不同应变率下Kevlar49纤维束拉伸力学性能的实验研究

利用M TS810材料试验机及旋转盘式杆杆型冲击拉伸试验装置对Kevlar49纤维束进行了准静态拉伸及冲击拉伸实验研究, 首次在应变率为10-4/ s～ 103/s 范围内得到了Kevlar49纤维束完整的应力应变曲线。实验结果表明, Kevlar49纤维束的力学性能是与应变率相关的。在低应变率下Kevlar49纤维束对应变率不太敏感, 但比玻璃纤维束高; 在高应变率下Kevlar49纤维束对应变率敏感, 但不如玻璃纤维束强烈; 中应变率区是Kevlar49纤维束由应变率不太敏感到应变率敏感的过渡区。      

高强高模聚乙烯纤维力学性能的应变率和温度效应

利用MTS810材料试验机、旋转盘式杆-杆型冲击拉伸装置和温度控制箱,在温度20℃~110℃、应变率为0.001/s~700/s范围内,对高强高模聚乙烯纤维束进行了准静态和高应变率冲击拉伸实验,得到了不同温度、不同应变率时纤维束的应力-应变曲线。结果表明:高强高模聚乙烯纤维束的初始弹性模量具有应变率和温度相关的特性,随应变率提高而增加,随温度提高而下降;在常温下,破坏应力从准静态到动态,具有明显的应变率相关性,随应变率提高而增加,但在20℃~110℃范围内、高应变率下,对应变率变化不敏感;失稳应变也具有应变率和温度相关的特性,随应变率提高而减小,随温度提高而增大。在高应变率下,断裂应变能密度主要由初始弹性模量和失稳应变共同决定,受温度效应和应变率效应的综合影响。     

标距和应变率对Kevlar49单束拉伸力学性能的影响

为探究Kevlar 49单束的尺寸效应及应变率敏感性, 首先, 利用MTS万能试验机对不同标距(25、50、100、150、200和300 mm)的Kevlar 49单束进行了准静态(应变率为1/600 s-1)拉伸测试; 然后, 利用Instron落锤冲击系统对标距为25 mm的试样进行了动态(应变率为40~160 s-1)拉伸测试; 最后, 利用Weibull模型进行统计分析, 量化了不同标距和应变率下Kevlar 49单束拉伸强度的随机变化程度。结果表明: Kevlar 49单束的拉伸力学性能与标距和应变率有相关性; 拉伸强度随标距的增加而减小, 但随应变率的增加而增大; 峰值应变和韧性均随标距和应变率的增加而减小; 提取的Weibull参数可用于数值模拟及工程应用。      

E玻璃纤维增强环氧树脂基复合材料轴向拉伸力学性能的应变率效应

采用MTS-810材料试验机、Zwick-HTM5020高速拉伸试验机及分离式Hopkinson拉杆（SHTB）实验装置,并结合数字图像相关性（Digital image correlation,DIC）分析方法,对E玻璃纤维增强环氧树脂基复合材料棒材在10^-3~2 400 s^-1应变率范围内的轴向拉伸力学性能进行了较系统的实验研究,获得了不同应变率下材料的应力-应变曲线,揭示了应变率对材料的拉伸强度和断裂应变的影响规律。通过显微分析拉伸试样的断口形貌,揭示了试样的断裂机制及对应变率的依赖性。实验结果表明：E玻璃纤维增强环氧树脂基复合材料的力学性能具有强烈的应变率效应,归一化拉伸强度随着应变率对数线性增加,而归一化断裂应变则随着对数应变率线性减小;断口显微分析显示：E玻璃纤维增强环氧树脂基复合材料的轴向拉伸断裂模式依赖于应变率,低应变率加载下试样发生沿45°方向的剪切断裂,随着应变率增大,试样断裂模式逐渐过渡到沿轴向的拉伸断裂,特别是在高应变加载下,观察到大量的玻璃纤维丝被拉断,同时环氧树脂基体也发生严重的碎裂现象,这反映了基体材料与玻璃纤维之间相互约束作用在增强。     

