粘弹模量对金属基底-胶粘涂层结构中类Rayleigh波传播特性的影响

为了解粘接结构中胶粘涂层的粘弹特性,进而评价粘接质量,对半无限金属基底-胶粘涂层结构涂层中的类Rayleigh波传播特性进行了研究。从粘弹的基本理论出发,通过Laplace-Henkel变换技术建立起类Rayleigh波的频率方程,将粘接涂层视为Kelvin体计算并分析了其粘弹模量对波的频散及衰减关系的影响。理论计算发现粘弹体的粘滞项对频散低阶模式影响不大,但随着粘滞的增大对高阶模式可产生影响。同时粘滞还影响各阶模式波的衰减,特别是Sezawa模式对衰减较Saw模式更为敏感,在某些结构中还会出现局部的极小值现象,结果表明:对粘涂层结构涂层中类Rayleigh波传播特性的研究可为粘弹模量的定量评估及粘接质量的评价提供一定的理论方法。     

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。     

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。      

无限流体中孔隙介质圆柱周向导波的传播特性

为研究无限大流体约束的孔隙圆柱中周向导波的传播规律,分析孔隙参数对导波传播特性的影响,建立了无限流体中孔隙介质圆柱的理论模型,利用孔隙介质弹性波动理论,建立了周向导波频散方程,通过数值模拟计算得到无限流体中孔隙介质圆柱的频散曲线,探讨了圆柱半径和孔隙参数对导波传播特性的影响,并对导波的衰减特性进行了分析;通过数值计算,得到了周向导波的时域波形,讨论了孔隙参数对波形的影响.结果表明,孔隙介质圆柱半径的改变影响圆柱尺度,孔隙度的改变影响孔隙介质中体声波的波速,都对周向导波频散曲线产生一定的影响,所得到的频散曲线特征及衰减曲线与时域波形吻合.研究结果对开展无限流体中孔隙介质圆柱的超声无损评价提供了一定的理论参考.     

脱粘缺陷对粘接结构频散特性的影响

采用全局矩阵理论,通过引入内聚强度弱化模型和弹簧模型,计算了脱粘缺陷对粘接结构频散特性的影响。分析了内聚强度缺陷、界面缺陷及混合缺陷等因素对粘接结构频散特性的影响,发现频散曲线的演化规律与材料声学性质密切相关。不同材料和不同组合,其规律有很大不同。针对各向同性粘接结构,可能通过考察特定频率段内频散模态的变化,实现对脱粘缺陷的检测。     

基于Biot-喷射流统一模型Maxwell流体饱和孔隙介质中的弹性波

推广Biot-Tsiklauri声学模型的同时借鉴Dvorkin和Nur的工作,建立了具有任意孔径分布并顾及喷射流动机制的非牛顿流体饱和孔隙介质声学模型,研究了非牛顿流体(Maxwell流体)饱和孔隙介质中的弹性波的衰减和频散特性.着重讨论充孔隙Maxwell流体的非牛顿流效应对弹性波的频散和衰减的影响.研究表明,饱和流体的非牛顿流效应和喷射流动机制均是引起弹性波波频散和衰减的重要因素.依据非牛顿流体(Maxwell流体)饱和各向同性孔隙介质的Biot-喷射流声学模型,喷射流动只影响纵波的频散和衰减,而饱和流体的非牛顿流效应不仅影响纵波,而且还影响横波的频散和衰减.     

曲线坐标系下孔隙介质圆柱纵向表面波的传播特性

为了研究孔隙介质圆柱纵向表面波的传播规律,分析其频散和衰减特性,在正交曲线坐标系下建立了表面波的频散方程,通过数值计算得到频散曲线,将纵向导波最低模态与表面波进行对比,并分析了曲率半径及孔隙参数对表面波频散和衰减的影响。结果表明,当频率足够大时,导波最低模态的频散曲线与表面波近似;在同一频率下,表面波的相速度随曲率半径的增大而增大,随孔隙度的增大而减小;表面波的衰减随孔隙度的增大而增大。研究结果为开展孔隙介质圆柱结构的超声无损评价提供了一定的理论参考。     

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。     

非均匀饱含黏性流体孔隙介质中声波传播及井孔声场分析

声波在饱含流体孔隙介质中的传播特性与流体的黏滞性及孔隙介质的非均匀性密切相关.本文在Biot理论基础上,考虑了孔隙流体的剪切应力及孔隙结构的非均匀性,采用含黏性流体孔隙介质中的波动理论,研究了孔隙介质中四种体波的频散和衰减特性,分析了慢横波对快纵波转换散射的影响,进一步推导了孔隙地层井孔中的模式波及其声场的解析解,研究了非均匀孔隙介质中井孔模式波和波列的特征.研究结果表明,含黏性流体孔隙介质中存在慢横波,慢横波的频散很强,其传播特征受到介质孔隙度、渗透率及孔隙流体黏度的影响.在非均匀孔隙介质中,与慢横波相关的剪切应力平衡过程不仅导致快纵波的频散和衰减,还会影响井孔伪瑞利波及斯通利波的传播特征.本文的工作完善了孔隙介质中声波传播的物理机制,为孔隙地层井孔声波的解释与应用提供了理论指导.     

