Thermal and mechanical properties of cotton fabric-reinforced geopolymer composites

As natural fibres, cotton fabrics (CF) offer good opportunities as reinforcement material for geopolymer composites as they have good intrinsic mechanical properties. This article presents thermal and mechanical properties of CF-reinforced geopolymer composites containing up to 4.1 wt% CF. Thermo-gravimetric analysis was conducted to characterise their thermal performance and their mechanical properties, such as flexural strength, fracture toughness, flexural modulus and impact strength were evaluated. Results show that the enhancement of mechanical properties was achieved at an optimum fibre content of 2.1 wt%. Results of thermal analysis show that fly-ash based geopolymer can prevent the degradation of cotton fabric at elevated temperatures.     

Effect of fabric orientation on mechanical properties of cotton fabric reinforced geopolymer composites

This paper presents the thermal, mechanical and fracture behaviour of fly-ash based geopolymer composites reinforced with cotton fabric (0–8.3 wt.%). Results revealed that fly-ash based geopolymer can prevent the degradation of cotton fabric at elevated temperatures. The effect of cotton fabric orientation (i.e., horizontal or vertical) to the applied load on flexural strength, compressive strength, hardness and fracture toughness of geopolymer composites is also investigated. The results showed that when the fabrics are aligned in horizontal orientation with respect to the applied load, higher load and greater resistance to the deformation were achieved when compared to their vertically-aligned counterparts.     

Wood fibre-reinforced cement composites

The use of wood fibres as reinforcement for a structural composite material has been investigated. Although wood fibres have relatively poor mechanical properties compared with synthetic fibres, they have the advantages of low density, low cost and low energy demand during manufacture. A number of possible matrix materials were considered and Portland cement was chosen for further investigation. An examination was made of the effect of the pulping technique used to prepare the fibre on the strength of the composite and on the stability of the wood fibres in the cement matrix. A chemically pretreated high-temperature thermomechanical pulp and a pulp produced by the kraft process were selected for further study. The effect of the water-cement ratio of the matrix and the weight of fibre in the composite on the strength of the composite and the rate of increase in strength and fracture energy of composite are reported for composites containing these pulps. The results indicate that the kraft pulp is suitable for applications where slurry dewatering can be employed during the forming operation and that the thermomechanical pulp is more suited to applications where low water-cement ratio slurry is used.     

Interfacial properties of fibre-reinforced composites

Factors which control the strength and toughness of a fibre-reinforced composite in terms of the role of interfacial parameters such as the interfacial shear strength, the interfacial coefficient of friction and the matrix shrinkage pressure are reviewed. Techniques for determining the interfacial parameters, the stress distribution along the embedded fibre, and the mode of failure of the pull-out tests utilized in these techniques are considered.     

Thermoplastic fibre-reinforced composites for dentistry

The effect of a deterioration of matrix and matrix-fibre interface, caused by moisture, on the flexural properties (modulus, strength and ultimate strain) of unidirectional E-glass fibre-reinforced thermoplastics [polycarbonate, poly(ethyleneterephthalate glycol) and nylon 12] was investigated. Specimens of span/thickness ratio varying from 15 to 30, prepared from pultruded thermoplastic prepregs, were tested in directions parallel to and perpendicular to the fibre orientation as moulded and after immersion in water at 85°C for 100 h. No significant reduction in longitudinal properties, controlled by fibre behaviour, resulted from exposure to water. A significant reduction in transverse properties, controlled by the matrix and interface behaviour, was observed. Polycarbonate/bare E-glass fibre composite annealed at 275°C for 1 h before immersion in water exhibited a superior resistance to moisture attack. This phenomenon is attributed to the transcrystalline interphase near the fibre surface and good wetting of fibres by the matrix. Resistance against moisture favours this material for further clinical application as an orthodontic wire.     

