Fretting of glass fibre reinforced composites

Favourable specific mechanical properties of polymer matrix composites make them an attractive material for application in many engineering structures for which they offer substantial improvements over metals. The paper deals with fretting behaviour of unidirectional glass epoxy composites/metal contacts. Fretting is a plague for many industries: failures, loss of matter, loss of function can be induced by fretting. It occurs in all quasi-static contacts and appears as a complex wear phenomenon where a lot of parameters have been studied. From the interface tribology concept, the velocity accommodation mechanisms are discussed for different fibre orientations versus the contact surface of the glass fibre reinforced epoxy material. Results were analysed in two steps. From friction logs, Running Conditions Fretting Maps (RCFM) were first plotted in order to give an analysis of contact conditions and determine the associated material responses. The tribological degradations were then analysed. Differences between the different fibre orientations are mainly discussed on the basis of the stiffness of the anisotropic material and the velocity accommodation in the contact.     

Matrix design for pseudo-strain-hardening fibre reinforced cementitious composites

Pseudo-strain-hardening behaviour under direct tensile loading in short fibre reinforced cement composites designed with quantitative guidance from micromechanics has been demonstrated experimentally, and conditions for the ductile behaviour of such engineered cementitious composites (ECC) have been formulated theoretically. In this paper special focus is placed on the influence of matrix properties on composite pseudo-strain-hardening. An experimental program is undertaken to study the dependence of the matrix properties on its mix compositions governed by water/cement and the sand/cement ratios. The theoretical and experimental knowledge thus obtained are combined to propose an innovative procedure for the design of composites using different types of matrix. The study is motivated by the need to develop a new class of ECCs with improved elastic modulus by the addition of fine aggregates to the cementitious matrix. Finally, a new composite is designed, and shown experimentally to exhibit the desirable features of pseudo-strain-hardening behaviour and improved elastic modulus.     

Single fibre fragmentation test for assessing adhesion in fibre reinforced composites

The single fibre fragmentation test for measuring the properties of the fibre–matrix interface in fibre-reinforced composites is reviewed. Special emphasis has been paid to the recent stress transfer models in single fibre composites and its application to the development of a suitable data reduction technique for the fragmentation test. The complexities of the correlation of the micromechanical results to the properties of the macrocomposites have been highlighted.     

Phenomenological fracture model for biaxial fibre reinforced composites

This paper proposes a new mathematical fracture model (FM) applicable to a biaxial reinforced composite material. The mathematical model provides predictions about the limit state of composite material. It is applicable both in uniaxial and biaxial requests. The mathematical model is validated by comparing its predictions with the experimental data obtained by authors. The studied composite material is composed by carbon fibre in epoxy matrix. The process used for obtaining the composite materials plates is vacuum forming.     

Fatigue behaviour of duplex metal coated SiC fibre reinforced Ti-15-3 matrix composites

Abstract

Duplex metal (Cu/Mo and Cu/W) coated SiC(SCS–6) fibre reinforced Ti-15-3 matrix composites have been prepared using a hot isostatic pressing process. The effect of the duplex metal coatings on the fatigue behaviour of unnotched SiC(SCS–6) fibre reinforced Ti-15-3 matrix composite has been studied. The fatigue resistance of this fibre reinforced composite is improved by use of the duplex metal coatings. The Cu/Mo and Cu/W duplex metal coating layers prevent debonding of the SCS coating layer from the SiC fibre surface, thus also effectively preventing a reduction in strength of the fibre. During the fatigue test, fibre bridging behind the matrix crack tip reduces the crack growth rate of the matrix; this mechanism is difficult to achieve with the pristine fibre composite. Evolution of the fatigue damage can be quantitatively evaluated by means of a fatigue damage parameter. Matrix crack propagation is the dominant factor responsible for the increase in damage parameter of the composites.     

