Fragmentation analysis of glass fibres in model composites through the use of Raman spectroscopy

Raman spectroscopy has been used to monitor deformation micromechanics in a model discontinuous fibre composite comprising a single glass fibre in an epoxy resin. The glass fibre was coated with a diacetylene-containing urethane copolymer that was subsequently cross-polymerised thermally. During composite deformation, the stress-induced Raman band shifts of the polydiacetylene sequences in the cross-polymerised coating were used to map the distributions of strain along glass fibres inside the epoxy resin matrix. The fragmentation of the fibre has been followed in detail and the behaviour analysed using classical shear-lag analysis. Values of the interfacial shear stress along fibre fragments were determined from the measured fibre strain distributions and were shown to be limited by the shear yield stress of the matrix. The effect of adhesion between the coating and the fibre upon the strain distributions has been investigated in detail. The fibre strain distributions can only be determined accurately when the adhesion is good. However, in the case of poor adhesion, although the strain distribution in the coating follows that of the matrix, the fragmentation process can still be monitored.     

Evaluation of interfacial properties between carbon fibres and semicrystalline thermoplastic matrices in single-fibre composites

The interfacial properties between pitch-based carbon fibre and semicrystalline thermoplastic matrices have been investigated by using the fragmentation test on single-fibre composites. For this purpose, fibres with seven different degrees of surface oxidation were prepared. From the fragmentation test, it was found that oxidization of carbon fibre reduces the fibre fragment length. Further, the length is also influenced by the nature of resin used as matrix. The morphology of crystallites formed on the fibres has been studied. Based on these results, the interfacial properties of carbon fibre and thermoplastic resins are discussed.     

Visco-elasticity of aramid fibres

Tests are described to measure the creep and relaxation response of aramid fibres with the specific aim of determining whether the visco-elastic response is linear or non-linear. Hitherto, creep and relaxation tests have been carried out in different circumstances and at different loads, which has led to disagreement about the type of response that aramid fibres exhibit. Tests are carried out at stresses between 10% and 80% of the short-term strength of the fibres under controlled temperature and humidity conditions, and it is shown that both creep and relaxation are non-linear at stresses below 40% of the breaking load, but both are linear at stresses above this level. This result explains the contradictions in earlier work and also indicates that there may be two different processes underway in the visco-elasticity of aramids.     

Role of microvoids in aramid fibres

The distribution of microvoids in high-strength aramid fibre has been established. The tensile and compressive behaviours of both untreated and silver sulphide-impregnated Kevlar 981 fibre are reported and the results are discussed in terms of the influence of microvoids on the mechanical performance.     

Silica-based aerogel composites reinforced with different aramid fibres for thermal insulation in Space environments

Silica aerogel composites reinforced with different aramid fibres have been synthesized and compared considering their potential use in thermal protection systems of Space devices. These composites were prepared from tetraethoxysilane and vinyltrimethoxysilane and the network was strengthened with aramid fibres. The results showed that the physical and chemical properties of the fibres were relevant, leading to composites with different properties/performance. In general, the obtained values for bulk density were low, down to 150 kg m^?3. Very good thermal properties were achieved, reaching thermal conductivities bellow 30 mW m^?1 K^?1, and thermal stability up to 550 °C in all cases. Short length fibres produce stiffer composites with lower thermal conductivities, while among longer fibres, meta-aramid-containing fibres lead to nanocomposites with best insulation performance. Standard tests for Space materials qualification, as thermal cycling and outgassing, were conducted to assess the compliance with Space conditions, confirming the suitability of these aerogel composites for this application.

Graphical abstract

     

The interfacial properties of aramid/epoxy model composites

Many attempts have been made to measure, evaluate and improve the level of interfacial adhesion in aramid/epoxy composites. Different surface treatments have been developed in order to promote chemical bonding between the fibre and the matrix but it is found that most of the surface treatments developed have shown little or no improvement in the level of interfacial adhesion. The interfacial properties of a model composite are often determined by measuring the interfacial shear strength using micromechanical test methods that employ different loading configurations. However, the values of interfacial shear strength determined using different test methods are found to be dependent upon the variation of localized stress in the samples due to the different loading configurations and often give different results. Using Raman spectroscopy it is shown that the strain-dependent shift of the 1610 cm^–1 aramid Raman band can be used to determine the point-to-point variation of axial fibre strain along aramid fibres embedded in epoxy resin matrices from which the interfacial properties can be derived. The interfacial properties of aramid/epoxy model composites have been determined using Raman spectroscopy where the properties of the fibre, including different surface treatments, and the matrix have been changed systematically. The results are reviewed here and compared to those obtained using conventional micromechanical test methods. It is also demonstrated that the Raman technique can be used to characterize the interfacial properties of aramid/epoxy model composites deformed using different micromechanical test methods. In this way the interfacial properties can be determined at different loading levels enabling the progressive failure of the fibre/matrix interface to be monitored and defined accurately.     

Evaluation of interface fracture energy for single-fibre composites

Raman and luminescence spectroscopy have been used for the first time to determine the interface fracture energy for single-fibre composites. By using the measured fibre stress distributions in single-fibre fragmentation composite specimens and a simple energy-balance scheme, the energy for the initiation of interfacial debonding has been estimated for carbon (T50) and α-alumina (PRD-166 and Nextel 610) fibres embedded in epoxy resins. It has been found that the interface fracture energy shows good sensitivity to changes in the level of fibre/matrix adhesion due to surface treatment and sizing of the fibres. It is also found that the values of interface fracture energy correlate well with measured values of interfacial shear strength determined for the same fibre/matrix systems.     

