Effect of interfacial silane network structure on interfacial strength in glass fibre composites

The effect of crosslinked structure of silane condensates on the interfacial strength between silane-treated glass fibre and epoxy resin was examined using a dialkoxysilane, γ-aminopropyldimethoxymethylsilane (APDS), and a trialkoxysilane, γ-aminopropyltrimethoxysilane (APTS). Glass fibres were treated with aqueous solutions of APDS, ATPS or mixtures of both to change the silane crosslink density, and then washed with methanol to leave only chemically bonded silane at the surface. As an index of interfacial strength, the interfacial stress transmissibility was determined using a modified single-filament test. The interfacial strength decreased in the order APDS, mixtures of APTS and APDS, and APTS, showing that siloxane crosslinking of the silane condensates reduces the interfacial strength. This result suggests that the crosslinked structure depresses the penetration of the resin into the interphase and hinders reaction between silane organofunctional groups and the resin. In addition, treatment of the glass fibre with the various APDS-APTS mixtures reduced the interfacial strength in all cases, compared with the strength after APDS treatment alone. This indicates that the silane interphases formed in the present system do not lead to the formation of interpenetrating networks with the resin molecules.     

Effect of humidity during manufacturing on the interfacial strength of non-pre-dried flax fibre/unsaturated polyester composites

The influence of moisture content in the environment during manufacture of a novel cobalt-free UP matrix reinforced with flax fibres, on the fibre–matrix adhesion was studied. Flax surface energy was experimentally determined by measuring contact angles on technical fibres, using the Wilhelmy technique and the acid–base theory. The mechanical strength of the interface under different humidity conditions was characterized by the critical local value of interfacial shear stress, τ_d, at the moment of crack initiation, which was assessed by single-fibre pull-out tests. Differential scanning calorimetry and X-ray photoelectron spectroscopy analysis gave further insight into the topic. The results suggest that the effect of humidity during manufacturing on the composite interface might be limited. However, longitudinal composite strength decreased somewhat for composites produced in humid conditions, showing that there is some detrimental effect of high levels of moisture during cure on the fibre mechanical performance, likely caused by some fibre degradation.     

An improved engineering test method for measurement of the compressive strength of unidirectional carbon fibre composites

A modified test coupon has been developed for measurement of the compressive strength of unidirectional composite materials. The coupon proposed has a machined gauge length 8 mm long with integrally moulded end tabs of 0°/+/−45° construction. Strengths were measured on the new coupon and found to lead to improvements of over 50% compared with the CRAG standard method. In addition, all failures occurred within the gauge lengths instead of at the end tabs. Failure stresses were in keeping with the stresses achieved in the 0° layers of laminates loaded to compressive failure.It is therefore strongly recommended that laboratories make use of the waisted coupon proposed in this work to generate unidirectional compressive strength design data for composite materials. In addition, it is recommended that the standards bodies seriously consider modifying their approved test method for unidirectional compressive strength and consider adopting the coupon design proposed in this work.     

Surface characterization of carbon fibres and interfacial properties of carbon fibre composites

The surface characteristics of carbon fibres treated by electrochemical oxidation have been determined, and the effect of these properties on the interfacial adhesive strength (interlaminar shear strength (ILSS) and transverse flexural strength) of epoxy-matrix composites investigated. The surface properties of the carbon fibres were dependent on the nature of the electrolyte used during the electrochemical oxidative treatment. Use of a basic electrolyte resulted in an improvement of composite ILSS and transverse flexural strength due to an increase in the number of surface functional groups. On the other hand, using an acid electrolyte resulted in the ILSS increasing with the number of surface functional groups, while the transverse flexural strength decreased for composites in which the carbon fibres had a high surface oxygen content. It is concluded that the weak boundary layer derived from fissures on the surface of carbon fibres treated in an acid electrolyte decreased the transverse flexural strength.     

Thermomechanical behaviour of interfacial region in carbon fibre/epoxy composites

A study of the thermomechanical stability of the fibre-matrix interphase in carbon/epoxy composites has been carried out. The thermodynamic work of adhesion has been evaluated at room temperature by wetting measurements. The interfacial shear stress transfer level τ for sized and desized carbon fibre has been measured as a function of temperature by means of a single-fibre fragmentation test. As the test temperature increased τ values were found to decrease, with values being higher for the desized carbon fibre. The dependence of interfacial shear stress transfer on bulk matrix mechanical properties (modulus and shear strength) has also been discussed. Dynamic mechanical measurements performed on single-bundle composites confirmed the better thermomechanical stability of the desized fibre interphase.     

碳/碳复合材料纤维束/基体界面强度的表征

为了获得准确表征纤维束界面强度的方法,对影响纤维束界面强度的因素进行了研究.通过顶出方法对纤维束界面强度进行表征,利用SEM和Micro-CT研究了纤维束界面层微结构对纤维束界面强度的影响,并通过有限元分析方法对纤维束界面的断裂行为进行分析.结果表明：在纤维束界面层含有不同程度的脱粘和裂纹缺陷结构,影响了纤维束的界面强...     

