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.     

The effect of doubly tapered strut morphology on the plastic yield surface of cellular materials

Although the literature on the mechanics of cellular materials is vast, there is no theoretical model to account for the effects of axial yielding of struts aligned to the applied loading direction on the plastic yield surface under multiaxial loading conditions. An anisotropic hexagonal model having tapered strut morphology is developed to show these effects on the plastic yield surface under multiaxial tensile loading condition. This model covers several types of cellular structure such as two-dimensional (2D) hexagonal and square cellular materials, and three-dimensional (3D) hexagonal and rhombic cellular materials of rod-like columnar structure. A tetrahedral element with tapered strut morphology is also used for a foam model to illustrate these effects on the yield surface under axisymmetric loading condition. Plastic collapse due to bending moment in the inclined struts is a dominant mode. However, under multiaxial tensile loading, the collapse due to axial yielding of struts parallel to the loading direction is found to be an important mode. The shape of plastic yield surface was found to depend not only on relative density but also on the strut morphology.     

Plastic collapse of cellular structures comprised of doubly tapered struts

In this paper, micro-structural models are developed to examine the effects of tapered strut morphology on the plastic collapse of cellular structures. The analytical models are for materials that fail by plastic yielding and cover several types of columnar structure (e.g. hexagonal and square honeycombs, and hexagonal and rhombic cellular materials of rod-like columnar structure). The results indicate that the plastic collapse of hexagonal cellular materials is dependent not only on the relative density but also on the strut morphology. The presence of taper in struts can increase or decrease the plastic collapse strength of cellular materials, depending on the strut morphology.     

拉压强度不同材料厚壁球壳的安定性分析

对拉压强度不同材料厚壁球壳进行了安定分析，得出了球壳加载应力场、残余应力场及安定极限的解析解，所得的解能反映材料拉压强度差效应。最后，利用所得的解研究了材料拉压强度差效应对球壳安定极限的影响，结果表明，考虑材料拉压强度差效应时球壳安定极限将明显提高。     

Three-dimensional vibration analysis of thick, tapered rods and beams with circular cross-section

A three-dimensional (3-D) method of analysis is presented for determining the free vibration frequencies and mode shapes of thick, tapered rods and beams with circular cross-section. Unlike conventional rod and beam theories, which are mathematically one-dimensional (1-D), the present method is based upon the 3-D dynamic equations of elasticity. Displacement components u_r, u_θ, and u_z in the radial, circumferential, and axial directions, respectively, are taken to be sinusoidal in time, periodic in θ, and algebraic polynomials in the r and z directions. Potential (strain) and kinetic energies of the rods and beams are formulated, the Ritz method is used to solve the eigenvalue problem, thus yielding upper bound values of the frequencies by minimizing the frequencies. As the degree of the polynomials is increased, frequencies converge to the exact values. Convergence to four- digit exactitude is demonstrated for the first five frequencies of the rods and beams. Novel numerical results are tabulated for nine different tapered rods and beams with linear, quadratic, and cubic variations of radial thickness in the axial direction using the 3-D theory. Comparisons are also made with results for linearly tapered beams from 1-D classical Euler–Bernoulli beam theory.     

复合材料中厚蒙皮多墙翼面结构损伤的有限元分析

复合材料中厚蒙皮多墙翼面结构中蒙皮承受全部弯矩,因而蒙皮的损伤机理和破坏载荷对结构的设计有着极为重要的意义与价值.通过自行编制UMAT子程序将三维逐渐损伤技术嵌入ABAQUS有限元软件程序中,随后基于复合材料中厚蒙皮多墙翼面结构静态弯曲试验,建立了结构局部的三维有限元模型,用子程序模拟了复合材料蒙皮损伤演化过程,并预测了初始破坏强度.模拟预测蒙皮损伤产生的位置和长度与试验观察到的蒙皮破坏部位和长度完全一致,预测的初始破坏强度和试验的误差为4.48%,由此证明了该有限元模型的准确性.从而为复杂的中厚蒙皮多墙翼面结构在实际工程中的应用提出了一种更为有效可靠的研究方法.     

Hashin准则的应力应变形式在复合材料渐进损伤计算中的对比

Hashin准则在复合材料渐进损伤分析中得到广泛应用,它的原始形式用应力表达,而在渐进损伤实际计算中用到了应变形式。推导了这两种形式下渐进损伤计算中需要的复合材料非线性本构关系及对应的切线刚度矩阵,对比分析了它们之间的不等价性。将这两种Hashin准则形式与不同损伤演化模型相结合,应用于开孔板的渐进损伤数值模拟,并与试验数据相对比。将Linde损伤演化模型分别与Hashin准则变形式和应力形式组合,前者的计算结果与试验值符合更好;将Sleight损伤演化模型与Hashin准则应力形式相结合,选取不同的衰减控制参数进行数值模拟,控制参数取0.8时比取其他值与试验结果符合更好。理论推导和数值模拟结果对复合材料渐进损伤实际应用中合理选取损伤准则形式和损伤演化模型有指导意义。     

Optimization and implementation of cellular manufacturing system in a pump industry using three cell formation algorithms

