高体积百分比颗粒增强聚合物材料的有效粘弹性性质

聚合物材料通常表现为粘弹性性质．为了改进聚合物材料的力学性能,通常将某种无机材料以颗粒或纤维的形式填充到聚合物中,从而得到增强、增韧的聚合物基复合材料．提出了一个新的细观力学模型,用于预测颗粒增强聚合物复合材料的有效粘弹性性质,尤其针对高体积百分比的颗粒夹杂复合材料,该方法基于Laplace变换和双夹杂相互作用的弹性模型．计算了玻璃微珠/ED-6复合材料的有效松弛模量以及恒应变率下的应力应变关系．计算结果表明在高体积百分比下该文方法比基于Mori Tanaka方法预测的粘弹性效应明显减弱．     

复合材料弹性结构的高精度多尺度算法与数值模拟

１．引言 由于复合材料结构物理参数的非均匀各向异性以及细部几何构形的复杂性,在计算它的位移场、应力、应变场时,传统的有限元法因网格生成困难和计算规模太大而难以实现．７０年代初,Ｉ．Ｂａｂｕｓｋａ,Ｊ．Ｌ．Ｌｉｏｎｓ等人针对复合材料弹性结构提出了均匀化方法,见文［１］,数值实验表明,均匀化方法对描述复合材料弹性结构的有效材料常数及刚度性质是有效的,但它不能刻画应力和应变场的局部变化,因而作为复合材料强度理论的判断依据,均匀化理论显然是不够的．为此,Ｊ．Ｌ．Ｌｉｏｎｓ,Ｏ．Ａ．Ｏｌｅｉｎｉｋ等分别就…     

具有非均匀界面相的颗粒和纤维增强复合材料弹性静力学问题的解析解

通过将以位移表示的平衡方程转化为黎卡提方程,得到了具有非均匀界面相的颗粒和纤维增强复合材料非均匀界面相内弹性场的解析解·　所得的解析解是弹性模量呈幂次方变化的非均匀界面相解的通用形式·　任意给定1个幂指数,可以得到具有非均匀界面相的颗粒和纤维增强复合材料体积模量的解析表达式·　通过改变幂指数及幂次方项的系数,此解析解可适用于具有多种不同性质的非均匀界面相·　结果表明:界面相模量和厚度对复合材料模量有很大的影响,当界面相存在时,粒子将出现一种"尺寸效应"·     

应用边界元法模拟纤维增强复合材料平面弹性问题

将含有随机分布多种夹杂相复合材料的二维弹性力学问题归结为复连通区域的边界积分方程,进而转化成矩阵方程进行求解和分析．根据同类夹杂相外在边界上的面力与位移之间关系矩阵完全相同的特点,使得最后的矩阵方程阶数得到大规模减少,这正是此处提出改进的边界元方法的主要思路．数值算例表明,对于此类问题,与常规的边界元分域解法相比更加有效．以该方法为基础,可以详细给出纤维增强复合材料二维条件下的宏观等效力学性质．     

随机DEA期望值模型的一些性质

为判别决策单元在随机DEA期望值模型下的随机有效性,首次提出了随机期望无效、随机期望弱有效、随机期望有效以及随机期望超有效的概念．并给出了三个命题用于判别不同显著性水平下随机期望效率与期望效率的关系．在此基础上,得到了两个重要的性质：（1）当期望效率保持不变时,随机期望效率为显著性水平的增函数;（2）当显著性水平保持不变时,随机期望效率为期望效率的增函数．最后,利用随机模拟和一个算例对上述结论进行了验证．     

一种颗粒随机分布复合材料弹性位移场均匀化方法的理论分析

针对计算随机颗粒分布复合材料弹性位移/力学场时,采用样本求力学性能期望值需要花费大量时间和内存的问题,给出了一种计算颗粒随机分布复合材料弹性位移场的均匀化方法,并且获得了均匀化位移场与期望位移场之间的一种理论误差.首先由复合材料的特性定义了均匀化理论的随机场和概率空间,然后结合单胞内颗粒随机分布复合材料的特性做了一些合理假设得到了在整个颗粒随机分布复合材料组成区域上的期望位移场与均匀化位移场之间的一种理论估计,最后对此法所具有的优点、适应范围,缺点、以及需要改进的地方做了进一步讨论.     

