连续碳纤维单丝的应变电阻效应

碳纤维是一种十分重要的新型材料,是先进复合材料中最重要的增强材料之一,同时又是一种功能材料. 研究了连续碳纤维单丝的应变电阻效应,分析了在拉应力作用下其电阻率的变化规律.结果表明:连续碳纤维单丝的电阻应变灵敏系数为1.38,比常用电阻应变片的要低,原因是碳纤维的电阻率在拉应力作用下是减小的.另外连续碳纤维受拉时,其电阻的变化由纤维几何尺寸及电阻率的变化引起,但主要是由几何尺寸的变化而引起的.同时,测试了连续碳纤维单丝的力学性能.     

2D-C/SiC复合材料偏轴拉伸力学行为研究

通过对2D-C/SiC复合材料试件进行不同偏轴角度的拉伸实验,研究了偏轴角度对材料拉伸力学特性的影响。通过应变片分别测得了材料加载方向和纤维束方向上的应力-应变行为,对比分析了偏轴角度对上述应力-应变行为的影响;并结合试件断口扫描电镜照片,阐释了纤维束方向上拉伸和剪切损伤间的相互耦合效应。实验结果表明,材料的拉伸模量和强度随偏轴角度的增大出现明显下降;材料纤维束方向上的拉伸损伤和剪切损伤具有显著的相互促进作用。最后,以材料0°拉伸和45°拉伸实验数据为基础,建立了材料的偏轴拉伸应力-应变行为预测模型,模型预测结果与实验结果吻合较好。     

接触电阻受载时的变换规律研究

讨论了依附于树脂基体中碳纤维间在微米尺度下的接触导电现象,分析了不同受载阶段下碳纤维搭接处的电阻响应规律。碳纤维接触处受到外加载荷局部有应变产生时,其接触处电阻的力阻响应规律可分为不同的两个阶段:第1阶段搭接处电阻表现出随着压应变的增大而同步线性地减小;第2阶段中纤维间的接触电阻发展为由隧道效应产生的非接触电阻,随着拉伸载荷的增大,搭接处电阻同步增大。     

二维编织陶瓷基复合材料偏轴拉伸力学性能预测

基于二维编织C/SiC复合材料的细观结构,建立了碳纤维丝/热解碳界面/SiC基体和纤维束/表层SiC基体两种尺度下的细观单胞模型,通过有限元方法计算碳纤维丝/热解碳界面/SiC基体模型的等效弹性常数和强度,然后代入纤维束/表层SiC基体模型中计算,并引入Tsai-Wu失效准则,考虑不同失效模式的损伤,建立了二维编织C/SiC复合材料的渐进损伤模型,模拟了其偏轴拉伸应力-应变行为。针对该模型,阐述了二维编织C/SiC复合材料单胞模型在复杂应力状态下其纤维束的损伤过程。数值模拟结果与实验数据吻合较好,验证了模型的有效性,为该种材料的力学性能分析提供了一种有效方法。     

基于电阻信号的CFRP纤维断裂识别实验及理论模型

碳纤维是决定CFRP复合材料性能的主要因素之一,因此为保证复合材料结构的健康状态、识别纤维的损伤与断裂就显得至为关键。文中通过加载电流并采集电阻信号的方式来识别CFRP复合材料中碳纤维的健康状态,通过实验和理论分析的方法进行了深入研究。实验针对2种不同铺层的试件进行测量:单向层合板[0]8和正交层合板[0/90]4。对试件端部进行了粘接铜箔处理以使试件稳定导电,使用KEITULEY2700数据采集仪测量试件电阻。实验内容包括静态电阻测量、拉伸受力状态下材料的电阻特性分析,并建立和推导了电阻-应变理论分析模型,进行了通电疲劳实验。研究结果表明,碳纤维的铺层角度对CFRP结构电阻有较大影响,[0]8铺层材料电导率和破坏强度约为[0/90]4铺层材料的2倍;CFRP材料受外载荷作用时,损伤程度与其电阻变化有对应的关系,其电阻信号变化可以作为反映材料损伤情况和识别纤维断裂的主要参数;实验结果与所建立的电阻-应变模型拟合结果吻合较好。文中研究的CFRP纤维断裂识别和航空复合材料结构健康监控及实时监测具有重要的意义。     

双向等轴拉伸载荷下二维编织陶瓷基复合材料的应力-应变行为预测

将二维编织结构简化为(0°/90°)s正交铺层结构。采用含损伤变量的剪滞分析理论,解得双向等轴拉伸载荷下,0°层和90°层开裂后各层的应力分布;基于随机基体裂纹演化理论,随机纤维损伤和最终失效理论,确定了0°层和90°层沿纤维方向的应力-应变关系,以及切线拉伸模量与施加载荷之间的关系;然后,将切线拉伸模量代入正交铺层结构的剪滞分析中,进而预测出二维编织陶瓷基复合材料在双向等轴拉伸载荷下的应力-应变关系。预测结果表明:在双向等轴拉伸载荷下,二维编织陶瓷基复合材料的横向和纵向应力-应变曲线基本相同,与单向加载时的应力-应变曲线相近。     

