反复荷载作用下碳纤维复合材料力电性能试验研究

碳纤维及其复合材料已广泛应用于土木工程领域,本文针对手工浸渍的环氧树脂基碳纤维布复合材料在反复荷载作用下的力电性能进行了系统的试验研究和分析.研究表明,在反复荷载作用下该类型复合材料的电阻变化率与应变关系曲线表现出与单调荷载作用下不同的新的特征.本文通过对试验结果的分析,进一步验证和深化了对碳纤维复合材料导电机理的认识,为进一步研究土木工程中环氧树脂基碳纤维复合材料的力电性能以及发展可实际应用的地震损伤自监测智能结构奠定了基础.     

基于纤维束/环氧树脂复合材料试验的单向层合板横向拉伸强度预测方法

实验研究表明,纤维束/环氧树脂复合材料试件的横向拉伸强度与工程上常用的单向层合板横向拉伸强度在趋势上具有很好的相关性,但是数值上存在一定差距。本文使用两种碳纤维和两种环氧树脂制备了三种纤维束/环氧树脂复合材料和单向层合板,并分别测量了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度,以及环氧基体的拉伸强度。在实验基础上,应用Griffith断裂强度理论建立了纤维束/环氧树脂复合材料和单向层合板的横向拉伸强度的关系模型,通过两种复合材料实验的结果拟合了该模型中的参数。利用第三种复合材料实验进行校验,发现该模型预测的单向层合板横向拉伸强度与实测强度之间达到很好的一致性,相对偏差为9%。采用本文提出的方法,可以用较为简单的纤维束/环氧树脂复合材料和环氧基体拉伸试验预测单向层合板的横向拉伸强度。     

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。     

单丝断裂能量法研究温度对碳纤维/环氧界面性能的影响

针对两种不同上浆剂碳纤维/高温固化环氧树脂体系, 采用基于WND(Wagner-Nairn-Detassis)能量模型的单丝断裂法, 测试分析了从室温到130 ℃范围内单丝复合体系界面断裂能的变化规律, 研究了碳纤维上浆剂对界面耐热性能的影响, 并结合复合材料层板的短梁剪切性能, 分析了微观和宏观界面性能的关联性。结果表明: 在测试温度范围内, 碳纤维/环氧体系的界面断裂能随温度升高呈先下降而后基本不变的趋势, 去除上浆剂后界面断裂能及其随温度的变化程度与未去除上浆剂的情况存在差异, 说明上浆剂对界面耐热性有重要作用。碳纤维/环氧树脂层板层间剪切强度随温度升高线性下降, 与界面断裂能的变化规律不一致, 这与两种测试方法的原理及界面破坏位置的不同有关。      

碳纤维/环氧树脂复合材料缠绕接头拉伸失效机制

通过试验及数值模拟对碳纤维/环氧树脂复合材料缠绕接头轴向拉伸失效机制进行研究。基于ABAQUS有限元软件,通过连续介质损伤模型及内聚区模型,分别对碳纤维/环氧树脂复合材料缠绕接头各部件及界面进行模拟,编写用户自定义材料子程序(UMAT),建立复合材料的渐进损伤模型,最终得到碳纤维/环氧树脂复合材料缠绕接头的应力分布和载荷-位移曲线,并与试验结果对比确定结构的失效机制。结果表明:有限元分析所得碳纤维/环氧树脂复合材料缠绕接头损伤部位及失效模式与试验吻合,失效载荷与试验值相差较小,证明仿真分析方法的有效性。通过对比失效模式发现,拉伸载荷作用下,链环是主承力部件,其弧形端部是应力集中处,纤维断裂即从此处开始发生并向外扩展,导致链环断裂及整体结构破坏。      

弱电流对碳纤维/环氧树脂界面黏结性能的影响

针对碳纤维/环氧树脂体系,设计了在碳纤维单丝复合体系加载弱电流的方法,并通过单丝断裂实验,研究了弱电流对两种碳纤维/环氧树脂界面黏结性能的影响。结果表明:在0.60~0.67mA直流电加载一定时间后,T300B和T700SC碳纤维体系的界面黏结强度均下降,界面载荷传递效率降低,随着电流加载时间的延长,降低程度变化不明显;T300B碳纤维体系界面性能对弱电流的敏感性更高,可能与两种碳纤维的表面特性不同有关;通过对不同固化度试样加载弱电流后的界面性能进行分析,认为碳纤维/环氧树脂界面性能受弱电流影响的机理与界面区应力状态改变以及导电产生的焦耳热效应有关。     