Plasticity theory for fibre-reinforced composites

Recent experiments reported in [1] show that for a boron-aluminium fibre-reinforced composite plastic yielding is effectively independent of tension in the fibre direction over a wide range of values of this tension. This confirms a long-standing conjecture by the author and colleagues. The results have major implications for the formulation of plasticity theories for fibre-reinforced materials. This paper reviews the theory of anisotropic plasticity based on the usual assumptions of plasticity theory, together with the property that yielding is not affected by a superposed tension in the fibre direction. Yield conditions, flow rules and hardening rules are formulated for uniaxial reinforcement; brief consideration is given to a material reinforced by two families of fibres.     

Strain-rate sensitive tough fibre-reinforced composites

A new type of composite was designed and tested which has greater fracture toughness under impact loading conditions than conventional fibre-reinforced composites. This composite is strain-rate sensitive and can be more than twice as tough as conventional composites having the same matrix and fibre. The key concept used was to coat the reinforcing fibres with a thin layer of viscous fluid in order to maximize the shear stress acting on the fibres during the fibre pull-out. At a given strain-rate the shear stress can be optimized by changing the fluid viscosity and thickness of the coating. The optimum results are obtained when the frictional force is equal to the fibre strength.Composites were made with uniaxial and randomly oriented E-glass fibres in a polyester resin. Samples with uncoated fibres were used as reference. The viscous fluids used included Dow Corning 200 Fluid with viscosities of 10⁵ cP and 10⁶ cP, Zelec U.N., petrolatum and silicone vacuum grease.Notched uniaxial samples with uncoated fibres (fibre volume fraction of 0.06) showed an energy absorption of 16.8 kJ m^–2 (3.2 ft lb in.^–1) in the lzod test. The uniaxial samples coated with Dow Corning 200 Fluid showed an energy absorption from 6.7 kJ m^– (1.28 ft lb in.^–1) to 41.4 kJ m^–2 (7.87 ft lb in.^–1) depending on the thickness of the coating. The samples with random uncoating fibres (fibre volume fraction of 0.20) had an energy absorption of 14.2 kJ m^–2 (2.71 ft lb in.^–1) while the samples with coated fibres ranged from 13.7 to 31.6 kJ m^–2 (2.60 to 6.02 ft lb in.^–1).     

Tensile strength of discontinuous fibre-reinforced composites

A stochastic Monte-Carlo approach, based on Eyring's chemical activation rate theory, is used to study the factors controlling the tensile strength of discontinuous fibre-reinforced composites. The model explicitly takes into account the local distribution of stress near fibre ends. Both the fibre and the matrix are allowed to break during fracture of the composite. The stress-strain curves and the modes of failure of the composite are found to be strongly dependent on the volume fraction and aspect ratio of the fibres. The importance of adhesion at the fibre/matrix interface is also studied. The results are compared with available experimental data.     

The strength of bagasse fibre-reinforced composites

The increasing board densities resulting from increased final platten pressure applied during production (or maximum moulding pressure) provides the reason for observations of similar increases in the Young's modulus and the tensile strength, when two phenol formaldehyde bonded boards which are reinforced with 80% and 90% by volume of bagasse fibres are examined. Scanning electron microscopy was carried out on the fracture zones to differentiate between the effects of varying formaldehyde content. However, even with differences in the sizes of the cavities formed at fracture and in the board densities, the observed modulus versus density and strength versus density curves follow exponential trends. By using the exponential trends, a single best linear tensile strength versus Young's modulus relationship is deduced for these varying fibre to matrix combinations.     

Thermo-visco-plastic behaviour of fibre-reinforced polymer composites

The effect of temperature and strain rate on the tensile behaviour on a series of polymeric matrix-unidirectional glass–fibre composites was studied. Dynamic mechanical analysis (DMA) experiments, as well as tensile tests at three different strain rates and three different temperatures below T_g were performed on off-axis specimens of three different orientations. The strong temperature and strain rate dependence, exhibited by the materials examined, was further described theoretically by applying a formulation of finite elastoplasticity. Constitutive laws based on the material anisotropy, were applied, in combination with constitutive equations of hypoelasticity, written in their objective form. Moreover, empirical equations for the hardening coefficients, arising from the thermal activation theory, were proposed to formulate the temperature and strain rate effect.     

