应用锤击法的大型复合材料桁架结构段自由振动分析

复合材料桁架结构以其轻质和优异的力学性能应用于大型航天航空飞行器结构,其承载能力和振动特性是决定其应用效果的关键因素。本文考察了大型碳纤维/环氧复合材料方形截面桁架结构段的自由振动特性。采用锤击法实验测试得到了复合材料桁架结构段自由振动的模态和频率,并与有限元数值模拟结果进行对比分析,论证了锤击法测试振动特性在复合材料桁架结构上应用的可行性和准确性。     

A comprehensive review on material selection for polymer matrix composites subjected to impact load

Polymer matrix composites (PMC) are extensively been used in many engineering applications. Various natural fibers have emerged as potential replacements to synthetic fibers as reinforcing materials composites owing to their fairly better mechanical properties, low cost, environment friendliness and biodegradability. Selection of appropriate constituents of composites for a particular application is a tedious task for a designer/engineer. Impact loading has emerged as the serious threat for the composites used in structural or secondary structural application and demands the usage of appropriate fiber and matrix combination to enhance the energy absorption and mitigate the failure. The objective of the present review is to explore the composite with various fiber and matrix combination used for impact applications, identify the gap in the literature and suggest the potential naturally available fiber and matrix combination of composites for future work in the field of impact loading. The novelty of the present study lies in exploring the combination of naturally available fiber and matrix combination which can help in better energy absorption and mitigate the failure when subjected to impact loading. In addition, the application of multi attributes decision making (MADM) tools is demonstrated for selection of fiber and matrix materials which can serve as a benchmark study for the researchers in future.     

Damage mechanics and energy absorption capabilities of naturalfiber reinforced elastomeric based bio composite for sacrificial structural applications

The present study deals with the experimental, finite element (FE) and analytical assessment of low ballistic impact response of proposedflexible'green' composite make use of naturally available jute and rubber as the constituents of the composite with stacking sequences namely jute/rubber/jute (JRJ), jute/rubber/rubber/jute (JRRJ) and jute/rubber/jute/rubber/jute (JRJRJ). Ballistic impact tests were carried out by firing a conical projectile using a gas gun apparatus at lower range of ballistic impact regime. The ballistic impact response of the proposed flexible composites are assesses based on energy absorption and damage mechanism. Results revealed that inclusion of natural rubber aids in better energy ab-sorption and mitigating the failure of the proposed composite. Among the three different stacking se-quences of flexible composites considered, JRJRJ provides better ballistic performance compared to its counterparts. The damage study reveals that the main mechanism of failure involved in flexible com-posites is matrix tearing as opposed to matrix cracking in stiff composites indicating that the proposedflexible composites are free from catastrophic failure. Results obtained from experimental, FE and analytical approach pertaining to energy absorption and damage mechanism agree well with each other. The proposed flexible composites due to their exhibited energy absorption capabilities and damage mechanism are best suited as claddings for structural application subjected to impact with an aim of protecting the main structural component from being failed catastrophically.     

内压作用下复合材料修复裂纹管道的失效分析

为准确评估在内压作用下纤维复合材料修复含裂纹管道的有效性及失效压力,建立复合材料修复后裂纹管道的失效数值模型。该数值模型通过扩展有限元法模拟管道裂纹的扩展,利用cohesive单元模拟胶层的脱粘失效,复合材料的失效通过最大应力失效准则进行判定。通过静水压爆裂试验对所提失效数值模型进行验证,实验结果与数值计算结果具有较好的一致性。失效数值分析结果表明:当内压增大至一定值后,未修复管道的初始裂纹沿轴向及壁厚方向逐渐扩展,进而使得管道内壁单元扩展成真实裂纹,此时真实裂纹贯穿整个壁厚方向,即认为裂纹管道发生爆裂失效,爆裂失效压力随初始裂纹半长呈指数形式下降。复合材料修复裂纹管道的不同修复工况呈现相同的失效模式:在单调递增的内压作用下,管道内表面首先出现黏结裂纹,而后其外表面裂纹张开趋势急剧上升,使得复合材料层应力急剧上升,达到极限强度而失效。且对于不同的初始裂纹尺寸,存在对应的复合材料缠绕层数临界值。     

