硅的热氧化动力学理论

本文是对硅的热氧化动力学理论的发展。现有的硅热氧化动力学理论不能同时描述任意厚度氧化层的生长规律、外加电场对氧化速率的影响和氧化速率与氧分压的依赖关系。本文在考虑氧化层中空间电荷的影响以及离化氧分子O₂~-和离化氧原子O~-同时参与与Si—Si键反应的基础上,提出了一个更为普遍的氧化动力学模型,它可克服现有理论的不足。文中给出了理论与实验结果的比较,并讨论了本动力学模型与其它动力学模型的关系。     

基于有限元的金属基短纤维复合材料MMC的一种统计蠕变模型

在有限元分析的基础上建立了一个单向应力状态下金属基短纤维复合材料(MMC)的统计蠕变模型.首先建立细胞模型并进行有限元分析,得到了单向应力状态下材料细观尺寸及载荷方向对宏观蠕变响应的影响规律.通过在细胞模型中增加一界面层(考虑材料特性和厚度)来研究基体和纤维的界面对MMC宏观蠕变响应的影响.基于细胞模型的数值结果,提出了一适用于纤维平面随机分布的随机统计模型,该模型考虑了纤维的断裂.通过试验获得纤维的统计分布规律.分析结果表明随机统计模型可以满意地描述试验结果.进一步讨论了材料细观尺寸,纤维的断裂特性以及界面层的材料特性和厚度对MMC宏观蠕变响应的影响.     

基于相场方法的铁基合金高温氧化与生长应力分析

高温氧化性能是评价热防护材料的一项重要指标,然而由于氧化过程是一个含微结构演化的复杂过程,其定量计算分析一直是研究的难点．基于材料热力学理论,建立了能够考虑微结构演化的相场方法来模拟材料的高温氧化,从而解决了抗氧化性能与氧化生长应力定量计算分析的问题．采用所建立的相场方法,对Fe-Cr-Al-Y合金的高温扩散过程、氧化性能和生长应力演化进行了计算,数值计算结果与文献中的实验结果吻合良好,计算结果还揭示了最大生长应力和外界环境氧浓度之间的线性关系．所发展的相场方法为研究复杂环境下的高温氧化提供了一种有力的计算分析手段．     

气泡间距对受污染球形气泡界面性质和尾流的影响

为了理解气泡间相互作用对受污染气泡水动力学特性的影响,基于改进的停滞帽模型,以表面活性剂作为污染介质,详细研究了不同气泡间距下气泡的界面参数、周围流场和尾涡特性.通过求解气泡界面与流域间的吸附和解析方程,考虑局部流动以及Marangoni效应的影响,形成稳定的污染界面.借助Langmuir方程将界面切应力与界面表面活性剂浓度相关联,实现气泡界面切应力的求解.研究发现,改变两气泡的间距,不会显著影响气泡1的界面参数,而对气泡2的界面参数影响巨大.气泡1尾涡向气泡2上游界面的逼近是气泡2界面参数改变的主要原因,该尾涡对气泡2界面上表面活性剂分布的影响与对流作用相反,其可以把流向气泡2尾部的表面活性剂拖回气泡上游界面,从而影响气泡2的界面参数分布,并出现了低影响与高影响阶段.而且气泡1的尾涡长度和涡中心垂直位置的值受气泡2上游界面浓度和气泡间距的共同影响,气泡2各尾涡参数值随上游界面浓度的增加而减小直至为零.     

考虑剪力滞效应的先简支后连续-箱梁徐变效应分析

为了分析混凝土箱梁的徐变效应与剪力滞效应的相互影响,基于箱梁剪力滞效应计算的能量变分方法,推导了混凝土箱梁徐变受剪力滞效应影响的次内力和应力计算公式.结合先简支后连续箱梁算例,计算了受剪力滞效应影响后的徐变弯矩和应力,分析了考虑徐变效应影响的剪力滞系数.结果表明,与不考虑剪力滞效应的结果相比,剪力滞效应增大了箱梁徐变效应.对算例箱梁,考虑剪力滞效应后,对中支点截面处徐变的影响最大,次内力增大42.56%,腹板处应力减小8.5%.     

一个两流体系统中mKdV孤立波的迎撞*

本文从文[2]的基本方程出发,采用约化摄动方法和PLK方法,讨论了三阶非线性和色散效应相平衡的修正的KdV(mKdV)孤立波迎撞问题.这些波在流体密度比等于流体深度比平方的两流体系统界面上传播.我们求得了二阶摄动解,发现在不考虑非均匀相移的情况下,碰撞后孤立波保持原有的形状,这与Fornberg和whitham[6]的追撞数值分析结果一致,但当考虑波的非均匀相移后,碰撞后波形将变化.     

