Elastic response of a carbon nanotube fiber reinforced polymeric composite: A numerical and experimental study

Premature failure due to low mechanical properties in the transverse direction to the fiber constitutes a fundamental weakness of fiber reinforced polymeric composites. A solution to this problem is being addressed through the creation of nanoreinforced laminated composites where carbon nanotubes are grown on the surface of fiber filaments to improve the matrix-dominated composite properties. The carbon nanotubes increase the effective diameter of the fiber and provide a larger interface area for the polymeric matrix to wet the fiber. A study was conducted to numerically predict the elastic properties of the nanoreinforced composites. A multiscale modeling approach and the Finite Element Method were used to evaluate the effective mechanical properties of the nanoreinforced laminated composite. The cohesive zone approach was used to model the interface between the nanotubes and the polymer matrix. The elastic properties of the nanoreinforced laminated composites including the elastic moduli, the shear modulus, and the Poisson’s ratios were predicted and correlated with iso-strain and iso-stress models. An experimental program was also conducted to determine the elastic moduli of the nanoreinforced laminated composite and correlate them with the numerical values.     

碳/酚醛防热复合材料烧蚀行为的数值模拟

碳/酚醛复合材料被广泛地应用于钝头体表面,是飞行器优秀的热防护材料。为了准确地预测其烧蚀性能,本文从复合材料的组成物纤维和基体的角度出发,基于能量、质量守恒和热分解方程,考虑了烧蚀过程中材料热属性的非线性变化和烧蚀面的退缩,分别计算了纤维和基体的烧蚀性能,预测了烧蚀过程中防热复合材料的温度分布、密度变化、质量损失规律及热属性和线烧蚀率等。结果表明：碳/酚醛复合材料的烧蚀是各种因素相互作用、相互影响的高度非线性过程;烧蚀过程中材料结构具有不均匀的温度分布,烧蚀面区域材料密度衰减最大并且材料的质量损失和损失率几乎呈线性增加;纤维和基体的烧蚀行为存在明显差异,分别预测两者的烧蚀性能,可以为热防护材料的设计提供更加准确的参考和依据。     

碳纤维／改性双马来酰亚胺复合材料RFI成型工艺数值模拟

通过在改性双马来酰亚胺树脂中添加热塑性酚酞聚芳醚砜（PES）,制备了在室温下具有良好成膜性的改性双马来酰亚胺树脂膜。改性双马来酰亚胺树脂的差示扫描量热（DSC）、凝胶时间、动态和恒温黏度研究表明,该树脂膜在初始反应温度130℃时,改性双马来酰亚胺的黏度为139．3mPa·s,黏度≤1000mPa·s和黏度≤3000mP...     

复合材料雷击放电效应三维数值模拟

建立长空气间隙及复合材料层合板三维有限元模型,采用棒-板长间隙正极性流注生长概率模型为基于经典流注理论的随机放电模型。计算空气间隙区域内各网格点电位,列出电极周围所有流注待发展路径,计算每条流注待发展路径的生长时间,确定具有最小生长时间路径为优先选择路径。流注到达复合材料层合板后选流注与板交叉点作为雷击附着点进行复合材料层合板雷击直接效应分析,获得复合材料层合板在雷电压作用下电势、温度及热应力分布。计算结果表明,流注瞬间产生的高电势、温度及热应力主要沿顶层电导率最大方向对称扩展,所研究的初步结果可作为定性描述;为准确模拟复合材料的雷击放电效应过程,需进一步考虑复合材料的雷击烧蚀及热力学等破坏机理。     

Late-stage fatigue damage in a 3D orthogonal non-crimp woven composite: An experimental and numerical study