In recent years cellular manufacturing has become an effective tool for improving productivity. Attainment of full benefits of cellular manufacturing depends firstly on the design of the machine cells and part families and secondly on the method of operation which take full advantages of cell properties. Inappropriate methods of loading and scheduling can even lead to the failure of cellular manufacturing systems (CMS), however efficiently the cell is designed. This paper examines three array-based clustering algorithms, namely rank order clustering (ROC), rank order clustering-2 (ROC2) and direct clustering analysis (DCA) for manufacturing cell formation, with a real-life example to demonstrate the effectiveness of various clustering algorithms. The machine cell formation methods considered in this comparative and evaluative study belongs to the cluster formation approach of solving the MCF problem. The most effective method is selected and used to build the cellular manufacturing system. The comparison and evaluation are performed using four published performance measures and compares the improvements with the existing conventional system and the cellular manufacturing system. The above algorithms were written in the C++ language on an Intel/Pentium III-PC-compatible system.     

基于准布儒斯特角法研究抛光过程中光学材料的表面质量

光学硬脆材料在机械加工过程中不可避免地形成表面/亚表面损伤,对器件性能、使用寿命等具有至关重要的影响。相较于磨削、研磨两种前道工艺,抛光阶段材料的损伤逐渐减少至极其微量的程度,而最终残留的损伤将伴随材料的使用全周期,研究抛光阶段表面质量的变化过程对掌握抛光的工艺质量十分必要,但测量难度高。针对这一问题,本文在总结抛光阶段损伤形式的基础上,首先仿真分析了准布儒斯特角法检测表面质量的原理和优势,随后以Nd∶GGG激光晶体为研究对象,利用椭偏仪对不同抛光工艺下样品表面质量的变化进行了实验研究。通过与白光干涉法测量的表面形貌进行对比,准布儒斯特角偏移量和相位角变化曲线斜率准确地反映了抛光过程中表面质量的变化,展现了该方法的无损伤和高灵敏度特性,以及辅助研究抛光工艺的可行性。最后,对准布儒斯特角法在表面质量检测方面面临的问题进行了分析和展望。     

钢球滚子弧面凸轮分度机构的接触应力分析

运用回转张量法推导出凸轮廓面方程和啮合方程，并进一步计算了该机构啮合曲面的接触应力。     

Quantitative analysis of variable-angle total internal reflection fluorescence microscopy (VA-TIRFM) of cell/substrate contacts

Variable-angle total internal reflection fluorescence microscopy (VA-TIRFM) allows controlled variation of the illumination depth with the potential of measuring both membrane/substrate separation distances and sizes of focal contacts. VA-TIRFM images are collected from well-spread bovine aortic endothelial cells (BAEC) stained with a membrane-bound carbocyanine dye. Quantitative determination of absolute membrane/substrate separation distances and individual focal contact area are attempted using a simplified model of TIRFM optics. For angles slightly greater than the critical angle of 64°, both the dorsal and ventral membranes were illuminated, while images excited above 66° illuminated only focal contacts. Above 74° the fluorescence of focal contacts was dominated by background noise. Direct application of the simplified optical model without accounting for background intensity was unsatisfactory. However, correction for background fluorescence and nonlinear regression of the untransformed data over the working range yielded focal contact separation distances of 24 ± 13 nm. Focal contact areas estimated by TIRFM (1·3 ± 0·7 μm²) agreed closely with areas observed by immunofluorescence staining of vinculin (1·5 ± 0·3 μm²).     

齿根三维粗糙表面的应力集中系数计算研究

考虑粗糙表面影响因素的齿轮齿根处应力集中系数计算是齿轮弯曲疲劳寿命精准预估的难题,以磨削喷丸后的直齿轮为研究对象,研究粗糙表面下的齿根应力与应力集中计算问题.采用白光干涉仪Wyko NT9100对磨削喷丸齿根粗糙表面进行测量,得到粗糙表面形貌数据,基于空间坐标变换原理,使用Python对有限元软件进行二次开发,通过调整...     

Surface activity of cancer cells: The fusion of two cell aggregates

A key feature that distinguishes cancer cells from all other cells is their capability to spread throughout the body. Although how cancer cells collectively migrate by following molecular rules which influence the state of cell-cell adhesion contacts has been comprehensively formulated, the impact of physical interactions on cell spreading remains less understood. Cumulative effects of physical interactions exist as the interplay between various physical parameters such as (1) tissue surface tension, (2) viscoelasticity caused by collective cell migration, and (3) solid stress accumulated in the cell aggregate core region. This review aims to point out the role of these physical parameters in cancer cell spreading by considering and comparing the rearrangement of various mono-cultured cancer and epithelial model systems such as the fusion of two cell aggregates. While epithelial cells undergo volumetric cell rearrangement driven by the tissue surface tension, which directs cell movement from the surface to the core region of two-aggregate systems, cancer cells rather perform surface cell rearrangement. Cancer cells migrate toward the surface of the two-aggregate system driven by the solid stress while the surface tension is significantly reduced. The solid stress, accumulated in the core region of the two-aggregate system, is capable of suppressing the movement of epithelial cells that can undergo the jamming state transition; however, this stress enhances the movement of cancer cells. We have focused here on the multi-scale rheological modeling approaches that aimed at reproducing and understanding these biological systems.