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.     

Tension and flexural strength of silicon carbide fibre-reinforced glass ceramics

The use of silicon carbide-type fibres to reinforce lithium aluminosilicate glass ceramics results in composites with exceptional levels of strength and toughness. It is demonstrated that composite strength and stress-strain behaviour depend onin situ fibre strength, matrix composition, test technique and atmosphere of test. Both linear and non-linear tensile stress-strain curves are obtained with ultimate strengths at 22° C approaching 700 MPa and failure strains of 1%. Flexure tests performed at up to 1000° C in air are compared with data obtained in argon to demonstrate a significant dependence of strength and failure mode on test atmosphere. Finally, glass ceramic matrix composite performance is compared with a silicon carbide fibre-reinforced epoxy system to demonstrate the importance of matrix failure strain on strength and stress-strain behaviour.     

Tensile strength of discontinuous fibre-reinforced composites

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

Influence of Fabric Parameters on Microstructure,Mechanical Properties and Failure Mechanisms in Carbon-Fibre Reinforced Composites

The effects of fibre/matrix bonding, fabric density, fibre volume fraction and bundle size on microstructure, mechanical properties and failure mechanisms in carbon fibre reinforced composites （plastic and carbon matrix） have been investigated. The microstructure of unloaded and cracked samples was studied by optical microscopy and scanning electron microscopy （SEM）, respectively whereas the mechanical behaviour was examined by 3- point bending experiments. Exclusively one type of experimental resole type phenolic resin was applied. A strong fibre/matrix bonding, which is needed for high strength of carbon fibre reinforced plastic （CFRP） materials leads to severe composite damages during the pyrolysis resulting in low strength, brittle failure and a very low utilisation of the fibres strain to failure in C/C composites. Inherent fabric parameters such as an increasing fabric density or bundle size or a reduced fibre volume fraction introduce inhomogenities to the CFRP＇s microstructure. Results are lower strength and stiffness whereas the strain to failure increases or remains unchanged. Toughness is almost not affected. In C/C composites inhomogenities due to a reduced bundle size reduce strain to failure, strength, stiffness and toughness. Vice versa a declining fibre volume fraction leads to exactly the opposite behaviour. Increasing the fabric density （weight per unit area） causes similar effects as in CFRPs.     

Effects of fibre volume fraction on the stress transfer in fibre pull-out tests

The elastic stress transfer taking place across the fibre/matrix interface is analysed for the fibre pull-out test by means of both micromechanics and finite element (FE) analyses. A special focus has been placed on how fibre volume fraction, V_f, affects the interface shear stress fields in the model composites containing both single and multiple fibres. In a so-called ‘three-cylinder model’, where a fibre, a matrix and a composite medium are coaxially located, the constraint imposed on the central fibre due to the surrounding fibres is properly evaluated. It is shown in the FE analysis that the differences in stress distributions between the composite models containing single and multiple fibres become increasingly prominent with increasing V_f. The principal effect of the presence of surrounding fibres in the multiple-fibre composite model is to suppress effectively the development of stress concentration near the embedded fibre end and thus eliminate the possibility of debond initiation from this region for all V_f considered. This is in sharp contrast to the single-fibre composite model, in which the interfacial debond can propagate from the embedded end if V_f is larger than a critical value. These findings are essentially consistent with the results from micromechanics analysis on the same specimen geometry. The implications of the results for the practical fibre pull-out test as a means of measuring the interface properties are discussed.     

Effects of fabric parameters on the tensile behaviour of sustainable cementitious composites

The mechanical behaviour of fabric-reinforced composites can be affected by several parameters, such as the properties of fabrics and matrix, the fibre content, the bond interphase and the anchorage ability of fabrics. In this study, the effects of the fibre type, the fabric geometry, the physical and mechanical properties of fabrics and the volume fraction of fibres on the tensile stress–strain response and crack propagation of cementitious composites reinforced with natural fabrics were studied. To further examine the properties of the fibres, mineral fibres (glass) were also used to study the tensile behaviour of glass fabric-reinforced composites and contrast the results with those obtained for the natural fabric-reinforced composites. Composite samples were manufactured by the hand lay-up moulding technique using one, two and three layers of flax and sisal fabric strips and a natural hydraulic lime (NHL) grouting mix. Considering fabric geometry and physical properties such as the mass per unit area and the linear density, the flax fabric provided better anchorage development than the sisal and glass fabrics in the cement-based composites. The fabric geometry and the volume fraction of fibres were the parameters that had the greatest effects on the tensile behaviour of these composite systems.     

