A design model to predict optimal two-material composite structures

A method is presented for the prediction of optimal configurations for two-material composite continuum structures. In the model for this method, both local properties and topology for the stiffer of the two materials are to be predicted. The properties of the second, less stiff material are specified and remain fixed. At the start of the procedure for computational solution, material composition of the structure is represented as a pure mixture of the two materials. This design becomes modified in subsequent steps into a form comprised of a skeleton of concentrated stiffer material, together with a nonoverlapping distribution of the second material to fill the original domain. Computational solutions are presented for two example design problems. A comparison among solutions for different ratios of stiffness between the two materials gives an indication of how the distribution of concentrated stiffer material varies with this factor. An example is presented as well to show how the method can be used to predict an efficient layout for rib-reinforcement of a stamped sheet metal panel.     

Automated structural design and analysis of advanced composite wing models

The design of a composite wind tunnel model has demonstrated the ability to tailor the response of a composite structure to provide desired static and dynamic characteristics. This was possible because the strength and stiffness properties of composite structures can be controlled through selection of materials and lamination patterns. To take maximum advantage of the capability, efficient computer procedures are being developed for the design and analysis of composite structures. A finite element procedure and a direct Rayleigh-Ritz procedure, specialized for the preliminary analysis of wing-type structure, are discussed. The use and accuracy of these procedures have been demonstrated on a low cost, low risk basis in the design and analysis of a composite wind tunnel model and in test-theory correlation for static and dynamic response. Material selection, intermediate design decisions, fabrication, testing for natural modes and frequencies, and testing for influence coefficients for the wind tunnel model are discussed.     

A stiffness adjustment mechanism maximally utilizing elastic energy of a linear spring for a robot joint

This paper proposes a mechanism that adjusts mechanical stiffness around a robot joint and utilizes whole elastic energy of an elastic element. The proposed mechanism consists of a lead screw mechanism, a linear spring, and wires. The lead screw mechanism moves a nut of the lead screw mechanism to change a bending point of the wire, which connects the linear spring and the lead screw mechanism. Then, moment arm and ratio of joint rotation to extension of the spring are varied. As a result, joint stiffness is adjusted. Because this mechanism does not apply tension to the spring for the stiffness adjustment, whole elastic energy of the spring can be utilized for joint rotation. This utilization can minimize weight and size of the elastic element. Additional advantages of the proposed mechanism are mechanical simplicity, wide range of adjustable stiffness, and no energy consumption for keeping constant stiffness. We analyze characteristics of the proposed mechanism and compare with other mechanisms in detail. Device development and experimental results are provided for demonstrating the effectiveness of the proposed mechanism.     

3D chip stack technology using through-chip interconnects

The current technology in micro-and nano-electronics is insufficient to meet future demands for several applications. Most state-of-the-art solutions rely on so-called embedded technologies, which are both expensive and complex. One solution to the problem of integrating mixed technologies is the concept of 3D stacking. Our approach implements an epitaxial etch-stop layer for thickness control of the thinning process. Using this etch-stop layer, we can create a precise alignment of back-side vias to the landing pads in the first metal layer of the active CMOS, resulting in small via diameters and high connection densities between individual-layers of the 3D stack. Furthermore, we can use other materials, like GaAs (gallium arsenide), in combination with an epitaxial lift-off process. We use a copper-tin soldering process based on the solid-liquid interdiffusion (solid) process to create the electrical and mechanical connection between the single chip layers. Using this process, we created true multilayer stacks and tested them with respect to the static electrical properties of ohmic contacts and interchip vias. We directly incorporated these results in the design of test circuits that create tests for stuck-at failures of the interchip connections after stack assembly. This article presents a technology overview of how to achieve the goal in a 3D fabrication process. It also shows measurements for characterizing interconnects.     

Design and control of tensegrity robots for locomotion

The static properties of tensegrity structures have been widely appreciated in civil engineering as the basis of extremely lightweight yet strong mechanical structures. However, the dynamic properties and their potential utility in the design of robots have been relatively unexplored. This paper introduces robots based on tensegrity structures, which demonstrate that the dynamics of such structures can be utilized for locomotion. Two tensegrity robots are presented: TR3, based on a triangular tensegrity prism with three struts, and TR4, based on a quadrilateral tensegrity prism with four struts. For each of these robots, simulation models are designed, and automatic design of controllers for forward locomotion are performed in simulation using evolutionary algorithms. The evolved controllers are shown to be able to produce static and dynamic gaits in both robots. A real-world tensegrity robot is then developed based on one of the simulation models as a proof of concept. The results demonstrate that tensegrity structures can provide the basis for lightweight, strong, and fault-tolerant robots with a potential for a variety of locomotor gaits.     

