On Micromechanical Damage Modelling Of Fibre Reinforced Composites

The numerical analysis of engineering structures is usually based upon the assumption of a homogeneous as well as a continuous medium. Macroscopic homogeneity is maintained also for heterogeneous, fibre reinforced composite structures by replacing the inhomogeneous medium through a model of a mathematically homogenized material. The constitutive behaviour is then described in terms of volume averaged quantities that smear the heterogeneities of the microscale. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

三轴压缩下岩石/混凝土的三维细观损伤模型

考虑三轴压应力作用下的无限大体深埋椭圆形裂纹的变形场,给出了闭合椭圆形微裂纹的能量释放率.采用能量平衡方法分析了闭合椭圆形微裂纹的扩展条件,得到了代表性体积单元中具有任意空间取向的单个闭合椭圆形微裂纹及其扩展引起的附加柔度张量.分析了闭合椭圆形微裂纹的偏折扩展,得到了微裂纹偏折扩展引起的附加柔度张量.采用Taylor方法考虑微裂纹系统对代表性体积单元变形的影响,引入概率密度函数,得到了三轴压缩下岩石/混凝土的三维细观损伤模型.分析了混凝土的单轴压缩特性,结果表明该模型能够很好地描述实验现象.     

An Integral-Equqtion Approach for Replacement Modelling Over Finite Time Horizons

An integral-equation approach to calculating the exact and asymptoticestimates of expected costs in stochastic replacemnet problemsis advocated. The method is based upon the known solution tothe generalized renewal integral equation and upon the key renewaltheorem. The basic failure-bsed and age-based replacement policiesare taken as illustrative examples for the method, and the relationshipto more conventional methods of calculation is highlighted.The benefit of the integral-equation format is that it providesa relatively clear procedure for evaluating more complex stochastitreplacement situations.     

Micromechanical Approach of Martensitic Transformation Problem in Steels

To determine the mechanical behavior of material involving the martensitic phase transformation (for example, steels like 100Cr6), a representative volume element (RVE) model including phase transformation criterion is desireable at micromechanical approach. A framework combining the Eshelby's inclusion theory as well as continuum mechanics with phase-transformation (PT) critical condition at RVE model is presented briefly. And application of this model to estimate the critical aspect ratio of martensitic plate or lath inside homogeneneous stress field is also included, where the RVE can be under uniaxial tension/compression or pure shear loading case. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

A Multiscale Approach for the Modelling of Failure

In the present contribution a multi–scale – or rather two–scale – framework for the modelling of propagating discontinuities is introduced. The method is based on the Variational Multiscale Method. The displacement field is additively decomposed into a coarse– and a fine–scale part. This kinematic assumption implies a separation of the weak form in two equations, corresponding to the coarse–scale and the fine–scale problem. Both scales are discretized by means of finite elements. On the fine–scale, due to a much finer discretization, a heteregeneous mesostructure and propagating mesocracks can be considered. The propagation of the mesocracks is simulated independently of the underlying finite element mesh by discontinuous elements. The performance of the multi–scale approach is shown by a numerical example. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

An Operational Research Approach to Planned Maintenance: Modelling P.M. for a Vehicle Fleet

A methodology is advanced for modelling the problem of planned preventive maintenance, thereby relating plant performance to maintenance activity. Ideas are discussed and developed within the context of a P.M. system for the maintenance of a vehicle fleet. Three specific questions are addressed: first, the omission of any desirable activities from the P.M. schedule, secondly, the presence of redundant items in the schedule, and thirdly, the consequences of different frequencies of application of the P.M. schedule. A fleet model is obtained relating the P.M. period to measures such as proportion of breakdown repairs and the cost per vehicle year.     

Micromechanical creep model for pure copper

A Unit Cell made of copper is simulated and investigated under creep conditions within the framework of micromechanics. Geometrical 3D model of the copper microstructure is represented as a Unit Cell with grains of random crystallographical orientation and geometry. Such simulation enables algorithm of Voronoi tessellation. The stress-strain behavior of grains in the general case is anisotropic due to the ordered crystalline structure. The anisotropic model for a material with a cubic symmetry is implemented in Abaqus and used to assign behavior of grain interior in elastic and creep regions. Material parameters for elastic model are taken from elastic tests of single crystal copper [1]. Power law material parameters for creep model are taken from creep test performed for single crystal copper [2]. The model parameter ξ is validated numerically. Creep results are presented for the case of proportional loading during the primary and secondary creep. Statistical analysis of creep curves received for 55 different realizations of Unit Cell geometry is carried out. As a result confident interval and mathematical expectation of creep data are calculated. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Premixed Turbulent Combustion Modelling Using Level Set Approach