尾座式电动飞机复合材料机翼结构优化设计

以复杂地形地区物资快速投送平台设计为背景,开展尾座式电动飞机复合材料机翼设计研究。对机翼结构载荷进行分析,对机翼结构构件布置和铺层进行设计,形成复合材料机翼结构设计方案。建立复合材料机翼结构有限元模型,开展典型工况条件下的静力分析,得到机翼结构变形、应力和Tsai-Wu失效因子分布。采用分步优化策略,以机翼结构铺层厚度和铺层角度为设计变量,开展机翼结构质量优化计算,优化结果表明,优化后的机翼结构在满足强度和刚度要求下质量减轻约47.77%,可为尾座式电动飞机结构设计提供重要参考。     

An efficient SPH methodology for modelling mechanical characteristics of particulate composites

Particulate composites are one of the widely used materials in producing numerous state-of-the-art components in biomedical, automobile, aerospace including defence technology. Variety of modelling techniques have been adopted in the past to model mechanical behaviour of particulate composites. Due to their favourable properties, particle-based methods provide a convenient platform to model failure or fracture of these composites. Smooth particle hydrodynamics (SPH) is one of such methods which demonstrate excellent potential for modelling failure or fracture of particulate composites in a Lagrangian setting. One of the major challenges in using SPH method for modelling composite materials depends on accurate and efficient way to treat interface and boundary conditions. In this paper, a master-slave method based multi-freedom constraints is proposed to impose essential boundary conditions and interfacial displacement constraints in modelling mechanical behaviour of composite materials using SPH method. The proposed methodology enforces the above constraints more accurately and requires only smaller condition number for system stiffness matrix than the procedures based on typical penalty function approach. A minimum cut-off value-based error criteria is employed to improve the compu-tational efficiency of the proposed methodology. In addition, the proposed method is further enhanced by adopting a modified numerical interpolation scheme along the boundary to increase the accuracy and computational efficiency. The numerical examples demonstrate that the proposed master-slave approach yields better accuracy in enforcing displacement constraints and requires approximately the same computational time as that of penalty method.     

表面官能化对碳纳米管/环氧复合材料玻璃化转变温度及韧性的影响

通过碳纳米管的不同表面官能化,构造其与环氧树脂的不同界面。采用动态机械性能分析研究不同表面官能化碳纳米管对环氧树脂复合材料玻璃化转变温度的影响;采用摆锤冲击试验研究环氧树脂复合材料的韧性。结果表明:与纯环氧树脂相比,氨基化碳纳米管/环氧树脂复合材料的玻璃化转变温度升高,而羧基化碳纳米管/环氧树脂复合材料的玻璃化转变温度反而有所下降;碳纳米管/环氧树脂复合材料的冲击强度相比纯环氧树脂均提高了近一倍。复合材料性能的这些变化规律主要归因于不同表面官能化碳纳米管与环氧树脂基体间形成了不同的界面。     

Light weight HTPB-clay nanocomposites(HCN)with enhanced ablation performance as inhibition materials for composite propellant

In the present study,organically modified Montmorillonite clay with polar moiety,the Cloisite 30B,is used for preparation of Hydroxyl terminated polybutadiene(HTPB)-clay nanocomposites(HCN)by dispersion of nanoclay in polymer matrix under high shear mixing.The nanocomposites thus prepared are evaluated in composite propellants as inhibitor material for their functional utility.Several inhibition formulations containing 5 wt％-15 wt％of nanoclay,with or without the conventional filler Sb2O3,were prepared.All these formulations were evaluated for their physical,mechanical,thermal,and ablative properties.Ablation rate and density of the compositions containing Cloisite 30B is around 23％and 5％lower respectively in comparison of the base composition.Strain capability of these compositions is twofold higher than that of base composition.These compositions have also been evaluated for their smoke generation tendency by measuring infra red(IR)attenuation in the wavelength range 1.3 μm-5.6 μm and 8 μm-13 μm and thereby compared with the base composition.The corresponding results confirmed that the compositions containing Cloisite 30B as filler have much lower IR attenuation than compositions with conventional filler,Sb2O3.Replacement of 5％Sb2O3 by nanoclay showed 8％reduction in IR attenuation rate which further reduced to 16％on replacement of 15％of Sb2O3.Interfacial bonding of HCN based inhibitors is also comparable or even better than conventional inhibitors.Precisely,the nanoclay composites with Cloisite 30B as filler exhibit all desirable properties of an inhibitor.     

