基于晶体塑性理论的疲劳裂纹起始数值模拟

在疲劳荷载作用早期,材料在晶体尺度出现裂纹的萌生,为了研究疲劳短裂纹的发展,利用Monte Carlo法建立了多晶体晶粒集合的Voronoi有限元模型,并在Ansys Usermat子程序接口下编写了晶体塑性本构方程子程序,修正了以拉伸硬化为主的疲劳裂纹起裂的计算机模拟方法,并结合TANAKA和MURA的位错偶极子模型,模拟了晶粒集合在疲劳荷载作用下的裂纹萌生,最后与文献实验数据对比,这一修正的方法更加符合实验结果和宏观现象.     

带非线性支撑的转子有限元模型求解方法

用数值方法研究了非线性支撑的柔性转子系统的动学行为,提出了一种将有限元与非线性支撑结合的模型和求解方法。利用有限元法(FEM)构建转轴和转盘部分的模型,通过矩阵进行组合;利用离散元方法对包含滚动轴承和挤压油膜阻尼器(SFD)的支撑部分进行建模,此部分包含4个单元,分别为轴承内圈、外圈、SFD内圈和支撑鼠笼。有限元部分和离散元部分通过轴端节点相连,仿真过程中轴端位移传递给非线性支撑部分,支撑部分通过位移计算得到的非线性力反过来作用于有限元转子轴端部分。为了耦合求解有限元转子和非线性支撑组成的数学模型,提出了一种综合的迭代求解方法,克服传统的有限元求解方法对轴端隐性非线性支撑的求解局限性。由于转轴部分采用了Timoshenko梁单元建模,对比与简单转子模型,可以考虑陀螺力矩和轴的柔性特征,更能体现非线性支撑对振动真实影响。在建立的20个轴单元的有限元转子模型中,非线性响应更多体现在靠近非线性支撑的节点1和节点21处,响应频谱中靠近轴端的节点能体现出滚动轴承的2倍和3倍变柔振动频率。     

英语中的中国元素对提高中国文化软实力的积极意义

"中国英语"融合了中国传统和现代文化元素,是一种以标准英语为基础的特殊英语变体。中国元素体现在英语语言的各个方面及社会生活的各个领域,它对提高中华民族文化软实力具有重要意义。     

Numerical Simulation of Rotor Flow Field Based on Overset Grids and Several Spatial and Temporal Discretization Schemes

A numerical method based on solutions of Euler/Navier-Stokes (N-S) equations is developed for calculating the flow field over a rotor in hover. Jameson central scheme, van Leer flux-vector splitting scheme, advection upwind splitting method (AUSM) scheme, upwind AUSM/van Leer scheme, AUSM+ scheme and AUSMDV scheme are implemented for spatial discretization, and van Albada limiter is also applied. For temporal discretization, both explicit Runge-Kutta method and implicit lower-upper symmetric Gauss-Seidel (LU-SGS) method are attempted. Simultaneously, overset grid technique is adopted. In detail, hole-map method is utilized to identify intergrid boundary points (IGBPs). Furthermore, aimed at identification issue of donor elements, inverse-map method is implemented. Eventually, blade surface pressure distributions derived from numerical simulation are validated compared with experimental data, showing that all the schemes mentioned above have the capability to predict the rotor flow field accurately. At the same time, vorticity contours are illustrated for analysis, and other characteristics are also analyzed.     

Mesh-dependence of Material with Softening Behavior

The issue of mesh-dependence emerges when the conventional continuum damage model is applied to handling the softening behavior. In order to circumvent the mesh-dependence, the non-local theory is introduced into the conventional damage model and the finite element formulas are derived for two-dimensional gradient-enhanced damage model. A new element is proposed in which the basic unknown quantities are displacement, non-local equivalent strain and the gradient of non-local equivalent strain. The element and constitutive equation proposed in this article are added to the finite element software ABAQUS through user subroutine UEL. Numerical results show that the gradient-enhanced damage model can eliminate the mesh-dependence and is effective for dealing with the issue of softening behavior.     

Galactic Evolution of the Light Elements: A New Set of B Observations

The boron 2500 spectral region has been observed with the Goddard High Resolution Spectrograph (GHRS) of the Hubble Space Telescope (HST) in a new set of metal-poor stars and analyzed by spectrum synthesis technique, adopting the most recent model atmospheres. By taking into account the Li and Be abundances available from the literature for this same set of objects, the resulting patterns of their light elements abundances cannot be easily justified with the currently known stellar structure scenarios. The finding of real differences in the B content between stars with very similar stellar characteristics suggest that also production effects, rather than depletion and/or mixing only, should be taken into account as a possible and valuable explanation.     

升阶谱元素在正交各向异性复合材料剪切板中的应用

本文对近年发展的一种新的有限元元素—升阶谱元素进行了探讨,并把改进过的这种元素具体应用于当代直升机广泛采用的正交各向异性复合材料层合板(厚板)问题,实例表明:采用改进过的升阶谱元素,根据应力梯度的变化来选择元素的阶次是能够收敛于正确解的,从经济角度和实用角度来看,该元素性能比较优越。     

热-机耦合分析的有限元法及其应用

基于有限元分析的基本原理,建立了热-机耦合问题分析的有限元基本方程,并对工件挤压过程及摩擦制动问题进行了分析。算例表明,热-机耦合分析方法能很好地模拟受热结构的温度场和应力应变场,为结构设计和工艺设计提供依据。     

运输环境中某战术导弹结构振动特性分析

应用三维有限单元和导弹实体简化模型对一种战术导弹的结构振动特性进行分析，并动用ＡＮＳＹＳ有限元分析程序，计算得到了“自由－固支”条件下，导弹竖立状态的固有模态，取得了与试验和数据基本一致的结果，证明了所建模型物正确性，进而对运输环境中导弹水平支承条件下的固有频率和振型进行了分析计算，所得结果为导弹的使用可靠性分析提供了依据。     

三维角联锁结构复合材料等效弹性性能有限元分析

在复合材料承载时变形协调的前提下,采用CAD软件绘制三维织物单元体几何模型,然后导入结构分析有限元软件包,对工程中常用的带衬经三维角联锁结构复合材料的弹性性能进行分析,并将0°和90°方向弹性性能预测值及实验值与取向平均法预测值进行比较。结果表明,采用的有限元方法所得预测值及实验值与取向平均法预测值之间具有很好的一致性。