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。     

Two-phase viscoelastic jetting

A coupled finite difference algorithm on rectangular grids is developed for viscoelastic ink ejection simulations. The ink is modeled by the Oldroyd-B viscoelastic fluid model. The coupled algorithm seamlessly incorporates several things: (1) a coupled level set-projection method for incompressible immiscible two-phase fluid flows; (2) a higher-order Godunov type algorithm for the convection terms in the momentum and level set equations; (3) a simple first-order upwind algorithm for the convection term in the viscoelastic stress equations; (4) central difference approximations for viscosity, surface tension, and upper-convected derivative terms; and (5) an equivalent circuit model to calculate the inflow pressure (or flow rate) from dynamic voltage.     

Undulatory swimming in viscoelastic fluids

The effects of fluid elasticity on the swimming behavior of the nematode Caenorhabditis elegans are experimentally investigated by tracking the nematode's motion and measuring the corresponding velocity fields. We find that fluid elasticity hinders self-propulsion. Compared to Newtonian solutions, fluid elasticity leads to up to 35% slower propulsion. Furthermore, self-propulsion decreases as elastic stresses grow in magnitude in the fluid. This decrease in self-propulsion in viscoelastic fluids is related to the stretching of flexible molecules near hyperbolic points in the flow.     

Detachment of stretched viscoelastic fibrils

New experimental results are presented about the final stage of failure of soft viscoelastic adhesives. A microscopic view of the detachment of the adhesive shows that after cavity growth and expansion, well adhered soft adhesives form a network of fibrils connected to expanded contacting feet which fail via a sliding mechanism, sensitive to interfacial shear stresses rather than by a fracture mechanism as sometimes suggested in earlier work. A mechanical model of this stretching and sliding failure phenomenon is presented which treats the fibril as a nonlinear elastic or viscoelastic rod and the foot as an elastic layer subject to a friction force proportional to the local displacement rate. The force on the stretched rod drives the sliding of the foot against the substrate. The main experimental parameter controlling the failure strain and stress during the sliding process is identified by the model as the normalized probe pull speed, which also depends on the magnitude of the friction and PSA modulus. In addition, the material properties, viscoelasticity and finite extensibility of the polymer chains, are shown to have an important effect on both the details of the sliding process and the ultimate failure strain and stress. Electronic supplementary material Appendix B is only available in electronic form at and are accessible for authorised users.     

Wide-range viscoelastic measurement using resonating sensors

We have developed a specially designed sensor with its associated instrumentation using a so called acoustical near-field technique based on small resonating horns. Contrary to the conventional measuring devices used in the rheology industry, this sensor enables wide-range and continuous viscosity measurements, including liquid to solid material transition. The sensor is composed of a small piezoelectric element which enables the horn to resonate. Shear waves are generated by means of the thinner part of the sensor (dipped into the material) confining the acoustic field to the tip to ensure local measurements. The frequency is scanned across the resonating frequencies in order to perform these measurements with the maximum of speed and efficiency. The shear moduli G' and G' of the material is deduced from the electrical impedance measurement at the resonance frequencies. In case of rapidly changing materials, such as quick setting cements or polyesters, we have developed a specific fast instrumentation based on the impulse response of the sensor, which realizes very fast measurements, typically 30 measurements per second. Modelization and experiments are reported.     

Viscoelastic micropolar continuum and viscoelastic waves

Generalized Kelvin model is applied to isotropic viscoelastic micropolar continuum. Constitutive equations in integral and differential form, generalized Lamé equations, and wave equations for displacement and microrotation are derived for this model. For damped viscoelastic waves, which are realized in the continuum under examination, the wave vectors and vectors of decay are explicitly given as functions of elastic moduli, viscosity coefficients, angular frequency, density, and microinertia coefficient. Analogous relations are derived for further eleven simpler mass models. Results are compared for three models in current use. For two of them (classical elastic and micropolar elastic medium) are the derived results in agreement with the ones usually used, for the third model (viscoelastic continuum) are the usual formulas a limiting case of the relations derived in this paper.     

Magnetohydrodynamic flow of a viscoelastic fluid

The non-linear differential equation for the magnetohydrodynamic Poiseuille flow of Phan-Thein-Tanner (PTT) conducting fluid is derived. Using the homotopy analysis method (HAM), the series solution is developed and its convergence is discussed. Also, the results are presented graphically and the effects of non-dimensional parameters on the flow field are analyzed. The results obtained reveal many interesting behaviors that warrant further study on the equations related to non-Newtonian fluid phenomena, especially the shear-thinning phenomena. Shear thinning reduces the wall shear stress.     

Transverse vibrations of viscoelastic shallow shells

A method for the time history of motion of shallow shells of viscoelastic material under arbitrary time-dependent transverse load is presented. The method is based upon the concept of iso-amplitude contour lines on the surface of the shell. It is shown that the time behavior can be found by using the frequency of free vibration of the associated elastic shallow shell. As an illustration of the technique, the problem of a shallow dome upon an elliptic base is discussed, all details of which are explained by graphs.     

Anomalous viscoelastic response of nematic elastomers

We report a combined theoretical and experimental study of linear viscoelastic response in oriented monodomain nematic elastomers. The model predicts a dramatic decrease in the dynamic modulus in certain deformation geometries in an elastic medium with an independently mobile internal degree of freedom, the nematic director with its own relaxation dynamics. Dynamic mechanical measurements on monodomain nematic elastomers confirm our predictions of dependence on shear geometry and on nematic order, and also show a very substantial mechanical loss clearly associated with director relaxation.