索－膜结构中的矩形平片协调五力模型

本文提出了一种边界位移线性变化的矩形平片的五力模型，它既具有不包含刚体位移及未知量减少至最低限度的优点，又保证了相邻平片之间位移的连续性，适用于索－膜组合体系的力学分析及形状判定问题，也适用于空间组合网架的力学分析问题。     

Anisogrid composite lattice structures - Development and aerospace applications

The paper is an overview of the recent Russian experience in development and applications of Anisogrid (Anisotropic Grid) composite lattice structures. Anisogrid structures have the form of cylindrical (in general, not circular) or conical shells and consist of a dense system of unidirectional composite helical, circumferential and axial ribs made by continuous filament winding [1] and [2].High weight and cost efficiency of Anisogrid structures is provided by high specific (with respect to density) strength and stiffness of unidirectional ribs used as the basic load-carrying elements of the structure and by automatic winding process resulting in low-cost integral structures. Anisogrid structures proposed about 30 years ago are under serial production in Central Research Institute of Special Machinery (CRISM) which develops lattice interstages, payload attach fittings (adapters) and spacecraft structures for Russian space programs. By now, about 40 successful launches have been undertaken with Anisogrid composite lattice structures.The paper provides the information about fabrication processes, design and analysis methods, mechanical properties of the basic structural elements and application of Anisogrid composite design concept to aerospace structures.     

Formation of Network and Cellular Structures by Viscoelastic Phase Separation

Network (sponge) and cellular structures are often seen in various types of materials. Materials with such structures are generally characterized by light weight and high mechanical strength. The usefulness of such materials is highlighted, for example, by the remarkable material properties of bone tissue, which often has a highly porous structure. In artificial materials, plastic and metallic foams and breads have such structures. Here, we describe a physical principle for producing network and cellular structures using phase separation, and its potential applications to the morphological control of materials spanning from soft to hard matter.     

Dynamic and static modelling of piezoelectric composite structures using a thermal analogy with MSC/NASTRAN

As a first step towards the development of simplified integrated computational tools for the dynamic and static modelling of smart structures, this work validates both theoretically and experimentally the implementation of a multilayered three-dimensional model based on the analogy between thermal strains and piezoelectric strains under MSC/NASTRAN. To assess the piezoelectric–thermal analogy for different loading conditions, the numerical results obtained from this model are first compared to the results obtained from a finite element reference model based on a three-dimensional piezoelectric formulation. An experimental assessment is also conducted on a clamped AS4/3501-6 carbon/epoxy composite beam structure excited in the vicinity of the clamped end using an embedded piezoelectric actuator. Results obtained from the dynamic response of the structure show that the properties of the insulating layer appear to have an important effect and thus demonstrate the need for their modelling. In the last part of the paper, as a tool for further development of the computational tools for smart structures, the piezoelectric–thermal analogy model is used with a large number of three-dimensional elements to describe the complexity of the strain and stress fields in the vicinity of the active region.     

Free vibration analysis of reinforced thin-walled plates and shells through various finite element models

Abstract

This article deals with free vibration analysis of thin-walled structures reinforced by longitudinal stiffeners using refined one-dimensional (1D) models.The 1D theory, which is used in the present article, has hierarchical features and it is based on the Carrera Unified Formulation (CUF). The displacement field over the cross section is obtained by means of Taylor (TE) or Lagrange (LE) expansions. Finite element (FE) method is applied along the beam axis to obtain weak form solutions of the related governing equations. The obtained results are compared with those from classical finite element formulations based on plate and shell (2D), beam (1D), and solid (3D) elements that are available in commercial software. When solid formulation is used to build the FE solutions, stringers and skin are modeled with only 3D elements while, in the 2D-1D FE models, shell and beam elements are used for skin and stringers, respectively. Three benchmark problems are analyzed: a flat plate, a curved panel, and a thin-walled cylinder. When TE models are used, different orders of expansion, N, are considered, where N is a free parameter of the formulation. As far as Lagrange expansions are concerned, four-node (LE 4) and nine-node (LE 9) elements are used to build different meshes on the cross section. The results show that the present 1D models are able to analyze the dynamic behavior of complex structures and can detect 3D effects as well as very complex shell-like modes typical of thin-walled structures. Moreover, the 1D-CUF elements yield accurate results with a low number of degrees of freedom.     