残余界面应力对粒子填充热弹性纳米复合材料有效热膨胀系数的影响

根据黄筑平等人提出的基于“3个构形”的表/界面能理论,研究了热弹性纳米复合材料的有效性质,重点讨论了残余界面应力对纳米尺度夹杂填充的热弹性复合材料有效热膨胀系数的影响．首先,给出了由第一类Piola-Kirchhoff界面应力表示的热弹性界面本构关系和Lagrange描述下的Young-Laplace方程;其次,采用Hashin复合球作为代表性体积单元,推导了在参考构形下复合球内部由残余界面应力诱导的残余弹性场,并进一步计算了从参考构形到当前构形的变形场;最后,基于以上计算得到了热弹性复合材料有效体积模量和有效热膨胀系数的解析表达式．研究表明,残余表/界面应力对复合材料的热膨胀系数有重要影响．     

含涂层空心球复合泡沫塑料的模量预测及讨论

利用Mori-Tanaka平均应力的概念和Eshelby等效夹杂的原理,研究了含涂层空心球复合泡沫塑料的弹性性质,从理论上导出了这类复合材料的有效模量预测公式．此外,基于这些理论公式,讨论了涂层厚度和泊松比变化对复合泡沫塑料有效模量性质的影响．     

NGLM:一类全局收敛的Newton-GMRES方法

本文提出了一类具有全局收敛性质的Newton-GMRES方法—NGLM方法．该方法是对经典Newton—GMRES方法的推广．NGLM方法的全局策略是当在非精确Newton方向上后退不能成功时，转而在一个子空间上运用信赖域方法确定迭代步长．理论分析与数值实验均表明，NGLM方法改善了Newton—GMRES方法的强健性．     

Stokes波高阶谐波系数递推的数值计算*

本文用W.H.Hui提出的方法,在半物理平面内重新表述了Stokes波的数学模型和边界条件,提出了两种更有效的数值计算方法来获得Stokes波高阶谐波系数,并可递推至无穷.通过小参数转换,重新得到了Cokelet(1977)的波速和半波高的摄动展开式.     

Nonlinear stability and chaos in electrohydrodynamics

Using the method of multiple scales, the nonlinear instability problem of two superposed dielectric fluids is studied. The applied electric filed is taken into account under the influence of external modulations near a point of bifurcation. A time varying electric field is superimposed on the system. In addition, the viscosity and variable gravity force are considered. A generalized equation governing the evolution of the amplitude is derived in marginally unstable regions of parameter space. A bifurcation analysis of the amplitude equation is carried out when the dissipation due to viscosity and the control parameter are both assumed to be small. The solution of a nonlinear equation in which parametric and external excitations are obtained analytically and numerically. The method of generalized synchronization is applied to determine the equations that describe the modulation of the amplitude and phase. These equations are used to determine the steady state equations. Frequency response curves are presented graphically. The stability of the proposed solution is determined applying Liapunov's first method. Numerical solutions are presented graphically for the effects of the different equation parameters on the system stability, response and chaos.     

Towards modelling the curing process in particle-filled electro-active polymers

A large actuation voltage is required to produce a desired mechanical deformation in dielectric elastomers. To reduce the amount of the actuation voltage, several mechanisms can be applied and the inclusion of high dielectric permittivity fillers in the matrix material in the uncured stage is one of them. Moreover, to obtain a maximum advantage from the high dielectric permittivity fillers, an electric field is applied during the curing process which helps the particles to align in a preferred direction. The stiffness gaining during curing is modelled by an appropriate constitutive relation where the temporal evolution of the material parameters is considered. In this paper we present a phenomenologically-inspired large strain framework for simulating the curing process of polymers that can work under the use of an electro-mechanically coupled load. (© 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