A novel FEM model for biaxial non-crimp fabric composite materials under tension

This paper presents a novel finite element based approach able to represent the complex architecture of the non-crimp fabric (NCF) composite materials. By means of the stiffness averaging method, implemented in the research oriented FEM (finite element method) code B2000, the developed model is able to simulate the NCFs mechanical performances. Applications to simple coupons loaded in tension are presented in order to demonstrate the capability and the effectiveness of the presented approach. Nevertheless, the proposed methodology can be applied and extended to all NCF geometries. First, for validation purposes the numerical results detained for a specific configuration have been compared with experimental results available from literature. Then, a parametric study has been carried out to investigate the influence of the bundle waviness on the tension stiffness. Finally, due to the degradation of the in-plane mechanical properties, the presence of the stitching has been investigated.     

Combined asymptotic finite-element modeling of thin layers for scalar elliptic problems

Thin layers with material properties which differ significantly from those of the adjacent media appear in a variety of applications, as in the form of fiber coatings in composite materials. Fully modeling of such thin layers by standard finite element (FE) analysis is often associated with difficult meshing and high computational cost. Asymptotic procedures which model such thin domains by an interface of no thickness on which appropriate interface conditions are devised have been known in the literature for some time. The present paper shows how the first-order asymptotic interface model proposed by Bövik in 1994, and later generalized by Benveniste, can be incorporated in a FE formulation, to yield an accurate and efficient computational scheme for problems involving thin layers. This is done here for linear scalar elliptic problems in two dimensions, prototyped by steady-state heat conduction. Moreover, it is shown that by somewhat modifying the formulation of the Bövik–Benveniste asymptotic model, the proposed formulation is made to preserve the self-adjointness of the original three-phase problem, thus leading to a symmetric FE stiffness matrix. Numerical examples are presented that demonstrate the performance of the method, and show that the proposed scheme is more cost-effective than the full standard FE modeling of the layer.     

Development of CFRP racing motorcycle rims using a heuristic evolutionary algorithm approach

The scope of this paper is the application of evolutionary optimization methods to the development of composite fibre reinforced plastics (CFRP) racing motorcycle rims. The mass and the moment of inertia of a front and a rear CFRP rim are minimized subject to manufacturing, strength, and stiffness constraints. The stacking sequence of the composite laminates is optimized by applying a sophisticated parameterization concept making an excellent compromise between a huge variety of structure properties and a reasonable number of optimization parameters. The mechanical properties are simulated using the finite element analysis package ANSYS . Resulting displacement and Tsai–Wu index values are combined with the mass of the rim in order to assign a fitness value to each different design solution. The smart formulation of the fitness function allows the exploration of solutions close to the required strength and stiffness properties. The proprietary software DynOPS is utilized as an optimization engine. It links an evolutionary algorithm to arbitrary simulation programs and controls the entire optimization process. The sophisticated parameterization concept, together with the fitness function formulation, are the basis for the development of CFRP motorcycle rims decisively lighter than state-of-the-art magnesium alloy rims.     

平面联接固定结合部动力学模型的一种建模方法

机械结构结合部精确建模是对含结合部的机械结构进行动态分析和优化设计的关键问题之一。文章主要研究块体结构和平板由螺栓联接后构成的固定结合部的动力学建模问题。将结合部区域的刚度特性用结合部的材料机械特性参数等效表达,提出了虚拟接触材料的结合部刚度表达模型。通过模态实验和模型修正识别出不同结合面工况下虚拟接触材料的弹性模量,建立了含结合部动力特性的有限元模型。该模型可方便用于含结合部结构的有限元计算,经试验验证满足实际工程应用要求。     

Lattice structure lightweight triangulation for additive manufacturing

Additive manufacturing offers new available categories of geometries to be built. Among those categories, one can find the well developing field of lattice structures. Attention has been paid on lattice structures for their lightweight and mechanical efficiency ratio, thus leading to more optimized mechanical parts for systems. However this lightness only holds true from a mass related point of view. The files sent to additive manufacturing machines are quite large and can go up to such sizes that machines can freeze and get into malfunction. This is directly related to the lattice structures tendency to be of a high geometric complexity. A large number of vertices and triangles are necessary to describe them geometrically, thus leading to larger file sizes. With the increasing use of lattice structures, the need for their files to be lighter is also rising. This paper aims at proposing a method for tessellating a certain category of such structures, using topologic and geometric criteria to generate as few as possible triangles, thus leading to lightweight files. The triangulation technique is driven by a chordal error that controls the deviation between the exact and tessellated structures. It uses interpolation, boolean as well as triangulation operators. The method is illustrated and discussed through examples from our prototype software.     