单箱双室简支箱梁剪切变形及剪力滞双重效应分析

基于各个翼板选取不同的最大剪切转角差为剪力滞广义位移,应用能量变分原理分别推导出了考虑和不考虑剪切变形时单箱双室截面控制微分方程组,结合边界条件给出了箱梁纵向应力和竖向挠度的初参数解,从力学、数学角度上证实了剪切变形和剪力滞效应是两个相对独立的力学行为,进一步阐述了二者对箱梁的影响,即剪切变形对箱梁截面纵向应力无影响,但是对竖向挠度有很大的影响.数值算例表明,利用该文解和数值解分析跨中截面剪力滞系数横向分布规律,二者吻合程度良好,其横向分布规律与单室箱梁类似,唯独不同之处是边腹板处的剪力滞效应比中腹板处的剪力滞效应略微大一些;挠度计算表明,剪切效应使得该箱梁在集中和均布荷载作用下跨中挠度分别增大4.6%和2.7%.     

梯度磁电弹材料界面的Stoneley波波速研究

研究均匀弹性半空间(y ≤ 0)和功能梯度磁电弹半空间(y≥0)界面的Stoneley波的波速.基于弹性介质和磁电弹介质的本构方程、运动方程和界面连续条件,得到了弹性半空间(y≤0)和磁电弹半空间(y≥0)界面处的Stoneley界面波波速方程,并讨论了梯度系数对波速的影响.研究结果对于界面波器件的研制提供理论依据.     

低压对变温环境下高聚物黏结炸药界面损伤的抑制

该文旨在探究低压对变温环境下高聚物黏结炸药(polymer-bonded explosive,PBX)界面损伤的影响.首先基于Voronoi法生成PBX二维几何模型,并考虑炸药晶体颗粒为弹塑性、黏结剂为双层黏塑性以及采用零厚度内聚力模型反映界面黏结状况,研究了温度变化时PBX界面黏结性能的改变;再基于热-力耦合处理方法,研究了低压对变温环境下PBX界面损伤的抑制作用,拟合了降温阶段界面法向应力随低压变化的曲线.结果表明,升温阶段主要是由界面切向应力导致初始损伤,降温阶段主要是界面法向应力导致界面损伤,降温比升温更容易导致界面损伤;无论升温或降温,一定的低压载荷能够抑制界面损伤,但压力过大可能导致新的损伤;为抑制界面损伤,降温过程需要的压力应高于升温过程需要的压力,这与降温阶段的界面损伤较大是一致的.     

颗粒弥散增强型核辐射屏蔽材料强度模型研究

基于微观力学的均匀化理论,旨在从核辐射屏蔽材料的微观结构、物理特性的角度出发,通过多尺度方法研究了材料宏观的机械力学性质.主要研究对象为颗粒弥散增强的孔隙基体材料,推导出了此类复合材料（金属基材料、非金属类材料）的强度准则模型,可预测微观孔隙率与颗粒相体积分数对材料宏观强度的影响.在塑性极限分析法的理论框架下,在介观上成功引入了速度场跳动来描述两相界面间的力学特性,利用刚性核的球体胞元模型进行求解.最后,选用了界面速度为0的速度场对模型进行研究,并初步探讨了界面效应对材料性能的影响.     

Modelling of laminate structures in elasto-plasticity with a mixed finite element

The main advantage of the mixed finite element (displacements-stresses) is that, because of its continuity, it gives a good approximate stress field, without needing a high degree of interpolation in displacements. This degree of approximation is essential for elastic-plastic computations. However, the total continuity of stresses is too strong in a laminate structure along the interfaces. We show a method of achieving the correct level of continuity without losing the advantage of a good approximation. Some examples of laminate plane structures with plastic areas along the interfaces are given.     

