三维体目标间拓扑方向关系描述和推理

空间关系描述了地理信息系统(GIS)中实体间的位置、距离、方位、拓扑等的度量。为提高空间关系描述的惟一性和空间关系推理的准确性,将拓扑关系与方向关系集成描述,构建拓扑方向关系的描述表达模型。目标对象与参照物在X、Y、Z 3个坐标轴上投影间的Allen区间关系分别为R1、R2和R3,提出利用Allen区间关系对(R1,R2,R3)描述三维拓扑方向区域,用定义法研究拓扑方向关系定性推理,通过一些典型例子说明拓扑方向关系推理过程和结果,推理结果用组合推理表表示。     

考虑界面效应的复合泡沫塑料弹性性能数值仿真预测与试验研究

通过对不同空心陶瓷微珠含量的环氧基复合泡沫塑料进行准静态拉伸实验, 研究了填充微珠的体积分数对复合泡沫塑料弹性模量和泊松比的影响。基于其细观结构特征, 利用三维立方单胞有限元模型模拟了细观应力/应变场; 将内聚力单元引入细观有限元模型, 以此来模拟空心微珠与基体材料之间界面相的力学行为。将有限元预测结果以及两种传统的细观解析法与实验数据对比, 发现基于界面理想粘接假设的有限元模型和传统细观解析法均过高估计了复合泡沫塑料的弹性模量和泊松比; 复合泡沫塑料的弹性性能强烈地依赖于界面相的力学性质, 只有考虑界面效应的细观有限元模型才能给出较为精确的预测, 从而验证了文中细观建模方法的合理性。     

层合板层间断裂分析的组合界面单元及其特性

提出了一种由刚性元和零厚度的内聚力单元组合而成的新型界面单元,该界面单元嵌在板壳结构界面之间,可用来模拟界面损伤的起始和演化,能考虑板壳的平动和转动对分层损伤的作用。该界面单元具有有限厚度,八个结点,每个结点有五个自由度,通过刚性元将板壳单元结点的位移和结点力转换到内部零厚度的内聚力单元上,界面损伤通过内聚力单元的损伤演化体现出来。采用板壳单元和新型界面单元建立有限元模型,对混合弯曲(MMB)试验和双悬臂梁(DCB)弯曲试验进行了计算模拟,计算结果能很好地模拟结构的界面损伤过程。相比传统的用内聚力单元和三维实体单元组成的模型,建模方便,在精度相当的前提下,可以使单元尺寸增大一倍,减少裂尖内聚力区域(cohesive zone)内的单元数量,缩小计算规模,提高计算效率。     

高层建筑结构振动台模型试验与原型对比的研究

振动台模型试验是研究结构地震破坏机理和评价结构整体抗震能力的重要方法之一,如何正确处理试验模型与原型的相似关系以及如何由模型反应正确推导原型反应是很关键的问题。基于振动台试验,以12层的高层建筑框架结构为模型,对其进行了模型设计和试验,并运用大型非线性有限元分析软件MSC.Marc分别对原型结构和模型进行了地震作用下的动力响应分析。由模型地震反应按照动力相似关系推导出原型结构的地震反应,并将其与原型结构的计算值进行对比分析。通过对原型与模型自振特性、加速度、位移和剪力的对比,验证了模型与原型相似关系的准确性,振动台模型试验可以真实的反映原型结构的地震响应,据此,根据模型的试验结果可以对原型结构进行抗震性能的评估。     

基于鼓包法的薄膜材料界面结合能的测定

薄膜材料的应用范围越来越广泛,薄膜材料的界面结合能对其使用过程具有重要影响。鼓包法是测量薄膜材料界面结合能的一种重要的方法。通过有限元软件ABAQUS模拟鼓包实验,并用ABAQUS中的内聚力模型表征薄膜与基底之间的界面性能,最终得到薄膜材料的界面结合能。计算结果表明:薄膜开裂时的压强和挠度的乘积与界面结合能呈线性关系,且斜率随鼓包模型的孔径变化而变化。通过不同孔径下的模拟最后得到Ni薄膜界面结合能的表达式,且表达式中的参数在实验过程中简单易测。     

