考虑刚度特性的零热膨胀复合材料高许用温变设计

材料的宏观零热膨胀可以通过两种不同的正热膨胀材料在单胞尺度上的复合实现。该类材料虽然能在较大温度波动环境下保持较高的几何稳定性,但两种材料之间过大的热应力很容易导致材料失效,从而限制其许用温度变化范围(简称许用温变)。为此,提出以单位温升的最大热应力作为零热膨胀材料许用温变的衡量指标,通过解析和有限元数值仿真两种方法,对三种典型弯曲变形机制的零热膨胀材料进行许用温变和刚度特性分析,揭示了单胞可设计参数对其性能的影响规律。结果表明:在满足零热膨胀条件下,通过合理的单胞结构设计和选材设计,可以实现刚度与许用温变双目标共赢。      

Temperature dependence of electrical resistance of alloys caused by dynamic and static disorders

The results are presented of the experimental (in situ) measurements of electrical resistance and thermal expansion of electron compositions and their mechanical copper- and zinc-based mixtures, as well as of stainless steels within the temperature range of 300–1000 K. From the correlation analysis of these properties, the thermal deformation of the atomic lattice is shown to play the key role in formation of the electron scattering cross section. The role of the static disorder in the scattering cross section formation in alloys becomes noticeable for lattices with a high atom packing density.     

Fabrication and mechanical characterization of 3D woven Cu lattice materials

3D metallic lattices designed to have two distinctly different material architectures have been woven with metallic Cu wires. A vacuum soldering technique was employed to metallurgically bond the wire nodes and form stiff 3D lattice materials. The structures and mechanical properties of the as-woven and soldered lattices were characterized by optical microscopy and micro-scale mechanical property experiments. The measured in-plane shear stiffness shows good agreement with predictions from finite element (FE) models that account for variations in the manufacturing and solder bonding. The study indicates that stiffness is influenced by the percentage of bonded nodes and the location of bonding. The 3D woven lattice materials manufactured in this study exhibited a very high percentage (80%) of bonded nodes and a unique combination of stiffness and density as compared to that typically reported for ultra lightweight lattice materials.     

含圆币型微裂纹三维编织复合材料的热膨胀系数预报

本文利用Eshelby-Mori-Tanaka 法和改进的刚度平均化方法, 考虑各种编织参数下纤维与微裂纹的相互作用, 理论预报了复合材料的有效热膨胀系数, 比较了不同微裂纹密度对热力学性能的影响, 为工程设计和材料工艺改进提供了理论指导。     

Anomalies in stiffness and damping of a 2D discrete viscoelastic system due to negative stiffness components

The recent development of using negative stiffness inclusions to achieve extreme overall stiffness and mechanical damping of composite materials reveals a new avenue for constructing high performance materials. One of the negative stiffness sources can be obtained from phase transforming materials in the vicinity of their phase transition, as suggested by the Landau theory. To understand the underlying mechanism from a microscopic viewpoint, we theoretically analyze a 2D, nested triangular lattice cell with pre-chosen elements containing negative stiffness to demonstrate anomalies in overall stiffness and damping. Combining with current knowledge from continuum models, based on the composite theory, such as the Voigt, Reuss, and Hashin-Shtrikman model, we further explore the stability of the system with Lyapunov's indirect stability theorem. The evolution of the microstructure in terms of the discrete system is discussed. A potential application of the results presented here is to develop special thin films with unusual in-plane mechanical properties.     

Modeling the process-induced dimensional variations of general curved composite components and assemblies

Dimensional variations are induced during the processing of composite materials. General curved components are commonly used in composite structures. Their performance is affected by the dimensional variations associated with the manufacturing process. This paper presents a piece-wise approach for predicting the dimensional variations of general curved composite components and assemblies. For a general curved composite component, it is first divided into a number of pieces of simple geometry. For each piece, the dimensional variation, i.e. spring-in, is calculated using the effective coefficients of thermal expansion. Based on the dimensional variation of each piece, the dimensional variations of the general curved component are calculated sequentially. This approach was validated against the finite element analysis. It shows that it offers excellent accuracy while avoiding time-consuming numerical computations. Besides general curved components, this approach can also be applied to composite assemblies. It provides the foundation for the tolerance analysis/synthesis of composites.     

