Effect of fibre content and alkali treatment on mechanical properties of Roystonea regia-reinforced epoxy partially biodegradable composites

The present paper investigates the effect of fibre content and alkali treatment on tensile, flexural and impact properties of unidirectional Roystonea regia natural-fibre-reinforced epoxy composites which are partially biodegradable. The reinforcement Roystonea regia (royal palm) fibre was collected from the foliage of locally available royal palm tree through the process of water retting and mechanical extraction. The poor adhesion between fibre and matrix is commonly encountered problem in natural-fibre-reinforced composites. To overcome this problem, specific physical and chemical treatments were suggested for surface modification of fibres by investigators. Alkali treatment is one of the simple and effective surface modification techniques which is widely used in natural fibre composites. In the present study both untreated and alkali-treated fibres were used as reinforcement in Roystonea regia epoxy composites and the tensile, flexural and impact properties were determined at different fibre contents. The alkali treatment found to be effective in improving the tensile and flexural properties while the impact strength decreased.     

Mechanical and electrical performance of Roystonea regia/glass fibre reinforced epoxy hybrid composites

The present paper investigates mechanical and electrical properties of Roystonea regia/glass fibre reinforced epoxy hybrid composites. Five varieties of hybrid composites have been prepared by varying the glass fibre loading. Roystonea regia (royal palm), a natural fibre was collected from the foliage of locally available royal palm tree through the process of water retting and mechanical extraction. Roystonea regia, E-glass short fibres were used together as reinforcement in epoxy matrix to form hybrid composites. It has been observed that tensile, flexural, impact and hardness properties of hybrid composites considerably increased with increase in glass fibre loading. But electrical conductivity and dielectric constant values decreased with increase in glass fibre content in the hybrid composites at all frequencies. Scanning electron microscopy of fractured hybrid composites has been carried out to study the fibre matrix adhesion.     

One-step amino-functionalization of milled carbon fibre for enhancement of thermo-physical properties of epoxy composites

Development of new chemical approaches for preparation of engineered carbon-based fillers is critical for high-performance applications. Herein, an efficient method for covalent functionalization of polyacrylonitrile-based carbon fibre through azo radical addition under mild condition is demonstrated. In this way, isobutyronitrile radicals in situ produced from thermal decomposition of 2,2′-azobisisobutyronitrile (AIBN), were covalently grafted on milled carbon fibre (MCF) surface, assisted by microwave irradiation, as evidenced by FTIR, Raman, and TGA analysis. The grafted isobutyronitriles on MCF surface (n-MCF) were applied for further MCF amino-functionalization (a-MCF) via nucleophilic reaction of an amine-rich compound. Then, both pure MCF and a-MCF were incorporated into epoxy matrix; and its curing process and thermo-physical properties were investigated using DSC, rheometry, DMA, TGA, and flexural analysis. The T_g and flexural strength of epoxy/a-MCF composites, compared to epoxy/MCF, increased by ∼3.5% and ∼10.2%, resulting from good adhesion between a-MCF and epoxy matrix which confirmed by SEM observations.     

Effect of resin system on the mechanical properties and water absorption of kenaf fibre reinforced laminates

The objective of this study is to compare the mechanical and water absorption properties of kenaf (Hibiscus cannabinus L.) fibre reinforced laminates made of three different resin systems. The use of different resin systems is considered so that potentially complex and expensive fibre treatments are avoided. The resin systems used include a polyester, a vinyl ester and an epoxy. Laminates of 15%, 22.5% and 30% fibre volume fraction were manufactured by resin transfer moulding. The laminates were tested for strength and modulus under tensile and flexural loading. Additionally, tests were carried out on laminates to determine the impact energy, impact strength and water absorption. The results revealed that properties were affected in markedly different ways by the resin system and the fibre volume fraction. Polyester laminates showed good modulus and impact properties, epoxy laminates displayed good strength values and vinyl ester laminates exhibited good water absorption characteristics. Scanning electron microscope studies show that epoxy laminates fail by fibre fracture, polyester laminates by fibre pull-out and vinyl ester laminates by a combination of the two. A comparison between kenaf and glass laminates revealed that the specific tensile and flexural moduli of both laminates are comparable at the volume fraction of 15%. However, glass laminates have much better specific properties than the kenaf laminates at high fibre volume fractions for all three resins used.     

