Critical fibre length and tensile strength for carbon fibre-epoxy composites

The tensile strength of epoxy resin reinforced with a random-planar orientation of short carbon fibres decreases with increasing temperature. This decrease may be estimated by the strain rate and temperature dependence of both the yield shear strength at the fibre-matrix interphase and the critical fibre length obtained by taking the distribution of fibre strength into consideration. The experimental value at room temperature is smaller than the calculated value. It is inferred that this result is attributed to the stress concentration caused by ineffective fibres produced during preparation which were shorter than the critical fibre length.     

Effect of interfacial bonding strength on tensile ductility of multilayered steel composites

The effect of the bonding strength of the laminate interface on ductility in the tensile deformation of multilayered steel composites was investigated. Multilayered steel composites consisting of alternating layers of as-quenched martensitic and austenitic steels were prepared with various bonding strengths, ranging from weak bonding obtained by alpha-cyanoacrylate adhesive to strong bonding obtained by cold rolling with a subsequent heat treatment. Tensile tests and peel tests were conducted to investigate the relationship between tensile behavior and bonding strength at the interface. It was demonstrated that tensile ductility could be markedly enhanced as the bonding strength increased, and that three types of tensile fracture behavior were identified depending on the bonding strength of the interface.     

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.     

Mathematical model for tensile behaviour of hybrid continuous fibre cement composites

A mathematical model has been developed for the tensile behaviour of cement composites reinforced with two types of continuous and unidirectional aligned fibres. The model is based on the theory for single fibre composites proposed by Aveston, Cooper and Kelly. Theoretical curves have been obtained for the tensile properties of the polypropylene/glass fibre-reinforced cement composites by means of substituting parameters into the developed equations, and these curves were compared with experimental results for a limited range of fibre combinations. It is shown that to attain optimum hybrid effects for toughness and the first maximum point of the stress-strain curve, the correct fibre volume combinations should be included.     

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.     

The tensile strength of short fibre-reinforced composites

Tensile strength is one of the most important mechanical properties of structural short fibre composites, and its prediction is essential for composite design. This paper develops a strength theory for three-dimensionally oriented short fibre-reinforced composites. The contribution of direct fibre strengthening to the composite strength is derived using a maximum-load composite failure criterion. Other strengthening mechanisms, such as residual thermal stress, matrix work hardening and short fibre dispersion hardening are also incorporated into the calculation of composite strength. In the derivation of direct fibre strengthening, the strain and stress of short fibres with different inclination angles were first derived, and the direct fibre strengthening was calculated from the maximum total load these short fibres can carry in the composite loading direction. This revised version was published online in November 2006 with corrections to the Cover Date.     

Synthesis of continuous silicon carbide fibre with high tensile strength and high Young's modulus

Polycarbosilane as the precursor of continuous SiC fibre was synthesized by thermal decomposition of polydimethylsilane. The structure of the polycarbosilane is concluded to be similar to that of polysilapropylene by the measurements of i.r. spectra, NMR spectra and chemical analyses. Its formation mechanisms are initially the formation of carbosilane by thermal decomposition of polydimethylsilane and then the increase in molecular weight by dehydrogenation-condensation of the carbosilane. Molecular structure and molecular weight distribution of the polycarbosilane depend on the reaction temperature.     

Influences of interfacial bonding strength and scatter of fibre strength on tensile behaviour of unidirectional metal matrix composites

The influences of interfacial bonding strength and scatter of strength of fibres on tensile behaviour of unidirectional metal matrix composites, whose matrix has low yield stress in comparison to the strength of fibres, were studied using the Monte-Carlo simulation technique using two-dimensional model composites. The following results were found. The strength of composites increases with increasing bonding strength, especially when the bonding strength exceeds the shear yield stress of the matrix and then remains nearly constant. The strength of composites is very sensitive to bonding strength when the scatter of fibre strength is large, but not when it is small. The fracture mode varies from non-cumulative to cumulative with increasing scatter of fibre strength for both cases of weak and strong interfacial bondings. The fracture surface becomes irregular when bonding strength becomes low and scatter of fibre strength becomes large. The applicability of the Rosen and Zweben models and the rule of mixtures to predict the strength of composites was examined.     

Fatigue of continuous fibre composites

Fatigue curves for composites are derived from those of the components, with as little arbitrariness as possible. Even with this restriction, the expected fatigue behaviour of the composites turns out to be of a rather diverse nature, depending on modulus ratio, volume fraction, and residual stress. The results concern pulsating tension as well as fluctuating load at an arbitrary mean stress. By comparison with available experimental data, our results lead to a better understanding of composite behaviour.     

The longitudinal tensile strength of unidirectional fibrous composites

When a fibre-plastic composite in which the fibres are brittle, continuous, and unidirectional is subjected to longitudinal tension under essentially static loading conditions, there exists a range of possible composite strengths. This paper presents a model which may be used to predict that range of possible composite strengths. An important feature of the model is that it considers both static and dynamic stress concentration effects on intact fibres which result from a fibre failure. A computer simulation technique is used to generate a set of generalized scatter limits for the average fibre stress at composite failure from the model. The generalized scatter limits may be used to predict the range of strengths for a composite material. The model results are used to predict the ranges of strength for composite materials prepared from three types of carbon fibre and these are compared with experimental results.     