Critical fibre length and tensile strength for carbon fibre-epoxy composites

The tensile strength of epoxy resin reinforced with a random-planar orientation of short carbon fibres decreases with increasing temperature. This decrease may be estimated by the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the critical fibre length obtained by taking the distribution of fibre strength into consideration. The experimental value at room temperature is smaller than the calculated value. It is inferred that this result is attributed to the stress concentration caused by ineffective fibres produced during preparation which were shorter than the critical fibre length.     

The strength of hybrid glass/carbon fibre composites

The tensile mechanical properties of hybrid composites fabricated from glass and carbon fibres in an epoxy matrix have been evaluated over a range of glass: carbon ratios and states of dispersion of the two phases. The failure strain of the carbon phase increased as the relative proportion of carbon fibre was decreased, and as the carbon fibre was more finely dispersed. This behaviour is commonly termed the hybrid effect, and failure strain enhancement of up to 50% has been measured. Only part of the effect may be attributed to internal compressive strains induced in the carbon phase by differential thermal contraction as the composite is cooled from its cure temperature. The laminae or ligaments of carbon fibre dispersed in the glass fibre phase show a multiple failure mode, and when the constitution is favourable catastrophic failure does not occur until a considerable number of ligament fractures have accumulated. Failure is thus progressive, and the material is effectively tougher than equivalent all-carbon fibre composites.     

Effect of fibre conditioning on the interfacial shear strength of glass-fibre composites

A study of the interaction of water-sized E glass fibres, supplied with and without an aminopropylsilane coupling agent, with vinyl and epoxy resins is reported. Interfacial shear strength measurements, made by means of the multifragmentation technique, have demonstrated that molecularly thin layers are effective adhesion promoters, as indicated by (a) the silane contamination on the nominally non-coupled fibres and (b) the aqueous extraction of the coupled fibres. Epoxy resins adhere through amino coupling reactions, but for the vinyl ester resin the maximum adhesion probably occurs through aluminium hydroxide sites exposed through extractive hydrolysis, and acidic residues in the resin.     

Influence of fibre surface oxidation treatment on mechanical interfacial properties of carbon fibre/ polyarylacetylene composites

Abstract

In the present work, the mechanical interfacial properties of carbon fibre (CF) reinforced polyarylacetylene (PAA) resin composites were modified through the surface oxidation treatment of carbon fibres by ozone. Both X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy showed that oxidation treatment could increase the amount of elemental oxygen on the fibre surface markedly by introducing more oxygen groups. Atomic force microscopy (AFM) images indicated that weak surface regions of fibres had been etched and removed, and the degree of fibre surface roughness was increased. The interlaminar shear strength (ILSS) and the interfacial shear strength (IFSS) of CF/PAA composites were both improved notably (no less than 50%). It could be concluded that an improvement of fibre surface chemical activity, better wettability of resin on the carbon fibre surface, and stronger mechanical joining between fibres and resin all resulted in the modification of interfacial properties of carbon fibre reinforced PAA composites. The influences of temperature, ozone concentration, and treatment time on the oxidation results were studied, and optimal treatment parameters determined.     

Effects of fibre length on tensile strength of carbon/glass fibre hybrid composites

The tensile strength of epoxy resin reinforced with random-planar orientation of short carbon and glass fibres increased as the length of the reinforcing fibres increased, and the increase in tensile strength remained almost unchanged after the fibre length reached a certain level. The tensile strength of composites at any fibre length could be estimated by taking the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the mean critical fibre length into consideration. The tensile strength of the hybrid composite could be estimated by the additive rule of hybrid mixtures, using the tensile strength of both composites.     

A method for the chemical anchoring of carbon nanotubes onto carbon fibre and its impact on the strength of carbon fibre composites

This article proposes an alternative way to use carbon nanotubes to improve the performance of carbon fibre-reinforced composites. A chemical process, based on esterification of surface groups, is used to anchor nanotubes onto carbon fibre surface. Anchored nanotubes form a network surrounding the carbon fibres. After CNT anchoring, the tow is impregnated with an epoxy resin and tensile tests are performed on this minicomposite sample. By enhancing matrix properties and fibre/matrix interface, the CNT network has a significant influence on the composite strength.     

Compressive strength of unidirectional and crossply carbon fibre/PEEK composites

The commonly accepted production methods of composite systems generally result in departure of the plies properties from transverse isotropy due to stresses acting during fibre—matrix bond formation. This anisotropy coupled with the composite structure affects compressive loading; the ultimate stresses as well as the direction, in- or out-of-plane, of kink propagation. A unidirectional and a crossply carbon fibre/PEEK composites were compression tested at ambient and elevated temperature as well as exposed to various chemical environments. Significant disruptions in fibre—matrix interface in the crossply composite were indicated. The compression tests showed that failure occurred through in-plane and out-of-plane fibre bucking and kinking in the unidirectional and crossply composites, respectively. Failure of the longitudinal plies in the crossply laminate occurred at significantly higher compression stress than for the unidirectional composite. Compressive failure mechanisms in unidirectional and multi-directional laminates are considered.     