Optimal design for the flexural behaviour of glass and carbon fibre reinforced polymer hybrid composites

A study on the flexural behaviour of hybrid composites reinforced by S-2 glass and T700S carbon fibres in an intra-ply configuration is presented in this paper. The three point bend test in accordance with ASTM D790-07 at various span-to-depth ratios was simulated using finite element analysis (FEA). For the purpose of validation, specimens of selected stacking configurations were manufactured following the hand lay-up process and tested in a three point bend configuration. The validated FEA model was used to study the effects of fibre volume fractions, hybrid ratio and span-to-depth ratio. It is shown that flexural modulus increases when the span-to-depth ratio increases from 16 to 32 but is approximately constant as the span-to-depth ratio further increases. A simple mathematical formula was developed for calculating the flexural modulus of hybrid composites, given the moduli of full carbon and full glass composites, and the hybrid ratio. Flexural strength increases with span-to-depth ratio. Utilisation of hybridisation can improve the flexural strength. A general rule is in order to improve flexural strength, the fibre volume fraction of glass/epoxy plies needs to be higher than that of carbon/epoxy plies. The overall maximum hybrid effect is achieved when the hybrid ratio is 0.125 ([0_G/0_7C]) when both V_fc and V_fg are 50%. The strength increases are 43.46% and 85.57% when compared with those of the full carbon and glass configurations respectively. The optimisation shows that the maximum hybrid effect is 56.1% when V_fc = 47.48% and V_fg = 63.29%.     

The potential of harakeke fibre as reinforcement in polymer matrix composites including modelling of long harakeke fibre composite strength

Mechanical properties of aligned long harakeke fibre reinforced epoxy with different fibre contents were evaluated. Addition of fibre was found to enhance tensile properties of epoxy; tensile strength and Young’s modulus increased with increasing content of harakeke fibre up to 223 MPa at a fibre content of 55 wt% and 17 GPa at a fibre content of 63 wt%, respectively. The flexural strength and flexural modulus increased to a maximum of 223 MPa and 14 GPa, respectively, as the fibre content increased up to 49 wt% with no further increase with increased fibre content. The Rule of Mixtures based model for estimating tensile strength of aligned long fibre composites was also developed assuming composite failure occurred as a consequence of the fracture of the lowest failure strain fibres taking account porosity of composites. The model was shown to have good accuracy for predicting the strength of aligned long natural fibre composites.     

The use of wire mesh–epoxy composite for enhancing the flexural performance of concrete beams

This paper presents a study of an ongoing research project on the use of new composites for enhancement of the performance of concrete beams. A plain concrete beam was externally bonded with wire mesh–epoxy composite using one to five wire mesh layers. The flexural performance of the beam specimens bonded with wire mesh layers was compared with the beam specimens bonded with carbon fibre as well as a hybrid of wire mesh–epoxy–carbon fibre composite. The test results show that the use of wire mesh with epoxy is an efficient way to improve the flexural performance of concrete beam specimens. The increase in wire mesh layers significantly enhances the flexural strength, cracking behaviour and energy absorption capability. In comparison with carbon fibre, wire mesh–epoxy composite is more efficient in flexural strength and ductility. In addition, it was found that a concrete beam bonded with a hybrid wire mesh–epoxy–carbon fibre composite has significantly more energy absorption capability compared to specimens bonded with only carbon fibre.     

The flexural stiffness characteristics of viscoelastically prestressed polymeric matrix composites

A novel composite material is described, where tension, applied to polymeric fibres, is released prior to moulding them into a matrix. On matrix solidification, compressive stresses imparted by the viscoelastically strained fibres improve mechanical properties. Previous studies showed that these viscoelastically prestressed composites had improved impact and tensile properties compared with control (unstressed) counterparts. In the current study, three-point bend tests on composites using nylon 6,6 fibre reinforcement in epoxy and polyester resins have demonstrated that the viscoelastic prestressing effect increases flexural stiffness. From deflections at 5 s and 900 s, using a freely suspended load on large span/thickness ratio (L/h) samples, the flexural modulus was increased by ～50% relative to control counterparts. Stiffness-increasing mechanisms relating to pre-tensioned fibre and matrix prestress effects are discussed. For small L/h samples (using controlled rate deflection up to ～5 s), the flexural modulus and resulting increase from viscoelastic prestressing were lower. This is attributed to shear effects and possibly fibre–matrix load transfer mechanisms. By exploiting time–temperature superposition, all samples were aged to the equivalent of 100 years at 20 °C and subsequent bend tests revealed no significant change in the modulus increase resulting from viscoelastic prestressing.     