Sandwich panels with Kagome lattice cores reinforced by carbon fibers

Stretching dominated Kagome lattices reinforced by carbon fibers were designed and manufactured. The sandwich panels were assembled with bonded laminate skins. The mechanical behaviors of the sandwich panels were tested by out-of-plane compression, in-plane compression and three-point bending. Different failure modes of the sandwich structures were revealed. The experimental results showed that the carbon fiber reinforced lattice grids are much stiffer and stronger than foams and honeycombs. It was found that buckling and debonding dominate the mechanical behavior of the sandwich structures, and that more complaint skin sheets might further improve the overall mechanical performance of the sandwich panels.     

零膨胀材料设计与模拟验证

零膨胀材料对提高航空航天结构和电子设备等的热几何稳定性有重要意义。采用拓扑优化技术设计各相材料在单胞域的分布形式, 以获得零膨胀材料的微结构形式。给出了由二相实体材料和空心构成的各向同性零膨胀材料的设计方案, 讨论了初始设计依赖性问题, 分析了该依赖性的存在原因。采用有限元技术代替实际测试, 分析了所设计材料的试件在均匀温度变化下的变形, 验证了所设计材料的零膨胀(低膨胀) 性质, 说明通过拓扑优化技术设计材料的微结构是设计零膨胀材料的有效方法。     

Equivalent mechanical properties of auxetic lattices from discrete homogenization

Auxetic materials having a network like structure are analyzed in terms of their deformation mechanisms and equivalent homogenized mechanical properties thanks to the discrete asymptotic homogenization method. This systematic and predictive methodology is exemplified for five different 2D periodical lattices: the re-entrant hexagonal, hexachiral, cross chiral, rafters and the re-entrant square. The equivalent moduli and Poisson’s ratio are expressed in closed form versus the microbeam geometrical parameters and rigidities. As a novel result, the predicted homogenized properties depend on the slenderness of the beam, hence providing more accurate results in comparison to the literature. The studied lattices allow to explore the two main mechanisms responsible for negative Poisson’s ratio, the re-entrant and the rolling-up mechanism. Non-standard overall behaviors, such as traction-shear coupling occurring for the cross chiral lattice, are evidenced. Negative values of the Poisson’s ratio are obtained in a certain range of the configuration parameter of each lattice. Comparisons of the obtained homogenized moduli with finite element simulations show a very good accuracy of the predicted effective mechanical behavior.     

Experimental investigation of the thermal properties of tailored expansion lattices

Composite bimaterial lattice structures which possess both low, tailorable thermal expansion and nearly optimal stiffness have been proposed for applications which require high structural stiffness in environments which include large temperature fluctuations, such as the surfaces of high-speed aerospace vehicles. An experimental validation of the thermal properties of these lattices when they are constructed of practical materials with easily manufactured bonded joints is contained herein. Bonded lattices, comprising aluminum and titanium alloys, have been manufactured with press-fit dovetail joints and tested in a variety of thermal environments. Results for equilibrium heating, rapid transient heating and thermal cycling leading to shakedown are presented and shown to be consistent with theoretically and numerically attained results.

Mechanical behaviour of CFRP sandwich structures with tetrahedral lattice truss cores

A method of manufacturing carbon fibre reinforced polymer (CFRP) tetrahedral lattice truss core sandwich structure by thermal expansion silicon rubber mould was developed. The sandwich structure was manufactured integrally without secondary bonding and the silicon rubber mould can be made mass-production with low cost in this approach. The intrinsic property of the CFRP was fully exploited because of carbon fibres aligned in the axial orientation of the truss member. The mechanical properties of CFRP tetrahedral lattice truss core sandwich structures were investigated by flatwise compression and shear test. The experimental results indicate that CFRP tetrahedral lattice truss core sandwich structures have higher weight-specific compressive strength than some metal truss cores, and are competitive with conventional honeycombs.     