On the equivalence of the electromagnetic problem of diffraction by an inhomogeneous bounded dielectric body to a volume singular integro-differential equation

The paper is concerned with the smoothness of the solutions to the volume singular integrodifferential equations for the electric field to which the problem of electromagnetic-wave diffraction by a local inhomogeneous bounded dielectric body is reduced. The basic tool of the study is the method of pseudo-differential operators in Sobolev spaces. The theory of elliptic boundary problems and field-matching problems is also applied. It is proven that, for smooth data of the problem, the solution from the space of square-summable functions is continuous up to the boundaries and smooth inside and outside of the body. The results on the smoothness of the solutions to the volume singular integro-differential equation for the electric field make it possible to resolve the issues on the equivalence of the boundary value problem and the equation.     

Transient response of two collinear dielectric cracks in a piezoelectric solid under inplane impacts

The paper is focused on the dynamic analysis of two collinear dielectric cracks in a piezoelectric material under the action of in-plane electromechanical impacts. Considering the dielectric permeability of crack interior, the electric displacements at the crack surfaces are governed by the jumps of electric potential and crack opening displacement across the cracks. The permeable and impermeable crack models are the limiting cases of the general one. The Laplace and Fourier transform techniques are further utilized to solve the mixed initial-boundary-value problem, and then to obtain the singular integral equations with Cauchy kernel, which are solved numerically. Dynamic intensity factors of stress, electric displacement and crack opening displacement are determined in time domain by means of a numerical inversion of the Laplace transform. Numerical results for PZT-5H are calculated to show the effects of the dielectric permeability inside the cracks, applied electric loadings and the geometry of the cracks on the fracture parameters in graphics. The observations reveal that based on the COD intensity factor, a positive electric field enhances the dynamic dielectric crack growth and a negative one impedes the dynamic dielectric crack growth in a piezoelectric solid.     

Linear analysis and energy budget of viscous liquid jets in both axial and radial electric fields

In their countless industrial applications, axisymmetric and non-axisymmetric instabilities are respectively responsible for electrospraying and electrospinning. A linear method and energy budget have been applied in this study to investigate the instability of viscous jets under both the axial and radial electric fields; the liquid was taken to be a leaky dielectric and the gas a perfect dielectric; the effect of a parabolic velocity profile was considered and compared to that of a uniform velocity, and the energy analysis explained the physical mechanisms to an extent. The liquid viscosity and parabolic velocity profile had a combined effect on jet instability. Work induced by the parabolic velocity profile consisted of two parts: the energy transferred from the basic flow to the disturbances, and the influence of the corresponding shear stresses. At low viscosities, these influences were positive enough to prevail over the viscous dissipation, enhancing axisymmetric instability. However, the parabolic velocity profile functioned differently in small and large wavenumber regions, and the response to the axial electric fields varied in different regions, accounting for the dual effects of axial electric fields on axisymmetric instability. Also, under the interplay between the strong axial electric fields and the parabolic velocity profile, two distinct unstable regions emerged for the non-axisymmetric mode. The effects of the radial electric fields were less sensitive, whether or not the parabolic velocity profile was considered. In summary, the parabolic velocity profile was significant–especially for charged jets with weak viscosity and strong axial electric intensity. The effects of axial electric fields in the atomization zone, and the effects of fluid permittivity coupled with the axial electric fields, were also investigated here.     

Closed-form solutions for two collinear dielectric cracks in a magnetoelectroelastic solid

The problem of two collinear electromagnetically dielectric cracks in a magnetoelectroelastic material is investigated under in-plane electro-magneto-mechanical loadings. The semi-permeable crack-face boundary conditions are adopted to simulate the case of two collinear cracks full of a dielectric interior. Utilizing the Fourier transform technique, the boundary-value problem is reduced to solving singular integral equations with Cauchy kernel, which then are solved explicitly. The intensity factors of stress, electric displacement, magnetic induction, crack opening displacement (COD) and the energy release rates near the inner and outer crack tips are determined in closed forms for two cases of possible far-field electro-magneto-mechanical loadings respectively. Numerical results for a BaTiO₃–CoFe₂O₄ composite are carried out to show the effects of applied mechanical loadings on the crack-face electric displacement and magnetic induction, the stress intensity factor and the COD intensity factor, respectively. The obtained results reveal that when the applied mechanical loading is stress, applied electromagnetic loadings have no influences on the stress intensity factor. When the applied mechanical loadings is train, the applied positive electromagnetic loadings decrease the intensity factors of stress and COD, and the applied negative ones increase the intensity factors of stress and COD. The variations of energy release rates are also given to show the effects of the geometry of two collinear dielectric cracks.     

