Influence of fibre type on flexural behaviour of self-compacting fibre reinforced cementitious composites

This paper investigates the flexural properties of self-compacting fibre reinforced cementitious composites that contain high fly ash volume. Seven types of fibres were compared at the same volume fraction and in similar matrices containing high-volume fly ash and having a high compressive strength of around 85 MPa at 28 days. Third-point bending test was conducted on beam specimens to obtain their load–deflection curves, and investigate their fracture behaviour, flexural strength, deflection and toughness. The results showed that using straight steel and micro-polyvinyl alcohol fibres produced composites demonstrating stable deflection-hardening with multiple-cracking phenomenon. This behaviour resulted in high flexural strength, along with large maximum deflection and toughness values, which are important for applications in cementitious composites. This study indicates that fibres with both sufficiently high aspect ratio and high tensile strength are necessary for achieving deflection-hardening in self-compacting cementitious composites with high-strength matrices containing high-volume fly ash.     

Unidirectional carbon fibre reinforced polyamide-12 composites with enhanced strain to tensile failure by introducing fibre waviness

Unidirectional (UD) carbon fibre reinforced polymers offer high specific strength and stiffness but they fail in a catastrophic manner with little warning. Gas-texturing and non-constrained annealing were used to introduce fibre waviness into UD polyamide 12 composites produced by wet-impregnation hoping to produce composites with a more gradual failure mode and increased failure strain. Both methods increased the variation of fibre alignment angle compared to the control samples. The composites containing wavy fibres exhibited a stepwise, gradual failure mode under strain controlled uniaxial tension rather than a catastrophic failure, observed in control samples. Gas-texturing damaged the fibres resulting in a decrease of the tensile strength and strain to failure, which resulted in composites with lower tensile strength and ultimate failure strain than the control composites. Non-constrained annealing of carbon fibre/PA-12 produced wavy fibre composites with ultimate failure strain of 2%, significantly higher than 1.6% of the control composite.     

Nickel- and copper-coated carbon fibre reinforced tin-lead alloy composites

Nickel and copper were deposited over brominated, surface treated, and pristine P-100 carbon fibres using cementation and electroplating techniques. The fibres were brominated by bromine vapour for 48 h and then desorbed at 200 °C in air for 12 h. The anodic oxidation treatment of the fibres involved electrochemical etching in a dilute sodium hydroxide electrolyte for 3 min. Electroplated coated fibres showed better tensile properties than cementation coated fibres. In addition, nickel coating exhibited better bonding with the carbon fibres compared to copper coating. The effect of bromination and surface treatment was improved adhesion between coating and fibres. Nickel- and copper-coated fibres, which were brominated, anodically oxidized, and pristine, reinforced tin-lead alloy composites were fabricated by squeeze casting. The composites containing coated treated carbon fibres had higher tensile and shear strength than the ones containing coated pristine carbon fibres. Moreover, the composite with coated brominated carbon fibres had better tensile strength and shear strength than the surface treatment. The results also showed the composites containing nickel-coated fibres had higher tensile and shear strength than the ones containing copper-coated fibres.     

Oil Palm Fibre Reinforced Phenol Formaldehyde Composites: Influence of Fibre Surface Modifications on the Mechanical Performance

Oil palm fibres have been used as reinforcement in phenol formaldehyde resin. In order to improve the interfacial properties, the fibres were subjected to different chemical modifications such as mercerisation, acrylonitrile grafting, acrylation, latex coating, permanganate treatment, acetylation, and peroxide treatment. The effect of fibre coating on the interface properties has also been investigated. Morphological and structural changes of the fibres were investigated using scanning electron microscopy and IR spectroscopy. Mechanical properties of untreated and treated fibres were studied. Changes in stress–strain characteristics, tensile strength, tensile modulus and elongation at break of the fibres upon various modifications were studied and compared. The incorporation of the modified fibres resulted in composites having excellent impact resistance. Fibre coating enhanced the impact strength of untreated composite by a factor of four. Tensile and flexural performance of the composites were also investigated. Finally, inorder to have an insight into the failure behaviour, the tensile and impact fracture surfaces of the composites were analysed using scanning electron microscope.     

