Room vs. temperature studies of model composites: modes of failure of carbon fibre/epoxy interfaces

Laser Raman spectroscopy (LRS), transmission polarised optical microscopy (TPOM) and scanning electron microscopy (SEM) have been used to characterise the interface of model single-fibre composites. The composites consisted of single-carbon fibres embedded in epoxy resin. Local stress measurements as a function of applied strain were performed using LRS at both room temperature (RT) and 60?°C. Consecutively, the coupons were strained to failure and field emission SEM was used to study the fracture surfaces. In a parallel study, identical systems were subjected to incremental tension and fracture events were recorded as a function of applied strain. At RT, TPOM was used to provide additional insight in the local stress transfer. The stress transfer was found to depend on the combined effect of interfacial chemistry and thermal stresses. Thus, in the case of sized fibres, there is a distinct change in the interfacial failure mode at high temperature, whereas in the case of unsized fibres, the stress transfer is dominated by thermal stresses: at high temperature it is weak, due to the relief of the thermal stress field.     

Interphase characterization of composites under fatigue loadings

The effect of glass-fiber epoxy interface in cross-ply reinforced composites on the fatigue behavior by using load-increasing fatigue test is studied throughout this paper. The damage as measured by stiffness reduction is more significant for the composites with poor bonded fibers as was found for EP sized ones, dependent from test conditions. The loss energy is shown to be a sensitive tool to characterize the nature of fiber–matrix adhesion. The loss energy for composites with poor adhesion between fiber and matrix results in significantly higher amounts of consumed energy during a single stress-strain loop than those composites containing well-bonded fibers.     

Physico-chemical characterization of the fibre/matrix interaction in polyethylene fibre/epoxy matrix composites

The interphase in polyethylene fibre/epoxy matrix composites is studied with FT-IR microspectroscopy using a set-up to investigate the matrix as close to the fibre as a few μm or less. It is shown that moisture present on the fibre surface is able to influence the polymerization reaction of the epoxy/anhydride matrix in an irreversible manner. This effect is enhanced for composites from the more hydrophilic polyvinylalcohol fibre. The fibre/matrix interaction in these thermoplastic fibre composites is also studied with DSC through the characterization of the fibre melting. A decreased 'DSC interaction parameter' is found if the composition of the interphase is changed by moisture. For a composite with an epoxy/amine matrix, on the other hand, the DSC interaction parameter is unaffected by moisture from the fibre surface.     

Interphase formation and fiber matrix adhesion in carbon fiber reinforced epoxy resin: influence of carbon fiber surface chemistry

The chemistry and morphology of the carbon fiber surface are important parameters which govern the properties of the interfacial region and the adhesion between carbon fibers and polymeric matrix in carbon fiber reinforced polymers. In the presented paper the surface chemistry of the fibers is varied while the surface morphology is left unchanged. We analyze chemical functionality and morphology of carbon fiber surfaces showing different degrees of activation, together with the adhesion of these fibers to an epoxy matrix and the width of the interfacial region between fiber and matrix. An increase of the oxygen and nitrogen concentration of the fiber surface, in particular in form of carboxyl functional groups, results in a significant increase of interfacial shear strength. Also the width of the interphase, as determined by scanning force microscopy in nanomechanical mode, depends on the activation degree of the carbon fibers. However, no direct correlation between interphase width, surface chemistry and fiber matrix adhesion is found, suggesting no direct influence of interphase width on adhesion properties.     

碳/环氧复合材料在航天有效载荷支撑结构中的应用

为了满足航天有效载荷轻型化、高比强度、高比模量的要求,分析了航天有效载荷支撑结构常用工程材料的优缺点;阐述了碳/环氧复合材料在航天有效载荷承力支撑结构中应用的可行性;介绍了该材料在航天有效载荷中的实际工程应用及相关设计方法。用优化设计后的碳/环氧复合材料支撑机构支撑80 kg重的有效载荷,在正弦振动和随机振动试验过程中碳纤支撑座没有发生破坏,而质量相对于铝合金结构降低了近30%。结果表明,碳/环氧复合材料应用于航天有效载荷支撑结构中是可行的。      

Effect of processing temperature on the micro- and macro-interfacial properties of carbon fiber/epoxy composites

Three different temperature schemes were applied on carbon fiber/epoxy composite to elucidate the effect on interfacial shear strength (IFSS) and inter-laminar shear strength (ILSS). It showed that carbon fiber/epoxy IFSS was significantly influenced by the processing temperature, while ILSS was only slightly changed. Moreover, the mechanical properties revealed no necessary relationship between the micro- and macro-interfacial strengths with the properties of epoxy matrix. Among all the temperature schemes, Pro2 (the one-platform curing scheme with relatively rapid heating rate) produced highest IFSS and ILSS. Fourier transform infrared spectroscopy analysis demonstrated that the sizing agent can chemically react itself and also react with epoxy resin at temperature 180?°C. The resin rheological data showed that different temperature schemes can considerably impact diffusion behavior of the resin molecules. Hence, the highest interfacial strengths for Pro2 scheme were ascribed to large extent of chemical reactions and good inter-diffusion between components, at the interface region.     