Strain rate and temperature dependence of tensile strength for carbon/glass fibre hybrid composites

The tensile strength of epoxy resin reinforced with a random planar orientation of short carbon and glass fibres increased as the strain rate increased, and the increase in tensile strength became slightly remarkable with increasing temperature. The strain rate-temperature superposition was held for each composite. The strain rate and temperature dependence of tensile strength of composites could be estimated based on the dependence of the mechanical properties of the matrix resin, the interfacial yield shear strength and the critical fibre length. The strain rate and temperature dependence of the tensile strength of the hybrid composite could be estimated by the additive rule of hybrid mixtures, using the strain rate and temperature dependence of the tensile strength of both composites. The experimental values at a higher rate were lower than the calculated values. It was hypothesized that this may have been caused by the ineffective fibres formed during preparation of the specimen.     

Creep strength of discontinuous fibre composites

A unidirectional, discontinuous fibre composite is considered under conditions of steady state creep in the direction of reinforcement. The composite consists of noncreeping, discontinuous, perfectly aligned, uniformly distributed fibres which are perfectly bonded to a matrix obeying a power relation between stress and strain rate. Expressions for the interface stress, the creep velocity profile adjacent to the fibres and the creep strength of the composite are derived. Previous results for the creep strength,σ _c obtained for composites of the same type are briefly reviewed and compared with the present result. It is shown that all results reduce to the same general expression $$\sigma _c = \alpha V_{f^{\sigma _0 } } \left( {\frac{{\dot \in }}{{\dot \in _{0 } }}} \right)1/n_{\rho ^{1 + 1/n} }$$ in whichρ is the fibre aspect ratio, \(\dot \in\) is the composite creep rate,V _f is the fibre volume fraction,σ ₀,ε ₀ andn are the constants in the matrix creep law. The creep strength coefficient α is found to be very weakly dependent onV _f and practically independent ofn whenn is greater than about 6.     

Uniaxial tensile strength of early age fibre concretes

This study deals with the determination of uniaxial tensile strength of concretes reinforced withA-fibres (slag-basalt fibres) and glass-fibres at the age of 0,3 and 6 hours. For the determination of the tensile strength of fibre-reinforced concrete in its early age of setting and hardening a method was applied which was developped for the investigation of plain concretes. Tests were performed to obtain the relationships between the fibre volume fractions and uniaxial tensile strengths of concretes at different, ages. The kinetics of strength increase has been studied. It is shown that the kinetics of strength increase is to a high degree dependent on the kind of fibre reinforcement.     

The strength of hybrid glass/carbon fibre composites

The tensile mechanical properties of hybrid composites fabricated from glass and carbon fibres in an epoxy matrix have been evaluated over a range of glass: carbon ratios and states of dispersion of the two phases. The failure strain of the carbon phase increased as the relative proportion of carbon fibre was decreased, and as the carbon fibre was more finely dispersed. This behaviour is commonly termed the hybrid effect, and failure strain enhancement of up to 50% has been measured. Only part of the effect may be attributed to internal compressive strains induced in the carbon phase by differential thermal contraction as the composite is cooled from its cure temperature. The laminae or ligaments of carbon fibre dispersed in the glass fibre phase show a multiple failure mode, and when the constitution is favourable catastrophic failure does not occur until a considerable number of ligament fractures have accumulated. Failure is thus progressive, and the material is effectively tougher than equivalent all-carbon fibre composites.     

The abrasive wear behaviour of continuous fibre polymer composites

The dry abrasive-dominant wear behaviour of several composite materials consisting of uni-directional continuous fibres and polymer matrices was investigated. Seven materials were examined: neat epoxy (3501-6), carbon fibre epoxy (AS4/3501-6), glass fibre/epoxy (E-glass/ 3501-6), aramid fibre/epoxy (K49/3501-6), neat polyetheretherketone (PEEK), carbon fibre/PEEK (APC2) and aramid fibre/PEEK (K49/PEEK). The wear behaviour of the materials was characterized by experimentally determining the friction coefficients and wear rates with a pin on-flat test apparatus. First, the effects of the operation variables apparent normal pressure, sliding velocity and apparent contact area were observed. The dimensionless wear rate increased linearly as the apparent normal pressure increased and decreased as the apparent contact area increased. Second, through microscopic observations of the worn surfaces and subsurface regions, basic wear mechanisms were identified as a function of fibre orientation. Observations of fibre-abrasive particle interactions allowed for the differentiation of the dominating wear mechanisms. Finally, a network of data was compiled on the wear behaviour in terms of the three material parameters: fibre orientation, fibre material and matrix material. This enabled the systematic selection of an ideal low wear composite material which would consist of a PEEK matrix reinforced with aramid fibres oriented normal to the contacting surface and carbon fibres oriented parallel to the contacting surface.     

Influence of microstructure on axial strength of unidirectional continuous fibre reinforced aluminium matrix composites

Abstract

Systematic empirical investigations on the relationship between microstructural features and mechanical performance of unidirectionally reinforced continuous fibre Al matrix composites (CFAMCs) carried out by the present authors in recent years are summarised. The employment of a high strength matrix alloy and the development of a strong fibre/matrix interface are beneficial to maximise the strengthening effect of the fibre reinforcement. Processing defects, such as second brittle phases in the matrix, non-infiltration defects, matrix solidification shrinkage voids, excessive interfacial reactions, the presence of reaction products on the interface, weak interfacial binding, and excessively high fibre volume fraction reduce composite strength to different extents via a number of different mechanisms. Criteria for the microstructure design of CFAMCs for optimum fibre strengthening efficiency are proposed.     

Tensile strength and failure mechanics of fibre composites

This paper is concerned with the prediction of the tensile strengths of fibre composites where the fibres are aligned in the direction of tensile loads, and are flawed to some extent. A theory is derived for predicting the strengths and failure mechanisms of such composites. The theory agrees reasonably well with experiments, and may be qualitatively applicable to composites containing randomly aligned fibres.