Optimal design of general stiffened composite circular cylinders for global buckling with strength constraints

A design strategy for optimal design of composite grid-stiffened cylinders subjected to global and local buckling constraints and strength constraints was developed using a discrete optimizer based on a genetic algorithm. An improved smeared stiffener theory was used for the global analysis. Local buckling of skin segments were assessed using a Rayleigh-Ritz method that accounts for material anisotropy. The local buckling of stiffener segments were also assessed. Constraints on the axial membrane strain in the skin and stiffener segments were imposed to include strength criteria in the grid-stiffened cylinder design. Design variables used in this study were the axial and transverse stiffener spacings, stiffener height and thickness, skin laminate stacking sequence and stiffening configuration, where stiffening configuration is a design variable that indicates the combination of axial, transverse and diagonal stiffener in the grid-stiffened cylinder. The design optimization process was adapted to identify the best suited stiffening configurations and stiffener spacings for grid-stiffened composite cylinder with the length and radius of the cylinder, the design in-plane loads and material properties as inputs. The effect of having axial membrane strain constraints in the skin and stiffener segments in the optimization process is also studied for selected stiffening configurations.     

复合材料先进网格结构共固化工艺的温度场模拟

基于ANSYS软件开发了用于模拟复合材料固化过程中温度-固化度耦合场变化程序, 并分别采用二维和三维有限元模型对复合材料先进正交网格结构的软模辅助共固化工艺过程进行了数值仿真。计算结果分析表明: 二维有限元模型不考虑网格结构肋骨交叉点的影响, 能以较大时间步长进行共固化工艺的数值模拟, 且分析结果具有较高的精度; 三维精细有限元模型可考虑肋骨交叉点在共固化工艺中对复合材料先进网格结构内温度-固化度耦合场的影响; 肋骨交叉点附近的温度场明显高于结构的其他部位。      

Progressive failure predictions for rib-stiffened panels based on multicontinuum technology

An ongoing challenge within the structural analysis community is to accurately predict damage and progressive failure in large-scale structural components composed of composite materials. Multicontinuum technology (MCT) provides a means to produce constituent (fiber and matrix) level stress and strain information within the framework of commercial finite element analysis. Constituent level stress/strain information forms the basis for a progressive failure algorithm that has successfully been used to predict coupon failure in a wide range of composite materials and laminate configurations.

In this paper, MCT is used to analyze the failure of rib-stiffened panels associated with advanced composite grid-stiffened structure (AGS) designs. More specifically, MCT is used to predict and analyze the separation of the rib to skin interface for comparison with tee pulloff and tee bend test data. Predictions for the initial and final separation of the rib to skin interfaces are shown to be in good agreement with experimental test data. The results also lend insight into design and manufacturing considerations that are key to the strength and performance of the rib to panel interface.     

Failure Analysis of a Launch Vehicle Umbilical Shutter Mechanism During Vibration Qualification

Payload fairing (PLF) of a launch vehicle is exposed to harsh vibration environments due to jet noise during liftoff and in-flight aerodynamic noise. Accordingly, the systems mounted on the payload fairing are to be qualified for the vibration levels, predicted corresponding to the envelope of acoustic spectrums at critical instants of atmospheric flight. This paper presents a detailed study of a failure observed on the payload cooling umbilical system, mounted on the cylindrical portion of the PLF structure, during its design qualification vibration testing. The umbilical shutter inadvertently opened during the test. The vibration responses on the shutter, the dynamic behavior of the system, and the forces and moments on the mechanism are analyzed, and the physics of failure is understood. The design marginality is identified, and the shutter locking mechanism reconfigured to achieve the desired level of robustness in the system.     