Relation between interfacial shear strength and compressive strength of carbon fiber/epoxy resin composite strands

The mechanism with which the fiber-matrix interfacial strength exerts its influence on the compressive strength of fiber reinforced composites has been studied by measuring the axial compressive strength of carbon fiber/epoxy resin unidirectional composite strands having different levels of interfacial shear strength. The composite strands are used for experiments in order to investigate the compressive strength which is not affected by the delamination taking place at a weak interlayer of the laminated composites. The interfacial strength is varied by applying various degrees of liquid-phase surface treatment to the fibers. The efficiency of the compressive strength of the fibers utilized in the strength of the composite strands is estimated by measuring the compressive strength of the single carbon filaments with a micro-compression test. The compressive strength of the composite strands does not increase monotonically with increasing interfacial shear strength but showes lower values at higher interfacial shear strengths. With increasing interfacial shear strength, the suppression of the interfacial failure in the misaligned fiber region increases the compressive strength, while at higher interfacial shear strengths, the enhancement of the crack sensitivity decreases the compressive strength.     

Liquid crystalline polymer networks based on a nematic epoxy resin with azoxy group

The paper presents research results on curing two recently synthesized liquid crystalline epoxy materials with selected amines. The process of cross-linking, the final product of curing, and the pure monomers were examined using polarizing optical microscopy (POM), differential scanning calorimetry (DSC), and dielectric spectroscopy (DS). Chemical structure of the products was confirmed using spectroscopic methods. The authors attempted to demonstrate how selection of curing conditions (such as the amine used as curing agent, the curing temperature or preparation of the surface in contact with the sample) influences optical properties of the cured product.     

切向气流和激光作用下碳纤维/环氧材料的损伤特性

无气流和切向气流马赫数分别为0.50,0.85条件下,开展了碳纤维/环氧材料激光辐照损伤特性研究实验,对碳纤维、环氧树脂和复合材料热失重曲线、温度历史曲线以及实验后复合材料损伤形进行分析,结果表明：由于切向气流阻止材料燃烧且对材料表面起冷却作用,无气流条件下材料的热损伤区域远大于激光辐照区域,与切向气流条件相比,材料后表面温升时间长、温升幅值高;在切向气流环境下,由于气流作用使得材料的损伤包括烧蚀损伤和断裂损伤;从损伤形貌和后表面温度历史、温升速率比较来看,在切向气流马赫数为0.50~0.85的速度范围内,碳纤维/环氧材料在切向气流和连续激光（102 W/cm2量级）联合作用下的损伤差异不明显。     

A study on the DC-electrical and thermal conductivities of epoxy/ZnO composites doped with carbon black

Thermo-electrical characterizations of hybrid polymer composites, made of epoxy matrix filled with various zinc oxide (ZnO) concentrations (0, 4.9, 9.9, 14.9, and 19.9 wt%), and reinforced with conductive carbon black (CB) nanoparticles (0.1 wt%), have been investigated as a function of ZnO concentration and temperature. Both the measured DC-electrical and thermal conductivities showed ZnO concentration and temperature dependencies. Increasing the temperature and filler concentrations were reflected in a negative temperature coefficient of resistivity and enhancement of the electrical conductivity as well. The observed increase in the DC conductivity and decrease in the determined activation energy were explained based on the concept of existing paths and connections between the ZnO particles and the conductive CB nanoparticles. Alteration of ZnO concentration with a fixed content of CB nanoparticles and/or temperature was found to be crucial in the thermal conductivity behavior. The addition of CB nanoparticles to the epoxy/ZnO matrix was found to enhance the electrical conduction resulting from the electronic and impurity contributions. Also, the thermal conductivity enhancement was mostly attributed to the heat transferred by phonons and electrons hopping to higher energy levels throughout the thermal processes. Scanning electron microscopy and energy-dispersive spectroscopy were used to observe the morphology and elements’ distribution in the composites. The observed thermal conductivity behavior was found to correlate well with that of the DC-electrical conductivity as a function of the ZnO content. The overall enhancements in both the measured DC- and thermal conductivities of the prepared hybrid composites are mainly produced through mutual interactions between the filling conductive particles and also from electrons tunneling in the composite's bulk as well.     

