Strength distribution of Carborundum polycrystalline SiC fibres as derived from the single-fibre-composite test

The single-fibre—composite (s.f.c) test, in which a fibre is embedded in an epoxy matrix and the composite tested in tension, was employed to obtain the statistical strength distribution of Carborundum SiC ceramic fibres over the range of gauge lengths from 0.5 to 20 mm. The raw s.f.c. test data was organized into three independent forms: the number of fibre breaks versus applied stress; the fibre fragment length distribution at the end of the test; and the fibre strength versus fragment length during testing. The data was interpreted using two different models of the fibre/epoxy—matrix interface, and it was found that a constant shear stress model could not self-consistently fit all of the s.f.c. data, whereas an elastic interface model provided good fits to all of the data. The applicability of the elastic interface model was supported by the absence of interfacial debonding and the rough fibre/matrix interface, which promoted mechanical interlocking. The s.f.c. test derived strength of ₀ = 1500 MPa at a gauge length of 20 mm, with a Weibull modulus of m = 9, agreed fairly well with independent tension test results obtained on 254 mm length samples. Obtaining self-consistent fits to all of the manifestations of the s.f.c. data requires careful testing and analysis, but the present work demonstrates that the s.f.c. test can be a powerful tool for the accurate and independent assessment of fibre strengths at small gauge lengths.     

Experiments on shear deformation,debonding and local load transfer in a model graphite/glass/epoxy microcomposite

Recent statistical theories for the failure of polymer matrix composites depend heavily on details of the stress redistribution around fibre breaks. The magnitudes and length scales of fibre overloads as well as the extent of fibre/matrix debonding are key components in the development of longitudinal versus transverse crack propagation. While several theoretical studies have been conducted to investigate the roles of these mechanisms, little has been substantiated experimentally about the matrix constitutive behaviour and mechanisms of debonding at the length scale of a fibre break. In order to predict the growth of transverse and longitudinal cracks using the same micromechanical model, we microscopically observed the epoxy shear behaviour around a single fibre break in a three-fibre microcomposite tape. The planar specimens consisted of a single graphite fibre placed between two larger glass fibres in an epoxy matrix. The interfibre spacing was less than one fibre diameter (<6 m) in order to reflect the spacing between fibres found in typical composites. The epoxy constitutive behaviour was modelled using shear-lag theory where the epoxy had elastic, plastic, and debond zones. The criteria for debonding were modified from conventional shear-lag approaches to reflect the orientational hardening in the epoxy network structure. The epoxy, which is brittle in bulk, locally underwent a shear strain of about 60% prior to debonding from the fibre.     

Synthesis process and microstructure of carbon fibres/graphite/copper composites by spark plasma sintering

Carbon fibres/graphite/copper composites are prepared by mechanical alloying and spark plasma sintering at different temperature in this study, and the microstructure evolution is investigated during sintering. The curves of punch displacement, mould temperature, furnace chamber vacuum, and voltage against time were recorded and discussed. The results show that the carbon fibre/graphite/copper composite presents neat and pure interface. Copper element is found in carbon fibres and graphite, which may be due to discharge. The microhardness of the obtained composites is up to 160?HV. The highest relative density in this test is 96.5%. The microstructure evolution process during SPS is associated with sparking, joule heat, pressure, and deformation. The sintering process is uneven at initial stage.     

Hybrid effects in the bending stiffness of graphite/glass-reinforced composites

The flexural modulus of graphite/glass-reinforced hybrids exhibits deviations from the rule-of-mixtures base-line. The deviation increases as the segregation of the glass and graphite layers increases, and it reaches a maximum when the two fibre types are arranged in 3 layers. When the stiffer fibres (graphite) are in the outer layers the deviation is positive but it is negative with the less stiff fibres (glass) on the outside. The extent of deviation is shown to be predictable by the analysis.     