航天器发射阶段声振环境载荷控制技术研究进展

航天器在发射阶段承受各种恶劣而复杂的声振环境载荷,严重影响着航天器的可靠性。目前,关于改善发射阶段航天器所承受的力学环境、切断振动噪声传播路径的相关研究已得到世界各国航天技术研究机构的重视。其中,各种声振环境载荷以透射声或结构声辐射的形式传递到整流罩内部后,将会在整流罩内形成恶劣声学环境——高声压级的噪声环境。本文主要对整流罩内噪声的主、被动控制技术的国外研究进展进行系统地综述和分析,以期对我国相关领域的技术研究及应用有所启示。     

Study of filament wound grid-stiffened composite cylindrical structures

Grid-stiffened composite structures are known for their very high efficiency under compressive loading environment. The grid of stiffening ribs is the primary feature in these structures and filament winding is employed as the most convenient manufacturing technique. Three different types of circular cylindrical structures – unstiffened shell (with skin only), lattice cylinder (with ribs only) and grid-stiffened shell (with skin and ribs) – are considered for experimental study and a series of these structures have been manufactured adopting a simplified and cheap manufacturing process. Different aspects of manufacturing that include tooling and other processing aspects are presented in this paper. Axial compression tests have been carried out and the results are compared with finite element analysis. Based on the test results and comparison with finite element analysis, conclusions are drawn on the efficacies of this relatively new class of structures.     

Resin Flow of an Advanced Grid-Stiffened Composite Structure in the Co-Curing Process

The soft-mold aided co-curing process which cures the skin part and ribs part simultaneously was introduced for reducing the cost of advanced grid-stiffened composite structure (AGS). The co-curing process for a typical AGS, preformed by the prepreg AS4/3501-6, was simulated by a finite element program incorporated with the user-subroutines ‘thermo-chemical’ module and the ‘chemical-flow’ module. The variations of temperature, cure degree, resin pressure and fiber volume fraction of the AGS were predicted. It shows that the uniform distributions of temperature, cure degree and viscosity in the AGS would be disturbed by the unique geometrical pattern of AGS. There is an alternation in distribution of resin pressure at the interface between ribs and skin, and the duration time of resin flow is sensitive to the thickness of the AGS. To obtain a desired AGS, the process parameters of the co-curing process should be determined by the geometry of an AGS and the kinds of resin.     

Local buckling modelling of isogrid and anisogrid lattice cylindrical shells with triangular cells

The paper deals with a refined analytical model for the local buckling failure modes of composite anisogrid lattice cylindrical shells made of a regular system of triangular cells. Such structures are preliminarily designed with the aid of closed-form solutions specifying the minimum mass and the corresponding optimal design variables under a set of formulated constraint equations. These equations address the main failure mechanisms that can be typically experienced by the structure due to axial compressive loads, namely, the global buckling of the shell, the local buckling of helical ribs, and the material failure of helical ribs. However, the local buckling of helical ribs is normally based on a simplified and qualitative approach. Thus, the scope of the present work is to improve the prediction of this failure mode by means of a rather accurate modelling which accounts for the interaction of intersecting hoop and helical ribs, the influence of the number of hoop sections of the shell, and the effect of the prebuckling tensile force in hoop ribs. The proposed model - that has been verified with the aid of finite-element analysis - lastly suggests the possibility to improve the preliminary design solution with respect to the fully analytical approach.     

基于渐进损伤分析的复合材料螺栓连接强度包线法研究

针对中国民机采用T800级复合材料这一新材料体系而基础数据匮乏的现状,采用渐进损伤分析(PDA)替代试验以显著降低研究周期和成本。综合渐进损伤方法和工程算法各自的优点,提出以渐进损伤分析替代应力集中减缓因子(SCRFs)测定试验,进而建立强度包线,并进行多钉连接强度预测的数值策略。为验证该数值策略的可行性,针对典型铺层应力集中减缓因子,测定试样,并开展渐进损伤分析,获得了试验件强度预测值来计算应力集中减缓因子,采用旁路载荷修正的强度包线法,绘制了典型铺层复合材料多钉连接旁路载荷修正强度包线,预测多钉连接的失效载荷,并与试验结果进行对比。结果表明:采用该数值策略预测的强度包线、多钉连接的失效载荷和失效模式均与试验结果吻合良好,证明了该数值策略的可行性。     

Static strength of a composite butt joint configuration with different attachments

The static failure behaviour of a composite single-strap joint configuration using three different attachments was studied experimentally. The attachments used were: adhesive bonding; mechanical bolting; and bonded-bolted joining. The dimensions of the composite butt joint used were determined based on actual joint configurations in aircraft structures, such that the test results would be beneficial to both the academic exploration and practical engineering application for advanced composite joint design and analysis. A damage evolution process was presented for the bonded-bolted butt joints based on the observed stress versus displacement curves and associated failure modes from all the related butt joints with the three attachments. An approach was then proposed for estimating the ultimate tensile strength in the bonded-bolted joints.     