Late-stage fatigue damage of an E-glass/epoxy 3D orthogonal non-crimp textile composite loaded in the warp direction has been investigated using a combination of mechanical testing, X-ray micro computed tomography (μCT), optical microscopy and finite element modelling. Stiffness reduction and energy dissipated per cycle were found to be complementary measurements of damage accumulation, occurring in three stages: a first stage characterised by rapid changes, a more quiescent second stage, followed by a third stage where the (decreasing) stiffness and (increasing) energy dissipation change irregularly and then rapidly, to failure. Microscopy of specimens cycled into the transition between the second and third stages showed macroscopic accumulations of fibre fractures in sections of warp tows which lying adjacent to the surface weft tows which are crowned-over by the Z-tows. At these locations, the warp tow fibres are subjected to stress concentrations both from transverse weft tow matrix cracks and resin pocket cracks.     

复合材料波纹梁冲击试验与数值模拟

为了探究复合材料波纹梁的吸能性能,针对铺层形式分别为[(±45)3/(0,90)/(±45)3]、[(±45)8]和[(±45)7]的3种复合材料波纹梁元件,进行了动态冲击试验,得到了吸能载荷-位移曲线,并对其损伤破坏形貌进行了分析。以连续损伤力学为基础,结合改进的Hashin损伤判定准则以及损伤演化规律,提出了针对波纹梁耐撞性损伤分析的刚度退化模型,并基于有限元软件平台开发了适用于波纹梁渐进损伤分析的子程序。对3种不同结构形式的波纹梁进行了渐进失效数值分析,模拟得到了能量评估参数比吸能(SEA)和平均载荷值,并将模拟结果与试验结果进行了对比分析。比较分析了不同薄弱环节复合材料波纹梁的吸能能力。结果表明:波纹梁在冲击载荷作用下发生了渐进压溃失效;平均压溃载荷的相对误差不超过12%,能够满足工程应用要求;薄弱环节的设置需综合考虑复合材料性能和铺层方式等因素。     

Thermal expansion of carbon–epoxy laminates measured with embedded FBGS – Comparison with other experimental techniques and numerical simulation

This paper deals with a mixed experimental–theoretical approach aiming to evaluate information given by optical fiber Bragg gratings embedded in carbon–epoxy laminates submitted to thermal expansion. FBG instrumented elementary and structural specimens are considered. The study is undertaken through a comparison of values obtained on the one hand experimentally with high temperature resisting foil gages and thermo mechanical analysis (TMA) and on the other hand numerically through a non linear FEM simulation. Discrepancies are observed when specimens are of UD type and good agreements appear when specimens are of approaching isotropic stacking sequence. To some extent, transverse effects are suspected to have a non negligible influence on the strain information delivered by the embedded FBG.     

Lightning ablation suppression of aircraft carbon/epoxy composite laminates by metal mesh

Three-dimensional finite element (FE) models of carbon/epoxy composite laminates with copper mesh and aluminum mesh protection were established subjected to lightning strike, in which different mesh spacing was selected. Effectiveness of numerical method was verified and impulse current waveforms with different current peaks were applied according to aircraft lightning zones. Thermal-electrical material parameters varying with temperature were added into numerical models. Element deletion method was used to deal with lightning ablation elements of composite structures. The results show that ablation area and depth of composite laminates with metal mesh protection are significantly smaller, which proves good protection effectiveness of metal meshes on anti-lightning strike. The denser the mesh spacing, the better the anti-lightning strike will be. Protection of composite laminates with copper mesh has better effects than that of aluminum mesh. Considering the effect of mesh spacing variation on composite structural weight and anti-lightning strike, the ideal mesh spacing was obtained.     