索－膜结构中的矩形平片协调五力模型

本文提出了一种边界位移线性变化的矩形平片的五力模型，它既具有不包含刚体位移及未知量减少至最低限度的优点，又保证了相邻平片之间位移的连续性，适用于索－膜组合体系的力学分析及形状判定问题，也适用于空间组合网架的力学分析问题。     

Comparison of the energy absorption capability of crash boxes assembled by spot-weld and continuous joining techniques

The design of vehicle front structures for crashworthiness is nowadays commonly based on a series of rigid subsystems that constitute a nearly undeformable survival cell for the passengers, and deformable subsystems able to efficiently dissipate the vehicle kinetic energy. During frontal crash, which is by far the most dangerous impact situation, the front rails represent the main deformable components aimed to dissipate the kinetic energy of the vehicle, therefore their behaviour is crucial to obtain good vehicle performance, with stable and controlled energy dissipation. The design of the front rail, usually consisting of a thin-walled prismatic column, requires definition of the geometry, that is, of the shape and dimensions of the cross section, of the thickness of the material, and of the material itself.     

Fault Tolerant Suffix Trees

Classical algorithms and data structures assume that the underlying memory is reliable, and the data remain safe during or after processing. However, the assumption is perilous as several studies have shown that large and inexpensive memories are vulnerable to bit flips. Thus, the correctness of output of a classical algorithm can be threatened by a few memory faults. Fault tolerant data structures and resilient algorithms are developed to tolerate a limited number of faults and provide a correct output based on the uncorrupted part of the data. Suf- fix tree is one of the important data structures that has widespread applications including substring search, super string problem and data compression. The fault tolerant version of the suffix tree presented in the literature uses complex techniques of encodable and decodable error-correcting codes, blocked data structures and fault-resistant tries. In this work, we use the natural approach of data replication to develop a fault tolerant suffix tree based on the faulty memory random access machine model. The proposed data structure stores copies of the indices to sustain memory faults injected by an adversary. We develop a resilient version of the Ukkonen’s algorithm for constructing the fault tolerant suffix tree and derive an upper bound on the number of corrupt suffixes.     

The design of advanced aircraft structures to resist acoustic fatigue; work in progress

Details of the current research activities in the Department of Aeronautics and Astronautics, on the response of advanced aircraft structures to acoustic loading, are presented. Both box type and sandwich structures, employing aluminium alloy, carbon fibre reinforced plastic, and GLARE composite materials, are being investigated. In order to develop design guidelines for these complex structures, it is necessary to combine theoretical predictions, using the finite element method, with experimental measurements of the structural response to random acoustic loading. Both types of structure will be tested in the Progressive Wave Tube facility at Southampton. In addition, it is hoped that a more comprehensive damping guide will be produced for the type of structure used in advanced aircraft design.     

Hybrid approach in bird strike damage prediction on aeronautical composite structures

This paper deals with the problem of numerical prediction of bird strike induced damage on aeronautical structures. The problem of soft body impacts has been tackled by applying a hybrid Eulerian Lagrangian technique, thereby avoiding numerical difficulties associated with extensive mesh distortion. Eulerian modeling of the bird impactor resulted in a more realistic behavior of bird material during impact, which has lead to an enhanced response of the impacted structure. The work presented in this paper is focused on damage modeling in composite items of aeronautical structures. The bird impactor model and damage modeling approaches have been validated by comparison with experimental gas gun results available in the open literature, while the complete damage prediction procedure has been demonstrated on a complex airplane flap structure finite element model.     

