Energy-efficient actuation in infinite lattice structures

Buildup of internal self-stresses in hyperstatic adaptive structures resists actuation. A recent paper by Guest and Hutchinson (2003) shows that periodic infinite truss structures cannot be both statically and kinematically determinate structures; therefore, a rigid infinite lattice bar framework must be hyperstatic. This paper shows that it is possible to design adaptive periodic infinite truss structures that can achieve any state of uniform strain without energy cost by actuating only a subset of the bars in a coordinated fashion. We show that actuation of only 3 bars in two dimensions or 6 bars in three dimensions per unit cell is required. A mathematical apparatus is developed and an example of such a bitriangular lattice structure is given, along with accompanying illustrations. Supporting animations can be found at the authors’ website.     

Investigation of clustered actuation in tensegrity structures

As tensegrity research is moving away from static structures toward active structures it is becoming critical that new actuation strategies and comprehensive active structures theories are developed to fully exploit the properties of tensegrity structures. In this paper a new general tensegrity paradigm is presented that incorporates a concept referred to as clustered actuation. Clustered actuation exploits the existence of cable elements in a tensegrity structure by allowing cables to be run over frictionless pulleys or through frictionless loops at the nodes. This actuation strategy is a scalable solution that can be utilized for active structures that incorporate many active elements and can reduce the number of actuators necessary for complex shape changes. Clustered actuation also has secondary benefits, specifically reducing the force requirements of actuators in dynamic structures, reducing the number of pre-stress modes to potentially one global mode and relieving element size limitations that occur with embedded actuation. Newly formulated clustered equilibrium equations are developed using energy methods and are shown to be a generalization of the classic tensegrity governing equations. Pre-stress analysis, mechanism analysis and stability of clustered structures are discussed. Lastly, examples compare the mechanics of a clustered structure to an equivalent classic structure and the utility of clustering is highlighted by allowing for actuation throughout a class 1 (no bar-to-bar connections) tensegrity while not embedding the actuators into the structure.     

Single member actuation in large repetitive truss structures

Numerical results are presented for the energy required to actuate a single member in three large two-dimensional lattice structures. As expected, the actuation energy, relative to the energy required to stretch a bar, scales with bar stockiness to the power two for a hexagonal lattice, whose behaviour is bending dominated, but is not greatly affected by stockiness for a triangulated lattice, whose response is stretching dominated. For a kagome lattice, however, the relative actuation energy scales with stockiness to the power one. Simple models show that, for the kagome, the attenuation of the deformation with distance from the actuated bar itself depends on the stockiness, and this results in the unexpected energy scaling; they also show that the energy is approximately equally partitioned between bending and stretching.     

Actuation of the Kagome Double-Layer Grid. Part 2: Effect of imperfections on the measured and predicted actuation stiffness

The actuation stiffness of a set of steel Kagome Double-Layer Grid (KDLG) structures with brazed joints is measured experimentally and compared with predictions by the finite element method. The predicted actuation stiffnesses for the perfect KDLGs much exceed the measured values, and it is argued that the low values of observed actuation stiffness are due to the presence of geometric imperfections introduced during manufacture. In order to assess the significance of geometric defects upon actuation stiffness, finite element calculations are performed on structures with a stochastic dispersion in nodal position from the perfectly periodic arrangement, and on structures with wavy bars. It is found that bar waviness has the dominant effect upon the actuation stiffness. The predicted actuation stiffness for the imperfect structures are in satisfactory agreement with the measured values assuming the same level of imperfection between theory and experiment.     

Kagome=笼目

数值计算中,划分平面区域可以用正方形、正三角形、正六角形等,也可联合正三角形和正六边形,称为kagome划分.在格栅(lattice)结构中,也有一种叫kagome的格栅.     

Model for thermoelastic actuation of an axisymmetric isotropic circular plate via an internal harmonic heat source

This paper presents a reduced analytical modeling method for the initial optimal design of thermoelastic micromachined actuators. The key aspects of the model are a Green’s function formulation of the axisymmetric heat conduction equation that incorporates an internal heat source and the solution of the thermoelastically forced bending wave equation. Model results of a representative thermoelastic structure include transient temperature and sinusoidal steady state transverse displacement. Comparison with finite element analysis shows excellent agreement with favorable computational costs. Model constraints at low frequencies are identified and discussed. The computational efficiency of the analytical model makes it a more viable modeling method for design optimization.     