三维碳/碳复合材料板件拉伸强度预报

针对三维碳/碳复合材料板件开孔敏感性问题进行了试验与有限元分析。建立了三维碳/碳复合材料代表体积单元(RVE)和完好板件、开孔板件的有限元模型,给出了宏观应力到细观应力的转换方法,并基于此方法,实现了完好板件、开孔板件拉伸破坏过程的数值模拟。进行了三维碳/碳复合材料完好板件和开孔板件的单向拉伸试验,模拟计算结果与试验结果吻合良好。分析了不同宽径比(WTDR)对开孔板件拉伸强度的影响,其中当宽径比达到6时,开孔板拉伸极限强度相比完好板下降11.5%,可以认为如果宽径比不低于6,开孔对板件拉伸极限强度影响较小。该结论可以为碳/碳复合材料开孔板件设计提供指导。     

Thermal expansion behaviors of aluminum composite reinforced with carbon nanotubes

The coefficient of thermal expansion (CTE) of aluminum matrix composite reinforced with 1.0wt.% multi-wall carbon nanotubes (MWNTs) fabricated by cold isostatic pressing and hot squeeze technique was measured between 25 and 400 °C with a high-precision thermomechanical analyzer, and compared with those of pure aluminum and 2024Al matrix fabricated under the same processing. The results show that the CTE of the composite obviously reduces in relation to those of pure aluminum and 2024Al matrix due to the introduction of MWNTs. The addition of 1.0wt.% MWNTs to 2024Al matrix decreases the CTE by as much as 12% and 11% compared with those of pure aluminum and 2024Al matrix at 50 °C, respectively, which indicates that carbon nanotube reinforced metal matrix composite may be a promising materials with low CTE.     

Fatigue life prediction of carbon/epoxy laminates by stochastic numerical simulation

A three dimensional (3D) finite element model is developed to predict the progressive fatigue damage and the life of a plain carbon/epoxy laminate (AS4/3501-6) based on the longitudinal, transverse and in-plane shear fatigue characteristic. The model takes into account stress analysis, fatigue failure analysis, random distribution and material property degradation. Different cross- and angle-ply laminates including [0₈], [90₈], [0/90₂]_s, [0/90₄]_s, [0₂/90₂]_s, [30₁₆], [45/−45]_2s with the available experimental data are considered for the fatigue life simulation. In order to consider the random distribution of the laminate’s properties from element to element in the model, the laminate’s stiffness, and strength are randomly generated using a Gaussian distribution function. Sudden and gradual material properties degradation are considered during the fatigue simulation. The progressive fatigue damage and failure analysis is implemented in ABAQUS through user subroutines UMAT (user-defined material) and USDFLD (user-defined field variables). The predicted fatigue life of the simulation for different laminates is in good agreement with the experimental results.     

Effect of interleaving on the impact response of a unidirectional carbon/epoxy composite

Instrumented drop-weight impact response of a unidirectional carbon/epoxy composite and that of the corresponding laminate interleaved with poly(ethylene-co-acrylic acid) at the midplane were studied. The impact penetration energy of the unidirectional composite beam increased strongly upon interleaving. Realtime observations using a microscope in a static flexure test indicated a dramatic change of failure mode from the dominantly compressive fracture of the baseline laminate to the dominantly tensile fracture process upon interleaving, in accordance with the corresponding fractographic features of the impact specimens. These observations were attributed to the change in stress distribution upon introduction of the interleaf layer. The relevance of the present observations to the role of high-strain, low-modulus adhesive layers in the toughening of composites is discussed.     

The thermal response to deformation to fracture of a carbon/epoxy composite laminate

The thermoelastic response to deformation of carbon-fibre/epoxy-resin composite laminates has been considered theoretically, and compared with experimental results. It is found that the surface temperature is strongly dependent on the near-surface lay-up. The total thermal response to tensile testing to failure is also reported. The results indicate that a simple, but accurate, estimation of the yield strength in composites is possible by measuring the thermoelastic limit strength, and that microstructural ‘events’ are detectable by measuring the surface temperature with sufficiently high resolution.     

