以在关键部件增加板厚、顶盖和地板附加阻尼层、顶盖加加强筋等方式改变车身结构的局部模态来破坏车身结构模态和声腔模态的强耦合状态,降低车内的低频轰鸣声     

海洋细长结构参数激励不稳定区的确定方法

分别采用变形参数法,改进的变形参数法及傅立叶分析法确定海洋细长结构参数振动Hill方程的不稳定区,比较三种方法的优缺点,并给出了前三阶不稳定区图像,为张力腿、立管等海洋细长柔性结构的设计提供参考。     

基于声辐射模态有源解耦控制的溢出机理研究

基于声辐射模态有源控制,通过作动器位置布置可以使得控制过程解耦,通过对比解耦和非解耦两种控制方式,并对其控制结果进行讨论。发现解耦控制所需控制能量较小并且相对控制稳定,比非解耦控制有明显优势。并且两者在低频段都能取得良好的控制效果,在频率较高时出现不同程度的控制溢出。在解耦控制的基础上,通过分析各阶声辐射模态辐射效率特点,作动器布置对称形式与声辐射模态对称形式的对应关系,揭示了溢出的机理     

基于声辐射模态的粘弹性阻尼板声功率最小化研究*

为了减小薄板结构振动辐射的噪声,在重量约束下进行了声功率最小化研究。利用有限元振动模型来处理结构振动环节,利用声辐射模态模型来处理声辐射环节,用可行方向法进行了优化设计研究。以一矩形粘弹性阻尼板为例,得到了阻尼重量约束下板的声功率最小化模型,结果表明在满足重量约束条件下能够达到明显的降噪效果。     

复合环境激励下的声振耦合分析

基于有限元和边界元理论,建立了仪器舱典型结构的耦合模型,分析了声振耦合对结构响应的影响。当典型结构经受随机振动+混响声场复合环境激励时,采用随机分析的原理,用谱分析的分析方法定性的分析结构的动力响应情况,推导了节点响应谱密度的计算公式,解决了Sysnoise软件同时对声振复合环境进行数值模拟的问题。仿真结果表明：声振耦合影响结构的响应,且在各频率上影响程度不同,在低频时较弱,相当于附加阻尼的效果,随着频率的升高,耦合程度随之加强,改变了结构的响应分布;对声振复合环境数值模拟,在低频部分,随机振动激起的响应占主要部分,随着频率的升高,混响声场激起的响应占主要部分,混响声场激起的模态数也要多于随机振动激起来的。     

钢轨垂直振动模态导波频散曲线、波结构及检测应用*

频散曲线和波结构是钢轨导波检测时设计检测方法和分析回波波形的基础。首先介绍了半解析有限元计算任意截面弹性波导中导波传播的频散曲线和波结构的基本方法。然后应用该方法求解了U60型钢轨中垂直振动模态的导波频散曲线和波结构,并讨论了频散曲线和波结构在钢轨轨头缺陷导波检测中的应用。最后采用模态力锤实验及时频联合分析的方法验证了频散曲线数值计算结果。钢轨轨头横向裂纹垂直振动模态导波检测的有限元数值模拟和实验表明该模式导波是检测钢轨轨头缺陷的有效检测模式波型。     

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。      

火炮身管超声导波频散特性及传感器配置

超声导波检测技术具有检测距离远、效率高的优点,适用于检测火炮身管等圆管类结构,具有突出的军事应用价值和前景。首先对圆管超声导波理论进行推导,并对身管损伤的几种常见类型进行分析研究。采用数值计算的方法绘制频散曲线,通过分析其频散特性对导波检测频率进行优选,初步得到了最佳的检测频率范围;其次着重对传感器种类选择、数量及分布方式与导波传播特性的关系进行研究。实验表明,导波检测的最佳频率范围为0～300 kHz,且导波弯曲模态一般不宜作为检测模态;传感器数量的增加不仅增强了激励信号的强度,还有效抑制了身管中导波的频散。     

Numerical and experimental investigation of thickness effect on residual stress measurement

This article studies the effect of thickness on residual stress measurements both numerically and experimentally. First, the released strain of aluminium specimens with different thicknesses subjected to incremental hole drilling is analysed using an efficient finite element model. From these results, the cumulative calibration coefficients and their variations as a function of plate thickness are determined. Then, the residual stress profile of a 1-mm-thick aluminium plate subjected to incremental hole drilling is investigated by in-plane three-directional moiré interferometry. A comparative study between the principle stress data calculated using the special calibration coefficients that match the thickness and the general ASTM calibration coefficients that are designed for thick plates is carried out. The analysis results demonstrate that the thickness significantly influenced the release strain. Moreover, the mismatch between the calibration coefficients and the thicknesses results in a deviation in the stress data of more than 10%, and the deviation increases with decreasing thickness.