Microwave-absorbing characteristics of epoxy resin composites containing nanoparticles of NiZn- and NiCuZn-ferrites

NiZn- and NiCuZn-ferrite nanoparticles (50–70 nm) with the chemical formula Ni_0.5 Zn_0.5Fe₂O₄ (NiZn) and Ni_0.35Cu_0.15Zn_0.5Fe₂O₄ (NiCuZn) were synthesized by a combustion synthesis method. The nanocrystallite of these materials was characterized by structural and magnetic methods. Saturation magnetization increases from 83 emu/g (NiZn) to 91 emu/g (NiCuZn). Magnetic permeability and dielectric permittivity were measured on sintered samples (pellets and toroids) in the frequency range of 1 MHz–1.8 GHz. Reflection losses (R_L) for both samples were calculated from complex permeability and permittivity. Cu substitution in NiZn-ferrite enhances permeability and R_L.In order to explore microwave-absorbing properties in X-band, magnetic nanoparticles were mixed with an epoxy resin to be converted into a microwave-absorbing composite and microwave behaviors of both materials were studied using a microwave vector network analyzer from 7.5 to 13.5 GHz. Cu substitution diminishes absorption intensity in the range 11.5–12.5 GHz.     

Anisotropic orientation of the carbon fiber/liquid crystalline epoxy resin composites

Anisotropic orientation of carbon fiber (CF)/liquid crystalline epoxy (LCE) resin composite was readily induced during curing on a CF surface along a long molecular axis of CF. Orientation of LCE was confirmed with polarized optical microscope (POM) and wide angle X-ray diffractometer (WAXD). In addition, anisotropic ordering of LCE was correlated with curing rate, dynamic mechanical properties and thermal expansion behaviors of CF/LCE composite. Curing of LCE was accelerated in the presence of CF and the rubbery modulus of the CF/LCE composites cured at low temperature was enhanced by long-range, long axis orientational ordering of the LCE resin along a CF surface. Fully cured CF/LCE composite showed a negative coefficient of thermal expansion in the fiber direction. These results obtained in this study are interpreted in terms of structural changes occurring during curing.     

Composite electroplating of Cu-SiO2 nano particles on carbon fiber reinforced epoxy composites

The main purpose of this work is to co-deposit nano-SiO₂ particles into the copper coatings on carbon fiber reinforced epoxy (C/EP) composite surface by electrodeposition method in order to improve the micro hardness of coatings. C/EP composites are copper plated with sulfuric acid based solution, and the effects of nano-SiO₂ and C₆H₁₂O₆ in the electrolyte contents on the copper coatings are investigated. It is found that crystalline grains of coatings are markedly refined by nano-SiO₂ in the acidic sulfate copper plating bath and the ceramic particles cause an increase in hardness of coatings though nano-SiO₂ results in a decline of deposition rate and a decrease in electrical conductivity of electroplating layers. Otherwise, C₆H₁₂O₆ in the plating bath is indispensable to the layer formation even though nano-SiO₂ added. These results demonstrate that the hardness of coatings will be increased with appropriate contents of co-deposited SiO₂ and C₆H₁₂O₆ in the plating bath.     

Preparation and characterization of carbon nanotubes/epoxy resin nano-prepreg for nanocomposites

The surface carbon nanotubes (CNTs) were modified to generate functional reactors by using the sonicication method to distribute CNTs evenly among epoxy resin, which was prepared into nano-prepreg with carbon fibers. Additionally, based on various proportions of modified and unmodified CNTs, the mechanical properties and conductivities of the composite, as well as, the characteristics of material subjected to various temperature conditions were investigated. Experimental results indicate that increasing CNT content enhances the mechanical strength and electrical properties. At various temperatures, the mechanical strength drops with increase in temperature because different expansion coefficients differ between fiber and epoxy resin. Finally, the failure surface of nanocomposite was examined using scanning electron microscopy (SEM). Finally we provide a discussion of the failure mechanism of the material.     

Preparation and characterization of multiwall carbon nanotube/polypyrrole coaxial fibrils

Multiwall carbon nanotube (MWNT)/polypyrrole (PPy) fibrils were fabricated by template-free in situ electrochemical deposition of PPy over MWNTs, and characterized by electron microscopy and electrical measurements. Scanning and transmission electron microscopy studies reveal that PPy coating on the surface of nanotube is quite uniform throughout the length, with the possibility of forming unique Y-junctions. Current (I)-voltage (V) characteristics at various temperatures show nonlinearity due to tunneling and hopping contributions to transport across the barriers. AC conductivity measurements (300-4.2 K) show that the onset frequency scales with temperature, and the nanoscale connectivity in MWNT/PPy fibrils decreases with the lowering of temperature.     