Static analysis of reinforced thin-walled plates and shells by means of finite element models

In this paper, variable kinematic one-dimensional (1D) structural models have been used to analyze thin-walled structures with longitudinal stiffeners and static loads. These theories have hierarchical features and are based on the Carrera Unified Formulation (CUF). CUF describes the displacement field of a slender structure as the product of two function expansions, one over the cross-sectional coordinates, Taylor (TE) or Lagrange (LE) expansions were used here, and one along the beam axis. The results obtained using the refined 1D models have been compared with those from classical finite element analyses that make use of plates/shells and solids elements. The performances of classical and refined structural models have been compared in terms of accuracy and computational costs. The results show that the use of the LE over the cross-section allows the strain/stress fields to be evaluated accurately for all the structural components. The comparisons with the results obtained using the classical models highlight how, the use of 1D refined models, allows the number of degrees of freedom (DOF) to be reduced, meanwhile, the accuracy of the results can be preserved.     

Probabilistic analysis of truss structures with uncertain parameters (virtual distortion method approach)

A new approach for probabilistic characterization of linear elastic redundant trusses with uncertainty on the various members subjected to deterministic loads acting on the nodes of the structure is presented. The method is based on the simple observation that variations of structural parameters are equivalent to superimposed strains on a reference structure depending on the axial forces on the elastic modulus of the original structure as well as on the uncertainty (virtual distortion method approach). Superposition principle may be applied to separate contribution to mechanical response due to external loads and parameter variations. Statically determinate trusses dealt with the proposed method yields explicit analytic solution in terms of displacements while redundant trusses have been studied by means of an asymptotic expansion exhibiting explicit dependence on parameter fluctuations. Probabilistic characterization of the response may then be obtained both for statically determinate and statically indeterminate stochastic trusses.     

大跨高耸柔性结构的风致振动

我国的改革开放政策带来了大跨度桥梁和高耸结构建设的高潮。随着结构跨度和高度的增大,风对这些柔性结构的作用就不限于静力风荷载,而必须考虑各种风致振动问题,以及由此引起的动力风效应。 国际风工程界对结构风效应的研究已着重于非线性风致振动的精细化,结构气动参数的精确识别以及为风洞试验所需的紊流风场的真实模拟。结构抗风设计也正从确定性方式向基于可靠性理论的概率性评价过渡。 为了使我国结构风工程领域的研究全面赶上世界先进水平,并为解决进入21世纪的特大跨度桥梁和超高层建筑中的风工程问题作好准备,本文提出了一些迫切需要进行的研究热点。     

Troublesome Crystal Structures: Prevention,Detection, and Resolution

A large number of incorrect crystal structures is being published today. These structures are proving to be a particular problem to those of us who are interested in comparing structural moieties found in the databases in order to develop structure-property relationships. Problems can reside in the input data, e.g., wrong unit cell or low quality intensity data, or in the structural model, e.g., wrong space group or atom types. Many of the common mistakes are, however, relatively easy to detect and thus should be preventable; at the very least, suspicious structures can be flagged, if not by the authors then by the referees and, ultimately, the crystallographic databases. This article describes some of the more common mistakes and their effects on the resulting structures, lists a series of tests that can be used to detect incorrect structures, and makes a strong plea for the publication of higher quality structures.     

Three decades of composites activities at US Air Force Materials Laboratory

In the decades of 1960s–1980s, Air Force Materials Laboratory pioneered many programs and training of personnel that led to major advances in the understanding and utilization of composite materials and structures. The early involvement of industry resulted in rapid identification of the advantages and technical hurdles to be overcome. Through extensive use of postdoctoral appointments, ROTC at universities, visiting and exchange scientists, the Laboratory was able to reach many talented young people who in the years following have assumed leadership in teaching and research of composite materials worldwide. A recount of the activities during these three decades is timely because most of the “young” men and women who had spent time at the Laboratory will be ready to retire in the next few years.