Analysis of various thick-walled cross-ply composite cylindrical shells subjected to lateral pressures

A simple and accurate finite cylindrical element method was formulated by the authors to determine stresses and deformations in laminated thick-walled cylindrical composite shells. This method reduces the overall problem to a one-dimensional one. In the previous study it was found that significant tensile radial strains occur near the inner shell surface, when the shell is subjected to external pressures. These radial tensile strains are of the same magnitude as the failure strains for the composite shells for shell dimensions and pressures of practical use. In the present study various stacking sequences are investigated to determine the one resulting in the lowest radial tensile strains. Using this optimum stacking sequence, results are obtained for shells composed of E-glass/epoxy, T300/SP-286 graphite/epoxy, and Kevlar 49/epoxy. In addition, two hybrid composite systems consisting of graphite/epoxy and glass/epoxy are studied.     

基于配筋率包络指标的冷却塔群塔风致干扰准则

风荷载条件冷却塔群塔干扰效应为结构设计关键控制因素。为考虑群塔条件绕流形态改变引起的复杂三维风压分布形式及数值大小变化对结构内力及配筋的影响,现行国内外水工行业规范采用单一的群塔比例系数放大无干扰圆柱扰流简化的二维风压分布。为评价其精度、合理性和经济性,该项研究以某超大型冷却塔六塔典型布置为例,基于风洞试验、结构有限元分析和结构设计配筋方案,选择干扰效应表现明显的代表性受扰塔作为研究对象,分析了塔筒不同高度处的平均风压分布规律并与单塔结果作对比;计算了其在16个风向角20种设计荷载组合下的塔筒子午向外侧、子午向内侧、环向外侧、环向内侧理论配筋量并将其包络线与单塔结果作对比,提出基于配筋包络的沿塔筒高度变化的分项群塔比例系数用于工程实践。研究表明：基于水工荷载规范建议的单一群塔系数难于涵盖干扰效应导致的复杂三维风压分布变化,推荐沿塔高变化的分项比例系数可以兼顾结构设计过程的便捷、经济和合理性。     

The response of composite structures with pre-stress subject to low velocity impact damage

The effect of an initial pre-stress on the response of carbon-fibre/epoxy laminated plates subjected to low velocity impact is investigated. Prior to being impacted, the samples are loaded either uniaxially or biaxially using a specially designed test rig which enables tension or compression loading, independent on each axis. Impact tests were carried out for two impact energies for uniaxial and biaxial tension, pure shear and the zero pre-stress cases. The effect of the pre-stress on the permanent indentation depth, absorbed energy and peak impact loads is experimentally quantified.

The results indicate that the penetration/perforation depth, peak load and absorbed energy are essentially independent of the nature and magnitude of the pre-stress at low levels of impact energy (6 J), becoming more significant at higher levels of impact energy (10 J).     

Reinforcement effects of MWCNT and VGCF in bulk composites and interlayer of CFRP laminates

The reinforcement effects of two nanofillers, i.e., multi-walled carbon nanotube (MWCNT) and vapor grown carbon fiber (VGCF), which are used at the interface of conventional CFRP laminates, and in epoxy bulk composites, have been investigated. When using the two nanofillers at the interface between two conventional CFRP sublaminates, the Mode-I interlaminar tensile strength and fracture toughness of CFRP laminates are improved significantly. The performance of VGCF is better than that of MWCNT in this case. For epoxy bulk composites, the two nanofillers play a similar role of good reinforcement in Young’s modulus and tensile strength. However, the Mode-I fracture toughness of epoxy/MWCNT is much better than that of epoxy/VGCF.     

Creep induced rate effects on radial cracks in multilayered structures

This paper considers foundation and epoxy creep induced loading rate effects on radial cracks in multilayered structures. These include top layers of glass or silicon that are bonded to polycarbonate foundations with epoxy. The creep properties of the epoxy join and the polycarbonate foundation are determined using compression experiments and spring-dashpot models. The measured creep parameters are then incorporated into an analytical mechanics model, and finite element simulations are used to predict the effects of creep on the critical loads for radial cracking at different loading rates. The models suggest that the combined effects of creep and slow crack growth must be considered in the predictions of the critical loads required for radial cracking in the systems containing glass top layers. Since slow crack growth does not occur in silicon, the model considering the creep effect is used to predict the critical loads for radial cracking in the systems containing silicon top layers. In both of the structures, analytical solutions are obtained for bi-layer structures and finite element simulations are used for tri-layer structures. Our results show that the analytical solutions obtained by bi-layer structures provide good estimations for tri-layer structures when the epoxy thickness is less than 100 μm. The predictions obtained for both systems are shown to provide improved predictions by comparing with experimental results reported by Lee et al. [J. Am. Ceram. Soc., 2002, 85(8), 2019–2024]. In both systems, the modeling of join/substrate creep is shown to be important for the accurate prediction of loading rate effects on radial cracking.     