Percolation threshold of carbon nanotubes filled unsaturated polyesters

This paper reports on the development of electrically conductive nanocomposites containing multi-walled carbon nanotubes in an unsaturated polyester matrix. The resistivity of the liquid suspension during processing is used to evaluate the quality of the filler dispersion, which is also studied using optical microscopy. The electrical properties of the cured composites are analysed by AC impedance spectroscopy and DC conductivity measurements. The conductivity of the cured nanocomposite follows a statistical percolation model, with percolation threshold at 0.026 wt.% loading of nanotubes. The results obtained show that unsaturated polyesters are a matrix suitable for the preparation of electrically conductive thermosetting nanocomposites at low nanotube concentrations. The effect of carbon nanotubes reaggregation on the electrical properties of the spatial structure generated is discussed.     

Oxidation protection for carbon fibre composites

Carbon fibre-reinforced ceramic matrix composites are promising candidate materials for high-temperature structural applications such as gas turbine blades. In oxidizing environments at temperatures above 400°C, however, carbon fibres are rapidly oxidized. There is, therefore, a need to coat the composite in order to protect it against oxidation. This review identifies the requirements of an effective oxidation protection system for carbon fibre-reinforced ceramics and summarizes the work which has been carried out towards this goal over the last 50 years. The most promising coatings are those composed of several ceramic layers designed to protect against erosion, spallation and corrosion, in addition to possessing a self-healing capability by the formation of glassy phases on exposure to oxygen.     

The effect of fibre sizing on fibres and bundle strength in hybrid glass carbon fibre composites

Tests have been carried out on single carbon fibres supplied in the sized and unsized conditions, as well as impregnated tows and tows in a glass–carbon fibre hybrid composite of the same fibre. The results were analysed using a Weibull distribution for the strengths of the reinforcing fibres and composites. The tensile strength of the single fibres appeared to be unaffected by the sizing of the filaments. In the case of the impregnated tows, an increase in characteristic strength of 7% was observed for the unsized fibres. The strength of the impregnated tows in hybrid composites was seen to be 15% higher than those tested in air. This can be attributed to the “hybrid effect”. This revised version was published online in November 2006 with corrections to the Cover Date.     

Influences of interfacial bonding strength and scatter of fibre strength on tensile behaviour of unidirectional metal matrix composites

The influences of interfacial bonding strength and scatter of strength of fibres on tensile behaviour of unidirectional metal matrix composites, whose matrix has low yield stress in comparison to the strength of fibres, were studied using the Monte-Carlo simulation technique using two-dimensional model composites. The following results were found. The strength of composites increases with increasing bonding strength, especially when the bonding strength exceeds the shear yield stress of the matrix and then remains nearly constant. The strength of composites is very sensitive to bonding strength when the scatter of fibre strength is large, but not when it is small. The fracture mode varies from non-cumulative to cumulative with increasing scatter of fibre strength for both cases of weak and strong interfacial bondings. The fracture surface becomes irregular when bonding strength becomes low and scatter of fibre strength becomes large. The applicability of the Rosen and Zweben models and the rule of mixtures to predict the strength of composites was examined.     

Effect of Co60 gamma ray irradiation for carbon fibre on interfacial properties in epoxy resin composites

Abstract

In order to improve the interfacial adhesion between carbon fibre and resin matrix in composite materials, it is necessary to treat the surface of the carbon fibre. In this paper, γ-ray irradiation technique was used to modify polyacrylonitrile based carbon fibre. Laser Raman spectrum and X-ray photoelectron spectroscopy were used to investigate and analyse the structure and chemical composition near the surface of the carbon fibre. The influence of irradiation parameters on the interlaminar shear strength (ILSS) of carbon fibre reinforced epoxy composite materials and the bundle tension strength of carbon fibre was studied. The interfacial adhesion behaviour of composites was characterised using torsional braid analysis. The results show that after irradiation the ILSS of the composite was increased by 20%, while the glass transition peak of the specimen, determined from torsional braid analysis, shifts towards a higher temperature compared with an unirradiated specimen. The value of the glass transition temperature T _g is increased from 416.8 to 424.3 K. After irradiation there was no apparent change in the bundle tensile strength of carbon fibre. Investigations indicate that after irradiation the decrease of microcrystal size, the increase of surface free energy of carbon fibre surface and the active chemical function group formed from unsaturated carbon atoms improve the interface adhesion between the carbon fibre and the matrix in the composites.