A study of the mechanical behaviour on fibre reinforced hollow microspheres hybrid composites

This paper presents the results of an investigation into the effects of hollow glass microsphere fillers and of the addition of short fibre reinforcements on the mechanical behaviour of epoxy binding matrix composites. Properties like flexural stiffness, compressive strength, fracture toughness and absorbed impact energy, were studied. The specimens were cut from plates produced by vacuum resin transfer moulding having a microsphere contents of up to 50% and with fibre reinforcement up to 1.2% by volume. The tests performed with unreinforced composites show that flexural and compressive stiffness, maximum compressive stresses, fracture toughness and impact absorbed energy decrease significantly with increasing filler content. However, in terms of specific values, both flexural and compressive stiffness and impact absorbed energy increase with microsphere content. The addition of glass fibre produces only a slight improvement in the flexure stiffness and fracture toughness, while increasing significantly the absorbed impact energy. In contrast, the addition of a small percentage of carbon fibres produces an important improvement in both fracture toughness and flexure stiffness, when hybrid composites with 0.9% carbon fibre are compared to unreinforced foam, but did not improved absorbed impact energy.     

A study of the influence of the environment on the measurement of interfacial properties of carbon fibre/epoxy resin composites

The importance of understanding the interface and its reactions is paramount to determining optimum composite properties in service. However, many tests are not carried out at the temperatures and humidities which the component may be subjected to in service. The influences of resin type, hardener, cure time and temperature have been well established, although much work continues in ignorance of the precise details of the system being used. Additionally and perhaps more importantly, the temperature at which the composite part will be used and therefore the properties of the resin and the interface at this temperature should be ascertained for each system or well understood and therefore predicted. A specially designed environmental chamber has been built for the purpose of conducting high temperature fragmentation tests. Fractures of the resin at filament breaks in various systems and at various temperatures have also been studied. It is shown that small changes in the temperature of testing (relative to the T_g) can alter the properties of the interface as much as any changes to the fibre treatment and matrix cure. The changes are found to involve not only a shift in the values of the fragment length but also their distribution, as well as the fracture morphology at fragmentation. All these factors must be considered in any analysis of the 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.     

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.     

Multiscale molecular modeling of SWCNTs/epoxy resin composites mechanical behaviour

Atomistic and mesoscale simulations were conducted to estimate the effect of the diameter and weight fraction of single walled carbon nanotubes (SWCNTs) on mechanical behaviour and glass transition temperature (T_g) of SWCNTs reinforced epoxy resin composites. Atomistic periodic systems of epoxy resin and epoxy resin/SWCNTs were built with different weight ratios and were subject of an extensive multistage equilibration procedure. Molecular dynamics simulations were used to estimate glass transition temperature, Young modulus and solubility parameter of epoxy resin and epoxy resin/SWCNTs composites. Dissipative particle dynamics method and Flory–Huggins theory was employed to predict epoxy resin/SWCNTs morphologies. The results show that incorporation of SWCNTs with diameters ranging from 10 to 14 ? has beneficial effect on mechanical integrity and T_g. Overall, the agreement between predicted material properties and experimental data in the literature is very satisfactory.     

Multiscale molecular modeling of SWCNTs/epoxy resin composites mechanical behaviour

Atomistic and mesoscale simulations were conducted to estimate the effect of the diameter and weight fraction of single walled carbon nanotubes (SWCNTs) on mechanical behaviour and glass transition temperature (T_g) of SWCNTs reinforced epoxy resin composites. Atomistic periodic systems of epoxy resin and epoxy resin/SWCNTs were built with different weight ratios and were subject of an extensive multistage equilibration procedure. Molecular dynamics simulations were used to estimate glass transition temperature, Young modulus and solubility parameter of epoxy resin and epoxy resin/SWCNTs composites. Dissipative particle dynamics method and Flory–Huggins theory was employed to predict epoxy resin/SWCNTs morphologies. The results show that incorporation of SWCNTs with diameters ranging from 10 to 14 Ǻ has beneficial effect on mechanical integrity and T_g. Overall, the agreement between predicted material properties and experimental data in the literature is very satisfactory.     

Effect of coupling agent on natural fibre in natural fibre/polypropylene composites on mechanical and thermal behaviour

To enhance the adhesion between the natural fibre and the thermoplastic matrix, a coupling agent of maleic anhydride grafted polypropylene MAPP is applied. In literature, there are different guidelines of the optimum percentage required of MAPP. Therefore, a systematic work is carried out to optimise the MAPP percent with respect to the type of the natural fibre. Different parameters are investigated namely; Coupling agent ratio to the fibre (0%, 6.67%, 10%, 13.3%, 16.67%), coupling agent source, fibre type (flax, hemp, sisal), and fibre content (30%, 50%). Composite is produced using a kneader and the resulting material is assessed mechanically, thermally, microscopically and for water absorption. For different MAPP source and the natural fibre type, optimum MAPP to fibre ratio is found in average to range between 10% and 13.3% according to the investigated property (stiffness, strength and impact). Increase of MAPP is found to decrease the melting temperature. The thermal behaviour is also linked to the copolymer molecular weight.     