树脂复合减振钢板U型弯曲回弹实验与数值模拟研究

通过带法兰边的U型弯曲成形实验研究,考察了树脂复合减振钢板在不同压边力下的回弹特性.实验结果表明:压边力对树脂复合减振钢板回弹特性影响显著.较大的压边力有利于减小回弹缺陷.其次,考虑树脂层的粘弹性特性,采用非线性粘弹性模型来描述树脂层的力学变形行为,并采用Cohesive单元和固体壳单元分别对树脂层和表层钢板进行离散,进行了树脂复合减振钢板在不同压边力下的U型弯曲有限元数值模拟研究.和实验结果比较表明,所建立的有限元模型能够较好的模拟U型弯曲成形过程.最后,基于建立的有限元模型,考查了成形速度,树脂层厚度和表层钢板初始屈服应力对回弹的影响.参数分析结果表明:这三个参数对回弹角的影响显著.该研究对树脂复合减振钢板冲压工艺设计具有一定的指导意义.     

A hybrid electromagnetism-like algorithm for two-stage assembly flow shop scheduling problem

This paper presents a study on the two-stage assembly flow shop scheduling problem for minimising the weighed sum of maximum makespan, earliness and lateness. There are m machines at the first stage, each of which produces a component of a job. A single machine at the second stage assembles the m components together to complete the job. A novel model for solving the scheduling problem is built to optimise the maximum makespan, earliness and lateness simultaneously. Two optimal operation sequences of jobs are determined and verified. As the problem is known to be NP-hard, a hybrid variable neighbourhood search – electromagnetism-like mechanism (VNS-EM) algorithm is proposed for its handling. To search beyond local optima for a global one, VNS algorithm is embedded in each iteration of EM, whereby the fine neighbourhood search of optimum individuals can be realised and the solution is thus optimised. Simulation results show that the proposed hybrid VNS-EM algorithm outperforms the EM and VNS algorithms in both average value and standard deviation.     

Matching boundary conditions for lattice dynamics

We design a class of accurate and efficient absorbing boundary conditions for molecular dynamics simulations of crystalline solids. In one space dimension, the proposed matching boundary conditions take the form of a linear constraint of displacement and velocity at atoms near the boundary, where the coefficients are determined by matching the dispersion relation with a minimal number of atoms involved. Bearing the nice features of compactness, locality, and high efficiency, the matching boundary conditions are then extended to treat the out‐of‐plane wave problems in the square lattice. We construct multidirectional absorbing boundary conditions via operator multiplications. Reflection coefficient analysis and numerical studies verify their effectiveness for spurious reflection suppression in all directions. Compact and local in both space and time, they are directly applicable to nonlinear lattices and multiscale simulations. Copyright © 2012 John Wiley & Sons, Ltd.     

A general constitutive relation for linear elastic foams

A three-dimensional linear elastic constitutive relation is formulated based on a representative unit cell of foam using elasticity theory and micromechanics homogenization scheme. The displacement and strain fields of the unit cell are obtained from elasticity theory and used to derive the macroscopic strain field defined on the outer surface of unit cell through homogenization scheme. By assuming a uniform macroscopic stress on the unit cell surface and the existence of strain energy potential, the constitutive relation of linear elastic foams is obtained. The newly derived constitutive relation is a function of mechanical property of solid constituent, the geometry of cell struts, and the porosity of foams and is able to characterize the anisotropic behavior of foams due to non-uniform strut geometry. The linear elastic response of open-celled foams with both low- and medium-relative densities can be studied using the derived constitutive relation. The effective elastic modulus for uniform strut geometry is reduced from the constitutive relation and agrees well with Gibson and Ashby's semi-empirical equation, Warren and Kraynik's, and Zhu's analytical models within relative density ranging from 0 to 0.35. For non-uniform strut geometry, the calculated effective elastic moduli in three axial directions are different and the material displays anisotropic behavior. The bulk modulus shows less dependence on the variation of the strut geometry. Poisson's ratios are also reduced from the compliance matrix.     

The directed relation graph method for mechanism reduction in the oxidative coupling of methane

A detailed mechanism comprising the GRI-Mech natural gas combustion mechanism and recent updates to it that included soot formation and reactions involving acetylene by Frenklach and coworkers was chosen to describe the gas phase reactions in the oxidative coupling of methane (OCM). The complete mechanism was reduced for use under OCM conditions with the directed relation graph method. The mechanism reduction achieved reduction ratios of 0.55 for the number of species and 0.57 for the number of reactions when the differences in the concentrations of CH₄, O₂, C₂H₆, C₂H₄, and C₂H₂ between the original and reduced mechanisms were required to be less than 5%.