High strain rate compressional behavior of stitched and unstitched composite laminates with radial constraint

This paper is concerned with the high strain rate compressional behaviour of glass/epoxy (Hy-E 9134B, Fiberite, USA) composite laminates with or without stitching reinforcement by untwisted Kevlar-49 threads (1140 denier). The split Hopkinson pressure bar (SHPB) apparatus is used in performing the high strain rate tests. Test data are analyzed in a manner similar to that reported by Hauser Exp. Mech., 6 (1966) 395. Specimens are tested at strain rates up to 10⁴ s⁻¹. Unidirectional laminated parallelepiped samples are impacted along their fiber direction. Their high velocity compressive ductility is observed. Both [0°]₂₄ and [(0°/90°)₆]_S glass/epoxy circular specimens with disc diameters of 10 and 50·8 mm are transversely impacted by an input bar in order to study their high strain rate behavior. Moreover, two sets of stitched circular specimens with disc diameters of 10 and 50·8 mm are also examined. The effect of strain rate and radial constraint on the dynamic properties of stitched and unstitched GFRP laminated specimens and their associated damage patterns are described.     

Design of CNG tank made of aluminium and reinforced plastic

A compressed natural gas storage tank suitable for gas operated vehicles can be made of thin-walled aluminium liners with a glass/epoxy reinforcement overwind. The hybrid construction allows the higher tensile strength of the glass reinforcement to be utilized, whilst giving a structure which weeps before burst and self-health. The advantages of the chosen construction can only be maximized for the optimal wall thickness and optimal stiffness of the composite shell after optimal prestressing of the tank. The purpose of the design task is to optimize the construction. This paper gives the methodology involved in achieving this using analytical techniques. The reliability of the design method together with the advantages of the chosen structural solution are proved by the manufacture and testing of prototype tanks.     

Effect of Shell Coating Techniques on Dynamic Response of Photoconductive Core/Shell Nanorod Arrays

Efficient carrier collection in the core/shell nanowire (nanorod) arrays requires a high quality interface between core and shell materials. A highly conformal shell layer around nanorods can lead to fast dynamic response in photoconductive devices by a radial charge flow. Therefore, choice of the deposition technique for the conformal shell layer becomes crucial. In this study, the dynamic response of indium sulfide (In₂S₃) nanorods/silver (Ag) core/shell devices is compared in which Ag shell layers are deposited by different physical vapor deposition (PVD) techniques. In₂S₃ nanorods are fabricated by glancing angle deposition. The core/shell devices with Ag shell sputtered at a relatively high working gas pressure (≈3 × 10⁻² mbar) produce the highest photocurrent compared to other devices in which more directional incident flux (with working gas pressure of ≈3 × 10⁻³ mbar) is utilized for Ag shell layer. The reduced transit times indicate a conformal shell achieved by the high pressure sputtering technique that has a wide angular distribution flux. In addition, a more directional flux yet with a small angle (≈30°) incidence with respect to the substrate surface normal also helps increase the photocurrent. Such simple and scalable PVD techniques are shown to offer alternative fabrication approaches in producing high quality core/shell nanostructures.     

Measurement of the stress field created within the resin between fibres in a composite material during cooling from the cure temperature

In a long uniaxial glass fibre/epoxy resin composite that is constrained by a rigid outer shell not to change its overall external dimensions, the transverse components of principal stress generated during cooling from the temperature of cure have been measured at various distances from the specimen ends. The radial (compressive) and hoop (tensile) components of stress in the resin between four fibres whose axes define the edge of a prism with square section are in excess of 6 and 100 MPa, respectively. In an identical specimen, 4 days of exposure to distilled water at 80° C was found to give rise to a tensile hoop stress of magnitude sufficient to cause fracture of the rigid outer shell.     

Stress-dependent thermal relaxation effects in micro-mechanical resonators

Thermal relaxation is a key factor in determining the quality factor of micro and nano resonators, which controls the energy dissipation through the coupling of the mechanical and thermal domains. While the literature contains approximate, exact and computational models for quantitative analysis of thermo-elastic coupling, very few techniques are available to ‘tune’ it without changing the material, geometry or operating conditions. In this paper, we develop an analytical model that considers a pre-stress in a flexural resonator to modify the thermal relaxation time and thus increase the quality factor. The effects of length-scale, pre-stress and geometry on the quality factor have been analyzed. The model predicts that significant improvement in terms of dimensionless quality factors is possible by tuning the pre-stress.     