高聚物纤维材料的高应变率响应行为研究

利用MTS和旋转盘式杆－杆型冲击拉伸试验装置，在高应变率、大应变率范围条件下（Aramid:0.01/s-1000/s;PVA:0.01/s-1500/s），研究了应变率对芳纶纤维和高强PVA纤维束力学性能的影响；同时，利用扫描电子显微镜考察了纤维材料在不同应变率下的微观断裂机理。     

Kevlar芳酰胺纤维可用来制造高性能船只

杜邦公司的 Kevlar 芳酰胺纤维所具有的强度、重量和刚性是其它传统的纤维所不具备的。今后十年中,Kevlar 纤维将是制造高性能船只的重要材料。Kevlar49与玻璃纤维的抗张强度相近(玻璃纤维为3400兆帕斯卡[MPa],Kevlar为3600MPa),但比重差别颇大(玻璃纤维     

玻纤增强聚丙烯复合材料的应变率敏感特性

采用层合热压实验法将玻璃纤维基毡与聚丙烯薄膜复合制成不同玻纤含量的玻纤增强聚丙烯(GF/PP)复合板材,在不同的加载速率下进行拉伸测试,研究GF/PP复合材料的应变率敏感特性,分析Burgers模型对该材料本构关系拟合预测的可行性。结果表明:GF/PP复合材料在低应变率范围内对应变率是敏感的,随应变率的增加,其断裂应力和抗拉强度增大;随玻璃纤维含量的增加,其所对应的应变率效应反而有所下降。同时,Burgers模型能够有效地拟合预测出该材料的拉伸应力-应变曲线,与实验曲线相比,进一步验证了GF/PP复合材料的应变率敏感特性及其变化趋势。     

钢纤维超高强混凝土动态力学性能

采用分离式霍普金森压杆装置对不同纤维体积率的钢纤维超高强混凝土进行不同应变率的冲击压缩试验,结果表明钢纤维超高强混凝土是应变率敏感材料,并测出其应变率敏感阀值,当应变率超过阀值后,钢纤维超高强混凝土的强度、韧度与弹性模量都随纤维体积率的增加而显著提高,在高应变率下,超高强混凝土基体成粉碎性破坏,而钢纤维超高强混凝土呈现出“裂而不散”的破坏形态。     

单脉冲加载技术及其在钛合金拉伸行为研究中的应用

为精确评价钛合金热率相关的力学行为,利用冲击拉伸试验系统和基于单应力脉冲加载的冲击拉伸复元试验技术分别获得TC11钛合金在高应变率(102~103s-1)范围内的绝热应力-应变曲线和等温应力-应变曲线,实现拉伸响应的热力解耦;利用冲击拉伸加卸载试验技术实施变温度和变应变率测试,研究历史效应对于本构行为的影响。结果表明:TC11的初始屈服行为呈现温度软化和应变率强化特性,而等温塑性应变硬化行为表现出温度和应变率不敏感特征,瞬态绝热温升是导致材料动态应变硬化率降低的主要原因;高应变率加载时材料内的热功转换系数约为0.9,且其拉伸力学行为无明显的温度和应变率历史效应。实验结果为建立钛合金的本构模型奠定试验基础。     

碳纤维与芳纶纤维增强环氧树脂复合材料的动态力学性能

分别使用高速拉伸试验机和霍普金森拉杆装置对碳纤维、芳纶纤维增强环氧树脂平纹复合材料的动态力学性能进行测试。结果表明：在拉伸过程中，碳纤维和芳纶纤维呈脆性断裂特征，两种材料的弹性模量几乎不受应变率的影响，抗拉强度随着应变率的增加而升高，且与应变率的对数呈线性关系。     

纤维束动态拉伸力学性能的实验研究

对Ｐｏｌｙｖｉｎｙ－ａｌｃｏｈｏｌ（ＰＶＡ），Ｅ－ｇｌａｓｓ和Ｋｅｖｌａｒ４９纤维束的应变率相关性和能量吸收能力进行了对比，冲击拉伸实验结果表明，除ＰＶＡ纤维束的初始弹性模量外，三种纤维束的力学性能均具有应变率相关性，其中ＰＶＡ和Ｅ－ｇｌａｓｓ的应变率敏感性较Ｋｅｖｌａｒ４９强烈；Ｋｅｖｌａｒ４９的能量吸收能力高于ＰＶＡ并且明显优于Ｅ－ｇｌａｓｓ。     

