Jute/polypropylene composites I. Effect of matrix modification

Natural fibre-reinforced polymers can exhibit very different mechanical performances and environmental aging resistances depending on their interphase properties, but most studies have been focused on fibre surface treatment. Here, investigations of the effect of maleic anhydride grafted polypropylene (MAHgPP) coupling agents on the properties of jute fibre/polypropylene (PP) composites have been considered with two kinds of matrices (PP1 and PP2). Both mechanical behaviour of random short fibre composites and micro-mechanical properties of single fibre model composites were examined. Taking into account interfacial properties, a modified rule of mixture (ROM) theory is formulated which fits well to the experimental results. The addition of 2 wt% MAHgPP to polypropylene matrices can significantly improve the adhesion strength with jute fibres and in turn the mechanical properties of composites. We found that the intrinsic tensile properties of jute fibre are proportional to the fibre’s cross-sectional area, which is associated with its perfect circle shape, suggesting the jute fibre’s special statistical tensile properties. We also characterised the hydrophilic character of natural fibres and, moreover, humidity environmental aging effects. The theoretical results are found to coincide fairly well with the experimental data and the major reason of composite tensile strength increase in humidity aging conditions can be attributed to both improved polymer–matrix and interfacial adhesion strength.     

Design of an impact resistant glass fibre/epoxy composites using short silk fibres

The prevailing utilisation of light and strong structural materials has led to an increasing demand to engineering industries on developing different types of advanced composites. Thus, the development of simple and low cost woven glass fibre composites with an improvement on their tensile and impact properties is suggested. In this paper, the hybridization of a glass fibre reinforced composite is achieved by using low cost short silk fibres as a medium to enhance its cross-ply strength. The comparison on the tensile and impact properties of the composite reinforced by the short silk fibre (with the content from 0.3 to 0.6 wt%) with a pristine glass fibre composite sample was conducted. Fracture surfaces were analysed by using scanning electron microscopy (SEM). Experimental results indicated that the maximum Young’s modulus and ductility index (DI) of a silk reinforced composite increased by 50% and 75%, respectively as compared with the pristine one. Furthermore, the visual examination on drop-weight test samples proved that the impact resistance of the silk reinforced composite was better than that of the pristine sample as well. According to the results obtained, it was found that the addition of 0.4 wt% short silk fibre into glass fibre composite was shown to be the advisable reinforcement content to achieve better tensile and impact strengths.     

The effect of reprocessing on the mechanical properties of polypropylene reinforced with wood pulp,flax or glass fibre

Composites of polypropylene, substitutable for a given application and reinforced with: Medium Density Fibreboard fibre (MDF) (40 wt%); flax (30 wt%); and glass fibre (20 wt%), were evaluated after 6 injection moulding and extrusion reprocessing cycles. Of the range of tensile, flexural and impact properties examined, MDF composites showed the best mean property retention after reprocessing (87%) compared to flax (72%) and glass (59%). After 1 reprocessing cycle the glass composite had higher tensile strength (56.2 MPa) compared to the MDF composite (44.4) but after 6 cycles the MDF was stronger (35.0 compared to 29.6 MPa for the glass composite). Property reductions were attributed to reduced fibre length. MDF fibres showed the lowest reduction in fibre length between 1 and 6 cycles (39%), compared to glass (51%) and flax (62%). Flax fibres showed greater increases in damage (cell wall dislocations) with reprocessing than was shown by MDF fibres.     

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.     

Evaluation of continuous alumina fibre reinforced composites based upon pure aluminium

Abstract

Composites of super purity aluminium unidirectionally reinforced with Altex or Nextel 610 continuous alumina basedfibre have been made by liquid metal infiltration. The composites were well consolidated, with fibre volume fractions V_f of 0.4 and 0.6 for the Altex composites and 0.7 for the Nextel composite, the higher values being obtained where preforming involved the use of sized fibre tows. Matrix porosity was very low and there was no evidence of any deleterious reaction product having formed at the fibre/matrix interface. Monotonic longitudinal tensile tests of the composites gave Youngs modulus values between 125 and 250 GPa, in line with rule of mixtures (ROM) predictions and evidence of effective load transfer between fibres. The onset of yielding in longitudinal composites was commensurate with the yield stress of unreinforced super purity aluminium for V_f = 0.4 (～20 MPa), but increased to 225 MPafor V_f =0.7. The tensile strengths of the Altex composites were 760 and 930 MPa, values in accord with ROM predictions based upon equal load sharing of fibres up to the mean filament failure stress. Although the Nextel composite had a higher tensile strength of 1250 MPa, this was significantly lower than the ROM value of 1650 MPa and was better described by fibre ‘bundle’ theory. Predictions of the accumulation of fibre damage, by statistical analysis, indicated that filament breakage commenced at an applied stress of ～50 MPa for the Altex composite and ～ 500 MPa for the Nextel composite. Despite damage at the lower stress, however, the Altex composites were able to tolerate many more ‘double’ fibre breaks than the Nextel composite, the failure of which coincided with the onset of the first double break. Transverse tensile tests of the composites gave Young's modulus values between 80 and 170 GPa, in line with ROM predictions. The yield stress increased with increasing V_f, from 10 to 60 MPa, this behaviour being attributed to plane strain deformation caused by the virtually non-deformable fibres constraining matrix flow. The tensile strengths showed a similar trend, with 84 MPa for V_f =0.4 increasing to 168 MPa for V_f = 0.7.     

