Lightweight carbon fibre rods and truss structures

Lightweight carbon fibre rods and truss structures are of growing importance for modern transportation technologies. The struts of such frameworks are commonly designed as fibre-wound CFRP tubes. Here CFRP sandwich rods are an advantageous alternative. They have a lightweight foam core covered by a relative thin layer of composite material. In many real applications, however, the superior mechanical properties of such struts can only be utilized with appropriate load transfer elements. Two types of load transfer elements designed for high tensile and compressive loads with a simple screw connection will be described. The framework structures discussed in this paper refer to framework beams which have, in their simple version, cross-sections of an equilateral triangle. To realise a single point support at the ends of the beam-like truss, the rail struts must converge into a conical structure. The presented structures are connection designs without any metallic elements. An outstanding application of CFRP-trusses, rod connectors, and the mentioned formlocking load transfer elements can be found in the advanced lightweight structure of a recently developed semi-rigid airship, the Zeppelin NT.     

Effect of Material and Geometry on Crushing Behaviour of Laminated Conical Composite Shells

This paper examines the effects of the material and structural geometry on the crushing behaviour, energy absorption, failure mechanism and failure mode of circular conical composite shell. The static crushing behaviour of circular conical composite shell under uniform axial compressive load has been investigated experimentally. The cone vertex angles used were 0, 6, 12 and 18 degrees. The cone vertical length and bottom outer diameter were kept for all the cases as 100 and 110 mm, respectively. Failure modes were examined using several photographs taken during the crushing stages for each specimen. Results obtained from this investigation showed that the initial failure was dominated by the interfacial and shear failure, while the delamination and eventually fibre fracture were dominated the failure mechanism after the initial failure. It has also found that the static crushing behaviour of the circular conical shell is very sensitive to the change in the vertex angle. Reinforcement type greatly affects the energy absorption of the circular conical and cylindrical shells.     

On the energy absorption capability of axially crushed composite elliptical cones

The behaviour of composite elliptical thin walled cones subjected to quasi-static axial crushing is examined experimentally. Series of experiments were performed for composite elliptical cones with the same ellipticity ratio and different vertex angles ranging from 0° to 24°. Experiments showed that the catastrophic failure mode is avoided by deviating from the elliptical tubular shape to the elliptical conical one. A significant enhancement in load carrying capacity and energy absorption capability was seen in the case of elliptical conical shells.     

Performance analysis of fibre metal laminated thin conical frusta under axial compression

The crushing behaviour and energy absorption capacity of frustrated conical shells made-up of bare aluminium (AC) and E-glass fibre/epoxy resin composite overwrapped aluminium (CWAC) was studied under quasi-static axial compression condition. Using spinning process, the hollow frustrated conical specimens were fabricated with the help of wooden conical shaped mandrill with semi apical angles of 16° and 21°. Thin commercial aluminium sheets of average thickness 0.87 mm were obtained for making aluminium conical specimen. CWAC frusta were fabricated by wrapping glass fibre/epoxy resin over aluminium conical shell to form hybrid composite with required thickness by hand layup process. Quasi-static axial compression load was applied over top end of the specimen with cross head speed as 2 mm/min using Universal Testing Machine (UTM). From the experiment results, the load–deformation characteristics of different AC and CWAC frusta were investigated. Energy absorption capacities or crashworthiness and mode of collapse of all models of AC and CWAC are determined from load–deformation curve and the same was validated with finite element analysis package ABAQUS®.     

大型复合材料夹芯筒屈曲分析中芯材剪切变形与壳体锥度的影响

针对某卫星结构中的大型复合材料夹芯承力筒进行屈曲分析。首先分别建立柱壳段、锥壳段和框段的几何方程、物理方程与应变能表达式,以及给定载荷条件下的外力势能表达式。然后根据位移边界条件采用正交完备的三角级数构造承力筒的位移模态,最后根据最小势能原理求解临界载荷。通过对比计及与不计及芯材剪切变形和壳体锥度时临界载荷的计算结果,发现对于该承力筒而言,芯材剪切变形与壳体锥度对临界载荷计算结果的影响均小于10%。该分析结果为承力筒在设计阶段的简化计算提供了理论依据。     

复合材料层合扁锥壳的冲击屈曲

研究了计及横向剪切的对称铺设正交异性复合材料层合扁锥壳在三角脉冲载荷作用下的非线性动力屈曲问题；通过在复合材料层合扁锥壳非线性稳定性的基本方程中增加横向转动惯量项并引入三角脉冲冲击力，得到了层合扁锥壳的冲击控制方程，采用Galerkin方法得到以顶点挠度表达的冲击响应方程，并用Runge—Kutta方法进行数值求解，应用B—R准则确定冲击屈曲的临界荷载；讨论了壳体几何参数、物理参数和边界条件对复合材料层合扁锥壳冲击屈曲的影响。     