碳纤维毡/环氧树脂复合材料的力阻效应

以短切碳纤维毡和环氧树脂为原材料制成复合材料,考察了该材料在单向拉伸载荷下的力阻响应。实验结果表明,该材料具有正力阻效应(拉应变引起材料的电阻增大)。其中,单层碳纤维毡/环氧树脂复合材料的力阻灵敏度可达13.9,但在加载过程中其电阻表现出逐渐衰减趋势;多层碳纤维毡/环氧树脂复合材料的力阻性能更为稳定,但随着层数的增加灵敏度逐渐降低,5层复合材料的力阻灵敏度下降到5.7。多层复合材料的立体导电网络是其稳定性提升和灵敏度下降的主要原因。将碳纤维毡/环氧树脂多层复合材料敷设在梁结构表面形成智能表层,利用其力阻性能实现了梁结构在循环载荷下的变形监测以及在单调载荷作用下损伤监测。     

拉伸应变下镍包碳纤维填充导电橡胶取向及电阻响应

导电橡胶因其超高的弹性和良好的导电性而在柔性电子材料领域受到广泛关注。受载时导电填料重新排列会产生相应的电阻变化。本研究选取镍包碳纤维（Nickel coated carbon fiber,CF-Ni）填充于基胶聚二甲基硅氧烷（Polydimethylsiloxane,PDMS）中制备导电橡胶。基于MATLAB平台,观察了导电橡胶中CF-Ni纤维的静态分布,并构建了纤维取向率的计算方法;随后施加动态应变载荷,选用3D打印的样品,在拉伸应变下,对其进行了纤维在胶体中取向变化的同步观察。结果表明:CF-Ni纤维沿拉伸应变方向发生偏转,最大偏转角度可达20°; CF-Ni纤维的取向分布及拉伸应变下纤维的偏转会对导电橡胶的电学性能产生影响,在60%的应变量下,CF-Ni/PDMS表面电阻最大变化为600%。导电橡胶电阻对拉伸应变载荷的响应为CF-Ni/PDMS用于制备柔性传感器奠定了基础。      

环氧树脂基体中碳纤维单丝及其界面的温敏效应

为探索树脂基碳纤维复合材料(CFRP)温敏效应的机理,针对环氧树脂基体中碳纤维单丝及正交搭接的碳纤维界面进行了温敏效应实验。实验结果表明碳纤维单丝具有NTC效应,其温敏曲线线性稳定,碳纤维单丝被树脂浸润后,由于树脂膨胀带动纤维在其轴向的伸长,使得树脂基中碳纤维单丝的NTC效应有所减弱;树脂基中碳纤维界面表现出非线性变化的PTC效应,温度越高,其界面电阻随温度变化的趋势越显著,树脂基碳纤维复合材料的温敏特性是树脂基体中碳纤维单丝及其界面的温敏效应协同作用的结果,相对于碳纤维本身而言,碳纤维界面对温度变化更敏感,在CFRP温敏效应中占主导地位。     

2D-SiC/SiC复合材料损伤耦合力学行为

基于轴向和45°偏轴加载实验,分别获得2D-SiC/SiC复合材料在单一轴向应力和复合应力状态下纤维束轴向方向上的拉伸、压缩和面内剪切应力-应变行为,计算分析材料在复合应力状态下的损伤耦合力学行为。结果表明,在45°偏轴拉伸和压缩复合应力状态下材料损伤耦合力学行为的起始应力分别约为40MPa和-100MPa。复合应力状态下材料纤维束轴向方向上的拉伸损伤和面内剪切损伤进程间具有相互促进作用,面内剪切损伤对压缩损伤进程具有促进作用,但是压缩应力分量对面内剪切损伤进程具有明显的抑制作用;上述损伤耦合作用随着应力水平的增加而越发显著。由试件断口电镜扫描结果可知,复合应力状态下材料纤维束轴向方向上3个应力分量对材料内部0°/90°和45°3种取向基体裂纹开裂损伤进程的影响作用,是2D-SiC/SiC复合材料产生损伤耦合力学行为的主要细观损伤机制。     

Progressive damage analysis and strength prediction of 2D plain weave composites

A two-step, multi-scale progressive damage analysis is implemented to study the damage and failure behaviors of 2D plain weave composites under various uniaxial and biaxial loadings. In the progressive damage mode (PDM), a formal-unified 3D Hashin-type criterion is formed to facilitate analysis work and engineering application, with shear nonlinearity considered in the stiffness matrix of yarn. The periodic boundary conditions are developed for the off-axis loading simulations. The simulated stress–strain curves under on-axis uniaxial tension and compression show good agreements with experimental results. The influences of different 3D Hashin-type criteria are subsequently discussed. Moreover, the strength decrease at different off-axis angles and the failure envelopes under on-axis and 45° off-axis biaxial loadings are obtained, with the discussion of different failure characteristics under each loading condition.     