Tribological and mechanical behaviour of dual-particle (nanoclay and CaSiO<Subscript>3</Subscript>)-reinforced E-glass-reinforced epoxy nanocomposites

An E-glass-reinforced epoxy-based nanocomposite containing organomodified nanoclay (15–20 nm) and calcium silicate particles (75–149 μm) was developed through mechanical shearing mixing and hand layup techniques. Three weight fractions (2, 3 and 4%) of nanoclay were selected to study the effects of nanoclay on mechanical and wear behaviour of nanocomposites. Tensile and flexural properties of nanocomposites were evaluated and compared. The wear properties were evaluated for three speed (3.14, 4.19 and 5.24 m s^?1) and load (20, 50, and 80 N) conditions based on a design of experiment (L16 matrix) concept. The wear loss results were statistically analysed to study the significance of load, speed and nanoclay content. The morphologies of wear surface and fracture surface were examined with the aid of a scanning electron microscope (SEM) to identify the wear and fracture mechanisms. It was found that the wear loss increases with increasing nanoclay amount due to the particle agglomeration effects. Statistical analysis determines that the load is the most significant parameter affecting the wear resistance of nanocomposites. The mean and S/N ratio analyses rank the parameters significance in affecting wear resistance as follows: load > nanoclay content > speed. The wear mechanisms of nanocomposites are complex due to the observation of multiple features such as fibre thinning, matrix wear and fibre/matrix debonding as against abrasive wear in the pure epoxy. Tensile and flexural test results show that a good dispersion of nanoclay is achieved with 2 wt% amount in epoxy-based nanocomposites. The mechanical properties degrade above 2 wt% due to the excessive reinforcement, uneven distribution and the particle agglomeration effects. Fractography studies of tension-failed samples show that pure epoxy resin fails by multimode gauge explosive mode, whereas nanocomposites fail mainly by the matrix/fibre interface failure and fibre breakages.     

Improved fracture toughness of carbon fibre/epoxy composite laminates using dissolvable thermoplastic fibres

This paper reports on a novel toughening concept based on dissolvable phenoxy fibres, which are added at the interlaminar region in a carbon fibre/epoxy composite. The composites were prepared by resin infusion of carbon fibre fabric with the phenoxy introduced as a chopped fibre interleaf between the carbon fibre plies. The thermoplastic phenoxy fibre dissolved in the epoxy during curing at elevated temperatures and a phase separated morphology with phenoxy-rich secondary phase was formed upon curing. It was found that the average Mode-I fracture toughness value, G_1c increased tenfold with only 10 wt.% (with regard to the total matrix content) phenoxy fibre added. Other properties such as Young’s modulus, tensile strength and thermal stability were not adversely affected. The mechanical and thermal properties of the neat epoxy–phenoxy blends were also studied for comparison.     

Use of hemp and flax in composite manufacture: a search for new production methods

A new cost effective method of fabricating strong plywood-type composites from strips of hemp fibres is reported, which takes advantage of the first frosts in autumn. The extracellular ice formed in the plants detaches the fibre layers from the woody material. In a three-point bending test 20×20×100 mm³ fibre/epoxy test beams with a similar structure to that of plywood were found to be of comparable strength, the highest flexural strength being 65 MPa. A two-component epoxy resin (Araldite^®) was used as an adhesive. The mass fraction of the strips was 50–80%. The compressive stress during the manufacturing process was 0.1 or 8 MPa. The good appearance, manufacturing properties and workability of the biofibre composites make them suitable especially for floor and furniture manufacture.By pressing together 48 layers of hemp or flax mats which were originally intended for insulation purposes composites were produced that were even stronger than those made from strips. Hemp was spring harvested, which somewhat reduced the strength of fibre bundles. The great advantage of spring harvesting hemp fibre is that no artificial retting or drying is needed which makes the industrial raw materials, and therefore the final products, economically attractive. The highest flexural strengths of the test beams were around 140 MPa and stiffness 6 GPa with a fibre mass fraction of 50–60%. A 6 MPa compressive stress was applied during the manufacturing process.     

Characterization of functionally gradient epoxy/carbon fibre composite prepared under centrifugal force

Centrifugal force was employed in order to induce a spatial gradient of fibre distribution in the epoxy/carbon fibre system. The gradient structure of the epoxy/carbon fibre composite can be controlled by varying the rotation time and the material parameters, such as fibre length, fibre content and matrix viscosity. The spatial gradient distribution of carbon fibres in an epoxy matrix was achieved by the combined mechanism of packing and settling. The mechanical properties of the functionally gradient epoxy/carbon fibre composite were also investigated. At the same content of carbon fibre, the flexural strength of the functionally gradient composite was higher than that of conventional isotropic composite. This revised version was published online in November 2006 with corrections to the Cover Date.     

