碳纳米管纤维/环氧树脂复合材料的界面剪切强度及微观结构

研究了碳纳米管纤维的微观结构和拉伸性能,并进一步分析了其与环氧树脂形成界面剪切强度及微观结构。采用单丝断裂试验测试了碳纳米管纤维/环氧树脂复合材料体系的界面剪切强度,结合单丝断裂过程中的偏光显微镜照片、复合材料的拉曼谱图和断口扫描电镜照片,研究了碳纳米管纤维/环氧树脂复合材料界面的微观结构。结果表明: 碳纳米管纤维/环氧树脂复合材料的界面剪切强度约为14 MPa;在碳纳米管纤维和环氧树脂形成界面的过程中,环氧树脂可以浸渍纤维,形成具有一定厚度的复合相,这种浸渍过程和界面相的形成都有利于碳纳米管纤维与基体之间的连接。     

The strength of carbon fibres

The available information on the structure and properties of high strength carbon fibres is reviewed, and some new data are presented, showing the effects of boron doping and neutron irradiation on the properties of PAN-based carbon fibres.Theories relating the Young's modulus of the fibre to its microstructure are examined, and it is concluded that their relationship is qualitatively understood. Variations in electrical resistivity with different treatments may also be explained satisfactorily, again, in qualitative terms.The strength of carbon fibres is less well understood, however. It has been suggested that the fibre strength is governed by the presence or absence of stress-raising flaws, but while it is clear that such flaws can markedly reduce the strength, there is no clear estimate of the strength of an unflawed fibre.In this paper, we examine an alternative failure mechanism, initiated by shearing of the graphite crystallites in the fibre, and we conclude that such a mechanism may control the strength of the more graphitic fibres. Increases in the strength of carbon fibres may thus be achieved, not only by reducing the number and severity of the flaws, but also by applying the principles of solid-solution or dispersion hardening, and by reducing the graphite crystallite size (grain refining).     

Interfacial shear strength of a glass fiber/epoxy bonding in composites modified with carbon nanotubes

In recent years, carbon nanotubes (CNTs) grown on fibers have attracted a lot of interest as an additional reinforcing component in conventional fiber-reinforced composites to improve the properties of the fiber/matrix interface. Due to harsh growth conditions, the CNT-grafted fibers often exhibit degraded tensile properties. In the current study we explore an alternative approach to deliver CNTs to the fiber surface by dispersing CNTs in the fiber sizing formulation. This route takes advantage of the developed techniques for CNT dispersion in resins and introduces no damage to the fibers. We focus on unidirectional glass fiber/epoxy macro-composites where CNTs are introduced in three ways: (1) in the fiber sizing, (2) in the matrix and (3) in the fiber sizing and matrix simultaneously. Interfacial shear strength (IFSS) is investigated using single-fiber push-out microindentation. The results of the test reveal an increase of IFSS in all three cases. The maximum gain (over 90%) is achieved in the composite where CNTs are introduced solely in the fiber sizing.     

Interfacial reaction and strength of SiC fibres coated with aluminium alloys

Silicon carbide fibres (Nicalon) were coated with pure aluminium and aluminium alloys containing silicon. The coated fibres were annealed to produce an interfacial reaction zone between the coated layer and the fibre. The effect of this reaction zone on the tensile strength of the fibre was investigated. During the early stages of growth the reaction zone of the fibre is thin, and the strength of the fibre is controlled by inherent defects so that the fibre retains its original strength. After the early stages, notches are formed in the reaction zone of the fibre on loading at a small strain and the fibre fractures when a notch extends into the fibre. In this stage the fibre strength is dependent on the thickness of the reaction zone. An alloying addition of 1 or 5 at % Si to the aluminium matrix was found to be effective in retarding the growth rate of the reaction zone.On leave from Institute of Metal Research, Academia Sinica, Shenyang, China.On leave from Instituto Superior Tecnico, Departamento de Metalugia e Materais, Lisbon, Portugal.On leave from Hitachi Research Laboratory, Hitachi Ltd, Saiwai-Cho, Hitachi, Ibaraki 317, Japan.     

The recoil compressive strength of pitch-based carbon fibres

The recoil compressive strengths of pitch-based carbon fibres with folded-radial and flat-layer textures were compared. Using the recoil test method, it was shown that the compressive strengths of pitch-based carbon fibres with folded-radial textures are superior to pitch-based carbon fibres with flat-layer textures at all modulus levels. Analysis of the failed fibre ends revealed that the folded-radial texture appeared to inhibit shearing of the basal planes. This may account for the superior compressive strengths of pitch-based carbon fibres with folded-radial textures. Procedural differences in the recoil test were shown to influence the calculated recoil compressive strength of pitch-based carbon fibres. Microscopic analysis of the recoil test specimens revealed that some energy is absorbed in the area where the fibre is secured to the support tab.     