Experimental and theoretical study on the strain rate and temperature dependence of mechanical behaviour of Kevlar fibre

The tensile impact experiments on Kevlar 49 fibre bundles were carried out at strain rates 140, 440, 1350 s⁻¹ and at temperatures −60, −20, 15, 50 and 90°C. It was found that the tensile mechanical properties of Kevlar 49 fibre bundles depend both upon the strain rate and the temperature. A bimodal Weibull distribution statistical model of the strain rate and temperature dependence of fibres, and a test method of determining mechanical properties and Weibull parameters of fibres from the fibre bundle test were adopted to characterize the combined effects of strain rate and temperature on Kevlar 49 fibre strength distribution. The simulated results are in good agreement with the experimental data, which proves that the bimodal Weibull distribution function is suitable to represent the statistical strength distribution of Kevlar 49 fibre which has peculiar ‘skin–core’ physical structure, and that the test method is valid and reliable.     

Mechanical behaviour of glass and carbon fibre reinforced composites at varying strain rates

The choice of composite materials as a substitute for metallic materials in technological applications is becoming more pronounced especially due to the great weight savings these materials offer. In many of these practical situations, the structures are prone to high impact loads. Material and structural response vary significantly under impact loading conditions as compared to quasi-static loading. The strain rate sensitivity of both carbon fibre reinforced polymer (CFRP) and glass fibre reinforced polymer (GFRP) are studied by testing a single laminate configuration, viz. cross-ply [0°/90°] polymer matrix composites (PMC) at strain rates of 10⁻³ and 450 s⁻¹. The compressive material properties are determined by testing both laminate systems, viz. CFRP and GFRP at low to high strain rates. The laminates were fabricated from 48 layers of cross-ply carbon fibre and glass fibre epoxy. Dynamic test results were compared with static compression test carried out on specimens with the same dimensions. Preliminary compressive stress–strain vs. strain rates data obtained show that the dynamic material strength for GFRP increases with increasing strain rates. The strain to failure for both CFRP and GFRP is seen to decrease with increasing strain rate.     

Rate dependent deformation of carbon fibre reinforced Al–Li metal matrix composite

Abstract

The dynamic deformation characteristics and failure behaviour of laminated carbon fibre reinforced Al–Li metal matrix composite has been studied experimentally with the objective of investigating the dependence of mechanical properties on the applied strain rate and fibre volume fraction. A vacuum melting/casting process was used for manufacturing the tested composite. Impact testing was performed using a Saginomiya 100 metal forming machine and a compressive split Hopkinson bar over a strain rate range of 10^-1 s^-1 to 3×10³ s^-1. It is shown that the flow stress of the composite increases with strain rate and fibre volume fraction. The highest elongation to fracture values were found at low rate loading conditions, although a significant increase in ductility is obtained in the dynamic range. The composite appears to exhibit a lower rate of work hardening during dynamic deformation. Strain rate sensitivity and activation volume are strongly dependent on strain rate and fibre volume fraction. Fractographic analysis using scanning electron microscopy reveals that there is a distinct difference in the morphologies of the fractures, with corresponding different damage mechanisms, between specimens tested at low and high strain rates. Both strain rate and fibre volume fraction are important in controlling fibre fragment length and the density of the Al–Li debris. The relationships between mechanical response and fracture characteristics are also discussed.     

Testing of single fibre bundles of carbon/carbon composite materials

A method has been developed for excising single fibre bundles of carbon/carbon material from large billets, and of testing them in axial tension to failure. The fibre bundles are strain gauged so that the tensile modulus and complete stress/strain curves to failure can be obtained. Experimental results are presented and discussed.     