Effects of fibre content on mechanical properties and fracture behaviour of short carbon fibre reinforced geopolymer matrix composites

Geopolymer matrix composites reinforced with different volume fractions of short carbon fibres (C_f/geopolymer composites) were prepared and the mechanical properties, fracture behaviour and microstructure of as-prepared composites were studied and correlated with fibre content. The results show that short carbon fibres have a great strengthening and toughening effect at low volume percentages of fibres (3·5 and 4·5 vol.%). With the increase of fibre content, the strengthening and toughening effect of short carbon fibres reduce, possibly due to fibre damage, formation of high shear stresses at intersect between fibres and strong interface cohesion of fibre/matrix under higher forming pressure. The property improvements are primarily based on the network structure of short carbon fibre preform and the predominant strengthening and toughening mechanisms are attributed to the apparent fibre bridging and pulling-out effect.     

Laccase and alkali treatments of cellulose fibre: Surface lignin and its influences on fibre surface properties and interfacial behaviour of sisal fibre/phenolic resin composites

This paper is an attempt to investigate the influences of enzyme (laccase) and alkali treatments on the surface lignin of single cellulose fibre. The fibre surface characteristics and the interfacial behaviour of the sisal fibre/phenolic resin composites were also studied by SEM, AFM, XPS. The surface lignin greatly affected the surface physical and chemical properties of single cellulose fibres. The surface lignin concentration was up to 35% for the raw fibre without any treatment, and then it decreased to 24%, 20% and 18% for the fibres with laccase treatment, alkali treatment and laccase/alkali treatment, respectively. The removal of lignin from fibre surface could enhance the interfacial strength of composites, and thus increase the tensile strength and internal bonding strength by 43% and 51%, respectively, for the composites obtained from laccase/alkali treated fibres.     

Atmospheric plasma fluorination as a means to improve the mechanical properties of short-carbon fibre reinforced poly (vinylidene fluoride)

The impact of fluorination of carbon fibres on the properties of short fibre reinforced polyvinylidene fluoride (PVDF) composites was studied. As received and continuously atmospheric plasma fluorinated (APF) carbon fibres were cut to an average fibre length of 2 mm. Short fibre composites (SFC) containing 5, 10 and 15 wt.% carbon fibres were manufactured using a twin-screw mixer. Test specimens were produced by injection moulding. The mechanical properties of the SFC were studied using tensile and compression testing. As expected, the incorporation of short-carbon fibres into PVDF led to an increase in strength and stiffness. The tensile strength and Young’s modulus of the SFC containing APF-treated carbon fibres increased by up to 17% and 190%, respectively. Furthermore, the compressive strength and modulus of the SFC containing APF-treated carbon fibres also increased by 19% and 35%, respectively. APF of carbon fibres results only in a marginal increase in the bulk matrix crystallinity of PVDF as determined by DSC. Scanning electron micrographs of fracture surfaces from tensile tested specimens exhibited a typical brittle failure mode with low fibre loading fraction. Despite the presence of up to 5% of voids and visible resin rich regions at fracture surface, SFC containing APF-treated fibres suggest better bonding at the fibre/matrix interface which led to the much enhanced mechanical properties.     

Effect of fibre surface treatment on yielding and fracture behaviour of glass fibre-polypropylene composite

Two parallel studies of the mechanical properties of composites based on modified polypropylene is presented. The fillers used were either uncoated-glassy fibres or glassy fibres coated with aluminium. The tensile properties and fracture behaviour are investigated as a function of filler content over a range of temperatures and strain rates. The measured elastic modulus, tensile strength, and fracture parameters, G _c and K _c, showed dependencies on filler content. The observed enhancement is found to be remarkable in the case of composites containing uncoated surface treated glass fibres. The tensile yield stress of all the composites prepared showed strain rate and temperature dependence. Stress activation volume and activation energy of single-activated yielding processes are determined. The observed fracture morphology could explain the reinforcement behaviour of the measured mechanical properties.On sabbatical leave from the Physics Department of the University of Jordan, Amman, Jordan     

Effects of fibre/matrix adhesion on carbon-fibre-reinforced metal laminates—I.: Residual strength