The fabrication of ceramic-coated carbon fibre duplex elements

Attempts have been made to fabricate a bicomponent composite reinforcing element comprised of a central core of carbon fibre filaments surrounded by a cylindrical silicon carbide sheath. Such fibres are particularly attractive for composite reinforcement since they are potentially capable of exhibiting “duplex” type behaviour, thereby providing a possible means of minimizing anisotropy effects and increasing composite fracture toughness and ductility. Furthermore these elements should provide additional advantages such as eventually enabling multi-filament tows of high strength, low modulus carbon fibre to be formed into large compound fibres which combine high specific strength with a significantly improved overall Young's modulus arising from the stiffness of the ceramic sheath, which should also exhibit a high resistance to chemical attack. Methods of consolidating the multi-filament tow prior to coating have been investigated and suitable preliminary treatments evolved; tows have been coated with silicon carbide using a conventional vapour phase deposition technique to form elements basically conforming to “duplex” requirements. Initial tensile tests upon these elements are reasonably encouraging and reveal none of the side effects encountered previously with boron coatings; it is anticpated that much stronger silicon carbide tubes may be fabricated eventually by this technique using more closely controlled reaction conditions.     

Mixed resin and carbon fibres surface treatment for preparation of carbon fibres composites with good interfacial bonding strength

The objective of this work is to improve the interlaminar shear strength of composites by mixing epoxy resin and modifying carbon fibres. The effect of mixed resin matrix’s structure on carbon fibres composites was studied. Anodic oxidation treatment was used to modify the surface of carbon fibres. The tensile strength of multifilament and interlaminar shear strength of composites were investigated respectively. The morphologies of untreated and treated carbon fibres were characterized by scanning electron microscope and X-ray photoelectron spectroscopy. Surface analysis indicates that the amount of carbon fibres chemisorbed oxygen-containing groups, active carbon atom, the surface roughness, and wetting ability increases after treatment. The tensile strength of carbon fibres decreased little after treatment by anodic oxidation. The results show that the treated carbon fibres composites could possess excellent interfacial properties with mixed resins, and interlaminar shear strength of the composites is up to 85.41 MPa. The mechanism of mixed resins and treated carbon fibres to improve the interfacial property of composites is obtained.     

Interlaminar shear strength evaluation of curved composite samples

To assess the bonding quality between the different layers of composite rings fabricated by filament winding it is necessary to choose a useful shear test. A method for measuring the shear strength of thin curved samples is proposed herein. Stress distribution within the probe is calculated and evaluated by finite element method.     

Interfacial shear behavior of 3D composites reinforced with CNT-grafted carbon fibers

This paper presents the Molecular Dynamic (MD) models and the simulation results for the shear deformation process of an interface representative cell to develop an understanding of the roles of multi-walled carbon nanotube (MWNT) in enhancing fiber–matrix interfacial properties such as shear modulus and strength. Based on the MD results and the two simple formulae of rule of mixture, the shear modulus and strength of the carbon nanotube (CNT) grafted interface can be predicted. It is shown that there exists a good agreement between the predicted fiber–matrix interfacial shear strength with and without grafted CNTs and these measured from single-fiber micro bond test and single-fiber fragmentation test. The MD simulation also shows the MWNTs’ shear stress cross-section distribution is similar to that of a circular beam in shear with a non-zero shear stress on the neutral plane.     

The effect of nanostructure upon the compressive strength of carbon fibres

The relationship between the structure and the compressive strength of carbon fibres has been studied in detail. In order to determine the compressive strength, a combination of single-fibre composite tests and Raman spectroscopy was employed. It was found that the compressive stress–strain curves showed nonlinear behaviour, with modulus softening in compression. The compressive strengths for the fibres with a modulus ≥400 GPa were measured as ≤2 GPa and those with a modulus <400 GPa were >2 GPa. We have introduced a model to explain this behaviour that assumes that the fibres behave as composites consisting of both crystallites and amorphous carbon. It is suggested that the compressive strength is controlled by the critical stress for kinking the crystallites in the fibres. Hence, the compressive strength of carbon fibres is found to depend upon the shear modulus of the fibres and the orientation of the crystallites within them.     

The electrochemical evaluation of the surfaces of carbon fibres

Electrochemical methods are used to follow the changes in wetted fibre area produced by surface treatment, to estimate the fraction of that area in pores and to detect surface chemical groupings. The changes in area wetted by electrolyte correlate with the changes in interlaminar shear strength of the corresponding epoxy resin composites.     

