A constitutive model for transversely isotropic foams, and its application to the indentation of balsa wood

An elastic-plastic constitutive model for transversely isotropic compressible solids (foams) has been developed. A quadratic yield surface with four parameters and one hardening function is proposed. Associated plastic flow is assumed and the yield surface evolves in a self-similar manner calibrated by the uniaxial compressive (or tensile) response of the cellular solid in the axial direction. All material constants in the model (elastic and plastic) can be determined from a combination of a total of four uniaxial and shear tests. The model is used to predict the indentation response of balsa wood to a conical indenter. For the three cone angles considered in this study, very good agreement is found between the experimental measurements and the finite element (FE) predictions of the transversely isotropic cellular solid model. On the other hand, an isotropic foam model is shown to be inadequate to capture the indentation response.     

Application of a continuum constitutive model to metallic foam DEN-specimens in compression

The behavior of double-edge notched specimens of metallic foams in compression is studied numerically. To model the constitutive behavior of the metallic foam, a recently developed phenomenological, pressure-sensitive yield surface [1] is used. Compressive yielding in response to hydrostatic stress is incorporated through a dependence on the plastic Poisson ratio ν^p. Results are presented in terms of limit load F_lim, as a function of notch depth, a/W, and the plastic Poisson ratio ν^p. For incompressible plastic behavior, ν^p=0.5, the results show notch-strengthening due to constrained plastic deformation near the crack/notch-tip. For fully compressible plastic behavior (no lateral expansion on uniaxial compression, ν^p=0), no notch-effect is observed. The validity of using a continuum model for the analysis of metallic foam notched specimens is discussed.     

弹性机构动力平衡的有效新方法

从改善弹性机构动力特性的角度研究了机构弹性动力平衡的本质问题。提出了附加弹性元件的新方法,有效地解决了弹性机构震动力、震动力矩、输入扭矩波动和运动副反力的平衡难题。首次推导出具有弹性元件的弹性连杆机构动力学方程,从理论上阐述了此平衡方法的有效性。仅用附加一根扭簧的简单方法实现了平面四杆机构的弹性动力平衡,效果明显。     

线接触问题的弹性趋近量与滚子轴承的载荷分布

在对迄今为止所出现的各种线接触问题弹性趋近量分析方法进行全面阐述的基础上，就一些主要的弹性趋近量算式及其在滚子轴承载荷分布计算中的应用进行了求解和对比分析，分析表明三体接触时采用H．Zantopulos公式的结果最差，而在滚子轴承载荷分布的理论分析体系中，G.Lundberg、A.Palmgren、Luc Houpert以及改进的切片法等理论的计算结果相差不大，可以根据需要选用。     

Effective elastic constants of two-dimensional cellular materials with deep and thick cell walls

In order to analyse the elastic constants of cellular materials with deep and thick cell walls, finite element analysis using two kinds of unit cell approach (stiffness matrix method and compliance matrix method) is performed which is applicable to any orthotropic cellular materials. Comparison between results from the FEA, the theories presented in this paper and experiments of previous investigators indicate that the elastic constants of cellular materials with thick cell walls depend not only on the relative density but also on the joint stiffening effect. Approximate formulae under generalised plane strain conditions are also presented for the purpose of obtaining the effective elastic constants for cellular materials with deep and thick cell walls. A satisfactory agreement was found with experimental results obtained on a deep and thick cellular material. The results indicate that the previous models in which the wall of cellular materials is treated as a simple beam are not adequate to evaluate the effective elastic constants of cellular materials with deep and thick cell walls. In addition, considerable attention needs for the measurement of effective Young's modulus of square cellular materials in the two soft directions because it is strongly affected by misalignment errors.     

A flat punch pressed against an elastic interlayer under conditions of slip and separation

The problem of a square-ended rigid punch bearing on an elastic half-plane through the medium of an elastically dissimilar interlayer is considered, under plane deformation. The interlayer is in frictional contact with both the punch and half-plane. Regions of slip and separation along the interface are found, and it is shown how these phenomena affect the stress state in the immediate neighbourhood of the contact. This problem is of direct, practical interest to the design of interfacial bearing pads employed in supporting glass plates in structural applications.     

A novel evolutionary algorithm for determining unified creep damage constitutive equations

The determination of material constants within unified creep damage constitutive equations from experimental data can be formulated as a problem of finding the global minimum of a well defined objective function. However, such an objective function is usually complex, non-convex and non-differentiable. It is difficult to be optimised by classical gradient-based methods. In this paper, the difficulties in the optimisation are firstly identified. Two different objective functions are proposed, analysed and compared. Then three evolutionary programming algorithms are introduced to solve the global optimisation problem. The evolutionary algorithms are particularly good at dealing with problems which are complex, multi-modal and non-differentiable. The results of the study shows that the evolutionary algorithms are ideally suited to the problem. Computational results of using the algorithms to determine the material constants in a set of physically based creep damage constitutive equations from experimental data for an aluminium alloy are presented in the paper to show the effectiveness and efficiency of the three evolutionary algorithms.     

带孔弹性元件温度场的模拟实验方法

在一些特殊应用场合,传感器弹性元件处于温度梯度场而产生热应力,实验研究温度场及热应力分布是采取补偿措施的前提和基础,本文介绍了带孔弹性元件温度场研究的模拟实验方法,并根据获得数据整理出温度分布随几何参数变化的部分拟合方程。     

A mechanical model for the computation of phenolic foams in compression

In the field of packaging and cushioning applications, phenolic foams bring a suitable solution because of various qualities: incombustibility, cost, and mechanical behaviour in compression. The present paper is devoted to the development of a mechanical model for those foams, with a view to implementing it into a computer code. The yield criterion is derived from the famous Cam-Clay model provided with a standard flow rule, but the specificity of the model lies in the densification law. The densification equation is defined in an asymptotic form, and is shown to represent accurately the experimental tests in compression. The method of identification is applied to foam samples in a wide range of densities, thus providing a complete characterisation of this family of materials.     