Mechanical properties of a self-healing fibre reinforced epoxy composites

Inspired by biological systems in which damage triggers an autonomic healing response, a polymer composite material that can heal itself when cracked has been developed. In this work, compression and tensile properties of a self-healed fibre reinforced epoxy composites were investigated. Microencapsulated epoxy and mercaptan healing agents were incorporated into a glass fibre reinforced epoxy matrix to produce a polymer composite capable of self-healing. The self-repair microcapsules in the epoxy resin would break as a result of microcrack expansion in the matrix, and letting out the strong repair agent to recover the mechanical strength with a relative healing efficiency of up to 140% which is a ratio of healed property value to initial property value or healing efficiency up to 119% if using the healed strength with the damaged strength.     

Shrinkage of steel fibre reinforced cement composites

The paper presents results of an experimental investigation on the influence of steel fibres on the free shrinkage of cement-based matrices. Shrinkage tests were carried out on cement paste, mortar and two types of concrete mixes for a period of up to 520 days. Melt extract, crimped and hooked steel fibres were used for reinforcement at volume fractions ranging between 1 and 3%. The results indicate that fibres restrain the shrinkage of the various cement matrices to a significant extent, resulting in reductions of up to 40%. Crimped fibres are the most efficient in providing shrinkage restraint. The paper also presents a theoretical expression and an empirical expression which can be used to predict shrinkage strains of steel fibre reinforced cement matrices. The analysis requires a knowledge of the values of coefficient of friction, μ, at the fibre-matrix interface, which are also derived in this paper. The μ values for steel fibres in normal concrete, mortar and cement paste range between 0.07 and 0.12.     

Fracture toughness evaluation for short glass fibre reinforced composites

Valid plane-stress fracture toughness evaluation of short fibre reinforced composites relies essentially on the successful separation of the energy absorbed in the localized crack-tip region out of the total energy absorbed by the cracked material body at large. Three different experimental techniques, all stemming from the energetic interpretation of theJ integral, are utilized and their relative merits in the characterization of fracture initiation in short glass fibre reinforced injection-moulded nylon 6.6 examined. Various theoretical aspects concerning these experimental methods are outlined. The rationale behind using a single-edge-notched tension type specimen for theJ _c test is presented. TheJ _c value obtained from the compliance calibration method and the quasistatic energy method agree closely and can be considered to be independent of pre-crack length and specimen geometry when the pre-crack length to specimen width ratio (a/w) is larger than 0.45. The extrapolation method fails nevertheless to yield a physically consistentJ _c value, possibly due to its questionable theoretical representation. As no constraint on boundary conditions is necessitated during the course of crack extension, the quasistatic energy is physically more appealing.     

Short silk fibre reinforced nitrile rubber composites

The role of resorcinol-hexamethylenetetramine-silica bonding system in adhesion between silk fibre and nitrile rubber has been studied. Definite proportions of the three components of the bonding system vis-à-vis fibre loading are found to provide the optimum set of technical properties. Increase of fibre concentration in the composites causes an increase in hardness, tensile strength, tear strength, modulus, heat build-up, compression set at constant strain, abrasion loss, and restriction to solvent-swelling and a simultaneous decrease in some properties like resilience, compression set at constant stress and elongation at break. Ageing enhances the adhesion level and shows improved retention of strength properties of the composites. Increasing fibre loading in the mixes leads to a significant improvement in processing characteristics. Scanning electron microscopy studies of fracture surfaces obtained from tensile, tear, abrasion and heat build-up testing have been made in order to assess the failure mode.     

Novel basalt fibre reinforced glass matrix composites

A novel hot-pressing technique for the manufacturing of basalt fibre reinforced glass matrix composites was investigated. Two-dimensional (2D) fibre mats were sandwiched between borosilicate glass powder layers, thus configuring a much simpler processing route than that commonly employed for the production of fibre-reinforced glasses. Besides economic benefits, the use of fibre mats may lead to technologic advantages due to the possibility of readily coating the fibres with a suitable material (e.g. titanium oxide) by means of the sol-gel method. The coating of basalt fibre mats with TiO₂ is proposed for preventing the fibres from an excessive adhesion to the glass matrix. The developed composites containing 15 vol% of 2D-fibre reinforcement exhibited promising bending strength (∼90 MPa) and desirable “graceful” fracture behaviour without catastrophic failure. Thus the present study represents a convenient approach for production of advanced low-cost fibre reinforced glass matrix composites for structural applications.     