Parametric study on design of composite–foam–resin concrete sandwich structures for precision machine tool structures

In this paper, sandwich structures for micro-EDM machines are optimized by using parametric study varying composite geometries and parameters like stacking sequence, thickness and rib geometry. The structures are composed of fibre reinforced composites for skin material and resin concrete and PVC foam (Closed cell, Divinycell) for core materials. Column structure was designed by a beam with cruciform rib and performance indices such as static bending stiffness (EI) and specific bending stiffness (EI/ρ) for dynamic stability are examined by controlling the thickness and stacking sequence of composites. For the machine tool bed, which usually has a plate shape, was designed to have high stiffness in two directions at the same time controlling stacking sequence and rib geometry; that is, rib thickness and number of ribs. The sensitivity of design parameters like rib thickness and composite skin thickness was examined and the optimal condition for high stiffness structure was suggested. Finite element analysis was also performed to verify the static and dynamic robustness of the machine structure. L-shaped joint for combining bed and column of the micro-EDM machine was proposed and fabricated using adhesive bonding. The dynamic performance such as damping characteristics was investigated by vibration tests. From the results optimal configuration and materials for high precision micro-EDM machines are proposed.     

The effects of delamination deficiencies on compressive properties of composite grid-stiffened structures

This article provides a systematic approach to accurately predict the effects of delamination deficiencies on compressive properties of composite grid-stiffened structures. The deformation of the stiffener and the skin is investigated during a mechanical test. Cohesive elements in the finite element method were employed to calculate the ultimate load-carrying capacity. The delamination deficiencies in the interface between the skin and the stiffener have a significant effect upon its ultimate load-carrying capability compared with the delamination deficiencies in the skin and in the stiffener.     

纵向稀加筋复合材料园柱层壳的稳定分析

本文引用位移函数,将文献[1]中园柱层壳的经典理论微分方程组转化成一个微分方程式,在此基础上、将W.Nowacki方法[3]推广到复合材料园柱层壳,得到了稀加筋园柱层壳临界载荷的解析解答 给出了算例、若略去筋条时,其数值计算能退化成与文献[1]中相应的数值结果完全一致.      

A mechanically fastened composite laminate joint and progressive failure analysis

A comprehensive procedure for a mechanically fastened composite laminate joint (ASTM D5961 Proc. A, B) is demonstrated from fixture design to analysis of test results. The ASTM tests are applied to evaluate the standard laminate properties and the composite joints. Composite laminate mechanical joints were analyzed using the finite element method (FEM), and the results were compared to test results. A progressive failure analysis (PFA) was applied to the FEM to predict the overall failure behavior of the test specimens. Three laminate failure theories – maximum stress, maximum strain, and Tsai–Wu – were applied to the PFA to predict the test failure load, displacement and strength. The PFA method was suitable to predict the initial test range of test and maximum test load except for the excessive failure area.     

Rib separation in postbuckling stiffened shear panels

With the introduction of postbuckling lightweight composite structures, adhesively bonded joints with thin skins are susceptible to premature failure due to peel induced loading. This paper reports on the findings of a research programme to investigate the local and global responses of postbuckling rib-stiffened panels under shear loads. C- and I-section ribs, with and without taper in the flanges, have been studied experimentally and analysed numerically to assess their postbuckling behaviour and relative merit with respect to structural integrity of rib-stiffened shear panels. The rib-skin separation loads and locations for the different panels have been evaluated via mechanical tests, and the experimental data have been closely predicted using Finite Element Method models.     

Biaxial testing of high strength carbon fiber composite cylinders for pulsed magnet reinforcement

A methodology is introduced to test carbon-fiber-reinforced, hoop-wound composite cylinders for their biaxial mechanical properties under axial compression and hoop tension. The understanding of the behavior of these composites under biaxial loads is extremely important in the design of pulsed magnets. These composites are used as reinforcements for both the inner conducting layers and as an overall exterior reinforcement. Testing of actual pulsed magnets to ascertain design change effects of composite reinforcement schemes on the maximum attainable field can be expensive; hence, a standard biaxial testing method is desirable which is relevant to the design of pulsed magnets. In this investigation, an attempt was made to produce a standard testing procedure aimed at measuring the biaxial mechanical properties (elastic, plastic, and failure envelope) of composite materials. This methodology was applied to two different carbon/epoxy based composites. The results of these tests (elastic properties and failure points) are compared with theoretical predictions, specifically those due to Tsai-Wu.     