Damage to aircraft composite structures caused by directed energy system: A literature review

This paper presents a comprehensive review of the research studies on direct energy system effect on aircraft composite structures to develop a good understanding of state-of-the-art research and devel-opment in this area. The review begins with the application of composite materials in the aircraft structures and highlights their particular areas of application and limitations. An overview of directed energy system is given. Some of the commonly used systems in this category are discussed and the working principles of laser energy systems are described. The experimental and numerical studies re-ported regarding the aircraft composite structures subject to the effect of directed energy systems, especially the laser systems are reviewed in detail. In particularly, the general effects of laser systems and the relevant damage mechanisms against the composite structures are reported. The review draws attention to the recent research and findings in this field and is expected to guide engineers/researchers in future theoretical, numerical, and experimental studies.     

抗超高速撞击多层结构复合材料研究

本文介绍了抗超高速撞击多层结构复合材料的设计、制备和实验,并对结果进行了讨论,实验证明,以SiC陶瓷基复合材料为防护层、以空心微珠或多孔变密度复合材料为缓冲层、以编织物复合材料为结构层的多层结构复合材料,具有面密度低、抗撞击速度范围宽、抗撞击性能好的特点。     

Magnetic,thermal stability and dynamic mechanical properties of beta isotactic polypropylene/natural rubber blends reinforced by NiZn ferrite nanoparticles

The dispersion of magnetic nanoparticles in matrix is crucial to ensure optimum performance of the composite. The difficulty level of achieving good dispersion is further increase when a multi-phases of matrix is present. A pre-coating technique of magnetic nanoparticles with polypropylene using ball-mill prior to melt-blending process was employed to prepare a multi-phases thermoplastic natural rubber composite. The effect of filler loading (2 wt%-10 wt%) on morphology, structure, magnetic properties, thermal stability and dynamic mechanical properties of the composites were investigated. It was found that the NiZn ferrite nanoparticles act as nucleating agent to form beta isostatic polypropylene thermoplastic composites. The composites' magnetic properties are directly dependent on the filler concentration. The dispersion of magnetic fillers in polymer matrix plays role in affecting the magnetic properties and thermal stability. The preference of filler to locate at amorphous phase has distorted the chain orientation of natural rubber and polypropylene. Hence, the polymorphism and crystallinity of the matrix varied as the filler loading increased, affecting the dynamic mechanical properties. It was found that 8 wt% NiZn nanocomposite exhibits highest E’ and tanδ, indicating the dynamic mechanical properties of NiZn nanocomposite are affected by β-phase degree.     

先驱体转化致密SiC纳米复合材料的制备及其热电性能

以聚碳硅烷和锑改性聚硅烷为先驱体,利用先驱体转化SiC材料的富余自由碳高温石墨化的微观结构演变特点,采用热压烧结、先驱体浸渍—裂解法以及退火工艺制备出先驱体转化SiC纳米复合材料。采用SEM、TEM、XRD和Raman等测试手段表征和分析了相组成和微观结构,讨论了样品的热导率、电导率和塞贝克系数等热电参数随温度变化关系。研究表明,所得致密SiC纳米复合材料为n型热电材料。由于纳米石墨的作用,材料热导率抑制在4–8W/(m?K)范围。1600°C退火处理能够降低热导率,同时提高电导率和塞贝克系数绝对值,使先驱体转化法得到的SiC纳米复合材料无量纲热电优值ZT达到0.0028（650°C）,高于其他已报道的致密SiC/C复合材料和纳米复合材料体系。     