一个适合于复合材料层合板分析的应力杂交单元

本文根据修正的余能变分原理构造了一个适合于复合材料层合板特点的三角形单元。此单元能够考虑横向剪切变形的影响和局部扭曲效应。三角形单元的三个顶点取为节点,每个节点具有5个自由度,三个位移自由度,二个转角自由度。文中用此单元计算了几种层合板的固有频率,并将结果与解析解进行了比较。计算表明,此单元计算精度较高,应用方便。     

具有夹层的正交异性复合材料圆锥壳体的非线性轴对称屈曲分析

研究均布载荷作用下边缘可移夹支具有正交异性复合材料表层和软夹心的夹层圆锥壳非线性轴对称屈曲问题.利用变分原理导出具有正交各向异性表层夹层圆锥壳在均匀外压作用下的非线性轴对称屈曲问题的基本方程,采用修正迭代法[8]求得了可移夹支边界条件下壳体临界屈曲载荷的解析表达式,对几种典型的纤维增强复合材料表层夹层圆锥壳给出了数值结果和图表,讨论了几何参数和物理参数对临界屈曲载荷的影响。     

Buckling of conical composite shells

A semi-analytical approach is proposed to obtain the linear buckling response of conical composite shells under axial compression load. A first order shear deformation shell theory along with linear strain-displacement relations is assumed. Using the principle of minimum total potential energy, the governing equilibrium equations are found and Ritz method is applied to solve them. Parametric study is performed by finding the effect of cone angle and fiber orientation on the critical buckling load of the conical composite shells.     

The stability of composite material stiffened conical shells under axial compression

The stability of composite material stiffened conical shells under uniform axial compression and with classical clamped boundary conditions is investigated. The effects of stiffeners are uniformly distributed over the whole surface of the shell, and the shell is treated as an equivalent orthotropic shell. A method of solution is developed by using energy principles and Rayleigh-Rize approximations. It is shown that the approach proposed in this paper is practical, effective and satisfactory.     

温度-机械混合载荷作用下先进复合材料格栅加筋截顶圆锥壳体的屈曲分析

基于均匀化原理,推导了考虑沿格栅加筋圆锥壳体随母线变化的等效刚度阵和等效热膨胀系数,并采用前屈曲薄膜理论给出了在温度和均布外压载荷作用下格栅加筋圆锥截顶壳体稳定性分析的总势能表达式。基于最小势能原理得到了该壳体总体失稳的临界载荷值解析表达式,对典型复合材料格栅加筋截顶圆锥壳体的稳定性计算结果与有限元法所得结果相比较,验证了本文中方法的适用性。基于文中提出的方法,通过对不同温度条件下具有不同顶锥角复合材料格栅加筋截顶圆锥壳体热-力屈曲分析结果的讨论,指出温度对复合材料格栅加筋截顶圆锥壳体稳定性的影响程度将随其顶锥角增加而增大。     

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.     

Utilization of composite’s tensile properties for energy absorbing systems

This idea was inspired by the fact that huge energy absorption capability can be achieved if the longitudinal properties of composite structures are being utilized. This can only be done if the energy absorbing system postfailure scenario is mitigated to be tearing failure mechanism. In this paper, friction, crazing, circumferential expansion and tearing failure mechanisms were introduced and applied successfully to control the response of energy absorbing system to the applied load. The system was designed by slipping a solid cone into composite cone. The semi-cone angles used were 8°, 12°, 16° and 20°. The cone height and bottom diameter were kept constant for all cases as 100 mm and 76.2 mm, respectively. Force–stroke curves and deformation histories of typical specimens are presented and discussed. The results demonstrated that at first crush stage the energy is dissipated in the form of friction and crazing and the conical responded in an elastic manner, while the post-crush stage is dominated by tearing failure mechanisms.     

Crushing response of foam-filled conical tubes under quasi-static axial loading

Foam-filled thin-walled tubes are considered to be desirable energy absorbers under axial loading due to their higher energy absorption compared with empty tubes. This paper treats the axial crushing and energy absorption response of foam-filled conical tubes under quasi-static axial loading, using non-linear finite element models. Influence of important parameters such as wall thickness, semi-apical angle and density of foam filler was investigated and the results highlight the advantages of using foam-filled conical tubes as energy absorber. Results also indicate that the crush and energy absorption performances of conical tubes are significantly enhanced by foam filling. The primary outcome of the study is new research information and development of empirical relations which will facilitate the design of foam-filled conical tubes as energy absorbers in impact applications.     