Crystallization of fibres inside a matrix: a new way of fabrication of composites

Fibrous composites are normally fabricated by inserting premade fibres into a matrix and trying to tailor mechanical or physical properties of the material by a proper choice of fibre arrangement, fibre volume fraction, structure and properties of interface, etc. As a rule, this method satisfies all the needs fairly well. But in many cases, particularly when heat-resistant composites are involved, it leads to complications which cause composite experts to refrain from being involved in technically very attractive projects. So the need for alternative methods of composite fabrication obviously exists. The process described here is an example of such an alternative. It is based on the fibres growing from the melt within the volume of the matrix. The matrix should have prefabricated continuous cylindrical channels to be filled with the melt of the fibre material. The process is described using as a model a composite with a molybdenum matrix and single crystalline sapphire fibres. It is shown that the productivity of oxide fibre fabrication based on the process described can be some orders of magnitude higher than that based on the well known Czochralsky's and Stepanov's methods. The strength of the single-crystalline sapphire fibres obtained has been studied, as well as the high-temperature creep strength of composites containing such fibres. Some of the results of these experiments are reported here.     

In State of Art: Mechanical and tribological behaviour of polymeric composites based on natural fibres

In this article, a comprehensive literature review on the mechanical and tribological behaviour of polymeric composites based on natural fibres is introduced. The effects of volume fraction, orientations, treatments and physical characteristics of different types of natural fibres on the mechanical and tribological properties of several thermoset and thermoplastic polymers are addressed. The effects of the tribological operating parameters (applied load, sliding velocity and sliding distance) on the frictional and wear performance of natural fibre polymer composites are demonstrated. The collected date and analyses revealed that volume fraction, orientations, type of treatment and physical characteristics of the natural fibres significantly influence the mechanical and tribological behaviour of composites. The most influence key in designing natural fibre/polymer composite is the interfacial adhesion of the fibre with the matrix. NaOH chemical treatment found to be the most useful treatment method to enhance the interfacial adhesion of the natural fibres with the matrix, while other techniques exhibited either no effect or deterioration on the fibre strength. Frictional characteristics of the natural fibre composites are poor and solid lubricants are recommended to reduce the friction coefficient of the materials.     

Some magnetic and mechanical properties of fibre-reinforced nickel and nickel-iron alloys

Composites containing tungsten wires reinforcing nickel and nickel-iron alloy matrices have been fabricated by a filament winding-electroplating technique and a considerable improvement in the tensile strength was achieved relative to the unreinforced matrix. The presence of the fibres was found to have a significant influence on the magnetic properties of the composites measured in the direction of the fibre axes. In composites having a matrix with a negative magnetostriction, the maximum permeability decreased with increasing volume fraction, V _f, and was also dependent on the fibre diameter and the magnitude of the magnetostriction. In cases where the matrix had a positive magnetostriction the maximum permeability was observed to increase with increasing V _f, reaching a peak value at V _f0.1. It was suggested that the presence of stresses induced in the matrix during cooling from the heat-treatment temperature, due to the difference in the thermal expansion between the fibre and matrix, could explain this magnetic behaviour. By theoretical considerations, the peak was shown to coincide approximately with the volume fraction at which the maximum, uniaxial elastic stress was expected to form in the matrix. Above this volume fraction the uniaxial and transverse stresses became sufficiently high to cause plastic deformation in the entire matrix leading to the observed fall in the maximum permeability, although in all cases the value remained above that shown by the unreinforced matrix.     

Elastic-plastic thermal stresses and deformation of short-fibre composites

A micromechanics model is proposed to analyse residual stresses and deformations that develop in short-fibre composites upon an applied uniform temperature change. The model is based on Eshelby's equivalent inclusion method and treats the interaction among fibres at finite volume fractions through the Mori-Tanaka mean field theory. The model treats the matrix as an elastic/plastic material while the fibre is elastic and is able to account for the effects of the composite microgeometry. To this end, the effects of misoriented short fibres, the orientation of which is described by a density distribution function, are considered. Numerical results obtained from the proposed model indicate that the misorientation of short fibres has a significant effect on both the stress and deformation behaviour of short-fibre composites.     