The role of interfacial adhesion between fibre and matrix on the residual strength behaviour of carbon-fibre-reinforced metal laminates (FRMLs) has been investigated. Differences in fibre/matrix adhesion were achieved by using treated and untreated carbon fibres in an epoxy resin system. Mechanical characterisation tests were conducted on bulk composite specimens to determine various properties such as interlaminar shear strength (ILSS) and transverse tension strength which clearly illustrate the difference in fibre/matrix interfacial adhesion. Scanning electron microscopy confirmed the difference in fracture surfaces, the untreated fibre composites showing interfacial failure while the treated fibre composites showed matrix failure. No clear differences were found for the mechanical properties such as tensile strength and Young's modulus of the FRMLs despite the differences in the bulk composite properties. A reduction of 7·5% in the apparent value of the ILSS was identified for the untreated fibre laminates by both three-point and five-point bend tests. Residual strength and blunt notch tests showed remarkable increases in strength for the untreated fibre specimens over the treated ones. Increases of up to 20% and 14% were found for specimens with a circular hole and saw cut, respectively. The increase in strength is attributed to the promotion of fibre/matrix splitting and large delamination zones in the untreated fibre specimens owing to the weak fibre/matrix interface.     

Tensile behaviour of stainless steel fibre/epoxy composites with modified adhesion

In this study we investigate the tensile behaviour of unidirectional and cross-ply composites reinforced with ductile stainless steel fibres and modified adhesion to the epoxy matrix. Results show that annealed stainless steel fibres have a potential in designing tough polymer composites for structural applications. The stiffness of the UD composites made from these fibres is 77GPa combined with the strain-to-failure between 15% and 18% depending on the level of adhesion. Silane treatments were used to modify the adhesion. By treating the stainless steel fibres with different silane coupling agents, an increase of 50% in the transverse 3-point-bending strength was realised. Increasing the adhesion by 50% leads to a higher tensile strength and strain-to-failure in both UD and cross-ply laminates and a higher in-situ strength of the 90° plies. It also delays formation of matrix cracks and hinders growth of debonding.     

Procedural influences on compression and injection moulded cellulose fibre-reinforced polylactide (PLA) composites: Influence of fibre loading,fibre length,fibre orientation and voids

The influence of fibre loading (20, 30, 40 mass%), fibre fineness, and the processing procedure (compression moulding – CM and injection moulding – IM) on the tensile and impact strength of lyocell/PLA composites was examined. The results revealed a significantly higher tensile and impact strength for CM composites compared to IM composites. An increase in strength up to a fibre loading of 40% was determined for CM composites, while for IM composites the highest values were measured at a fibre loading of 30%. Composites were investigated for their void content, fibre orientation, fibre length and process-induced fibre damage. A better fibre/matrix adhesion and compaction of IM composites was found while fibre orientation as well as mechanical properties of extracted fibres show no significant differences between CM and IM composites. The different mechanical characteristics of CM and IM samples are attributed predominantly to the fibre aspect ratio and the distribution of voids.     

Mechanical property analysis of kenaf–glass fibre reinforced polymer composites using finite element analysis

Nowadays, natural fibres are used as a reinforcing material in polymer composites, owing to severe environmental concerns. Among many different types of natural resources, kenaf plants have been extensively exploited over the past few years. In this experimental study, partially eco-friendly hybrid composites were fabricated by using kenaf and glass fibres with two different fibre orientations of 0° and 90°. The mechanical properties such as tensile, flexural and impact strengths of these composites have been evaluated. From the experiment, it was observed that the composites with the 0° fibre orientation can withstand the maximum tensile strength of 49.27 MPa, flexural strength of 164.35 MPa, and impact strength of 6 J. Whereas, the composites with the 90° fibre orientation hold the maximum tensile strength of 69.86 MPa, flexural strength of 162.566 MPa and impact strength of 6.66 J. The finite element analysis was carried out to analyse the elastic behaviour of the composites and to predict the mechanical properties by using NX Nastran 9.0 software. The experimental results were compared with the predicted values and a high correlation between the results was observed. The morphology of the fractured surfaces of the composites was analysed using a scanning electron microscopy analysis. The results indicated that the properties were in the increasing trend and comparable with pure synthetic fibre reinforced composites, which shows the potential for hybridization of kenaf fibre with glass fibre.     