用溶胶-凝胶法制备碳纤维三维编织物增强氧化铝基复合材料的研究

本文研究了溶胶－凝胶（Ｓｏｌ－ｇｅｌ）法制备碳纤维三维编织物增强氧化铝（Ａｌ２Ｏ３）基复合材料的成型工艺及其力学性能，研究了两种主要起始物Ａｌ（ＮＯ３）３、ＡｌＣｌ３配制的氧化铝溶胶对复合材料成型工艺和力学性能的影响。分别以Ａｌ（ＮＯ３）３和ＡｌＣｌ３为起始物，制备得到Ⅰ＃、Ｉ＃复合材料。研究表明，以Ａｌ（ＮＯ３）３为起始物配制的溶胶粘度较小，利于材料的致密化。经过溶胶浸渍、凝胶、裂解１３个周期后，Ⅰ＃材料的密度和室温三点弯曲强度分别为１．８６ｇ／ｃｍ３和１４５．２ＭＰａ，而ＩＩ＃材料的密度和室温三点弯曲强度分别为１．６３ｇ／ｃｍ３和１０４．１ＭＰａ，材料均呈典型的韧性断裂模式。用扫描电子显微镜（ＳＥＭ）观察试样的断口形貌，发现断口表面有大量的纤维拔出，纤维表现了较好的增韧效果。     

金属泡沫复合材料夹芯结构的压痕响应实验

金属泡沫夹芯结构是近年来新出现的一种明显具有结构和功能一体化特点的新型轻质材料,它在临近空间飞行器、航海及汽车等领域有着广阔的应用前景。以以纤维增强复合材料面板、闭孔泡沫铝芯子为特征的复合材料夹芯结构为研究对象,对其在球形压头作用下的压痕响应、损伤模式、变形机制和失效机理进行理论分析和实验研究。研究发现,泡沫铝复合材料夹芯结构的压痕响应是夹芯结构的各个组成部分的响应、相互作用以及压头属性的综合作用结果。泡沫铝复合材料夹芯结构在球压头作用下的损伤模式为基体开裂、纤维断裂、层间分层、泡沫铝的屈服及剪切断裂五种失效模式。     

Effects of jute fibre content on the mechanical and dynamic mechanical properties of the composites in structural applications

Due to notable characteristics, sustainability concept and environmental issues, hybridisation natural with synthetic fibres to fabricate composites have been rapidly gaining market share in different applications (structural, military, aerospace and automotive vehicles). Compression, tension and fatigue tests of various stacking sequences of plain jute/carbon reinforced (PVB) polyvinyl butyral by hot hydraulic press technique were experimentally conducted. Six types of fabricated composites with various constituents (jute, carbon and their hybrids) were fabricated and tested. Notably, fatigue lifetime of hybrids increases with increasing the carbon content relative to the jute fibre content. On the other hand, Jute composites possess high strain compared to pure carbon composite, which gives an overall improvement in mechanical behaviours. Interestingly, H1 hybrid with Carbon/Jute/Carbon sequences offers similar fatigue stiffness behaviour of H3 hybrid with Carbon/Jute/Carbon/Jute sequences when subjected to cyclic loading. Carbon composite (C) exhibited the highest fatigue resistance, whiles jute composite (J) possessed the highest strain and semi brittle trends in both mechanical and fatigue performance. Results concluded that plain jute fibres could partially replace high-cost synthetic carbon fibres to produce more eco-friendly hybrids to be utilised in different composites industries.     

钢丝增强复合条带抗弹性能数值分析

为研究钢丝增强聚合物基复合条带的抗弹性能,基于冲击动力学理论分析其抗弹机理,再借助ANSYS/LS-DYNA软件和LS-PrePost前后处理器模拟有无增强钢丝2种工况下,条带对7.62 mm弹体的冲击响应。分析表明：条带的弹性模量、剪切模量和面密度会影响其能量吸收性能,能量吸收量随三者的增大而增大;由于聚合物基体的正交各向异性,应力云图显示出扁平椭圆状是合理的;在聚合物基复合条带中嵌入增强钢丝对条带整体的抗弹性能有积极的影响,但嵌入过量的钢丝会导致重量和成本的增加,因此需要对合理的钢丝配比率作进一步研究。研究结果可为钢丝增强聚合物基复合条带在野战防护结构中的应用提供参考。     

应用Isight的复合材料桁架结构优化设计

轻质复合材料及其结构以其优异的力学性能在航天航空飞行器上得到了广泛应用。考察玻璃纤维/环氧复合材料方形截面桁架在典型弯曲载荷工况条件下的非线性结构承载性能。采用Isight集成平台对桁架结构进行多参数优化设计,获得满足结构刚度和承载性能要求的最轻质桁架结构的几何参数,并分析最优化结构在载荷作用下的结构非线性响应行为。结果表明采用Isight平台对桁架结构进行多参数优化设计具有较高的效率和可信度。     