Electronic Shearography for Detecting Disbonds in Lattice/Skin Structures

Aluminum structures with an integral lattice (rib system) and skin currently are used for a variety of large space structures (Titan and Delta series) due to their structural efficiency. Various components of space structures will be manufactured out of graphite/epoxy composites in the future. Interstages of rockets, decks of small satellites (MightySat program), payload shrouds, solar cell substrates, rocket motor casings, etc., are being fabricated at the Air Force Phillips Laboratory. Tests have shown that the principal mode of initial failure in these structures involves separation between the skin and the ribs, during fabrication or loading. The redundancy of the rib system in these structures leads to alternate load paths that make it difficult to detect such defects. The fabrication, integration, and testing often are carried out in different parts of the country. It is therefore important to have a method of detecting and possibly quantifying the intergrity of the rib/skin interface. The objective of this paper is to demonstrate that electronic shearography (ES) can be used to detect disbonds in rib/skin ``Isogrid' structures. This laser-based interferometry technique provides fringe patterns that represent full-field displacement gradients. The ruggedness and portability of the system make it a prime candidate for in-service inspection of large structures. The observed fringe patterns change dramatically for disbonded ribs that form a basis for rapidly detecting disbonds over a large area. The expected fringe patterns can be quantified and compared with results from finite-element (FEM) analyses of the structure. A commercial FEM code was used with orthotropic material properties that are representative of the composites used.     

复合材料整体加筋板轴压后屈曲失效评估方法

在飞机结构设计中,为高效完成复合材料加筋板结构设计,亟需一种快速有效的复合材料整体加筋板后屈曲失效评估方法。采用商业有限元软件ABAQUS建立了加筋板有限元模型,研究了复合材料整体加筋板的轴压后屈曲失效评估方法,即“整体-局部”法。通过对单筋条壁板后屈曲的失效分析发现,“整体-局部”法可得到初始脱胶时的位移和载荷值以及较准确的破坏载荷值,是一种能有效确定加筋板后屈曲失效情况的快速评估方法,对提高结构设计效率和减轻飞机结构质量具有重要的工程意义。     

复合材料连接结构健康监测技术研究进展

连接结构是大型复合材料结构的关键环节,对保证复合材料结构的完整性具有重要作用。由于复合材料连接结构存在复杂的非线性耦合因素,使得复合材料连接结构的强度和破坏模式分析十分困难,因此,必须对复合材料连接结构的健康状态进行监测、诊断、评价和预测,通过在线监测获得的信息实时掌握结构的健康状况与对外界载荷的响应,并在此基础上对未来可能发生的缺陷和故障进行预报,以便能在合适时间段内采取措施,以保证复合材料结构的安全服役并取得最大的经济效益。以飞行器复合材料连接结构为背景,首先简要分析了复合材料胶接连接、机械连接和混合连接形式的损伤和失效模式,然后重点介绍了基于波传播法、阻抗法、智能涂层监测法、真空比较监测法、光纤传感监测法和混合集成监测法的复合材料连接结构健康监测(SHM)技术的研究进展,最后讨论了飞行器复合材料连接结构健康监测技术的发展趋势和面临的挑战。      

复合材料层合板及机翼格栅结构的动态特性优化设计

以复合材料层合板各单层连续变化的铺层角度为设计变量, 在有限元软件中对层合板结构的基频进行优化分析, 在四边简支和固支两种不同的边界条件下, 结构的基频分别提高了4.9%和16.2%, 并对优化前后结构的静力失效强度进行了对比分析。随后将这种优化方法应用到某无人机复合材料机翼格栅结构中, 针对格栅结构蒙皮和肋板共计24个纤维铺层角度进行了优化设计, 使结构基频提高了10.6%, 同时结构的承载能力也有了一定程度的提高。