Mechanical performance of heat/fire damaged novel flame retardant glass-reinforced epoxy composites

Novel glass-reinforced epoxy composites containing a phosphate-based intumescent and inherently flame retardant (cellulosic (Visil, Sateri) and phenol-formaldehyde (Kynol)) fibres have been fabricated. These components are added both as additives in pulverized form and as fibre interdispersed with intumescent as a fabric scrim for partial replacement of glass fibre. Fire testing has been performed using a cone calorimeter at an incident heat flux of 50 kW/m² and the results have shown that introduction of the intumescent/FR fibre to the matrix can significantly reduce the peak heat release values and smoke intensities evolved by composites. Mechanical testing in tensile and flexural modes of these samples has shown that inclusion of the intumescent/fibre system does not adversely influence their tensile and flexural properties. The effect of heat on mechanical properties has been observed by heating the samples in a furnace at 400 °C for 5 min and tested for their flexural and tensile strength retentions. The charred samples remaining after cone exposure were also tested for stiffness test. Some of the samples retained up to 21% of the initial stiffness after being exposed to high heat flux in the cone calorimeter whereas, the control sample was rendered unusable after cone calorimeter exposure.     

Influence of alkali treatment and fibre length on mechanical properties of short Agave fibre reinforced epoxy composites

Composites based on short Agave fibres (untreated and alkali treated) reinforced epoxy resin using three different fibre lengths (3 mm, 7 mm and 10 mm length) are prepared by using hand lay up and compression mould technique. The materials were characterized in terms of tensile, compressive, flexural, impact, water absorption properties and machinability behaviour. All mechanical tests showed that alkali treated fibre composites withstand more fracture strain than untreated fibre composites. As evidenced by the dynamic mechanical analysis (DMA) tests, the thermo-mechanical properties of the composite with alkali treated Agave fibre were considerably good as alkali treatment had facilitated more sites of fibre resin interface. The machinability and atomic force microscope (AFM) studies were carried out to analyze the fibre–matrix interaction in untreated and alkali treated Agave fibre–epoxy composites.     

Mechanical behaviour of jute cloth/wool felts hybrid laminates

This experimental work is aimed at the characterization of new fibre reinforced composites based on epoxy resin with both protein (wool) and lignocellulosic (jute) natural fibres. Wool-based and hybrid (wool/jute) composites with two different stacking sequences (intercalated and sandwich) were developed. Their microstructure has been investigated through optical and scanning electron microscopy, whereas their quasi-static mechanical behaviour has been evaluated in tension and bending. In addition, the impact behaviour under low-velocity impact at three different impact energies, namely 6 J, 8 J and 9 J has been addressed. The tensile and flexural tests have been monitored using acoustic emission (AE) in order to elicit further information about failure mechanisms. AE monitoring showed that development of damage was due to nucleation of matrix microcracks and subsequent debonding and pull-out phenomena in wool fibre composites and that only in hybrid composites a sufficient stress transfer across the jute fibre/matrix interface was achieved. The results confirmed the positive role of hybridization with jute fibres in enhancing both the tensile and flexural behaviour of wool-based composites, though highlighting the need for an improved adhesion between wool fibres and epoxy matrix.     

Post-fire flexural performance of epoxy-nanocomposite matrix glass fibre composites containing conventional flame retardants

This paper investigates the effect of varied nanoparticles (silicate nanoclays and double-walled carbon nanotubes) and micro-sized flame retardants (FRs) on the post heat/fire flexural performance of glass fibre-reinforced (GFR) epoxy composites. The fire reaction properties of GFR epoxy composites containing different combinations of nano- and micro-sized FRs were studied at varied incident heat fluxes (35–75 kW/m²). The flexural stiffness and modulus values of radiant heat-damaged GFR composites decreased rapidly with increasing incident heat flux. On another hand, the post-fire flexural properties of these specimens exposed for 30–90 s post-ignition at 50 kW/m² retained only 20% of their room temperature flexural properties. Despite significant improvements in the fire reaction properties, their post-fire flexural performance was least affected. This suggests that, while these flame retardants are effective in promoting char formation, the formed char networks are not consolidated enough to effectively constrain the fibre reinforcements.     