Interfacial shear stress distribution in model composites: the effect of fibre modulus

The micromechanics of reinforcement have been investigated for a continuous intermediate-modulus (IM) carbon fibre embedded in an epoxy resin (MY-750). The embedded single-fibre (fragmentation) geometry was employed as the loading configuration. A laser Raman spectroscopic method was used to obtain the fibre strain distribution along the embedded fibre fragments, at various levels of applied strain. The interfacial shear stress distribution along the fibre was derived through a balance of forces analysis.A number of parameters, such as the maximum interfacial shear stress at each level of applied strain and the fibre debonded length, were evaluated. The maximum interfacial shear stress of the IM fibre system was found to increase by 80%, compared with the high-modulus fibre system examined previously, while the distance from the fibre end where the interfacial shear stress maximizes was significantly shorter. The debonded length was found to increase only marginally up to an applied strain of 1.8%, followed by a dramatic rate of increase between 1.8% and 2.5% of applied strain.     

The abrasive wear behaviour of continuous fibre polymer composites

The dry abrasive-dominant wear behaviour of several composite materials consisting of uni-directional continuous fibres and polymer matrices was investigated. Seven materials were examined: neat epoxy (3501-6), carbon fibre epoxy (AS4/3501-6), glass fibre/epoxy (E-glass/ 3501-6), aramid fibre/epoxy (K49/3501-6), neat polyetheretherketone (PEEK), carbon fibre/PEEK (APC2) and aramid fibre/PEEK (K49/PEEK). The wear behaviour of the materials was characterized by experimentally determining the friction coefficients and wear rates with a pin on-flat test apparatus. First, the effects of the operation variables apparent normal pressure, sliding velocity and apparent contact area were observed. The dimensionless wear rate increased linearly as the apparent normal pressure increased and decreased as the apparent contact area increased. Second, through microscopic observations of the worn surfaces and subsurface regions, basic wear mechanisms were identified as a function of fibre orientation. Observations of fibre-abrasive particle interactions allowed for the differentiation of the dominating wear mechanisms. Finally, a network of data was compiled on the wear behaviour in terms of the three material parameters: fibre orientation, fibre material and matrix material. This enabled the systematic selection of an ideal low wear composite material which would consist of a PEEK matrix reinforced with aramid fibres oriented normal to the contacting surface and carbon fibres oriented parallel to the contacting surface.     

Hybrid effects in thin ply carbon/glass unidirectional laminates: Accurate experimental determination and prediction

Experimental results are presented which allow the hybrid effect to be evaluated accurately for thin ply carbon/epoxy–glass/epoxy interlayer hybrid composites. It is shown that there is an enhancement in strain at failure of up to 20% for very thin plies, but no significant effect for thicker plies. Hybrid specimens with thick carbon plies can therefore be used to measure the reference carbon/epoxy failure strain. The latter is significantly higher than the strain from all-carbon specimens in which there is an effect due to stress concentrations at the load introduction. Models are presented which illustrate the mechanisms responsible for the hybrid effect due to the constraint on failure at both the fibre and ply level. These results give a good understanding of how variability in the carbon fibre strengths can translate into hybrid effects in composite laminates.     

Unidirectional carbon fibre reinforced polyamide-12 composites with enhanced strain to tensile failure by introducing fibre waviness

Unidirectional (UD) carbon fibre reinforced polymers offer high specific strength and stiffness but they fail in a catastrophic manner with little warning. Gas-texturing and non-constrained annealing were used to introduce fibre waviness into UD polyamide 12 composites produced by wet-impregnation hoping to produce composites with a more gradual failure mode and increased failure strain. Both methods increased the variation of fibre alignment angle compared to the control samples. The composites containing wavy fibres exhibited a stepwise, gradual failure mode under strain controlled uniaxial tension rather than a catastrophic failure, observed in control samples. Gas-texturing damaged the fibres resulting in a decrease of the tensile strength and strain to failure, which resulted in composites with lower tensile strength and ultimate failure strain than the control composites. Non-constrained annealing of carbon fibre/PA-12 produced wavy fibre composites with ultimate failure strain of 2%, significantly higher than 1.6% of the control composite.