双矩形拱肋间的气动干扰效应研究

以某拱桥为例,通过数值模拟研究了串列双矩形拱肋的气动干扰效应,及其对两截面气动力系数的影响。在对计算模型进行验证的基础上,进一步研究了截面宽高比、间距比和来流风攻角对拱肋周围流场的影响,并结合压力云图和湍动能云图解释了气动力系数的变化规律,讨论了不同宽高比截面的漩涡脱落频率与结构自振频率之间的关系,分析了两拱肋升力时程的差异对整体扭矩可能产生的增大效应。结果表明：串列拱肋间的气动干扰效应显著。受上游截面尾流的影响,下游截面的阻力系数明显减小,其值与漩涡的形态、能量大小、移动轨迹等因素密切相关。上、下游截面的升力时程在幅值和相位上存在明显差异,导致拱肋整体的力矩增大,其效应随宽高比或间距比的增大而明显加强,随风攻角的增大而有所降低。漩涡脱落频率随宽高比的增大呈先增大后减小的趋势,而受间距比、风攻角的影响有限。对漩涡脱落频率与宽高比的变化进行多项式拟合,结合结构的模态频率可为拱肋的气动外形设计提供参考。     

92负刚度蜂窝结构压缩性能分析

为准确开展以黏弹性材料为载体的负刚度蜂窝结构数值模拟研究,提出一种基于黏弹性广义Maxwell模型的负刚度蜂窝结构压缩性能数值模拟法。进行了尼龙12的动态机械分析(DMA)测试,基于广义Maxwell模型拟合实测动态模量数据,得到反映尼龙12动态黏弹性的无量纲模量g_i和松弛时间τ_i。建立了负刚度蜂窝结构的有限元模型,基于实测的动态黏弹性参数,进行压缩性能的数值模拟研究,并同压缩试验结果进行比较,验证了数值模拟的准确性,利用数值模拟研究几何参数对结构压缩性能的影响规律。结果表明,基于黏弹性广义Maxwell模型的负刚度蜂窝结构数值模拟分析法可较准确模拟结构的压缩性能,为预测负刚度蜂窝结构的力学性能提供帮助。     

降低堇青石材料热膨胀系数的途径

堇青石材料热膨胀系数低,是优良的高温抗热震材料,但我国目前生产的堇青石材料热膨胀系数还有待进一步提高.综述了降低堇青石材料热膨胀系数的各种途径,包括调整材料的化学矿物组成、优化制备工艺和后期处理等,指出今后发展的方向是利用负膨胀材料开发低膨胀乃至零膨胀的堇青石材料,以期促进优质堇青石材料的研究与开发.     

Design Guideline for New Generation of High-Temperature Guarded Hot Plate

This paper complements the existing measurement standards and literature for high-temperature guarded hot plates (HTGHPs) by addressing specific issues relating to thermal conductivity measurement of technical insulation at high temperatures. The examples given are focused on the designs of HTGHPs for measuring thin thermal insulation. The sensitivity studies have been carried out on major influencing factors that affect the thermal conductivity measurements using HTGHPs, e.g., the uncertainty of temperature measurements, plate flatness and center-guard gap design and imbalance. A new configuration of center-guard gap with triangular shape cross section has been optimized to obtain the same thermal resistance as a 2 mm wide gap with rectangular shape cross section that has been used in the HTGHPs at NPL and LNE. Recommendations have been made on the selections of heater plate materials, high-temperature high-emissivity coatings and miniature temperature sensors. For the first time, thermal stress analysis method has been applied to the field of HTGHPs, in order to estimate the effect of differential thermal expansion on the flatness of thin rigid specimens during thermal conductivity tests in a GHP.     