Fibre spacing in steel fibre reinforced composites

A method is presented which makes it possible to evaluate the fibre content or to estimate the distribution of fibres in steel fibre reinforced composite materials from the analysis of X-ray pictures. The basic notion in this method is an apparent fibre spacing defined as the average spacing between the intersections of individual fibre projections upon a certain plane and an arbitrary base line drawn on that plane. Such apparent spacing may be estimated analytically, it may also be measured directly on the radiogram. The comparison of analytical and experimental data shows satisfactory agreement.

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Résumé On présente une méthode qui, par l'analyse des diagrammes de rayons-X, permet d'évaluer la teneur en fibre ou d'estimer leur distribution dans des composites renforcés de fibres d'acier. On suppose que la distribution des fibres est homogène et isotropique, toutes les fibres ayant même longueur et même diamètre. En tant que notion de base de cette méthode, on définit un espacement de fibres apparent comme l'espacement moyen entre d'une part les intersections des projections des fibres individuelles, et d'autre part une ligne de base arbitraire tracée sur le même plan. Un tel espacement apparent (s _{app. anl.} ) peut être évalué analytiquement comme une fonction de la longueur de la fibre (l), du diamètre (d) de la teneur en volume (β) et de l'épaisseur de l'échantillon (w)-équation (7). On peut aussi le mesurer directement sur le radiogramme commes _{app. exp.} [voirfig. 2, équation 12)]. La comparaison des résultats analytiques et expérimentaux (tableau I) montre une bonne concordance. On peut aussi se servir de la méthode pour évaluer l'épaisseur d'échantillons soumis aux rayons-X, ce qui prouverait une bonne lisibilité du radiogramme.

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Simple model for consolidation of metal matrix coated SiC fibre composites

Abstract

A simple approach to modelling the consolidation of matrix coated fibre composites is presented. It employs an existing porous material constitutive model for monolithic materials. It is argued that in the consolidation of metal coated SiC fibres, the deformation primarily occurs in an outer layer of the fibre coating, and the internal core remains undeformed, largely because of the generally hydrostatic compressive loading, and because of the incompressible nature of the material in creep. The consolidation process is therefore not vastly different to that occurs for monolithic metal fibres, and similar equations can therefore be used for the composite consolidation. The constitutive equations have been implemented into general purpose non-linear finite element software within a large deformation formulation by means of two different user subroutines, one providing a general implementation, and the other a cpu time efficient approach. The manufacture and testing of SiC continuous fibre, Ti-6Al-4V metal matrix composite specimens is described and the results of the tests compared with the model calculations, showing that good agreement can be achieved with a simple model. The dependence of volume fraction of fibres and temperature can be introduced empirically through the specification of just two material constants. The model is therefore useful in the development of consolidation processes.     

Generalised constitutive equations for densification of metal matrix coated fibre composites

Abstract

The present paper completes a study of constitutive equations for the consolidation processing of continuous fibre reinforced metal matrix composite materials. It builds on an earlier paper in which physically based constitutive equations were derived for the case of symmetrical, isostatic loading. In the present paper, constitutive equations are developed for in plane, general stress states. The total deformation of the consolidating composite is expressed as the sum of a conventional deviatoric creep term, together with a dilatational term, which was derived using a variational method previously published. The equations contain only two material parameters, which are the conventional creep coefficient and exponent for the fibre coating material (in this case, Ti-6Al-4V). The resulting equations have been implemented into finite element software enabling the simulation of practical consolidation processes. The model has been verified by comparing predicted results with those obtained from independent micromechanical models. A number of experimental tests have been carried out, and the model is used to predict the rates of densification for a range of experimental pressure and temperature histories. Good comparisons have been achieved.     