Crystallite size evolution of aramid fibres aged in alkaline environments

Aramid fibres are high-performance materials proposed in geotextiles for alkaline ground reinforcement. To study their durability in such environments, accelerated ageing has been carried out at pH 9 and at pH 11 for up to one and a half years. First, the lateral and longitudinal crystallite sizes have been determined before and after ageing under these conditions by Wide-Angle X-ray diffraction. Next, the tensile fracture surfaces have been observed after different ageing times by scanning electron microscopy. Finally, molecular weight changes have been evaluated by size exclusion chromatography. This study highlights the dependence of structural changes on the ageing conditions. A structural degradation scheme is proposed.     

Fibre/matrix stress transfer through a discrete interphase. Part 1: single-fibre model composites

This paper reports a study of the effect of an interphase on strain development in fibre-fragments. In order to form an interphase, an epoxy resin with known properties was applied to the surface of unsized reinforcing fibres and cured. These were then embedded in a matrix resin coupon, prior to fragmentation testing. The study included an examination of the effect of interphase thickness, by applying multiple coats of one of the resins, and the effect of the interphase properties, by varying the coating resins. It was found that the average fragment lengths at saturation were difficult to distinguish, as a result of the scatter introduced by the statistical distribution of fibre strengths. However, the strain interval between onset of fragmentation and saturation was found to be more sensitive to variations in the interphase properties.A finite element model was used to examine the strain development in the fragments in more detail. The mechanical properties of the fibre, interphase and matrix were accurately incorporated into the model, providing a realistic representation of the state of strain in the experimental samples. The predicted deformations around the fibre-break provided an explanation for the experimental observations.     

The bonding mechanism of aramid fibres to epoxy matrices

Despite considerable attempts to increase aramid-epoxy adhesion, to date, the adhesion levels achieved between aramid fibres and epoxy matrices are less than optimum for some applications. A combination of the aramid fibres' morphology, physical and chemical properties, and the interfacial mechanical stresses is responsible for the lack of success in increasing aramid-epoxy adhesion level. A key to improving the aramid-epoxy adhesion is a basic understanding of the interfacial mechanisms by which fibre and matrix interact. There is a considerable number of publications on aramid fibres and their composites. This paper reviews some of the literature relevant to aramid-epoxy bonding mechanisms.     

Fragmentation in alumina fibre reinforced epoxy model composites monitored using fluorescence spectroscopy

It has been found that well-defined fluorescence R₁ and R₂ lines can be obtained from PRD-166 alumina-zirconia fibres and that the fluorescence R lines shift with applied stress. They are found to shift to higher wavenumber when subjected to tensile deformation and to lower wavenumber in compression. The stress-sensitive fluorescence R₂ line has been used to map the distribution of stress along PRD-166 fibres embedded in an epoxy resin matrix cured under different conditions. It has been shown that the distributions of stress along the PRD-166 fibres at different levels of matrix strain are consistent with those predicted by conventional shear-lag analysis. The interfacial shear stress has been derived from the point-to-point variation of stress along the fibre. The fluorescence technique has also been used to map the stress distribution along a PRD-166 fragment in an epoxy matrix during a single-fibre fragmentation test where it is found that debonded regions propagate along the fibre fragments during loading, after initial fragmentation has occurred.     

Relationship between structure and mechanical properties for aramid fibres

The relationship between structure and mechanical properties for a series of twelve wellcharacterized aramid fibres has been determined. The fibres were produced under a variety of processing conditions and the fibre structure has been characterized using transmission electron microscopy. In particular, both the overall degree of molecular orientation in the fibres and the difference in structure between the fibre skin and core regions have been investigated in detail. The mechanical properties of the fibres have been evaluated using conventional mechanical testing and molecular deformation followed using Raman microscopy to monitor strain-induced band shifts. It has been shown that the mechanical properties of the fibres are controlled by the fibre structure. In particular, it is shown that the fibre modulus is governed by the overall degree of molecular orientation. It is also demonstrated that the fibre strength is controlled principally by the overall molecular orientation but may also be reduced by the presence of a highly-oriented skin region. It has been found that the rate of shift of the Raman bands per unit strain is proportional to the fibre modulus except for fibres with large differences in molecular orientation between fibre skin and core regions. For these fibres the rate of shift reflects the higher orientation of the skin.     

Plasma surface modification of advanced organic fibres: Part V Effects on the mechanical properties of aramid/phenolic composites

Aramid fibres have been treated in ammonia and oxygen plasma to enhance adhesion to resole phenolic resins. The plasma treatments resulted in significant improvements in interlaminar shear strength (ILSS) and flexural strength of composites made from these materials. Composites containing aramid fibres with epoxide groups reacted on to the ammonia plasma-treated fibre surface also showed further improvements in ILSS and flexural strength. Scanning electron and optical microscopic observations were used to examine the microscopic basis for these results, which have been compared with those obtained previously for aramid/epoxy and aramid/vinyl ester composites. For composites containing oxygen and ammonia plasma-treated fibres, the enhanced ILSS and flexural strength are attributed to improved wetting of the surface-treated aramid fibres by the phenolic resin. However, for those containing fibres with reacted epoxide groups on the ammonia plasma-treated fibre surfaces, the enhanced composite properties may be due to covalent chemical interfacial bonding between the epoxide groups and the phenolic resin. Effects of catalyst levels and cure cycle on the ILSS of composites laminated with untreated fabric has also been examined and optimum values have been determined. The catalyst concentration has an influence on the phase-separated water domain density in the matrix which in turn, affects the available fibre/matrix bonding area and hence the composite ILSS and flexural strength. This revised version was published online in November 2006 with corrections to the Cover Date.