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。     

单丝断裂双树脂法研究碳纤维/环氧树脂界面粘结性能

建立了单丝断裂双树脂体系法, 利用外层树脂的韧性使包埋于内层脆性树脂中的纤维单丝断裂达到饱和, 解决了断裂伸长率较低的树脂基体采用传统的单丝断裂法无法测得界面剪切强度的问题。分别采用界面剪切强度和界面断裂能作为表征参量, 考察了干态及湿热条件下两种T300级和两种T800级碳纤维/环氧树脂的界面性能, 并与单丝断裂单树脂体系的界面性能进行比较。结果表明: 单丝断裂双树脂体系与单树脂体系在表征碳纤维/环氧树脂的界面性能上定性规律一致; 双树脂体系界面断裂能和界面剪切强度均可评价界面的耐湿热性能, 且二者得到的变化规律一致; 湿热处理后界面粘结性能均呈下降趋势, 国外碳纤维体系的界面耐湿热性能明显优于国产碳纤维体系。     

树脂基搭接碳纤维智能层的力阻特性

提出一种具有局部搭接结构的树脂基碳纤维智能层, 将其敷设于结构表面以检测结构受载时的变形, 实现对结构大范围监测。基于该智能层, 采用单轴拉伸和三点弯曲的加载方式, 对构件进行应变和位移检测。实验发现, 碳纤维局部搭接结构是引起力阻效应的主要因素, 其单位应变的电阻率变化的灵敏度达到104, 相当于非搭接连续碳纤维复合材料力阻效应灵敏度的34倍。实验结果还进一步表明, 树脂基搭接碳纤维智能层力阻曲线光滑稳定, 其传感极限约为8500 με。建立了树脂基搭接碳纤维复合材料的电学模型, 揭示了这种力阻效应主要来源于搭接界面处层间电阻的变化, 并从纤维轴向力、搭接面积和层间剪应变三个方面解释了这种层间电阻变化的机制。      

单丝断裂双树脂法研究碳纤维/环氧树脂界面粘结性能

建立了单丝断裂双树脂体系法,利用外层树脂的韧性使包埋于内层脆性树脂中的纤维单丝断裂达到饱和,解决了断裂伸长率较低的树脂基体采用传统的单丝断裂法无法测得界面剪切强度的问题.分别采用界面剪切强度和界面断裂能作为表征参量,考察了干态及湿热条件下两种T300级和两种T800级碳纤维/环氧树脂的界面性能,并与单丝断裂单树脂体系的界面性能进行比较.结果表明:单丝断裂双树脂体系与单树脂体系在表征碳纤维/环氧树脂的界面性能上定性规律一致;双树脂体系界面断裂能和界面剪切强度均可评价界面的耐湿热性能,且二者得到的变化规律一致;湿热处理后界面粘结性能均呈下降趋势,国外碳纤维体系的界面耐湿热性能明显优于国产碳纤维体系.     

Measurement of stress relaxation in broken fibers embedded in epoxy using Raman spectroscopy

Raman spectroscopy was used to study the stress relaxation in broken fibers in a unidirectional composite. A single-fiber model composite consisting of a high modulus PAN-based carbon fiber and an epoxy resin matrix was loaded incrementally until the fiber got broken.Then the stress profile in the broken fiber was monitored under constant overall strain for 1000 hours by determining fiber stress through the stress dependence of the 2700 cm–1 Raman band peak position. Three experiments were done at different overall strains.It was observed that the stress profile in each broken fiber changed only a little even after 1000 hours whereas matrix normal stress in the fiber direction relaxed to about a quarter of the initial value in about 200 hours. It is shown that this result does not support linear viscoelastic solutions based on perfect bonding at interface since the present experiments had interfacial debonding and matrix shear yielding around fiber breaks.     

A carbonyl iron/carbon fiber material for electromagnetic wave absorption

A carbonyl iron/carbon fiber material consisting of carbon fibers grown on micrometer-sized carbonyl iron sphere, was synthesized by chemical vapor deposition using a mixture of C2H2 and H2. The hollow-core carbon fibers (outer diameter: 140 nm and inner diameter: 40 nm) were composed of well-ordered graphene layers which were almost parallel to the long axis of the fibers. A composite (2 mm thick) consisting of the carbonyl iron/carbon fibers and epoxy resin demonstrated excellent electromagnetic (EM) wave absorption. Minimum reflection losses of -36 dB (99.95% of EM wave absorption) at 7.6 GHz and -32 dB (99.92% of EM wave absorption) at 34.1 GHz were achieved. The well-dispersed and network-like carbon fibers in the resin matrix affected the dielectric loss of the EM wave while the carbonyl iron affected the magnetic loss.     