This paper is part of a Themed Issue on Measurement, modelling and mitigation of residual stress.     

The effect of size on structure and stress in grained films

ABSTRACT

Molecular dynamics simulations are carried out to study the evolution of stress during deposition of atoms on the (001) plane of an FCC crystal, using rectangular simulation box with x–y periodic boundary conditions. The system consists of grains and boundaries. In the current study, we consider different values of the grain-to-grain distances and different relative sizes of deposited atoms. The structure was determined by the kinematical scattering theory. The main factors influencing the stress in thin film systems are creating of dislocation and changing the crystalline structure in grains. The deposited atoms can significantly contribute to the total stress. Depending on their size, atoms contribute compressive or tensile stresses to the total stress.

This paper is part of a Thematic Issue on The Crystallographic Aspects of Metallic Alloys.     

Analysis of accuracy in prism coupling methods and a proposal of two simple ways for coupling to guided waves and/or for exciting surface plasmon resonance

The stationarity of the coupling spot in relation with different shapes of couplers is discussed. The accuracy in measuring the effective index by various couplers is analyzed. Two simple methods are proposed to couple light into guided waves and/or to excite surface plasmon resonance (SPR). One uses an optical block as a coupler and the incident beam falls onto it in two perpendicular directions. This method is particularly useful for exciting SPR at an interface between metal and anisotropic dielectric media, but also can be used to couple light into guided waves. Another method does not need any coupler. The incident beam is directly launched into the sample from the sustrate side. When a thin metal film is deposited on the substrate and covered either by air or another dielectric layer, SPR can be excited at the interface of metal/air of metal/dielectric. This method can be viewed as an alternative of Kretschmann configuration.     

Mathematical analysis and solution methodology for an inverse spectral problem arising in the design of optical waveguides

We analyse mathematically the problem of determining refractive index profiles from some desired/measured guided waves propagating in optical fibres. We establish the uniqueness of the solution of this inverse spectral problem assuming that only one guided mode is known. We then propose an iterative computational procedure for solving numerically the considered inverse spectral problem. Numerical results are presented to illustrate the potential of the proposed regularized Newton algorithm to efficiently and accurately retrieve the refractive index profiles even when the guided mode measurements are highly noisy.     

Approach for fatigue damage assessment of welded structure considering coupling effect between stress and corrosion

This paper presents a new approach to assess time-dependent corrosion fatigue damage of welded joint considering the coupling effect between mechanical factor and corrosion factor. The high stress region around weld will accelerate corrosion and be more likely to induce nonuniform corrosion of welded joint. And the effect of loading on corrosion behavior of the steel in NaCl solution was investigated. The synergistic effect between applied elastic stress and chemical attack on Q235 steel was evaluated by electrochemical experiments. A side longitudinal of ship structure is selected as a case study. Time-dependent stress concentration factor of welded joint as a function of corrosion deterioration was analyzed, and the iterative process of stress and corrosion degeneration of plate thickness was used to simulate coupling effect basing on the results of experiment. The hot spot stress approach was adopted to calculate the fatigue damage. It is revealed that the nonuniform corrosion could influence fatigue damage of welded joint, and that impact will be more and more significant with the growth of corrosion year.     

Computation of the stress intensity factors for repaired cracks with bonded composite patch in mode I and mixed mode

In this study, the finite element method is used to analyse the behaviour of repaired cracks with bonded composite patches in mode I and mixed mode by computing the stress intensity factors at the crack tip. The effects of the patch size and the adhesive properties on the stress intensity factors variation were highlighted. The plot of the stress intensity factors according to the crack length in mode I, shows that the stress intensity factor exhibits an asymptotic behaviour as the crack length increases. In mixed mode, the obtained results show that the Mode I stress intensity factor is more affected by the presence of the patch than that of mode II.     

Determination of thermal stress intensity factors for an interface crack in a graded orthotropic coating-substrate structure

The thermal fracture problem of an interface crack between a graded orthotropic coating and the homogeneous substrate is investigated by two different approaches. For the case that most of the material properties in the graded orthotropic coating are assumed to vary as an exponential function, the integral transform and singular integral equation technique is used to obtain some analytical results. In order to analyze the case with more complex material distribution, an interaction integral is presented to evaluate the thermal stress intensity factors of cracked functionally graded materials (FGMs), and then the element-free Galerkin method (EFGM) is developed to obtain the final numerical results. The good agreement is obtained between the numerical results and the analytical ones. In addition, the influence of material gradient parameters and material distribution on the thermal fracture behavior is also presented.