蜂窝锥壳结构动力学特性试验研究

通过模态试验对蜂窝结构锥壳结构的纵向固有频率和阻尼比进行了识别,并与数值仿真结果进行了对比。通过对比发现:复合材料以及蜂窝结构的应用能够降低该结构的固有频率并增加阻尼比;通过振动台试验,对锥壳-质量系统的动力学响应进行了研究,试验结果表明该锥壳结构对于85Hz以上的振动能够有效隔离,从锥壳下端至上端振动幅值衰减大于40%,具有理想的隔振效能。     

Influence of fibre taper on the work of fibre pull-out in short fibre composite fracture

A model has been formulated to determine the work of pull-out, U, of an elastic fibre as it shear-slides out of a plastic matrix in a fractured composite. The fibres considered in the analysis have the following shapes: uniform cylinder and ellipsoidal, paraboloidal or conical tapers. Energy transfer at the fibre–matrix interface is described by an energy density parameter which is defined as the ratio of U to the fibre surface area. The model predicts that the energy required to pull out a tapered fibre is small because the energy transfer at the fibre–matrix interface to overcome friction is small. In contrast, the pull-out energy of a uniform cylindrical fibre is large because the energy transfer is large. The pull-out energies of the paraboloidal and ellipsoidal fibres lay between those for the uniform cylindrical and the conical fibres. With the exception of the uniform cylindrical fibre which yields a constant energy density, tapered fibres yield expressions for the energy density which depend on the fibre axial ratio, q. In particular, the energy density increases as q increases but converges at large q. By defining the critical axial ratio, q ₀, as the limit beyond which u is independent of the fibre slenderness, our model predicts the value of q ₀ to be about 10. These results are applied to explain the mechanisms regulating fibre composite fracture.     

复合材料管状结构的能量吸收性能

复合材料结构作为防撞结构的能量吸收元件,已经逐步得到工程领域的广泛认同。复合材料结构的能量吸收能力是一种特殊要求的力学性能,它的研究目标和研究手段有其独特之处。综述了近年来复合材料管状结构能量吸收性能研究的最新进展,以及有待进一步研究的问题。     

X-ray tomographic observations of microcracking patterns in fibre-reinforced mortar during tension stiffening tests

For the last decades, new reparation or fabrication processes have been studied to replace traditional rebar by roving of different mineral or organic fibres to avoid corrosion issues. Such materials refer to the family of cementitious composite. Their tensile strength would directly depend on the proportion of reinforcement and strongly on the interfacial mechanical properties between fibres and cementitious matrix. From now, evaluation of interfacial properties was mostly limited to the use of force–displacement curves obtained from mechanical experiments. This work presents a new methodology using micromechanical tension stiffening tests combined with X-ray computed tomography (XRCT) observations, performed at the Anatomix beamline at Synchrotron SOLEIL, and specific image processing procedures. Multi-XRCT acquisitions with suitable scanning strategy are used to image the whole fibre-matrix interface along centimetric samples at four to five different levels of loading magnitude. Intensive image processing is then performed on tomographic images including digital volume correlation (DVC), image subtraction and Hessian-based filtering. This experiment allows to study damage mechanisms at small scale. The proposed methodology shows great potential to provide both qualitative and quantitative elements on interfacial mechanical behaviour such as crack growth and crack orientation. The interface between mortar and sufficiently small multi-fibre yarn used in this paper is shown to behave in certain condition as traditional rebar interface producing conical cracks in the surrounding matrix rather than debonding in mode 2, permitting a much higher energy dissipation during debonding. According to this study, conical cracks repartition and geometry are mostly influenced by the cementitious matrix. The spacing between cracks goes from 50 to 100 μm, and the angle between crack normal vector and yarn orientation goes from 35° to 50°.     

Buckling analysis and design of anisogrid composite lattice conical shells

Composite lattice anisogrid shells have now become a popular choice in many aerospace applications. Their use in various structural components, such as rocket interstages, payload adapters for spacecraft launchers, fuselage components for aerial vehicles, and parts of the deployable space antennas requires the development of more advanced finite-element models and analysis techniques capable of predicting buckling behaviour of these structures under variety of loadings. A specialised finite-element model generation procedure (design modeller) is developed and applied to the buckling analysis of the composite anisogrid conical shells treated as three-dimensional frames composed of the curvilinear ribs made of unidirectional composite material. Featuring a dedicated control procedure for positioning the beam elements, the design modeller enables a close approximation of the original twisted geometry of the curvilinear ribs. The parametric finite-element buckling analyses of the anisogrid conical shells subjected to axial compression, transverse bending, pure bending, and torsion showed the robustness and potential of the modelling approach. It was demonstrated that the buckling resistance can be significantly enhanced by either increasing the stiffness of a few hoop ribs located in the close proximity to the section with the larger diameter, or by introducing the additional hoop ribs in the same part of the conical shell. The effectiveness of the design analyses is demonstrated using particular examples. It has been shown that the resultant optimised designs can produce up to 22% mass savings in comparison with the non-optimised lattice shells.     

Nucleation and growth of surface microstructures on Nd+:YAG laser ablated elastomer/carbon composite

Pulsed laser, such as the second harmonic emission of a Nd⁺:YAG laser at 532 nm, is used for surface modification of composite material consisting of elastomer and carbon black. Morphology and chemical compositions of the composite were probed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), respectively. Microstructures such as conical, flat conical and dome-like structures were produced on the ablated surface at a pulse energy of 23 mJ/pulse and pulse width of 10 ns. For each of these microstructures, the origin of the ablated process is particularly addressed and a model describing their nucleation and growth process is proposed.