Structural Optimization of Automotive Body Components Based on Parametric Solid Modeling

Parametric modeling was used to build several models of an automotive front structure concept that utilizes carbon fiber composite materials and the corresponding molding processes. An ultra-lightweight aluminum body front structure was redesigned to include an all-composite front structure. Two alternative concepts were studied which represent the structure as a bonded assembly of shells. Closed sections result from two pieces – an inner and outer. Parametric modeling was found to be a useful tool for building and modifying models to use in optimization concept studies. Such models can be built quickly and both the sketch dimensions and location dimensions are particularly useful for making the adjustments necessary to fit the various body pieces together. The parametric models then must be joined together as one geometric solid model in order to obtain a surface mesh. Structural optimization input data can then be seamlessly and quickly created from the parametric-model-based finite element model to begin the tradeoff studies. This integrated process in which parametric modeling was coupled with structural optimization was used to carry out design studies on the lightweight body front structure. Several carbon fiber material combinations were studied to determine mass reduction potential of certain types of carbon fiber products considered to be lower cost than typical carbon fiber materials used in the past. Structural optimization was used to compare several composite constructions for the design of the bonded front structure. Eight cases were studied using various materials and composite lay-ups. Mass savings estimates from 45–64% over steel were obtained. The most reasonable design consisted of a combination of relatively low cost chopped carbon fiber and woven carbon fiber and using a 20 mm balsa core in the top of the shock tower area. This design had a maximum thickness of 7 mm and a mass reduction over steel of approximately 62%.     

Real-time control of geometry and stiffness in adaptive structures

Adaptive structures are those which can adjust their geometry, stiffness and damping on demand to] meet the changes in their loading environment. With the advents in the piezo-electric device and composite material technologies, such structures are already being considered seriously in aero-space industry. The success of these structures depends largely on the effective control of the structure through the actuators, and the sensors embedded in their load carrying members. This work outlines the basic theory for the geometry, stiffness and damping control. Necessary and sufficient conditions for stress free geometry control in statically determinate and indeterminate adaptive discrete structures are given. In particular, for the subset of discrete adaptive structures viz. adaptive truss structures the equations similar to the forward and inverse kinematic equations of open loop mechanical manipulators are derived. These equations describe the large geometry control under show motion (i.e., no inertia forces) assumption. Two criteria for choosing the optimum control from among the possible ones are established. A fast algorithm based on variable order variable step multistep method is given that can compute the controls for a large maneuver in real-time. Numerical results from the algorithm are presented. As an example of damping and stiffness alteration on demand, the vibration control in adaptive trusses by means of elongations and elongation-rates of the active elements is also given.     

Pen‐writable nanocarbon arrays fabricated using liquid‐crystalline materials for potential use in displays

Abstract— We have fabricated carbon nanotubes and nanofibers using liquid‐crystalline materials. By using these materials, the orientation of graphene layers can be manipulated using surface‐anchoring techniques common to liquid‐crystal displays. The bulk material properties of the graphene can be controlled by the spatial arrangement of the layers due to their anisotropic nature. We have also demonstrated a method to pattern arrays of these nanoforms using an automated printing technique.     

Strength of hierarchical materials

Many biological materials exhibit a hierarchical structure over more than one length scale. Understanding how hierarchy affects their mechanical properties emerges as a primary concern, since it can guide the synthesis of new materials to be tailored for specific applications. In this paper the strength and stiffness of hierarchical materials are investigated by means of a fractal approach. A new model is proposed, based both on geometric and material considerations and involving simple recursive formulas.     