Statistical prediction of fatigue failure of fibre reinforced composite materials

A statistical approach is proposed to evaluate the residual strength and life of unidirectional and angle-ply composite laminates subjected to in-plane tensile cyclic stresses. The method is based on the extension of previous static failure criteria describing independently the fibre failure and matrix failure modes, combined with the statistical nature of fatigue failure of fibre-reinforced composites. The static and fatigue strengths of composite laminates at any off-axis angle are evaluated using the fatigue failure functions for the three principal failure modes, which are determined from the fatigue behaviour of unidirectional composites subjected to longitudinal and transverse tension as well as in-plane shear stresses. The evaluations of the fatigue strength of unidirectional E-glass/epoxy laminates under off-axis fatigue loading and angle-ply S-glass/epoxy laminates under in-plane fatigue loading show good agreement between theoretical predictions and experimental results.     

三向碳碳材料在小偏轴角拉伸下的切口强度预报

在小偏轴角拉伸下,带切口的三向碳碳材料平面试件首先沿切口平面作自相似扩展。扩展到断裂的纤维束数与宽度方向的纤维束总数之比达到某一数值时,试件沿纤维束(或在另一些单层中垂直纤维束)方向剪断。断裂是灾难性的,所以这种切口强度预报与简单拉伸时的切口强度预报是相当的;区别仅在于在小偏轴角拉伸时,纤维束除受拉伸应力外,还作用有不可忽视的剪应力。本文对纤维束作了拉剪联合作用的强度测量,以得到其强度的概率分布的参数。据此预报了三向碳碳材料在小偏轴角拉伸时的切口强度,以及整个试件剪断时的断裂的纤维束数与纤维束总数之比值,理论预报与实验结果吻合良好。      

Off-axis fatigue behaviour and its damage mechanics modelling for unidirectional fibre–metal hybrid composite: GLARE 2

The fatigue behaviour of a unidirectional fibre–metal laminate GLARE 2 has been studied under various off-axis loading conditions. Tension–tension fatigue tests were first performed at room temperature on nine kinds of plain coupon specimen with a different off-axis angle. A non-dimensional effective stress defined on the basis of the classical static failure theory was applied as an off-axis fatigue strength parameter. A macroscopic fatigue damage mechanics model was then developed using the non-dimensional effective stress, and it was compared with the classical fatigue failure models for composites. The absolute off-axis fatigue strength decreases as the off-axis angle increases. The longitudinal fatigue strength of GLARE 2 is about two times as high as that of the high-strength aluminium alloy, while the transverse fatigue strength is almost one-half. The S–N relationships are almost linear for all off-axis angles in the intermediate range of fatigue life 10³<N_f<10⁵, and they are followed by fatigue limits. The off-axis fatigue data plotted using the strength ratio (i.e. the maximum fatigue stress normalized by the static strength) are approximately represented by a single master S–N curve. The non-dimensional effective stress succeeds in describing this characteristic of the off-axis fatigue behaviour. The damage mechanics model developed using the non-dimensional effective stress can favourably reproduce the directional nature of the constant amplitude off-axis fatigue behaviour of GLARE 2. This model has an advantage over the classical fatigue failure models for composites with respect to the numerical procedure for fatigue life analysis.     

Tensile fatigue behaviour of glass plain-weave fabric composites in on- and off-axis directions

The tension–tension fatigue behaviour of woven fabric composites was investigated in the on-axis and off-axis directions. Similar to the static tensile properties of woven fabric composites, the tension–tension fatigue behaviour of woven fabric composites was also observed to be anisotropic. The fatigue damage development in both the directions was studied using scanning electron microscopy and acoustic emission techniques. The influence of the applied fatigue frequency and the fatigue stress ratio in both the directions was investigated. At a fatigue frequency of 5 Hz, the hysteresis heating will occur more severely in the off-axis direction than in the on-axis direction. Increasing fatigue stress ratio will lead to a lower hysteresis heating.     