Epoxy composites reinforced with deacetylated Phormium tenax leaf fibres

Leaf fibres from Phormium tenax (harakeke, New Zealand flax) were treated with 1 wt.% aqueous NaOH at 30 °C to remove all of the acetyl groups, accounting for most of a 7% mass loss. Increasing the treatment time to 4 h showed no detectable increase in mass loss, but increasing the NaOH concentration to 5% doubled the mass loss by removing some of the xylans. Changes in fibre morphology were interpreted in terms of swelling of cell walls into lumens, followed by shrinkage on drying without reopening the lumens. Deacetylation retarded the rate of water uptake in unidirectional epoxy composites, but showed no detectable influence on the equilibrium moisture content, flexural modulus or strength. The results suggest that high levels of natural acetylation are not a useful feature of leaf fibres, at least in the context of reinforcement for epoxy composites.     

Mechanical properties of randomly oriented short Sansevieria cylindrica fibre/polyester composites

The tensile, flexural and impact properties of randomly oriented short Sansevieria cylindrica fibre/polyester (SCFP) composites are described for the first time in this work. Composites were fabricated using raw S. cylindrica fibres (SCFs) with varying fibre lengths and weight percents of fibre. When the length of the SCFs was increased, the tensile, flexural and impact properties of the composite were increased up to a 30-mm fibre length, and then a curtailment in properties occurred for higher fibre length composites. SCFP composites showed a regular trend of an increase in properties with fibre weight percent until 40% and afterwards a decrease in properties for composites with greater fibre weight percent. Tensile tests revealed that the tensile strength was about 76 MPa, the Young’s modulus was 1.1 GPa and the elongation at break was between 7% and 8.3%. The flexural strength and modulus were estimated to be around 84 MPa and 3 GPa, respectively. Impact tests exhibited a strength of approximately 9.5 J/cm². The analysis of the tensile, flexural and impact properties of short SCFP composites displayed a critical fibre length and optimum fibre weight percent of 30 mm and 40%, respectively. Scanning electron microscope (SEM) studies were carried out to evaluate the fibre/matrix interactions. The experimental tensile strengths were compared with the theoretical predictions and found to be in good agreement with Hirsch’s model. An X-ray diffraction (XRD) analysis of the composites exposed the presence of cellulose IV with a crystallinity index of 60% and crystallite size of 68 nm.     

Phase morphology of nanofibre interlayers: Critical factor for toughening carbon/epoxy composites

Electrospun thermoplastic nanofibres were employed to toughen carbon/epoxy composites by direct deposition on carbon fibre fabrics, prior to resin impregnation and curing. The toughening mechanism was investigated with respect to the critical role of phase morphology on the toughening effect in carbon/epoxy composites. The influences of solubility in epoxy and melting characteristics of thermoplastics were studied towards their effects on phase structure and delamination resistance. For the three different thermoplastic nanofibre interlayers used in this work, i.e. poly(ε-caprolactone) (PCL), poly(vinylidene fluoride) (PVDF) and polyacrylonitrile (PAN) nanofibre interlayers, only PCL nanofibres produced toughening. Although cylinder-shaped fibrous macrophases existed in all three interlayer regions, only PCL nanofibres had polymerisation-induced phase separation with epoxy, forming ductile thermoplastic-rich particulate microphases on the delamination plane. These findings clearly show that the polymerisation-induced phase separation is critical to the interlayer toughening by thermoplastic nanofibres. An optimal concentration (15 wt.%) of PCL solution for electrospinning was found to produce composites with enhanced mode I interlaminar fracture toughness (G_IC), stable crack growth and maintained flexural strength and modulus.     

Woven hybrid composites: Tensile and flexural properties of oil palm-woven jute fibres based epoxy composites

In this research, tensile and flexural performance of tri layer oil palm empty fruit bunches (EFB)/woven jute (Jw) fibre reinforced epoxy hybrid composites subjected to layering pattern has been experimentally investigated. Sandwich composites were fabricated by hand lay-up technique in a mould and cured with 105 °C temperatures for 1 h by using hot press. Pure EFB and woven jute composites were also fabricate for comparison purpose. Results showed that tensile and flexural properties of pure EFB composite can be improved by hybridization with woven jute fibre as extreme woven jute fibre mat. It was found that tensile and flexural properties of hybrid composite is higher than that of EFB composite but less than woven jute composite. Statistical analysis of composites done by ANOVA-one way, it showed significant differences between the results obtained. The fracture surface morphology of the tensile samples of the hybrid composites was performed by using scanning electron microscopy.