非一致性界面热流固耦合作用的整体求解

提出了非一致性界面热流固耦合作用整体求解的一种方法．热流体求解基于Boussinesq假设和不可压缩的Navier-Stokes方程．流体区域的运动采用任意Lagrange-Euler（ALE）方法．拟固体元方法实现流体区域的变形．使用几何非线性的热弹性动力学描述固体运动．为了保证界面处应力和传热的平衡,采用了基于Gauss积分点的数据交换方法,对热流固耦合最终形成的强非线性方程实现整体求解．数值实例分析表明该方法的健壮性和有效性．     

FEM simulation of oxidation induced stresses in a simplified thermal barrier coating system

To study the behaviour of the complex failure mechanisms in thermal barrier coatings, a simplified model system is used to reduce the number of system parameters. The artificial system consists of a bond-coat material (fast creeping Fecralloy or slow creeping MA956) as the substrate with a Y₂O₃ partially stabilized plasma sprayed zircon oxide TBC on top and a TGO between the two layers. A FEM simulation was developed to calculate the stress distribution inside the coating system. The simulation permits the identification of compression and tension areas which are established by the growth of the oxide layer and the stresses which occur during the heating and cooling processes. Full thermal cycles, including heating from RT to 1000°C, dwelling for 2 hours and cooling to RT, were simulated. The stress distributions provide an insight into the possible failure mechanisms in the coating and allow to draw conclusions for optimizing real thermal barrier coating systems. It is shown that the failure mechanism depends on the creep properties of the bond-coat. Therefore, optimizing the creep properties may adjust lifetime of the whole coating system. (© 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

轻质热防护系统多层材料组合结构的热应力分析

提出了轻质热防护系统外面板使用多层结构的概念,设计了2种热防护材料组合构成的3种铺层方案．通过模拟飞行器再入大气层时受到的机械和热载荷条件,数值计算得到了层间剪切力、底部温度和y方向位移．计算结果发现,层间剪切力发生在边缘部位且呈反对称分布;选用高热导率和高热容材料能够减少材料内的温度梯度,进而有效地降低结构的热应力和热变形;在均匀温度场情况下,两种材料的热膨胀系数之差越小,则层间剪切力越小．该研究表明不同的材料组合和铺层次序的多层结构,可以满足不同设计要求,具有优化设计潜力．     

Features of the behavior of a liquid film heated from above and placed over deep liquid

The features of rupture occurring in a liquid film lying on deep liquid are studied. A thermal load applied to the film free surface leads to its deformation and rupture. The process is studied using a mathematical model describing the motion of a thin viscous nonisothermal liquid layer. The model is based on the two-dimensional Navier-Stokes equations. To solve them, the boundary conditions on the film-gas and film-liquid interfaces are written in explicit form. The effect of the thermal load on the film rupture is analyzed numerically depending on the Marangoni number and the stresses on the film-liquid interface. The results of solving model problems are presented.     

Evaluación numérica del efecto del espesor de la placa de contacto en la acción de palanca en conexión de acero tipo «T»

This paper presents a numerical study of «T-Stub» steel connections using FEM analyses. In such connections prying action phenomenon may take place. Prying action effect on the bolts has been studied before but the location of the prying action forces has always been simplified. The effect of the thickness of «T-Stub» flanges on contact areas between flanges and support base is investigated in this paper. A 3D finite element model is used and interface elements are employed for the investigation. Nonlinear FE analyses are undertaken on connection with different flange thickness and bolt preload with two bolts. Nonlinear gap elements are used as interfaces. Discussion and conclusions on contact areas and stresses, prying action distributions and resultant of loads on bolts in the «T-Stub» connections are presented.     

Free-edge stress analysis of general composite laminates under extension, torsion and bending

In this study, based on the reduced form of elasticity displacement field for a long laminate, an analytical method is established to exactly obtain the interlaminar stresses near the free edges of generally laminated composite plates subjects to extension, torsion, and bending. The constant parameters being in the displacement field, which describe the global deformation of a laminate, are appropriately calculated by using the improved first-order shear deformation theory. Reddy’s layerwise theory is subsequently employed for analytical and numerical examinations of the boundary layer stresses within arbitrary laminated composite plates. Various numerical results are developed for the interlaminar normal and shear stresses along the interfaces and through the thickness of laminates near the free edges. Finally the effects of end conditions of laminates and geometric parameters on the boundary-layer stress are studied.     

Simultaneously developing flow and heat transfer of non-Newtonian fluids in equilateral triangular duct

A numerical investigation based on the Galerkin finite element method was carried out to solve the full three-dimensional governing equations for simultaneously developing steady laminar flow and heat transfer to a purely viscous non-Newtonian fluid described by a power law model flowing in equilateral triangular ducts. Two commonly used thermal boundary conditions, constant wall temperature (T boundary condition) and constant wall heat flux both axially and peripherally (H2 boundary condition) were examined. It is shown that the Nusselt number distribution along the walls is affected appreciably by the variation of the power law index. Results are presented and discussed for a wide range of power law indices and Prandtl numbers for T and H2 boundary conditions.