Finite element analysis of spiral strands with different shapes subjected to axial loads

In this paper a new mathematical geometric model of spiral one or two-layered oval wire strands are proposed and an accurate computational two-layered oval strand 3D solid model, which is used for a finite element analysis, is presented. The three dimensional curve geometry of wires axes in the individual layers of the oval strand consists of straight linear and helical segments. The present geometric model fully considers the spatial configuration of individual wires in the right and left hand lay strand. Derived geometric equations were used for the generation of accurate 3D geometric and computational models for different types of strands. This study develops 3D finite element models of two-layer spiral round, triangular and oval strands subjected to axial loads using ABAQUS/Explicit software. Accurate modelling and understanding of their mechanical behaviour is complicated due to the complex contact interactions and conditions that exist between individual spirally wound wires. Comparisons of predicted responses for the strands with different shapes and constructions are presented. Resultant stress and/or deformation behaviours are discussed.     

海缆铠装结构锚固性能有限元分析

基于有限元显式算法,通过预紧、加载载荷作用次序,研究海缆铠装层的锚固结构在工作载荷和破断力下的承载能力,并对影响锚固性能的参数进行敏感性分析.数值仿真结果显示：在破断力作用下,法兰内壁与钢丝接触位置产生明显压痕,钢丝断裂位置发生在钢丝在锚固出口位置.增加锥形锚固的内锥角可降低钢丝轴向拉伸量,减缓法兰的不利应力.同时.在较低摩擦因数下具有较大内锥角的接头盒依旧具有良好的机械性能.所设计的内锥形锚固维修接头盒能满足施工过程中的最大拉伸载荷要求.     

极小曲面力学超材料抗冲吸能特性分析

本文基于极小曲面结构构建了一类力学超材料,并研究了其准静态及动态力学特性.首先,通过对等效密度为30%、40%和50%的超材料样件进行准静态压缩试验,分析了不同等效密度下结构准静态力学特性变化规律,结果表明,结构模量及平台应力随等效密度的增长呈指数上升,其变化规律可用Gibson-Ashby模型进行精准拟合;其次,研究了不同冲击工况对极小曲面力学超材料动态力学特性的影响规律.根据动态力学特性影响因素及变化规律,分别构建了刚性-完美塑性-锁定模型和简化吸能特性预测模型,对冲击时的力学超材料强度及吸能特性进行预测.结果表明,基于三周期极小曲面的力学超材料具有良好的抗压抗冲吸能特性,且其动态力学性能可以通过建立的模型进行精确预测,为高性能防护结构设计提供了理论基础.     

形状记忆合金复合结构无线应变监测的新方法

将一定体积分数的形状记忆合金丝埋入复合材料形成混杂复合结构,可明显提高结构的应变吸收能量,改善复合材料结构的韧性.选择将这种混杂结构中的一些增强形状记忆合金丝作为电容器的电极,将构成的电容器接到振荡电路,结构的变形引起电容量的变化,从而导致振荡频率的改变,通过测量振荡电路的频率可实现结构应变的无线监测.理论分析和试验结果表明:这种方法是可行的.     

Substrate‐free cholesteric liquid‐crystal displays

Abstract— This paper demonstrates the first substrate‐free cholesteric liquid‐crystal displays. The encapsulated cholesteric displays are ultra‐thin (with a total thickness around 20 μm) and ultra‐lightweight (0.002 g/cm²). The displays exhibit unprecedented conformability, flexibility, and drapability while maintaining electro‐optical performance and mechanical integrity. All functional display layers are sequentially coated on a preparation substrate and then lifted‐off from the preparation substrate to form a free‐standing display. The display fabrication process, electro‐optical performance, and display flexibility are discussed.     

Micro-structured flow fields for small fuel cells

 A growing number of portable consumer electronics need a small, lightweight power supply with high capacity. Miniaturized fuel cells can meet this demand. A polymer electrolyte fuel cell (PEFC) typically consists of two electrodes, separated by a polymer electrolyte membrane, two current collectors and two sets of flow channels for the reactants all mounted together in a housing. The current collector or diffusion layer is usually made from carbon paper or carbon cloth. The carbon fibers lead the electric current and the pores allow for transport of the reactants. The focus of the investigations presented here is the miniaturisation of the macroscopic flow fields in fuel cells in order to achieve higher energy densities. Three concepts of fuel cells with micro-structured flow fields are presented, amongst them one complete fuel cell system for the use in portable applications. Since different packaging and assembly techniques with flexible materials are being used, the form of the cell can be tailored for specific applications. A stack of cells can be very flat resulting in power densities as high as 1 W/cm³. Received: 10 August 2001/Accepted: 24 September 2001 This paper was presented at the Fourth International Workshop on High Aspect Ratio Microstructure Technology HARMST 2001 in June 2001.