板壳结构选型优化设计

提出了最佳结构型式概念;证明了结构存在最佳结构型式的充要条件;建立了启发式算法;实例计算证明,选型优化不仅可巧妙地融合形状、拓扑、布局优化的优点,而且计算省、结果可靠。     

薄板结构的安定优化设计

本文通过对薄板的Ｂ样条函数插值及结构中残余应力场的温度参数模拟，研究了薄板安定分析的样条有限元列式，在分析结构安定分析的数学规划列式的基础上，解决了薄板结构在给定可变载荷作用下的安定最小厚度设计问题。由于温度参数的引用，使得研究的问题的自由度大幅度减少。算例表明本文所提出的方法及所编程序的可行和有效。     

Novel numerical implementation of asymptotic homogenization method for periodic plate structures

The present paper develops and implements finite element formulation for the asymptotic homogenization theory for periodic composite plate and shell structures, earlier developed in  and , and thus adopts this analytical method for the analysis of periodic inhomogeneous plates and shells with more complicated periodic microstructures. It provides a benchmark test platform for evaluating various methods such as representative volume approaches to calculate effective properties. Furthermore, the new numerical implementation (Cheng et al., 2013) of asymptotic homogenization method of 2D and 3D materials with periodic microstructure is shown to be directly applicable to predict effective properties of periodic plates without any complicated mathematical derivation. The new numerical implementation is based on the rigorous mathematical foundation of the asymptotic homogenization method, and also simplicity similar to the representative volume method. It can be applied easily using commercial software as a black box. Different kinds of elements and modeling techniques available in commercial software can be used to discretize the unit cell. Several numerical examples are given to demonstrate the validity of the proposed methods.     

The non-linear response of transversely-loaded folded plate structures

The folded-plate or “prismatic shell” structure has often provided in the past an economical solution to the problem of roofing large clear areas in a wide variety of building types including hangars for aircraft, auditoriums, gymnasiums, factories, supermarkets and similar major structures. Previous attempts to solve the stress analysis problem for this type of structure employed certain approximations or “linearizations”, which have proven to be satisfactory for the purpose of predicting stresses but not entirely satisfactory for the purpose of predicting deflections. Test results from ten aluminum model specimens, which indicate that deflections occurring in folded plate structures in response to the application of transverse loads are non-linear, are summarized herein. In addition, a new analytical approach based on the principle of minimum potential energy and utilizing the Rayleigh-Ritz method is proposed herein which specifically treats the described non-linear deflection response problem.     

Nonlinear Lamb waves in plate/shell structures

鉴于常规超声检测技术对分布式材料细微损伤和接触类结构损伤的检测效果不佳,近年来非线性超声技术逐渐引起广泛关注.超声波在板壳结构中通常以兰姆波的形式进行传播,然而由于兰姆波的频散及多模特性,使得非线性兰姆波的理论和实验研究进展缓慢.本文从经典非线性理论出发,总结了源于材料固有非线性诱发的非线性兰姆波的理论和实验两个方面的研究进展,并综述了兰姆波的二次谐波发生效应在材料损伤评价方面的若干应用;从接触声非线性理论出发,讨论了目前由于接触类结构损伤诱发的非线性兰姆波的研究现状.最后展望了非线性兰姆波的未来研究重点及发展趋势.     

板壳结构中的非线性兰姆波

鉴于常规超声检测技术对分布式材料细微损伤和接触类结构损伤的检测效果不佳,近年来非线性超声技术逐渐引起广泛关注.超声波在板壳结构中通常以兰姆波的形式进行传播,然而由于兰姆波的频散及多模特性,使得非线性兰姆波的理论和实验研究进展缓慢.本文从经典非线性理论出发,总结了源于材料固有非线性诱发的非线性兰姆波的理论和实验两个方面的研究进展,井综述了兰姆波的二次谐波发生效应在材料损伤评价方面的若干应用;从接触声非线性理论出发,讨论了目前由于接触类结构损伤诱发的非线性兰姆波的研究现状.最后展望了非线性兰姆波的未来研究重点及发展趋势.     

An equivalent classical plate model of corrugated structures

An equivalent classical plate model of corrugated structures is derived using the variational asymptotic method. Starting from a thin shell theory, we carry out an asymptotic analysis of the strain energy in terms of the smallness of a single corrugation with respect to the characteristic length of macroscopic deformation of the corrugated structure. We obtained the complete set of analytical formulas for effective plate stiffnesses valid for both shallow and deep corrugations. These formulas can reproduce the well-known classical plate stiffnesses when the corrugated structure is degenerated to a flat plate. The extension–bending coupling stiffnesses are obtained the first time. The complete set of relations are also derived for recovering the local fields of corrugated structures.     