On the thermal problem for composite beams using a finite element semi-discretisation

The paper deals with steady state thermo-elastic problems in beam-like structures and it is composed of three theoretical sections. The first part presents a two-dimensional finite element procedure to compute the temperature distribution within a beam cross section subjected to prescribed boundary conditions. It allows the beam cross section to be modelled taking into account any kind of thermal anisotropy or inhomogeneity.

The second part is devoted to the structural thermo-elastic problem in a beam having arbitrary non-homogeneous, anisotropic material properties over the cross section but constant along the axis; the extension of a well-known semi-discretisation procedure to take into account anisotropic thermal expansion coefficients is presented. In this way it is possible to compute strains and stresses related to temperature distributions on the cross section computed, using the method outlined in the first part of the paper.

The third part describes the procedure to evaluate thermal equivalent loads suitable for a three dimensional frame analysis.

Some examples are presented and the results are compared either with their theoretical counterparts or with numerical results obtained from a full three-dimensional finite element analysis.     

Damage-induced property changes in composites subjected to cyclic thermal loading

Damage in the form of transverse cracks resulting from thermal loading is studied as it relates to the dimensional stability of flat laminates and stiffness changes in cylindrical tubes. Graphite-epoxy specimens were subjected to cyclic thermal loading in the temperature range −250 to +250°F. It is shown that transverse cracking is the dominant damage mechanism in both types of structural elements. Fiber splitting is also quite common at the low test temperatures. Experimental results indicate that damage significantly reduces the inplane coefficients of thermal expansion of flat laminates and the torsional stiffness of the tubes. Theoretical predictions for coefficients of thermal expansion as a function of crack spacing in flat laminates followed the same trend as experimental results.     

Mechanical response of cellular solids: Role of cellular topology and microstructural irregularity

Rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular microstructures tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and cellular microstructure. The present study reports on the results of a series of computational experiments that explore the effect topology and microstructural irregularity (or non-periodicity) on overall mechanical response of cellular solids. Compressive response of various 2D topologies such as honeycombs, stochastic Voronoi foams as well as tetragonal and triangular lattice structures have been investigated as functions of quantitative irregularity parameters. The fundamental issues addressed are (i) uniqueness of mechanical response in irregular microstructures, and effects of (ii) specimen size, (iii) boundary morphology, (iv) cellular topology, and (v) microstructural irregularity on mechanical response.     

Application of triangular element invariants for geometrically nonlinear analysis of functionally graded shells

This paper is an attempt to construct a computationally effective curved triangular finite element for geometrically nonlinear analysis of elastic shear deformable shells fabricated from functionally graded materials. The focus is on the concise finite-element formulation under the demand of accuracy-simplicity trade-off. To this end, a nonconventional approach based on the invariants of the natural strains of fibers parallel to the element edges is used. The approach allows one to obtain algorithmic formulas for computing the stiffness matrix, gradient, and Hessian of the total strain energy of the finite element. Transverse shear deformation effects are taken into account using the first order shear deformation theory with the shear correction factor dependent on the material property distribution across the shell thickness. The performance of the proposed finite element is demonstrated using problems of functionally graded plates and shells under mechanical and thermal loads.     

Unstructured,curved elements for the two‐dimensional high order discontinuous control‐volume/finite‐element method

Quadrilateral and triangular elements with curved edges are developed in the framework of spectral, discontinuous, hybrid control‐volume/finite‐element method for elliptic problems. In order to accommodate hybrid meshes, encompassing both triangular and quadrilateral elements, one single mapping is used. The scheme is applied to two‐dimensional problems with discontinuous, anisotropic diffusion coefficients, and the exponential convergence of the method is verified in the presence of curved geometries. Copyright © 2014 John Wiley & Sons, Ltd.