Hamilton体系辛半解析法及在非均匀电磁波导中的应用

力学中的Hamilton体系需用对偶变量来描述,而电磁场正好有电场和磁场这一对对偶变量．尝试将力学中的Hamilton体系理论应用于电磁波导的分析,以横向电场和磁场作为对偶变量,将电磁波导的基本方程导向辛几何的形式．基于Hamilton变分原理, 导出横向离散的半解析系统方程, 保持体系的辛结构．以非均匀波导为例, 求解了方程的辛本征值问题, 计算结果与解析解相当吻合．     

基于内变量理论的电流变液本构关系

研究了电流变液的微结构本构关系.其理论框架是基于内变量理论和机理的分析.电流变液是由高介电常数的颗粒悬浮在某种液体中组成的.在电场作用下,极化的颗粒将沿着电场方向聚集在一起形成链状结构.颗粒聚集体的大小和方向将随外加电场和应变率的变化进行调整,因而可以通过建立起能量守恒方程和力平衡方程来确定颗粒聚集体的大小和方向的变化.那么,一个三维的、清晰的本构关系可以由相互作用能和系统的耗散能导出.具体考虑和讨论了在简单剪切载荷作用下的系统响应,发现电流变液的切变剪薄粘滞系数同系统Mason数之间近似于幂指数∝(Mn)-082的关系.     

微通道液体流动双电层阻力效应

采用数值方法求解双电层的Poisson-Boltzmann方程和液体运动的Navier-Stokes方程,研究微通道双电层对压强梯度液体流动的阻力效应． 量纲分析表明,双电层阻力大小可以用一个无量纲的电阻力数表示．它与液体的介电系数、固体表面的zeta电位平方成正比,与液体的动力粘性系数、电导率以及微通道的宽度平方成反比．在计算流动诱导的流动电位势和电阻力时,提出电流密度平衡条件,可以消除传统电流平衡条件导致的固壁附近产生局部回流的不合理物理现象．还给出不同电阻力数的微通道流量、流量损失率、速度剖面的数值结果,合理解释了双电层对微通道液体流动的阻力效应．     

Electrostrictive stresses near crack-like flaws

Slit cracks in purely dielectric material systems do not perturb any applied uniform electric field. Furthermore, when the dielectric is unconstrained and does not support any conducting plates or mechanical loads, there are no additional mechanical stresses generated in the material upon introduction of the crack. This situation applies to both electrostrictive and piezoelectric materials. However, flaws which have finite thickness such as thin elliptical or ellipsoidal voids will cause severe inhomogeneous concentration of the electric field. In turn, this can generate substantial mechanical stress from electrostrictive or piezoelectric sources. The effect of an elliptical through flaw in an infinite isotropic body is considered. It is found that, in the case of thin ellipses, the near flaw tip mechanical stresses approximate the singular stresses near a slit crack with an equivalent stress intensity factor. In that sense, the flaw may be considered as a slit crack and treated in terms of linear elastic fracture mechanics. However, except for impermeable and conducting flaws, the value of the equivalent stress intensity factor depends on the aspect ratio of the flaw. As the aspect ratio of the flaw diminishes, the magnitude of the equivalent stress intensity factor falls and disappears in the limit of a slit crack. The results are used to show that a flaw-like crack in a material with a very high dielectric constant can be treated by fracture mechanics as an impermeable slit crack when the flaw aspect ratio is an order of magnitude greater than the ratio of dielectric permittivities (flaw value divided by the value for the surrounding material).