Effects of moisture on the mechanical properties of glass fibre reinforced vinylester resin composites

Glass fibre reinforced vinylester resin composites incorporating varying amounts of fibres (63.5, 55.75, 48.48, 38.63 and 27.48 wt%) were characterized for their mechanical properties both as prepared and after treatment with boiling water for 2, 4, 6, 8 and 24 h. Weights of the samples were found to increase to a saturation at about 8 h with boiling water treatment. In keeping with the composite principle, the mechanical properties improved with fibre loading. However, the properties were relatively inferior when treated with boiling water for longer hours attributing to ingress of moisture by capillary action through the interface between the fibre and the resin matrix. Considering the rates of moisture absorption and correlating with the mechanical properties, it was observed that the deteriorating effects were predominant up to 4 h treatment with boiling water. Estimation of defect concentrations for 63.5 wt% of nascent fibre reinforced composites as well as those composites treated with boiling water for 24 h were 56.93% and 64.16% respectively. Similarly, 27.48 wt% nascent fibre reinforced composites and those composites with boiling water treatment showed the estimation of defect concentrations of 39.94% and 50.55% respectively. SEM study of the fractured surfaces showed heavy fibre pull-out in the tensile zone whilst shear fracture of the fibre bundles was predominant at the compressive zone of the samples tested for flexural strength properties.     

Properties of glass fibre cement — the effect of fibre length and content

The properties of glass fibre reinforced cement composites (grc) containing alkali-resistant fibres of lengths 10 to 40 mm and volume fractions 2 to 8% have been studied. At 28 days the optimum properties of the composite were achieved with 6 vol % fibre addition. These were 4 to 5 times the bending strength, 3 to 4 times the tensile strength and 15 to 20 times the impact strength of the unreinforced cement paste. Further increase in the fibre content increases the porosity of the composite resulting in the lowering of bending and tensile strengths. The stress and strain of the composite at matrix cracking increased with increasing fibre contents. No significant improvements in the modulus of the composite were observed over the range of fibre additions investigated. The trends in the properties of grc as affected by the variations in volume fraction and length of the fibre, and environmental conditions of curing of the composites, are qualitatively related to the degree of cement hydration, changes in porosity of the composites and fibre/matrix interfacial effects. The properties of grc change with time, (strengths tend to decrease) and long term studies are in progress.     

The impact strength of fibre composites

Two models have been developed which predict the crack initiation energy, notched impact strength and unnotched impact strength of fibre composites. One is applicable to composites containing short fibres and the other to composites containing long fibres. Data obtained with randomly oriented short fibre composites were consistent with the one model. The other model has been verified using composites containing uniaxially oriented long fibres and long fibres oriented randomly in a plane. The success of the model demonstrates that the high notched impact strength with long fibres is due to the redistribution of stress away from the stress concentrating notch, the extra stress that can be held by the fibre relative to the matrix and the work required to pull fibres out of the matrix during crack propagation. The parameters which have been shown to control the fracture energy are composite modulus, fibre length, fibre volume fraction, effective fibre diameter, fibre tensile strength and the coefficient of friction during fibre pull-out from the matrix. The matrix toughness on the other hand usually has no effect at all for composites containing fibres randomly oriented in two dimensions and only a minor effect in exceptional cases. The shear strength of the fibre-matrix bond has only an indirect effect in that it controls the number of fibres which pull out rather than fracture.     

Experimental and theoretical studies on the properties of injection moulded glass fibre reinforced mixed plastics composites

Recycled mixed post-consumer and post-industrial plastic wastes consisting of HDPE, LDPE and PP were injection moulded with short glass fibre (10–30% by weight) to produce a new generation composite materials. Intensive experimental studies were then performed to characterise the tensile, compression and flexural properties of glass fibre reinforced mixed plastics composites. With the addition of 30 wt.% of glass fibre, the strength properties and elastic modulus increased by as much as 141% and 357%, respectively. The best improvement is seen in the flexural properties due to the better orientation of the glass fibres in the longitudinal direction at the outer layers. The randomness and length of the glass fibre were accounted to modify the existing rule of mixture and fibre model analysis to reliably predict the elastic and strength properties of glass fibre reinforced mixed plastics composites.     