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.     

On the local elastic–plastic behaviour of the interface in titanium/silicon carbide composites

The purpose of this study is to conduct a high-resolution nonlinear finite element analysis of the elastic–plastic behaviour of titanium/silicon carbide composites subject to transverse loading. This class of metal matrix composites is designed for the next generation of supersonic jet engines and deserves careful assessment of its behaviour under thermo mechanical loads. Three aspects of the work are accordingly examined. The first is concerned with the development of a representative unit cell capable of accurately describing the local elastic–plastic behaviour of the interface in metal matrix composites under thermal and mechanical loads. The second is concerned with the determination of the influence of mismatch in the mechanical properties between the inhomogeneity and the matrix upon the induced stress fields and the plastic zone development and its growth. The third is concerned with unloading and the role played by the interface upon residual stresses. It is found that the maximum interfacial stress in the matrix appears in the case involving cooling from the relieving temperature with subsequent applied compressive loading. It is also found that the mismatch in mechanical properties between the matrix and the inhomogeneity introduces significant changes in the stress distribution in the matrix. Specifically, it is observed that the maximum radial and tangential stresses in the matrix take place at the interface. The plastic deformation of the matrix leads to a relaxation of these stresses and assists in developing a more uniform interfacial stress distribution. However, the matrix stresses and the resulting equivalent plastic strains still reach their maximum values at that interface. The results show similarities in the patterns of the interfacial stress distribution and plastic zone development for all ranges of fibre volume fractions and loading levels examined. However, they also show marked differences in both the magnitude and patterns of matrix stress distribution between the adjacent inhomogeneities as a result of interaction effects between the fibres.     

3D-Quantification of the distribution of continuous fibres in unidirectionally reinforced composites

Synchrotron microtomography is carried out for continuous C-fibre reinforced aluminium and a continuous C-reinforced polymer. The local volume fraction as well as the orientation distribution of the reinforcement are analysed three dimensionally for both composites using self-developed calculation methods. Representative elements for the analysis of the local volume fraction are determined by using two-point probability functions. The results show that regions with smaller volume fractions tend to form channels along the fibre bundles for both composites. Channels of high volume fractions, representing touching fibres (local volume fraction >55 vol%), are identified for the polymer matrix composite. The regions with high volume fraction >50 vol% tend to form clusters in the case of the metal matrix composite. The orientation of the reinforcement is followed throughout the volume of both composites. The results show preferential orientations within each bundle of the fibre reinforced metal. The orientation of the reinforcement is more homogeneous in the fibre reinforced polymer and the largest misorientations are found within the channels separating fibre bundles. The characterisation methods developed in this work can be used to evaluate quality criteria adopted in the stage of development of the composites.     

Thermal-mechanical fatigue behaviour of alumina fibre/aluminium-lithium composite

Metal matrix composites are candidates for elevated temperature applications. For this reason, it is important to understand their behaviour under thermal-mechanical fatigue conditions. Thermal cycling of a composite material creates thermal stresses in the composite because of thermal expansion mismatch between the fibre and the matrix. This can lead to plastic deformation of the matrix, interface damage, and fibre fracture. Mechanical cyclic loading of the composite during thermal cycling can aggravate the situation even more. A computer-controlled servo-hydraulic thermal-mechanical fatigue test system was used to perform tests on -Al₂O₃(FP)/AI-2%Li metal matrix composite specimens. The volume fraction of unidirectionally aligned fibres was 35%. The tests performed were free-expansion tests, fast and slow thermal fatigue tests, and isothermal fatigue tests. Large reductions in the composite strength were observed under thermal fatigue conditions. This degradation can be attributed to observed microstructural damage of the fibre/matrix interface and fibre fracture.     