接头局部增强的复合材料层合板螺栓连接试验与数值模拟

采用真空辅助成型工艺（VARI）制备了四种局部增强的复合材料层合板螺栓连接试件,通过试验及数值模拟对其力学性能进行了研究。数值研究中将复合材料层合板连接件的拉伸作为一个准静态问题,运用ABAQUS的显示分析算法及所编写用户材料子程序VUMAT对连接件进行了三维渐进失效模拟,同时在有限元模型中采用内聚力单元模拟了层合板与所设增强层的界面分层失效。数值计算结果与试验结果取得了较好的一致,验证了本文中数值方法的有效性。研究结果表明,不同的局部增强方案对复合材料螺栓连接性能的影响较大,设置[0/90/0/90]_S铺层的内置纤维增强层能显著提高层合板的螺栓连接性能。     

Impact characterization of glass/epoxy composite plates: An experimental and numerical study

This study presents the effects of impact energy, impactor mass and impact velocity on the maximum contact force, maximum deflection, contact time, absorbed energy, and overall damage area of glass/epoxy laminated composites, experimentally and numerically. The stacking sequence of the composite plates was chosen as [0°/30°/60°/90°]_S. The impact event was simulated and analyzed by using 3DIMPACT finite element code. The overall delamination area obtained from experimental study and numerical analyses were also examined. It is seen that the numerical results are in good agreement with experimental results.     

Influence of absorbed dose and deep traps on thermoluminescence response: a numerical simulation

Numerical simulations based on standard rate equations are carried out to study the dependence of the thermoluminescence (TL) response on the absorbed dose. The model, which includes thermally stimulated exoelectronic emission (TSEE), uses three electron traps--two active and one thermally disconnected (TD)--and one deep hole trap acting as a recombination centre. After instantaneous creation of a given dose of electron-hole pairs, one first follows isothermal recombination and trap filling before simulating the TL readout. Influence of TD traps and specific effects due to trap saturation are illustrated. A systematic study of the TL response is performed in wide ranges of the determining parameters. The dose dependence is found to be quadratic, linear or intermediate according to their relative values. Results are explained in terms of recombination-trapping competition, trap occupancy and in relation with the presence of TSEE.     

Nd:YAG激光辐照碳纤维复合材料的质量烧蚀

通过实验研究了连续Nd:YAG激光辐照下碳纤维环氧树脂复合材料的质量烧蚀规律.结果表明:当激光功率密度大于10 kW/cm2时,材料烧蚀质量与激光辐照能量成正比关系;在同样激光辐照能量下,半径为0.65 mm的激光辐照引起的复合材料的质量烧蚀率比半径为5.5 mm的激光辐照的结果大一倍;激光功率在燃烧阈值附近时,燃烧现象引起质量烧蚀率的波动.     

Large deformation response of a hyperelastic fibre reinforced composite: Theoretical model and numerical validation

The mechanical behaviour of an incompressible neo-Hookean material directionally reinforced with a generalised neo-Hookean fibre is examined in the finite deformation regime. To consider the interaction between the fibre and the matrix, we use a composite model for this transversely isotropic material based on a multiplicative decomposition of deformation, which decouples the uniaxial deformation along the fibre direction from the remaining shear deformation. The model is then verified numerically by a unit cell model with periodic boundary conditions. The strain energy stored in the unit cell is compared with the energy predicted by the proposed theoretical model and excellent agreement is reported.     

Experimental study of strain-rate-dependent behavior of carbon/epoxy composite

The strain rate dependent behavior of IM7/977-2 carbon/epoxy matrix composite in tension is studied by testing the resin and various laminate configurations at different strain rates. Tensile tests have been conducted with a hydraulic machine at quasi-static strain rates of approximately 10⁻⁵ s⁻¹ and intermediate strain rates of about 1 s⁻¹. Tensile high strain rate tests have been conducted with the tensile split Hopkinson bar technique at strain rates of approximately 400–600 s⁻¹. Specimens with identical geometry are used in all the tests. The standard split Hopkinson bar technique is modified to measure strain directly on the specimen. The results show that strain rate has a significant effect on the material response.     

Thermal expansion of copper matrix composite with spiral arrangement of carbon fibres

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.