Effects of plasma surface modification on the mechanical properties of carbon fiber and carbon fiber/epoxy composite

_The effect of surface treatment on mechanical properties of carbon fibers has been investigated by application of plasma polymerization of selected monomers in the vapor phase. The role of the fiber-matrix interface on carbon fiber-reinforced epoxy resin composites has also been studied. Composites have been prepared separately by the use of plasma-modified and unmodified carbon fibers in the epoxy resin matrix. The mechanical properties of carbon fibers (Hercules and Grafil) as well as of fiber/epoxy composites were examined by using single filament and three-point bending tests, respectively. It was observed that plasma polymerization treatment at selected plasma conditions led to significant improvement of interlaminar shear and flexural strength values of composites.     

PTCR effect in carbon black/copolymer composites

Some materials show an abrupt increase in resistivity when the temperature changes only over a few degrees. This phenomenon, known as PTCR effect (positive temperature coefficient of resistivity), has been largely studied in the last few years, due to its potential applications in industry. Particularly, it can be used in auto controlled heaters, temperature sensors, protection circuits and in security systems for power electronic circuits.In this work we present the study of the electrical properties of the percolating system carbon black particles filled with ethylene butylacrylate copolymer composite (EBA), in the temperature range from −100 to 100 °C and in frequencies between 10 Hz and 100 kHz. The PTCR effect was observed at temperatures slightly above the room temperature, for concentrations higher than that of the percolation critical concentration.The mechanism responsible for the change in resistivity, at this stage, is predominantly tunnelling, wherein the conductive filler particles are not in physical contact, and the electrons tunnel through the insulating gap between them. At low temperatures, such as below and close to the glass transition temperature, the DC conductivity obeys the Arrhenius law. The calculated activation energy values are independent of carbon black contents inside the copolymer matrix, suggesting that these particles do not interact significantly with the chain segments of the macromolecules in the EBA copolymer.     

真空紫外辐射对碳/环氧复合材料性能的影响

介绍了真空紫外线的模拟和射流式真空紫外辐射模拟设备的工作原理、技术参数及特点,并利用该设备及其它分析手段对环氧树脂浇铸体(EP648)和碳/环氧复合材料(C/E648)在真空紫外线辐照作用下的出气、质量损失率、层间剪切强度和表面状态的变化进行了研究。试验结果表明：真空紫外线辐照导致材料产生明显的出气效应,质量损失率呈现先递增后趋于平缓的趋势;17 280esh辐照剂量下,EP648的质量损失是C/E648的3.4倍,C/E648的层间剪切强度与辐照前相比下降了13.3%;环氧树脂破损较重,而碳纤维表面状态基本完好,并对内部的环氧树脂起到一定的保护作用。     

氢氧化铝/环氧树脂复合材料的电荷衰减特性

环氧树脂电气绝缘性能优良, 但是其在脉冲功率设备中充当绝缘子时, 表面容易带电且不易衰减, 当表面电荷集聚到一定的程度会造成局部放电甚至发展为沿面闪络。为了提高环氧树脂的沿面闪络性能, 用中心粒径为1 m的氢氧化铝(ATH)无机填料来改善环氧树脂复合材料的表面性能。分别制备了ATH填料质量分数为0%(纯环氧) , 20%, 40%, 60%, 80%和100%的ATH/环氧树脂复合材料试样。用电声脉冲法研究了ATH填料对环氧树脂复合材料电荷衰减性能的影响, 对比了试样直流极化场强为10 kV/mm和30 kV/mm的试验结果。结果表明：ATH/环氧树脂复合材料电荷的衰减常数不仅与填料的质量分数有关, 而且与试样的带电量有关。     

Enhanced interlaminar fracture toughness of unidirectional carbon fiber/epoxy composites modified with sprayed multi-walled carbon nanotubes

A recently reported solvent spraying technique was used herein for incorporation of multi-walled carbon nanotubes (MWCNTs) on unidirectional carbon fiber/epoxy prepregs. The role of the agglomerates reduction of oxidized MWCNTs on Mode-I interlaminar fracture toughness (G_IC) of laminated composites was investigated using double cantilever beam tests. Multiscale laminate composites were fabricated using MWCNTs without and with an acid oxidation, agglomerates reduction (AR) and a sequential treatment based on oxidation and AR. For comparison, specimens without MWCNTs were also prepared and tested. Fourier transform infrared analysis shows evidence of an important amount of oxygenated functional groups on the surface of as-received and oxidized MWCNTs. The results also show Mode-I fracture toughness improvements for all the laminated composites compared to reference samples. A substantial 52% increase in the average G_IC initiation was achieved for laminated composites reinforced with oxidized AR-MWCNTs prepared with only 0.05 wt.% MWCNTs.