On a type of radially retractable plate structures

The radially retractable plate structures (RPS) are a family of new retractable structures consisting of a set of cover plates connected by revolute hinges. The concept evolves from the foldable bar structures (FBS) by replacing the beams in an FBS with cover plates. With a single degree of freedom, the structures close to form a covered enclosure and expand to reveal a large central opening space, which makes them ideal for use as retractable roofs. In designing the plates of an RPS, two primary requirements have to be met. First, the boundary of these plates must be designed such that they form an enclosure without any gaps or overlaps in both closed and open configurations, and the plates do not interfere with each other during deployment. Second, all of the pivots of a beam must remain within the boundary of its corresponding RPS plate. This paper tackles the second problem. To meet the requirement, an analytical method is proposed. A feasible design area, defined by closed and open angles of the corresponding FBS, can be identified under this approach. The designers are no longer limited to use the empirical or numerical means to determine whether all of the pivots of a multi-angulated beam are within its corresponding plate. The analytical approach can be used regardless of the boundary’s profile. The approach can be extended into both symmetrical and non-symmetrical structures. Physical models built to validate our approach have shown that the analysis is correct.     

Model B-I for macro rectangular element and problems of influence surface for plate structures

Model B-I for marco rectangular element is presented for the first time in this paper. To establish the influence surface for resultant R of bending plates, a number of generalized distributive loads q are defined. It is shown by numerical examples that Model B-I and the formula for the generalized distributive loads advanced in this paper are featured by high accuracy, low memory space and flexibility in practical application, and that they are especially effective for plate structures subject to moving loads, such as the two-dimensional continuous plates of highway bridges and the flat stabs in piled jetty engineering.     

Stochastic boundary element method for reliability analysis of plate and beams composite structures

I.IntroductionTheengineeringstructuresareoftells,'1>:'rectcdtotheactionofthestochasticloadingthatvarieswiththetime,forexample,theengineeringstructuresactedonbytheearthquake,theoceanstructuresactedonbydynamicpressure,andthevehiclesofthetransportationinflue…     

板结构计算模型的新发展

现代复合材料层合板具有高强和轻型的突出优点,从而在军工和民用等诸多领域发挥着重要作用。这种板结构的特点是随着纤维走向的不同,层间材料的物理-力学特性发生剧烈变化。沿板厚方向变形的梯度比较陡峭,并在层间结合面处发生强不连续,呈现zig-zag （锯齿状）现象。这导致横向剪应变在板的静态和动态响应中发生重要作用,不计横向变形的经典组合板计算模型CLPT难以适应现代多层板计算分析的需要。考虑横向剪切变形影响的板的计算模型得到重视和发展。需要指出,现有各种考虑剪切变形影响的计算模型虽然有了很大的发展,但在全面和准确性上仍然存在一定的不足,难以适应现代多层组合板横向力和物理性能多变的情况。模型预测的沿板厚方向位移和应力的变化规律难以通过严格的检验。本文提出的以比例边界有限元为基础的正交各向异性板的数值计算模型,同时可适用于各种薄板与厚板的分析,对现代复合材料层合板的分析具有特殊的优越性。所得到的板的位移、正应力和剪应力沿板厚方向的变化,与三维弹性理论的标准解高度吻合。数值算例进一步表明,随着层间纤维走向的变化,板内位移场和应力场沿板厚方向剧烈变化所呈现的锯齿现象均可以精准地进行模拟。据此,本文建议方法对现代板分析的广泛适应性和高度准确性得到了充分论证。     

Parameter identification for cracks in elastic plate structures based on remote strain fields

A method for the detection of cracks in plate structures is presented. In contrast to most of the common monitoring concepts taking advantage of the reflection of elastic waves at crack faces, the presented approach is based on the strain measured at different locations on the surface of the structure. This allows both the identification of crack position parameters, such as length, location and angles with respect to a reference coordinate system and the calculation of stress intensity factors (SIF). The solution of the direct problem is performed on the basis of the BFM (body force method). The inverse problem is solved applying the particle swarm optimization (PSO) algorithm. The BFM is based on the principle of linear superposition which allows the calculation of the strain field in a cracked body. The strain at an arbitrary point in the structure is replaced by the strain provided by body force doublets in the uncracked structure. The doublets as well as external loads are parameters which have to be determined solving the inverse problem by minimizing a fitness function, which is defined by a square sum of residuals between measured strain distributions and computed ones for an assumed crack. The PSO algorithm applied to the fitness function operates on the basis of a swarm of candidate solutions. Once knowing loading and crack parameters, the SIF can be determined.     

On reinforcing effect of birefringent coatings on plate structures

A mathematical model is introduced and used to obtain a correction factor of the reinforcing effect of birefringent coatings on structures. The bending effect introduced to the coated structure due to the shifting of neutral surface is considered in the derivation of the correction factor. The photoelastic data corrected by this correction factor are found to be in good agreement with the theoretical calculations.