A systematic method for characterizing the elastic properties and adhesion of a thin polymer membrane

The elastic properties and adhesion of a thin silicone rubber membrane have been characterized. An effective instrumentation embodying a video-enhanced microscope, which meets force and displacement resolution of and 10 nm, was developed. It provides the capability of simultaneously measuring both the applied force and the resultant displacement of the thin polymer membrane. A linear theoretical elastic solution was applied to quantitatively interpret the measured central deflection of the membrane under a circular concentrical load. Young's modulus of the membrane can be easily determined once the applied force and the central deflection, together with the essential dimensions, are known. The membrane jumped into an adhesion contact when the punch approached the range of the inter-surface force across the punch-membrane gap. A pull-off event was observed at a nonzero contact circle when the tensile load reached a critical threshold. The experiment is basically consistent with a theoretical model based on linear elasticity and an energy balance. The new method can be used to measure mechanical behavior of prestressed ultra-thin biomembranes and thin walled biocapsules with a residual stress.     

A series solution for spatially coupled deflection analysis of thin-walled Timoshenko curved beam with and without elastic foundation

The exact solutions for the spatially coupled deflection and the normal stress at an arbitrary location of a crosssection of the thin-walled Timoshenko curved beam with symmetric and non-symmetric cross-sections with and without two types of elastic foundations are newly presented using series solutions for the displacement parameters. The equilibrium equations and the force-deformation relations are derived from the elastic strain energy including the effects of shear deformation and the axial-flexural-torsional coupling, and the strain energy considering the foundation effects. The explicit expressions for displacement parameters are derived by applying the power series expansions of displacement components to the simultaneous ordinary differential equations. Next, the element stiffness matrix is determined by using the force-deformation relationships. The normal stress at any arbitrary location of the cross-section for a curved beam is evaluated from the stiffness matrix. To verify the validity and the accuracy of this study, the displacements and the normal stresses of curved beams are presented and compared with the analytical solutions, the finite element results using the isoparametric curved beam elements based on the Lagrangian interpolation polynomial, and the detailed three-dimensional analysis results using the shell elements of SAP2000. This paper was recommended for publication in revised form by Associate Editor Maenghyo Cho Nam-Il Kim received his B.S. degree in Civil and Environmental Engineering from Sungkyunkwan University, Korea, in 1996. He then received his M.S. and Ph.D. degrees from Sungkyunkwan University in 1998 and 2004, respectively. Dr. Kim is currently a research professor at Civil and Environmental Engineering at Myongji University in Korea. Dr. Kim’s research interests include stability and vibration of steel and composite structures. Dong Ku Shin received his B.S. and M.S. degrees in Civil Engineering from Seoul National University, Korea, in 1983 and 1985, respectively. He then received his Ph.D. degree from Virginia Tech. at Blacksburg, VA, USA, in 1990. Dr. Shin is currently a professor of Civil and Environmental Engineering Department at Myongji University in Korea. Prof. Shin’s research interests include LRFD design of steel bridges and stability of composite structures.     

A model for micro-slip between flat surfaces based on deformation of ellipsoidal elastic bodies

Micro-slip is a phenomenon that occurs between contacting surfaces when a frictional load, less than that necessary to produce macro-slip, is applied to the contacting surfaces. A model is presented for micro-slip between a flat smooth surface and a flat rough surface. The rough surface is covered with uniformly distributed ellipsoidal elastic bodies. The results from two test cases show that anisotropy of the contacting surfaces influences the tangential stiffness at zero displacement, the length of the micro-slip zone and the energy dissipated in the contact.     

空间失稳目标线阵激光成像建模及参数优化

线阵激光成像雷达兼具激光测距和线阵成像的优点,是空间失稳目标远距离精密测量的重要工具,其获取的线阵图像既能用于运动估计也能用于空间目标三维测量。为模拟不同平面分辨率线阵激光成像雷达对空间失稳目标的数据采集,本文根据线阵激光成像雷达的成像机制及空间失稳目标的运动规律提出了一般性的空间失稳目标线阵激光成像建模方法,为不同地面验证方案提供了数据支持。同时,为优化雷达参数选取,本文通过计算单位区域特征获取率提出了一种基于数据采集完整性评估的线阵激光成像雷达参数优化方法,实验分析了与成像分辨率相关的性能参数对多种运动形式空间失稳目标数据采集的完整性。实验结果显示数据采集完整性与线阵规模及采样频率正相关,在线阵规模达到100元以上、采样频率200Hz以上,与空间目标自身运动形式几乎无关,建立的对应关系将为参数优化提供指导和参考依据。     

电子装备热控新技术综述(上)

随着微电子技术特别是军用电子和微波器件的飞速发展,以及军用高功率密度器件和微波器件的使用,电子设备功率密度越来越大.因此对冷却技术的要求就越来越高.文中综述了当前电子设备冷却技术的发展现状,该综述分两部分,这部分内容主要包括热传导技术和界面材料、热扩展技术、热管及衍生物以及一些高导热性能材料的研究现状.文章分析了现有冷却技术的优缺点,同时也指出了未来冷却技术的发展方向.