Matrix fractography of fibre-reinforced epoxy composites

This paper describes the origin and development of fractographic features found in the matrix of carbon fibre-reinforced epoxy composites and their significance in the analysis of the failure of structures fabricated from composite material.     

Microstructure of glass fibre-reinforced cement composites

The microstructure and fracture surfaces of glass fibre-reinforced cement (grc) composites have been examined by scanning electron microscopy. The toughness of grc stored in dry environments is attributed to the formation of a complex structure of subsidiary cracks, resulting in a large failure surface area, and the pull-out of the glass fibres. The reduction of the toughness of grc stored in wet conditions over a period of time reflects the reduction in the amount of subsidiary cracking and fibre pull-out. Observations indicate that this is due to changes in the microstructure of the matrix in and around the fibre bundles.     

Autoclaved beaten wood fibre-reinforced cement composites

The need to beat (or refine) wood pulp fibres before using as a reinforcement in cement composites has been re-examined. In the case of Pinus radiata unbleached kraft pulp, it has been confirmed that beating the fibres does contribute to attaining optimum mechanical properties for composites fabricated from such modified fibres.     

Silicon carbide fibre-reinforced resin matrix composites

Resin matrix composites reinforced with silicon carbide yarn and silicon carbide monofilament were fabricated and evaluated. Both composite systems exhibited excellent mechanical properties. Composite thermal expansion behaviour, fibre electrical resistance, and fibre thermal oxidation resistance are also reported. Advantages with respect to carbon fibre-reinforced resins are discussed.     

Interfacial degradation in fibre-reinforced cement-based composites

Abstracts are not published in this journal     

Recycled carbon fibre-reinforced polypropylene thermoplastic composites

Comingled carbon fibre (CF)/polypropylene (PP) yarns were produced from chopped recycled carbon fibres (reCF) (20 mm in length, 7-8 μm diameter) blended with matrix polypropylene staple fibres (60 mm in length, 28 μm diameter) using a modified carding and wrap spinning process. Microscopic analysis showed that more than 90% of the reCF were aligned along the yarn axis. Thermoplastic composite test specimens fabricated from the wrap-spun yarns had 15-27.7% reCF volume content. Similar to the yarn, greater than 90% of the reCF comprising each composite sample made, showed a parallel alignment with the axis of the test specimens. The average values obtained for tensile, and flexural strengths were 160 MPa and 154 MPa, respectively for composite specimens containing 27.7% reCF by volume. It was concluded that with such mechanical properties, thermoplastic composites made from recycled CF could be used as low cost materials for many non-structural applications.     

Interfacial degradation in fibre-reinforced cement-based composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

High-strength silicon carbide fibre-reinforced glass-matrix composites

Silicon carbide fibre-reinforced glass-matrix composites have been fabricated and tested. Two fibre forms, a 140 μm diameter monofilament and a 10 μm diameter filamentary yarn, were incorporated into a matrix of borosilicate glass. The hot-pressing fabrication procedure resulted in fully dense unidirectionally reinforced specimens with excellent flexural strength and fracture toughness over the temperature range 22 to 700° C. In addition, composite thermal expansion was found to be nearly independent of fibre orientation indicating that multiaxially reinforced composites should be readily fabricable without the occurrence of extensive cracking.     

The thermal conductivity of carbon fibre-reinforced composites

Measurements of the thermal conductivity between approximately 80 and 270 K of a series of unidirectional and bidirectional specimens of epoxy resin DX210/BF₃400 reinforced with Morganite high modulus (HMS) and high strength (HTS) carbon fibres are reported for in-plane and out-of-plane directions. The main features of the results conform with expectations based upon known structural properties of the fibres and predictions based upon current theoretical models. Employing the results for the composites in association with results for the pure resin, the account concludes with an assessment of some of the heat transmission characteristics of the fibres.