Could biopolymers reinforced by randomly scattered flax fibre be used in structural applications?

Thermoplastics reinforced by natural fibres are mainly used for fitting-up products in the automotive industry. The aim of this work is to study the tensile properties of natural fibre-biopolymer composites in order to determine whether or not, biocomposites may replace glass fibre reinforced unsaturated polyester resins. The materials used are flax fibre, polylactic acid (PLA), l-polylactide acid (PLLA), poly(3-hydroxylbutyrate) (PHB), polycaprolactone and starch thermoplastic (MaterBi® Z), poly(butylene succianate) (PBS) and poly(butylene adipate-co-terephtalate) (PBAT). The tensile properties of the flax fibres have already been determined [C. Baley, Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase, Comp Part A 2002;33:939–948]. The composites are manufactured using a film stacking technique. After studying the processing parameters, these are then adapted to each thermoplastic composites. Test samples are cut out from the composites to test their mechanical properties under tensile loading conditions. These tensile properties are then compared to those of similar polypropylene flax composites. Preliminary results show that the tensile properties are improved with the fibre volume fraction. The tensile strength and Young’s modulus of PLLA and PLA flax composites are greater than those of similar PP/flax fibre composites. The specific tensile strength and modulus of flax fibre/PLLA composite have proved to be very close to those of glass fibre polyester composites.     

The mechanical behaviour of PEEK short fibre composites

Short glass (GF) and carbon fibre (CF) reinforced poly-ether-ether-ketone (PEEK) composites were prepared by injection moulding and then microstructurally characterized. Their mechanical behaviour was determined by two different methods: a classical unidirectional tensile test and an immersion ultrasonic technique. The reinforcing effect of fibres is discussed in the context of the theory of reinforcement of Bowyer and Bader. Interfacial shear strength and critical fibre length at break are calculated for both PEEK/GF and PEEK/CF composites. Examinations of fracture surfaces of uniaxial tensile specimens revealed a higher adhesion of carbon fibres to PEEK matrix in regards to the adhesion concerning glass fibre-PEEK interfaces, which is in agreement with the results provided by the model. Compatibility of ultrasonic and tensile results is reported.     

Hybridisation effect on diffusion kinetic and tensile mechanical behaviour of epoxy based flax–glass composites

This paper aims at investigating the hybridisation effect on the diffusion kinetic and the tensile mechanical behaviour of flax–glass fibres reinforced epoxy composites. For this purpose, hybrid composites composed of flax and glass fibre laminates with different stacking sequences were consolidated by compression moulding and subjected to environment ageing. The obtained results show that the water uptake and the diffusion coefficient are clearly reduced by the addition of glass fibre layers in flax laminate. The ageing conditions performed show that the flax–glass hybridisation presents a positive effect in a wet environment at low temperatures (∼20 °C) in the Young’s modulus and the tensile strength. For example, the Young’s modulus fell by 50% and 41% for hybrid laminates with 6% and 11% of glass fibres, and by 67% for the Flax laminate. However, the flax–glass hybridisation was not necessarily a relevant choice when the hybrid laminates were exposed in a wet environment at high temperatures. Indeed, at 55 °C, this hybridisation had a negative effect on the tensile strength and on the specific tensile strength.     

Carbon nanotube grafted silica fibres: Characterising the interface at the single fibre level

Model polymer composites containing carbon nanotube (CNT) grafted fibres provide a means to investigate the influence of nanostructures on interfacial properties. Well-aligned nanotubes, with controllable length, were grown on silica fibres by using the injection chemical vapour deposition method, leading to a significant increase of the fibre surface area. In single fibre tensile tests, this CNT growth reaction reduced the fibre strength, apparently due to catalyst etching; however, the fibre modulus increased significantly. Contact angle measurements, using the drop-on-fibre method, indicated an excellent wettability of the CNT-grafted fibres by poly(methyl methacrylate) (PMMA). PMMA model composites were fabricated and studied using the single fibre fragmentation tests. A dramatic improvement (up to 150%) of the apparent interfacial shear strength (IFSS) was obtained for the composites containing CNT-grafted fibres. The improvement of IFSS was also influenced by the length and morphology of the grafted CNTs.