Effect of basalt fibre hybridisation on post-impact mechanical behaviour of hemp fibre reinforced composites

A major limitation to the spreading of natural fibre reinforced composites in semi-structural components is their unsatisfactory impact performance. As a potential solution, the production of synthetic/plant fibre hybrid laminates has been explored, trying to obtain materials with sufficient impact properties, while retaining a reduced cost and a substantial environmental gain. This study explores the effects of hybridisation of basalt fibre on post-impact behaviour and damage tolerance capability of hemp fibre reinforced composites. All reinforced laminates were impacted in a range of energies (3, 6, and 9 J) and subjected to both quasi-static and cyclic flexural tests with a step loading procedure. The tests have also been monitored by acoustic emission (AE), which has confirmed the existence of severe limitations to the use of natural fibre reinforced composites even when impacted at energies not so close to penetration and the enhanced damage tolerance offered by the hybridisation with basalt fibers.     

A comparison of the properties of hot compacted gel-spun polyethylene fibre composites with conventional gel-spun polyethylene fibre composites

In this paper we present an investigation of the ballistic and low strain rate mechanical properties of gel-spun polyethylene (GSPE) fibre composites. Composites were made using two different processing routes, a conventional prepreg route and the hot compaction process developed at the University of Leeds. Within these two groups different matrices and fibre arrangements were also investigated. Composites manufactured by the hot compaction process were found to have ballistic performance equivalent to that of the composites manufactured by the conventional prepreg route. The mechanisms of energy absorption in the two types of composite were, however, different. The hot compacted materials absorbed energy by fibre failure and back-face delamination while the composites made by the conventional prepreg route absorbed energy by fibre failure only. Interfacial strength played a very important role in the energy absorption of composites that underwent back-face delamination upon ballistic impact.     

The influence of fibre pretreatmert on the mechanical properties of epoxy-asbestos composites

This study is devoted to the mechanical properties of asbestos-epoxy composites. Chrysotile fibres were first pretreated by means of an interfacial polymerization technique effecting eventually a poly (hexamethylene adipamide) coating on the asbestos surface. The interface characteristics were significantly altered and due to the well-known compatibility between epoxy and polyamide phase considerable deviation from the behaviour encountered when using untreated fibres was confirmed. Accordingly, while varying pretreated fibre content and also concentration of the polyamide coating, the tensile properties of the resulted composites were examined.     

Influence of fibre properties on the mechanical behaviour of unidirectionally-reinforced ceramic matrix composites

Tensile tests on unidirectionally-reinforced ceramic matrix composites have been simulated using a micromechanical approach based on a shear-lag model and the chain-of-bundles theory. The model presented takes the probabilistic nature of the constituent properties into account. The results of the simulation have been compared with experimental data from literature in order to obtain information about the in-situ properties of the constituents and the influence of the fibre properties on the composite behaviour.     

A study of fibre and interface parameters affecting the fatigue behaviour of natural fibre composites

The tension–tension fatigue behaviour of different natural fibre reinforced plastics was investigated. The composites used were made of flax and jute yarns and wovens as reinforcements for epoxy resins, polyester resins and polypropylene.Fibre type, textile architecture, interphase properties, fibre properties and content were found to affect the fatigue behaviour strongly as illustrated with damping versus applied maximum load curves. It was found that natural fibre reinforced plastics with higher fibre strength and modulus, stronger fibre–matrix adhesion or higher fibre fractions possess higher critical loads for damage initiation and higher failure loads. In addition, damage propagation rates were reduced.Furthermore, unidirectional composites were less sensitive to fatigue induced damage than woven reinforced ones.     

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.     

Hybrid composites based on polyethylene and carbon fibres Part 2: influence of composition and adhesion level of polyethylene fibres on mechanical properties

Combining high performance polyethylene (HP-PE) and carbon fibres as reinforcing elements in so-called hybrid composite structures results in a unique class of structural materials possessing high damping and impact resistance. Mechanical properties of unidirectional HP-PE/carbon-epoxy hybrids have been studied, emphasizing basic mechanical characterization such as tensile, compressive and shear strength, initial as well as long-term modulus, vibrational damping and impact response. This paper describes the influence of overall composition and adhesion level of the HP-PE fibres on the mechanical properties of such hybrids.     

The effect of fibre surface roughness on the mechanical behaviour of ceramic matrix composites

The interface between the matrix and the fibre plays an important role in controlling the strength and toughness of ceramic matrix composites. It has been found experimentally that depending on manufacturing process, the interface may show substantial surface roughness, which has been modelled analytically with certain degree of success. The analytical models, however, do not take into account the interface geometry. Instead, only the magnitude of the surface undulation is included. In this paper, a direct simulation of fibre–matrix interface roughness by the finite element method is performed on an axisymmetric unit cell with a fully debonded interface. The simulation is employed to account for the three dimensional stress state, surface roughness and interface friction, which are normally simplified or idealised in theoretical studies. The model gives the highly non-uniform interface shear and pressure, which have direct implications on the interface damage and composite behaviour. Under the approximation made in the model, the positive transverse strains does not show up in the simulation despite the fact that two different surface roughness are used.     

Acoustic emission for monitoring the mechanical behaviour of natural fibre composites: A literature review

In recent years, natural fibres are increasingly used as reinforcements for the production of low-cost and lightweight polymer composites: other advantages include non-abrasive nature, high specific properties, and biodegradability. However, their limitations, including moisture absorption, poor wettability and large scattering in mechanical properties, and the not sufficient understanding of mechanisms controlling their mechanical behaviour and failure modes, still confine the use of natural fibre reinforced composites in non-structural applications. Acoustic emission (AE) proved useful for its capability of real-time monitoring over the whole material volume and high sensitivity to any process generating stress waves.This paper presents a literature review of AE applications in studies on natural fibre composites. The following fields of application are covered: (1) interface studies in single fibre composite (SFC) tests, (2) damage evolution and failure mechanisms detection and (3) crack propagation, including also current limitations of existing literature and future work.