Development of kenaf-glass reinforced unsaturated polyester hybrid composite for structural applications

The main aim of this paper is to develop kenaf-glass (KG) fibres reinforced unsaturated polyester hybrid composite on a source of green composite using sheet moulding compound process. Unsaturated polyester resin (UPE) and KG fibres in mat form were used at a ratio of 70:30 (by volume) with treated and untreated kenaf fibre. The kenaf fibre was treated with 6% sodium hydroxide (NaOH) diluted solution for 3 h using mercerization method. The hybrid composites were tested for flexural, tensile and Izod impact strength using ASTM D790-03, ASTM D618 and ASTM D256-04 standards respectively. The highest flexural, tensile and impact strength were obtained from treated kenaf with 15/15 v/v KG fibres reinforced UPE hybrid composite in this investigation.Scanning electron microscopy fractography showed fibre cracking, debonding and fibre pulled-out as the main fracture mode of composites and kenaf treated 15/15 v/v KG reinforced hybrid composite exhibited better interfacial bonding between the matrix and reinforcement compared to other combinations.     

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.     

Engineering and evaluation of hemp fibre reinforced polypropylene composites: Micro-mechanics and strength prediction modelling

The strength of a composite consisting of 40 wt% NaOH/Na₂SO₃ treated hemp fibre, polypropylene and 4 wt% MAPP was evaluated by means of mathematical modelling and mechanical testing. Interfacial shear strength, single fibre tensile strength and fibre length distribution within the composite were obtained, and theoretical composite strengths were determined by means of the Modified Rule of Mixtures and Bowyer–Bader models. The experimentally obtained composite tensile strength of 50.5 MPa was found to be one-third of the theoretical strength determined by means of the Bowyer–Bader model, and this difference was thought to be mainly due to the non-axial planar-random orientation of the fibres within the composite.     

Optimising industrial hemp fibre for composites

The optimisation of New Zealand grown hemp fibre for inclusion in composites has been investigated. The optimum growing period was found to be 114 days, producing fibres with an average tensile strength of 857 MPa and a Young’s modulus of 58 GPa. An alkali treatment with 10 wt% NaOH solution at a maximum processing temperature of 160 °C with a hold time of 45 min was found to produce strong fibres with a low lignin content and good fibre separation. Although a good fit with the Weibull distribution function was obtained for single fibre strength, this did not allow for accurate scaling to strengths at different lengths. Alkali treated fibres, polypropylene and a maleated polypropylene (MAPP) coupling agent were compounded in a twin-screw extruder, and injection moulded into composite tensile test specimens. The strongest composite consisted of polypropylene with 40 wt% fibre and 3 wt% MAPP, and had a tensile strength of 47.2 MPa, and a Young’s modulus of 4.88 GPa.     

The compression strength of composites with kinked,misaligned and poorly adhering fibres

Experiments carried out on pultruded fibre reinforced polyester resins show that, at moderate fibre volume fractions, the compressive strength of aligned fibre composites depends linearly on the volume fraction. The strength falls off when the fibre volume fraction,V _f=0.4 with Kevlar and high strength carbon fibres. The effective fibre strength atV _f<0.4 is much less than the tensile strength but it is close to the tensile strength with E-glass fibres and high modulus carbon fibres. Poor adhesion between fibres and matrix reduces the compressive strength, as does kinking the fibres when the fibre radius of curvature is reduced to below 5 mm. Misalignment of the fibres reduces the compressive strength when the average angle of misalignment exceeds about 10° for glass and carbon fibres. However, with Kevlar no such reduction is observed because the compression strength of Kevlar reinforced resin is only a very little better than that of the unreinforced resin.     