The paper deals with a level set approach application to SI engine combustion modelling, which is based on solving an additional transport equation to determine the flame front propagation. The presented work is an extension of the paper [6]. The influence of engine speed, air excess, swirl number, engine load as well as application of different turbulence model, in.uence of mesh coarseness and model fine-tuning constants are investigated and the results are presented. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monte Carlo Approach for Fatigue and Damage Calculation of Nonlinear Dynamical Systems

We construct analytical approximations for the Rice formula in the case where the stress is modelled as a non‐Gaussian stationary process. These approximations can be directly utilized by industrials in order to perform fatigue analysis of a structure under random loads.     

An Interactive Approach for Project Selection

This paper describes an interactive method for presenting a sequence of feasible sets of indivisible projects to a decision-maker. For each set as a whole, the decision-maker evaluates its utilities with respect to each of several attributes; the utilities are then combined to give a single utility for the set.A sequence of zero-one programmes is used to ensure that the only sets presented are those which are feasible and which are not contained in larger feasible sets. The sets are presented in descending order of size, and the presentation can be terminated by the decision-maker or by supplementary rules.The method for presenting sets can be used in other contexts in which complete enumeration of possible sets would otherwise be required.     

Spreadsheet Modelling Abuse: An Opportunity for OR?

Spreadsheet models are increasingly being used in decision making within organizations. With questions about the quality of these models, an investigation was conducted into the spreadsheet practices in ten firms, with an emphasis on the process of building spreadsheet models. The study showed that spreadsheet models were usually built in an informal, iterative manner, by people from all organizational levels. These people had received very little training in the building of models, which could help explain why at least 25% of the models contained errors. Other problems were also found. It was evident that the spreadsheet practices in the firms were inadequate. There is a need for increased training as well as setting and enforcing organizational spreadsheet standards. This could provide an opportunity for OR workers if carefully handled. Although the study was viewed as exploratory, it indicated a need for further study into the effect of formal design practices on the incidence of errors and model creation time.     

A Numerical Approach to Mechanosensation of Bone Tissue based on a Micromechanical Analysis of a Single Osteon

The subject of this work is to study the mechanosensation of bone tissue using a numerical approach. Therefore a finite element analysis of a single osteon was carried out. The calculated strains at the lacuna leads to a deeper insight into the mechanical conditions the osteocytes are exposed to. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

An Algebraic Approach for Extending Hamiltonian Operators

An algebraic approach for extending Hamiltonian operators is proposed. A relevant sufficient condition for generating new Lie algebras from known ones is presented. Some special cases are discussed and several illustrative examples are given.     

蚂蚁聚类算法在炮兵阵地选择中的应用

针对炮兵阵地配置的特殊要求,将模糊数学中的隶属度概念引入到炮兵阵地聚类算法之中,具有较强的可操作性,利于计算机实现,对炮兵指挥决策有一定的参考价值.     

Micromechanical modeling of twinning

There exist material models that incorporate mechanical twinning in a homogenized sense, or consider specific aspects, like grain refinement or texture evolution. However, since the RVE-technique became a standard method, it is possible to obtain more detailed predictions based on micromechanical models. In this work, an approach based on a nonconvex elastic potential and the corresponding results of FE calculations are presented. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

计算资产组合市场风险值的一种有效方法

市场风险值(VaR)是一种常用的度量风险的方法．本文采用极值理论中的阈值模型来计算VaR．基于中国上证指数和深成指数的收盘价，构造超越阈值的极值渐近概率分布，所得到的计算结果与传统方法相比较，有明显的优越性和更好的精度．     

一个NTU博弈的模糊延拓方法

对NTU合作博弈给出了一种模糊延拓方法,得到了模糊延拓后的模糊核心,并举例进行了说明。     

关于含裂纹的复合材料的一个细观断裂模型

本文研究复合材料的小范围屈服断裂过程.首先,提出了含裂纹复合材料的一个细观断裂模型,其中对裂纹顶端的外部地区采用连续介质各向异性描述,对裂纹顶端的小范围地区则采用多相的描述.多相区中考虑三种材料成分,即纤维、界面和基体,它们都可被看作是非线性的,并计入有限变形的影响.我们应用边界层——非线性有限元方法作出本问题的解.