Fatigue behaviour of continuous glass fibre reinforced composites

A study has been undertaken of fatigue in glass fibre reinforced composites. Two matrix resins were tested: a standard polyester and a polyurethane-vinyl-ester, which was designed to have a higher toughness. Three different types of glass fibre fabrics were used for reinforcement: a conventional woven roving and two stitch-bonded cloths. The glass cloths were combined into various lay-ups, in order to consider the effects of matrix, cloth and lay-up on the fatigue strength. Additionally, a study was undertaken to evaluate the micromechanisms that occurred during fatigue and how damage accumulated throughout the sample lifetime. This involved measuring stiffness changes during fatigue cycling, followed by microscopic study of the samples. It was found that similar damage micromechanisms occurred in each lay-up regardless of resin and cloth type, and these included matrix cracking, delamination and fibre breakage. However, differences were observed in the extent, location and rate of damage, and these were consistent with the variations seen in the fatigue strengths.     

Compressive behaviour of unidirectional flax fibre reinforced composites

The compressive strength of unidirectional flax fibre epoxy composites was studied. The compressive strength is influenced negatively by the presence of kink bands in the flax fibres. Improvement of the adhesion between the fibres and the epoxy resin can be achieved easily by removing the thin wax layer which covers the surface of the flax fibres. However, improving the adhesion between fibres and matrix only improves the compressive strength to a very limited extent. Stabilisation of the kink bands present in the fibres and improvement of the compressive properties of the fibres can be achieved by impregnating the fibres with melamine formaldehyde (MF) resin. This results in a large increase in the compressive strength of the resulting composite. The increase in compressive strength is proportional to the amount of MF resin present in the composite. However, the presence of the resin in the fibres lowers their tensile strength, and subsequently the tensile strength of the resulting composite.     

Interfacial separation of fibres in fibre reinforced composites

An analytical model is proposed to predict the ultimate tensile strength of fibre-reinforced composites when the failure is governed by fibre debonding.

The analytical analysis is based on the principle of the compliance method in fracture mechanics with the presence of an interfacial crack at the fibre/matrix interface. The model is developed on the basis of the assumption that both the matrix and the fibre behave elastically and the matrix strain at a zone far from the matrix-fibre interface is equal to the composite strain. Furthermore, it is assumed that a complete bond exists between the fibre and the matrix and that the crack faces are traction free.

It is shown that the separation strain energy release rate for fibre-reinforced composites can be obtained for cases with and without the existence of an interfacial crack. Numerical examples are presented and compared with results obtained in the literature by finite element analyses and from experimental tests. The comparison demonstrates the accuracy and the convergence of the model.     

Viscoelastic interlaminar shear modulus of fibre reinforced composites

An inverse parameter identification technique using a modified Iosipescu shear test (MIST) has been developed for determining the viscoelastic interlaminar shear modulus of composite laminates. The main component of the technique involves minimising the difference between an experimentally measured and a numerically determined creep response at various elevated temperatures by varying the interlaminar shear modulus terms in the numerical model. Consequently, the ‘optimum’ model for the viscoelastic interlaminar shear modulus can be found at each temperature. These individual models are then combined to form a single ‘master curve’ for which a time-shift function and a Prony-series is fitted. In the present studies, Hexcel F593–18 plain weave pre-preg laminates were investigated. Experimental creep tests were conducted at various temperatures between 40 °C and 150 °C. Through the application of the inverse parameter identification technique, it was determined that the viscoelastic interlaminar shear moduli of the composite material can be effectively modelled by a nine-term Prony series and a third-order polynomial time-shift function.     

纤维增强聚合物基复合材料的低温性能

对纤维增强聚合物基复合材料在低温领域的实际应用进行了分类介绍,通过对纤维增强聚合物基复合材料的低温性能、性能影响因素和作用机理、低温应用安全性等方面的研究工作进行总结,突出各类纤维增强聚合物基复合材料低温下的性能优势,阐明了材料性能的不足之处及相应改进措施.对于实际低温应用中纤维增强聚合物基复合材料的选择、性能设计优化,系统安全性的增强提供了参考作用.