Temperature and weldline effects on tensile properties of injection moulded short glass fibre PC/ABS polymer composite

The effect of weldline and temperature on tensile properties of injection moulded PC/ABS blend reinforced with different concentration levels of short glass fibres was investigated between 23 and 100 °C. The weldline was formed in the moulded specimens by direct impingement of two opposing melt fronts. It was found that weldline had no significant effect on tensile modulus with weldline integrity factor in the range of 1–0.98. Tensile modulus for both weld and unweld specimens increased linearly with increasing volume fraction of fibres, ϕ_f, and decreased linearly with increasing temperature. Tensile strength of both weld and unweld tensile specimens increased non-linearly with increasing ϕ_f reaching a maximum at ϕ_f≈ 0.16 for specimens without weldline and ϕ_f≈ 0.12 for specimens with weldline. A linear dependence with respect to ϕ_f was found for both weld and unweld tensile strengths for fibre volume fractions in the range 0–0.1. It was found that below the glass transition temperature of the matrix, tensile strength of the composite with and without weldlines decreased linearly with increasing temperature. The weldline integrity factor for tensile strength decreased with increasing ϕ_f, but showed no significant variation with respect to temperature.     

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.     

Tensile behaviour of FRC under high strain-rate

This paper presents experimental results on two types of concrete reinforced with steel and polyvinyl-alcohol (PVA) fibres subjected to dynamic tensile loading. The tests were carried out by using a Modified Hopkinson Bar apparatus on fibre reinforced concrete notched-specimens under three different strain-rates (50, 100, and 200 s⁻¹). From the experiments it was found that there is a significant enhancement in tensile strength with increasing strain-rates. The dynamic tests on steel FRC with the smaller loading rate (50 s⁻¹) showed a strength similar to the one measured from static tests; however, for increasing loading rates, a remarkable decrease of post-peak strength and ductility occurs. In specimens with PVA fibres, an enhancement of the tensile strength was also observed and a significant reduction of fracture energy and ultimate deformation occurred. Some experimental aspects are also discussed as the specimen shape, its dimension, the loading rate as well as the different post-peak behaviour from static and dynamic tests.     

Weibull statistical analysis of tensile strength data of short mullite fibre reinforced aluminium alloy composite

Abstract

A statistical evaluation by means of Weibull statistics was carried out on the tensile strength data of a short mullite fibre reinforced aluminium alloy composite, which was prepared by squeeze casting. The results show that the material has a high and reliable tensile strength. The area fractions of the fibres on the cut surface and on the fracture surface of specimens have been statistically analysed. The fibre distribution shows heterogeneity in the microsturcture. On the cut surface the average area fraction of fibres which make large angles with the normal of the cut surface (denoted as A _fl ) is slightly less than that of those fibres which make a small angle with the normal of the cut surface (denoted as A _fs ). However, on the fracture surface of the composite, A _fl is much bigger than A _fs , and the lower the tensile strength of the specimen, the bigger is A _fl on the fracture surface. Debonding of the interface between the large angle fibres and the matrix is an important cause of failure of the composite, and the non-uniform distribution of the large angle fibres is one of the main causes of the large scatter in the data.     

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.     

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.     

Dynamic effect on strength in SiCf/SiCm composite

Loading rate dependence of mechanical properties of SiC fibre-reinforced SiC composites (SiC_f/SiC_m) has been experimentally investigated as to the fibre volume fraction and coating materials for SiC fibre. The composites consisting of monolithic SiC and SiC fibre (Hi-Nicalon) coated with Boron-Nitride (BN) or Carbon (C) with fibre volume fractions of 20, 30 and 40% were fabricated by polymer infiltration–pyrolysis (PIP) process. The stress–strain response and strength were measured in tension over a wide range of strain rate,10⁻⁴∼200 s⁻¹. It was shown that the higher volume fraction, the larger tensile strength regardless of the kind of coating and strain rate. The interface friction stress evaluated by the fibre pullout length that is measured through microscopic observations of fractured specimens is larger in dynamic loading than in static loading. The BN-coated fibre gave the composite superior tensile strength to the C-coated fibre. This trend results from the variety of the interface friction stress associated with the coating thickness.     

Characterisation of cotton fibre-reinforced geopolymer composites

This paper describes the physical, mechanical and fracture behaviour of fly-ash based geopolymer reinforced with cotton fibres (0.3–1.0 wt%). Results show that the appropriate addition of cotton fibres can improve the mechanical properties of geopolymer composites. In particular, the flexural strength and the fracture toughness increase at an optimum fibre content of 0.5 wt%. However, as the fibre content increases, the density of geopolymer composites decreases due to an increase in porosity and tendency of fibre agglomeration.     

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.     

Studies on carbon fibre reinforced aluminium composite processed using pre-treated carbon fibres

Carbon fibre reinforced Al-12% Si alloy composite has been fabricated by pre-treating the fibres with K₂ZrF₆ followed by molten alloy infiltration and subsequent hot pressing of the preforms. The infiltration conditions were arrived at based on the measurement of tensile strength of the fibres extracted from the preforms. The fibre volume per cent of 20 was found to result in composite tensile strength of about 240 MPa as compared to tensile strength of 100 MPa for the unreinforced matrix. Characterization of the interface revealed the formation of ZrSi₂ and diffusion of potassium and aluminium into the fibre. The interfacial bonding was strong as is evinced by the absence of fibre pull-out on to the fracture surface.