The work of fracture of random fibre reinforced cement

From considerations of the pull-out behaviour of individual inclined wires from model composites, the work of fracture and post-cracking stress of random fibre reinforced composites have been estimated. Results are presented for cement composites reinforced with short, high-strength steel wires. It was found that when the fibres were short and/or the interfacial shear stress low, a substantial part of the composite work of fracture was due to plastic deformation of those fibres which were not perpendicular to the matrix crack face. Under such conditions random fibre composites showed superior properties than aligned composites of the same volume fraction of reinforcement.

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Résumé On a évalué le travail de rupture et la contrainte après fissuration de matériaux renforcés de fibres à répartition aléatoire à partir de l'étude de modèles de composites dans lesquels on a considéré le comportement à l'arrachement de filaments individuels inclinés. Les résultats donnés ici se rapportent à des composites à base de ciment renforcé avec de courts filaments d'acier de haute résistance. On constate que lorsque les fibres sont courtes et/ou la contrainte de cisaillement à l'interface faible, une part substantielle du travail de rupture est due à la déformation plastique des fibres qui ne sont pas perpendiculaires à la surface fissurée. Dans de telles conditions, les composites à fibres réparties au hasard montrent des proprietés meilleures que les composites à fibres alignées pour le même rapport de volume de renforcement.

     

Ductile unidirectional continuous rayon fibre-reinforced hierarchical composites

Endless rayon fibres (Cordenka®) were used to reinforce polyhydroxybutyrate (PHB) nanocomposites containing 2.5 wt.% nanofibrillated cellulose (NFC) to create truly green hierarchical composites. Unidirectional (UD) composites with 50–55% fibre volume fraction were produced using a solvent-free continuous wet powder impregnation method. The composites exhibit ductile failure behaviour with a strain-to-failure of more than 10% albeit using a very brittle matrix. Improvements at a model composite level were translated into higher mechanical properties of UD hierarchical composites. The Young’s moduli of rayon fibre-reinforced (NFC-reinforced) PHB composites were about 15 GPa. The tensile and flexural strength of hierarchical PHB composites increased by 15% and 33% as compared to the rayon fibre-reinforced neat PHB composites. This suggests that incorporation of NFC into the PHB matrix binds the rayon fibres, which does affect the load transfer between the constituents resulting in composites with better mechanical properties.     

Rheology of low carbon fibre content reinforced cement mortar

It is recognised that the addition of carbon fibres to a brittle cement matrix results in a less dense composite with enhanced ductility, improved impact resistance and increased toughness. In addition, the reinforcing effect of fibres in the cement often produces superior flexural strength and marked improvements in post-cracking behaviour. Further, carbon fibres influence the electrical properties of the composite which could, potentially, make it a smart material, with a range of applications. Despite attention directed towards the mechanical and electrical properties of carbon fibre reinforced cement (CFRC), there is a dearth of information of the influence of fibre additions on the rheological properties of the resulting composite. To this end, this paper describes an investigation using the Viskomat NT into the influence of carbon fibre additions (fibre length in the range 3–12 mm and volume in the range 0–0.5%) on the rheological properties of CFRC. Within the limitations of the instrument and testing procedure it is shown that CFRC’s conform to the Bingham model: increasing fibre volume and fibre length increase both the yield stress and plastic viscosity.     

Fracture mechanics of metal matrix-metal fibre composites

A model of the metal matrix-metal fibre composite has been constructed. The critical stress intensity coefficient has been estimated taking into account the increase of the energy absorbing capacity of a fibre surrounded by a plastic matrix due to the increase of the elongation at rupture of the fibre under these conditions. The results of experiments carried out on aluminium matrix-steel fibres composites support the validity of the model. The effect of conditions at the fibre-matrix interface on the fracture toughness of the composite has also been studied.     

Theory of multiple fracture of fibrous composites

The theoretical stress-strain behaviour of a composite with a brittle matrix in which the fibre-matrix bond remains intact after the matrix has cracked, is described. From a consideration of the maximum shear stress at the fibre-matrix interface, the extent of fibre debonding and the crack spacing in a partially debonded composite are derived. The energetics of cracking and the conditions leading to an enhanced matrix failure strain are then discussed and, finally, the crack spacing expected in composites containing fibres isotropically arranged in two or in three dimensions is derived for the case of very thin and hence very flexible fibres.