苯酚电聚合膜修饰炭纤维表面对 环氧基复合材料性能的影响

将连续炭纤维束用自制的空气梳分散成单丝状长带后, 通过采用循环伏安法的电化学方法将单体苯酚在炭纤维表面聚合成膜, 对炭纤维进行表面修饰, 以提高复合材料中炭纤维与树脂基体的界面粘结性能。红外光谱分析表明, 苯酚电聚合膜能够增加炭纤维表面的羟基、 醚键等活性官能团, 从而提高炭纤维与环氧树脂基体的界面粘结强度。与未进行表面修饰的炭纤维增强环氧树脂复合材料相比, 以聚苯酚膜修饰的炭纤维单丝带增强的环氧树脂基复合材料横向拉伸强度最大提高了90%, 纵向拉伸强度最大提高了45%, 层间剪切强度最大提高了110%。实验也表明, 将炭纤维束分散成炭纤维单丝带后能够更有效地增强复合材料的各项力学性能。      

Development of a carbonization-in-nitrogen method for measuring the fiber content of carbon fiber reinforced thermoset composites

Carbon fiber reinforced thermoset composites such as carbon fiber epoxy composites are widely used in aircraft and aerospace, and are being increasingly used in automotive applications because of their lightweight characteristics, high specific strength, and stiffness. The carbon fiber content in the composite plays a critical role in enhancing structural performance. The carbon fibers contribute to the strength and stiffness; therefore, the mechanical properties of the composite are greatly influenced by the carbon fiber content. Measurement of carbon fiber content is essential for product quality control and process optimization. In this work, a novel carbonization-in-nitrogen (CIN) method is developed to characterize the fiber content in carbon fiber thermoset composites. A carbon fiber composite sample is carbonized in a nitrogen environment at elevated temperatures, alongside a neat resin sample. The carbon fibers are protected from oxidization while the resin (the neat resin and the resin matrix in the composite sample) is carbonized under nitrogen environment. The neat resin sample is used to calibrate the resin carbonization rate and calculate the amount of the resin matrix in the composite sample. The new method has been validated on several thermoset resin systems, and found to yield accurate estimation of fiber content in carbon fiber thermoset composites.     

Surface and sub-surface degradation of unidirectional carbon fiber reinforced epoxy composites under dry and wet reciprocating sliding

The role of water on the sub-surface degradation of unidirectional carbon fiber reinforced epoxy composite is examined. The correlation between the debonding of carbon fibers at the fiber–epoxy interface, and the wear behavior of the carbon fiber composite are discussed based on an in-depth analysis of the worn surfaces. We demonstrate that a reciprocating sliding performed along an anti-parallel direction to the fiber orientation under dry conditions results in a large degradation by debonding and breaking of the carbon fibers compared to sliding in parallel and perpendicular directions. Immersion in water has a harmful effect on the wear resistance of the carbon fiber composite. The competition between crack growth and the wear rate of epoxy matrix and/or carbon fibers in the sliding track determines the level of material loss of the composite in both test environments.     

Comparison of nondestructive microfailure evaluation of fiber-optic Bragg grating and acoustic emission piezoelectric sensors using fragmentation test

Nondestructive evaluation of microfailure mechanisms in two-diameter SiC fibers/epoxy composites is investigated using a directly embedded fiber-optic sensor attached with an acoustic emission piezoelectric (AE-PZT) sensor. Interfacial shear strength by fragmentation test, and optical failure observation inside microcomposite can contribute to analyze two sensors quantitatively. Although fiber Bragg grating (FBG) sensor exhibits sudden wavelength shift due to plastic deformation by larger diameter SiC fiber breakage, AE-PZT monitors much more precise microfailure process, such as the fiber break or matrix cracking. Since the FBG sensor can measure the strain at only a single point, whether it can detect a fiber break in single-fiber composite specimen depends on its proximity to the failure location. In addition, micro-strain measurement at one single point may not provide enough information on the whole microfailure process including multiple fiber breakage and matrix crack. It can be considered that FBG sensor can be somewhat effective in measuring the continuous micro-strain change due to the internal disturbance such as resin curing, whereas AE-PZT sensor can be effective in detecting the microfailure by elastic wave propagation through the composite materials.     

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。     

Matrix cracking behaviors in carbon fiber/epoxy laminates filled with cup-stacked carbon nanotubes (CSCNTs)

This study investigated the damage accumulation behaviors in carbon fiber reinforced nanocomposite laminates under tensile loading. The nanocomposite laminates used in this study were manufactured from prepregs consisting of traditional carbon fibers and epoxy resin filled with cup-stacked carbon nanotubes (CSCNTs). Thermo-mechanical properties of unidirectional carbon fiber reinforced nanocomposite laminates were evaluated, and cross-ply laminates were subjected to tension tests in order to observe the damage accumulation behaviors of matrix cracks. A clear retardation of matrix crack onset and accumulation was found in composite laminates with CSCNT compared to those without CSCNT. Fracture toughness associated with matrix cracking was evaluated based on the analytical model using the experimental results. It was suggested that the dispersion of CSCNT resulted in fracture toughness improvement and residual thermal strain decrease, which is considered to cause the retardation of matrix crack formation.