Effect of organo-modified montmorillonite nanoclay on mechanical,thermal and ablation behavior of carbon fiber/phenolic resin composites

The mechanical, thermal and ablation properties of carbon phenolic (C-Ph) composites (Type-I) rein-forced with different weight percentages of organo-modified montmorillonite (o-MMT) nanoclay have been studied experimentally. Ball milling was used to disperse different weight (wt) percentages (0, 1,2,4,6 wt.％) of nanoclay into phenolic resin. Viscosity changes to resin due to nanoclay was studied. On the other hand, nanoclay added phenolic matrix composites (Type-II) were prepared to study the dispersion of nanoclay in phenolic matrix by small angle X-ray scattering and thermal stability changes to the matrix by thermogravimetric analyser (TGA). This data was used to understand the mechanical, thermal and ablation properties of Type-I composites. Inter laminar shear strength (ILSS), flexural strength and flexural modulus of Type I composites increased by about 29％, 12％and 7％respectively at 2 wt.％ addition of nanoclay beyond which these properties decreased. This was attributed to reduced fiber volume fraction (％Vf) of Type-I composites due to nanoclay addition at such high loadings. Mass ablation rate of Type-I composites was evaluated using oxy acetylene torch test at low heat flux (125 W/cm2) and high heat flux levels (500 W/cm2). Mass ablation rates have increased at both flux levels marginally up to 2 wt.％ addition of nanoclay beyond which it has increased significantly. This is in contrast to increased thermal stability observed for Type-I and Type-Ⅱ composites up to 2 wt.％addition of nanoclay. Increased ablation rates due to nanoclay addition was attributed to higher insulation effi-ciency of nanolcay, which accumulates more heat energy in limited area behind the ablation front and self-propagating ablation mechanisms triggered by thermal decomposition of organic part of nanoclay.     

Force chains based mesoscale simulation on the dynamic response of Al-PTFE granular composites

Force chains based mesoscale simulation is conducted to investigate the response behavior of aluminum-polytetrafluoroethylene (Al-PTFE) granular composites under a low-velocity impact. A two-dimensional model followed the randomly normal distribution of real Al particles size is developed. The dynamic compressive process of Al-PTFE composites with varied Al mass fraction is simulated and validated against the experiments. The results indicate that, force chains behavior governed by the number and the size of agglomerated Al particles, significantly affects the impact response of the material. The failure mode of the material evolves from shear failure of matrix to debonding failure of particles with increasing density. A high crack area of the material is critical mechanism to arouse the initiation re-action. The damage maintained by force chains during large plastic strain builds up more local stresses concentration to enhance a possible reaction performance. In addition, simulation is performed with identical mass fraction but various Al size distribution to explore the effects of size centralization and dispersion on the mechanical properties of materials. It is found that smaller sized Al particle of com-posites are more preferred than its bulky material in ultimate strength. Increasing dispersed degree is facilitated to create stable force chains in samples with comparable particle number. The simulation studies provide further insights into the plastic deformation, failure mechanism, and possible energy release capacity for Al-PTFE composites, which is helpful for further design and application of reactive materials.     

Micromechanical simulation and experimental investigation of aluminum-based nanocomposites

Ceramic reinforced metal matrix nanocomposites are widely used in aerospace and auto industries due to their enhanced mechanical and physical properties. In this research, we investigate the mechanical properties of aluminum/Nano-silica composites through experiments and simulations. Aluminum/Nano-silica composite samples with different weight percentages of silica nanoparticles are prepared via powder metallurgy. In this method, Nano-silica and aluminum powders are mixed and compressed in a mold, followed by sintering at high temperatures. Uniaxial tensile testing of the nanocomposite samples shows that adding one percent of Nano-silica causes a considerable increase in mechanical properties of nanocomposite compared to pure aluminum. A computational micromechanical model, based on a representative volume element of aluminum/silica nanocomposite, is developed in a commercial finite element software. The model employs an elastoplastic material model along with a ductile damage model for aluminum matrix and linear elastic model for nano-silica particles. Via careful determination of model parameters from the experimental results of pure aluminum samples prepared by powder metallurgy, the proposed computational model has shown satisfactory agreement with experiments. The validated computational model can be used to perform a parametric study to optimize the micro-structure of nanocomposite for enhanced mechanical properties.