Elasticity, shell theory and finite element results for the buckling of long sandwich cylindrical shells under external pressure

The buckling of a sandwich cylindrical shell under uniform external hydrostatic pressure is studied in three ways. The simplifying assumption of a long shell is made (or, equivalently, ‘ring’ assumption), in which the buckling modes are assumed to be two-dimensional, i.e. no axial component of the displacement field, and no axial dependence of the radial and hoop displacement components. All constituent phases of the sandwich structure, i.e. the facings and the core, are assumed to be orthotropic. First, the structure is considered a three-dimensional (3D) elastic body, the corresponding problem is formulated and the solution is derived by solving a set of two linear homogeneous ordinary differential equations of the second-order in r (the radial coordinate), i.e. an eigenvalue problem for differential equations, with the external pressure, p the parameter/eigenvalue. A complication in the sandwich construction is due to the fact that the displacement field is continuous but has a slope discontinuity at the face-sheet/core interfaces, which necessitates imposing ‘internal’ boundary conditions at the face-sheet/core interfaces, as opposed to the traditional two-end-point boundary value problems. Second, the structure is considered a shell and shell theory results are generated with and without accounting for the transverse shear effect. Two transverse shear correction approaches are employed, one based only on the core, and the other based on an effective shear modulus that includes the face-sheets. Third, finite element results are generated by use of the ABAQUS finite element code. In this part, two types of elements are used: a shear deformable shell element and a solid 3D (brick) element. The results from all these three different approaches are compared.     

Optimal laminations of thin underwater composite cylindrical vessels

This paper deals with the optimal design of deep submarine exploration housings and autonomous underwater vehicles. The structures under investigation are thin-walled laminated composite unstiffened vessels. Structural buckling failure due to the high external hydrostatic pressure is the dominant risk factor at exploitation conditions. The search of fiber orientations of the composite cylinders that maximize the stability limits is investigated. A genetic algorithm procedure coupled with an analytical model of shell buckling has been developed to determine numerically optimized stacking sequences. Characteristic lamination patterns have been obtained. FEM analyses have confirmed the corresponding significant increases of buckling pressures with respect to initial design solutions. Experiments on thin glass/epoxy and carbon/epoxy cylinders have been performed. The measured buckling pressures appear to be in good agreement with numerical results and demonstrate the gains due to the optimized laminations.     

The transient responses of two-layered cylindrical shells attacked by underwater explosive shock waves

An approximation solution is introduced for the dynamic response of a two-layered cylindrical shell of circular cross-section subjected to an underwater explosive shock wave. The solution is obtained within the framework of the Flugge thin shell theory and the reflected-afterflow-virtual-source (RAVS) method is used to account for the fluid–structure interaction. Detailed numerical computations are carried out, in dimensionless form, for the cases of infinitely long two-layered cylindrical shells. Time histories of nondimensional radial velocity, mid-surface strain, 0th mode radial displacement and 1st mode radial velocity are presented in graphical form and the effects of elastic modulus, shell radius and thickness on the transient response characteristics of the shells are investigated.     

Stress and failure analysis of woven composite plates with adhesive patch-reinforced circular hole

The application of a single-sided patch reinforcement to a woven E-glass fiber/epoxy composite panel with a central circular fastener hole is studied using three-dimensional finite element analysis. The width of the panel is 25.4 mm, while three hole diameters (3, 6 and 9 mm) are used in the study. The reinforcement patch is square in shape and is made of either E-glass fiber/epoxy or carbon fiber/epoxy laminae, with the patch-to-panel-thickness varying from 0.1 to 0.7. To simulate the ‘fastened’ condition, the patch-reinforced panel is bolted to a mild steel bar, which is fixed in the direction normal to the panel. One end of the panel is subject to unidirectional tensile load while the other end is under clamped boundary conditions. The through-thickness stress distributions and the failure loads of the patch-reinforced panels are evaluated by finite element analysis. Contact elements are used to account for interaction between contact surfaces. Experiments are also performed to verify the model.The relationships between the patch-to-panel-thickness and the strength of the panel and the material of patch reinforcement are considered and discussed.     

Curvature effects on pressure-induced buckling of empty or filled double-walled carbon nanotubes

Summary The curvature effects of interlayer van der Waals (vdW) forces on pressure-induced buckling of empty or filled double-walled carbon nanotubes (DWNTs) are studied for various radii, length-to-radius ratios, end conditions and internal-to-external pressure ratios. The analysis is based on a double-elastic shell model and assumes that the interlayer vdW pressure at a point between the inner and outer tubes depends not only on the change of the interlayer spacing, but also on the change of the curvatures of the inner and outer tubes at that point. Here the role of filling substances inside DWNTs is modeled by a uniformly distributed internal pressure. The present work aims to study the curvature effects on critical radial pressure. An explicit formula is obtained for the external buckling pressure of empty or filled DWNTs. The critical value of external pressure is estimated with various internal-to-external pressure ratios. It is shown that the curvature effects play a more significant role in buckling problems under radial pressure for small radii DWNTs than under pure axial stress. Our results show that loading transfer through vdW forces prior to buckling is important for the pressure-induced buckling of DWNTs rather than axially compressed buckling.     