Comparison of shear strength tests on AV119 epoxy-joined carbon/carbon composites

The results of an experimental investigation on carbon/carbon composites (C/C) bonded joints tested in shear at room temperature, under seven different configurations, are presented. The samples have been joined by an epoxy adhesive (AV119): this adhesive is not to be considered as a suitable joining material for C/C for high temperature applications, but just as a model joining material chosen to obtain several bonded samples in a short time. Advantages and disadvantages of each configuration are discussed from an experimental point of view. A finite element analysis is also performed to compare the stress distribution obtained within the joint for the different testing geometries. It is shown that the measured values of the apparent shear strength decrease with the maximum opening stress estimated within the middle of the joint.     

Interfacial shear strength of Ti/SiC fibre composites measured by synchrotron strain measurement

A single fibre Ti/SiC_f composite sample was loaded incrementally and unloaded in-situ on a beam line at the European Synchrotron Research Facility (ESRF) in Grenoble. It was possible to measure the strain along the fibre with a high spatial resolution (50 μm) within the titanium matrix. The interfacial frictional shear strength inferred from the longitudinal strain profile of the broken fibre was calculated to be around 200 MPa. The result is compared to the interfacial shear stress derived from other methods developed to evaluate the matrix/reinforcement interface in this system. An axisymmetric finite element model gave good agreement between the numerical and experimental results.     

Ab initio calculations of the walls shear strength of carbon nanotubes

The dependence of the energy of interwall interaction in double-wall carbon nanotubes (DWNT) on the relative position of walls has been calculated using the density functional method. This dependence is used to evaluate forces that are necessary for the relative telescopic motion of walls and to calculate the shear strength of DWNT for the relative sliding of walls along the nanotube axis and for their relative rotation about this axis. The possibility of experimental verification of the obtained results is discussed.     

Improving the strength of brazed joints to alumina by adding carbon fibres

The addition of short, bare, carbon fibres to a silver-based active brazing alloy (63Ag-34Cu-2Ti-1Sn) resulted in up to 30% improvement in the shear/tensile joint strength of brazed joints between stainless steel and alumina. The optimum fibre volume fraction in the brazing material was 12%. This improvement is attributed to the thinning and microstructural simplification of the alumina/braze reaction product (titanium-rich) layer, the softening of the brazing alloy matrix, the strengthening of the braze and the reduction of the coefficient of thermal expansion. The depth of titanium diffusion into the alumina was decreased by the fibre addition. The first two effects are due to the absorption of titanium by the fibres. This absorption resulted in less titanium in the brazing alloy matrix, a braze/fibre particulate reaction product (titanium-rich) on the fibres and the diffusion of titanium into the fibres. In contrast, the use of an active brazing alloy with a lower titanium content but without carbon fibres gave much weaker joints. This revised version was published online in November 2006 with corrections to the Cover Date.     

Effects of surface and sizing treatments on axial compressive strength of carbon fibres

Attempts have been made to estimate the fibre axial compressive strength of pitch-based graphitized fibres, and the effects of surface- and size-treatment on compressive strength was investigated. The estimated compressive strength of fibres decreases with increasing temperature. This decrease in compressive strength may be accounted for by a decrease in the radial compression force owing to a decrease in the residual thermal stress and a decrease in Young's modulus of the resin matrix. There is a linear relationship between the estimated compressive strength and radial compression force in a temperature range from room temperature to 80 °C. The real compressive strength of the fibres, determined by extrapolating this straight line until the radial compression force is zero, increases with increasing shear yield strength at the fibre-matrix interphase. In order to obtain reinforcing fibres with a higher compressive strength, it will be necessary to surface- and size-treat the fibres.     

On the estimation of the tensile strength of carbon fibres at short lengths

Generally, to determine the fibre-matrix interfacial properties in fibre reinforced plastics, it is necessary to know the tensile strength of the fibre at very short lengths, for which direct measurements are not possible. Accordingly, in this study, the determination of the tensile strength of high strength carbon fibres and their gauge length dependence are analysed by means of the Weibull model. The influence of the estimator chosen and of the sample size on the calculated value of the tensile strength of the fibre are first determined. Secondly, the accuracy of the three- and the two-parameter Weibull distributions is examined. Finally, it is shown that the most appropriate extrapolation at short length is performed by means of a linear logarithmic dependence on gauge length of the tensile strength. This method seems to be valid for untreated as well as for surface-treated high strength carbon fibres.