Dynamic tensile properties of carbon fiber composite based on thermoplastic epoxy resin loaded in matrix-dominant directions

The dynamic tensile properties of carbon fiber (CF) composite loaded in the matrix-dominant direction are experimentally determined. In this study, thermoplastic epoxy resin is used as a matrix of the CF composite. A dynamic tensile test is performed using a tension-type split Hopkinson bar technique. The experimental results show that there are not linear relationships between tensile strength and strain rate in case of the 10°, 30° and 45° specimens, although the tensile strength of CF composite, whose matrix is typical thermosetting epoxy resin, linearly increases with the strain rate for all fiber orientation angles. From the fracture surface observation, it is found that the ductile fracture of the matrix can be observed only when 10° off-axis specimen is tested under dynamic loading condition. It is inferred that the softening of the thermoplastic epoxy resin in the vicinity of interface area takes place with increasing strain rate.     

Single-bolt tension joint tests on pultruded GRP plate - effects of tension direction relative to pultrusion direction

The literature on single-bolt tension joints in structural grade pultruded GRP plate is reviewed and the differences between the main investigations are highlighted. Details are given of the test setup and the joint configurations used in a series 54 such tests on single-bolt joints in which the angle between the pultrusion and tension axes (the off-axis angle) and the joint geometry are varied. Ultimate strength, intial stiffness, initial bolt slip and bolt displacement at failure data are presented as functions of the joints' principal geometric ratios. The observed joint failure modes show that, for off-axis angles ≥ 30 °, bearing failure (a relatively benign failure mode) does not arise. Instead, tension mode failure predominates and cracks tend to propagate parallel to the rovings diagonally across the width of the joint. It is concluded that the rovings play a significant role in controlling the crack propagation and this has implications for joint layout and design in pultruded GRP plate.     

Off-axis strength differential effects in unidirectional carbon/epoxy laminates at different strain rates and predictions of associated failure envelopes

A difference between off-axis tensile and compressive strengths in a unidirectional carbon/epoxy laminate is examined at 100 °C for different fiber orientations and strain rates. By comparing their predictions with experimental results, the Tsai–Wu, Hoffman, Hashin–Rotem failure criteria that can distinguish between the off-axis strengths in tension and compression are evaluated for the accuracy of prediction of the off-axis strength differential (SD) effect and of the failure envelopes associated with off-axis loading at different strain rates. It is shown that the failure envelope associated with off-axis compression is unsuccessfully predicted by these failure criteria. The comparison suggests that the SD effects in the longitudinal, transverse and shear strengths should be taken into account for accurate prediction of the off-axis failure envelope. On the basis of this experimental implication, simple modifications to the representative failure criteria are attempted in which both the normal and shear SD effects are taken into account.     

Synergism of several carbon series additions on the microstructures and tribological behaviors of polyimide-based composites under sea water lubrication

The effects of several carbon series additions including graphite (Gr), carbon fiber (CF) and carbon nanotube (CNT) on the microstructures and tribological behaviors of polyimide-based (PI-based) composites under sea water lubrication were investigated systematically. Results showed that the incorporation of any filler improved the wear resistance of polyimide (PI) under sea water lubrication, but did not decrease the friction coefficient. Especially the combined incorporation of 10%Gr, 10%CF and 5%CNT (in volume) was the most effective in improving the anti-wear properties of PI. This suggested that there existed a synergetic effect among the three carbon series additions on improving the wear resistance of PI. During the friction and wear process, the carbon additions played different roles in improving the wear resistance of PI-based composites. CF with high compressive strength can carry the main load applied on the sliding surfaces to inhibit the wear of PI matrix. CNT can decrease the stress concentration around CF and further protect CF from being broken. Gr in the form of much thinner layer can not only improve the loading capacity, but also play the same role of CNT to avoid CF carrying too much load. More importantly, Gr, CF and CNT worked synergistically to condense the microstructure of PI-based composite and ameliorate the interfacial combination between all fillers and PI matrix, which well explained why the PI–10%Gr–10%CF–5%CNT composite had excellent tribological properties, even under heavy load or high sliding speed.     

Tensile properties and fatigue characteristics of hybrid composites with non-woven carbon tissue

The mechanical characteristics of hybrid composites with non-woven carbon tissue (NWCT) are investigated under static tensile and tension–tension fatigue loadings. The hybrid composites are made by stacking NWCT and CFRP prepregs. Thirteen kinds of composites are studied; i.e. NWCT composites, CFRP longitudinal [0]₈, transverse [90]₁₂, off-axis [45]₁₂ and angle-ply [±45]_3S, hybrid longitudinal ([0/0]₄, [/0/0/]₄), transverse ([90/90]₆, [/90/90/]₆), off-axis ([45/45]₆, [/45/45/]₆), and angle-ply ([+45/−45]_3S, [/+45/−45/]_3S). The symbol ‘/’ means that the NWCT is located between the CFRP layers. To estimate the stiffness of hybrid composites, the rule of mixtures is used. The effects of NWCT on the tension–tension fatigue life, the residual strength and stiffness of hybrid specimens are evaluated. The fatigue damage and failure mechanisms of the hybrid composites are analyzed with an optical microscope.