Thermosetting epoxy reinforced shape memory composite microfiber membranes: Fabrication,structure and properties

The thermosetting epoxy-based shape memory composite microfibers are successfully fabricated by means of coaxial electrospinning. The PCL/epoxy composite fiber shows core/shell structure, in which epoxy as the core layer is for an enhancing purpose. By incorporating epoxy and PCL, the mechanical strength of composite fibers is greatly reinforced. The deformation is via the heating and cooling process, and the shape memory effect can be demonstrated from the micro level to the macro level. The whole shape recovery performance takes only 6.2 s when triggered by the temperature being at 70 °C. The porosity of woven microfibers changes in response to temperature. In addition, the PCL/epoxy composite microfiber membranes are analyzed in an in vitro cytotoxicity test, which proves that PCL as the shell layer provides the composite microfibers potential capabilities in biomedical science.     

高压密封连接器的设计及研究

为了提高耐压腔室内引出导线在高压环境下密封的可靠性,保证井下工具内使用的电子设备能够在常压下工作,设计了一种注塑类高压密封连接器.使用有限元软件对内部填充材料为橡胶、环氧树脂时的连接器密封性能作了分析,并进行了实验验证.分析和实验结果显示:当填充材料为橡胶时,其密封性能较好,外壳体的半锥角取88°较为合适,但在高压作用下橡胶易被挤出;当填充材料为环氧树脂时,可避免材料挤出现象发生,但在一定密封压力范围内会出现液体泄漏;当2种材料组合使用时,随着环氧树脂比例的增加,填充材料的外凸距离减小,环氧树脂体积比为40%较为合理.高压密封连接器主要靠前5扣螺纹承载,最大应力出现在外壳体的第1扣螺纹与第2扣螺纹之间的小径处;插针的最大应力出现在橡胶与环氧树脂交界处偏上位置.研究成果对高压密封连接器的设计以及内部填充材料的选择具有一定的参考意义.     

波传播法求解低频激励下水中加端板圆柱壳的振动

采用波传播法研究了低频下水中壳体的振动与响应。水中壳体由有限长加流体载荷的圆柱壳和两端的圆形端板组成,其中外部流体载荷用无限长模型进行近似处理。为了模拟推动系统的激励及船体上某一点激励,分别考虑了不同位置的轴向载荷和径向载荷,讨论了单个周向模态下的位移在总位移中的比重。主要研究了4种载荷,即作用在端板中心的轴对称载荷、作用在端板与圆柱连接处的轴向载荷、作用在连接处的径向载荷和作用在壳体中间的径向载荷,比较得出了轴对称和非轴对称、同一点不同方向载荷、同一方向不同位置载荷的响应位移的不同。此外,还研究了两端端板对不同载荷下水中壳体响应的影响,得出了端板主要抑制了壳体的较高阶模态下径向位移的结论。解析法结果与有限元法结果进行了比较,验证了该方法的正确性。     

壳间连接介质对双层壳声辐射性能的影响

研究了不同壳间连接介质的加筋双层壳的振动声辐射特性.基于Flügge壳体理论和Helmholtz波动方程,求解了双壳体声-流体-结构耦合方程,计算了有限长双层壳体在径向点激励下的声功率和振动速度级.结果表明,水层的耦合作用随频率的增高而降低,托板的耦合作用随频率的增高而增加,托板在内、外壳的振动传递中起着较大的作用.为了减小托板对振动及声辐射的影响,提出了阻尼托板结构,即在托板上添加阻尼材料,对含阻尼托板的圆柱壳声学特性进行了数值研究.结果表明,在中高频段,阻尼有效抑制了振动能量的传递,壳体的辐射声压明显降低,这对水下结构的减振降噪设计具有重要的参考意义.     

芯材增强对夹层圆柱壳动力学性能影响的有限元分析

将表层、增强材料与芯材分开,应用有限元分析软件ANSYS,采用8节点SOLID45实体单元,对增强型夹层圆柱壳建立物理模型,进行自由振动及瞬态动力学过程分析。考虑树脂材性、尺寸以及分布等参数的变化,分析了点阵增强和齿槽增强对夹层圆柱壳动力学性能的影响,将两种增强方式进行了对比。结果显示,树脂柱及树脂齿槽均可改变圆柱壳的振动特性,对降低瞬态荷载下的动力响应有积极作用。其中树脂材性的影响较小,而点阵和齿槽的尺寸与分布对圆柱壳动力学性能的影响较为明显,分析显示,点阵增强对于提高结构固有频率比齿槽增强更好一些,而齿槽增强对于降低端部受冲击荷载时的动力位移比点阵增强更好一些。