Investigation into stress transfer characteristics in alumina-fibre/epoxy model composites through the use of fluorescence spectroscopy

In composite materials, fibre/fibre interaction phenomena due to fibre failure are crucial in determining the composite fracture behaviour. Indeed, the redistribution of stress from a failed fibre to its intact neighbours, and stress concentration induced in the neighbouring fibres, determine the extent to which a break in one fibre will cause more breaks in others. In this paper, we have used fluorescence spectroscopy to study the stress transfer and redistribution induced by fibre fracture in two-dimensional Nextel-610 fibres/epoxy-resin micro-composites. The stress along the fibres was mapped at different load levels, and specimens with different inter-fibre distance were used to study the fibre content effect. The interfacial shear stress distribution along broken and intact fibres was derived by means of a balance of shear-to-axial forces argument. The experimental stress concentration factors (SCF) were smaller than values predicted from our model based on the cell assembly approach. As expected the 2D configuration allows access to the upper bound of the SCF in real composites. For the several specimens tested, a region of matrix yielding was observed behind the fibre fracture and no-debonding at the interface was detected. The measured SCF values agree well with those reported in recent study for carbon-fibre/epoxy model composites.     

Compressive and tensile behaviour of carbon fibres

Attempts have been made to discuss the fibre axial tensile and compressive behaviour of several carbon fibres prepared from different precursors, and surface- and/or sizing-treatments. With all fibres, the number of breaks increased with increasing tensile or compressive strain, and remained constant beyond a certain strain. The constant number of breakages based on the precursor differed remarkably in compression, whereas those based on the surface- and/or sizing-treatments differed remarkably in tension. In tension, the PAN-based fibre could be broken with a somewhat greater ease than the pitch-based fibre, while in compression, the pitch-based fibre could be broken with somewhat greater ease than the PAN-based fibre. © 1998 Chapman & Hall     

Multiple Data Set (MDS) weak-link scaling analysis of jute fibres

Jute technical fibres were tested in tension at 10 different gauge lengths between 6 mm and 300 mm (50 or 100 tests at long or short gauge lengths respectively). The Young’s modulus, strain to failure and ultimate tensile strengths were determined individually and then Weibull distribution parameters were estimated using the maximum likelihood method to quantify the variation. Single Data Set (SDS, standard) and Multiple Data Set (MDS) weak-link scaling (WLS) predictions were assessed using Anderson–Darling Goodness of Fit Numbers (GOFN). The use of MDS predictions provides better correlation with the experimental data than the standard weak-link scaling method. The authors recommend the use of MDS weak-link scaling for this problem with at least two points (but preferably three) with fibre lengths at two extremes (and, if used, the third point near the mean fibre length).     

Experimental determination of elastic properties of impact damage in carbon fibre/epoxy laminates

This paper describes an investigation of in-plane elastic properties of impact damaged regions in composite laminates. Quasi-isotropic carbon fibre/epoxy laminates were impacted and the impact damage examined by ultrasonic C-scanning, optical microscopy and thermal deplying. After impact damage observations, specimens were cut from the laminates and tested in tension and compression. The elastic modulus of the impact damage was, in both tension and compression, mainly controlled by the amount of fibre breakage. Interestingly, layers with broken fibres could sustain some load in compression, which led to higher elastic modulus in compression than in tension. The effect of delaminations on the elastic modulus was quite small in both tension and compression. The through-the-thickness variation of in-plane stiffness was studied by successively removing plies. The variation in stiffness was negligible, probably as a result of the very uniform distribution of delaminations and fibre breakage through the thickness of the laminates.     

Effect of silane coupling agent on mechanical performance of glass fibre

Mechanical performance of commercially manufactured unsized and γ-aminopropylsilane sized boron-free E-glass fibres has been characterised using single-fibre tensile test. Both apparent fibre modulus and fibre strength were found to strongly depend on fibre gauge length. The average strength of sized fibres was found 40–80 % higher than unsized fibres at different gauge lengths. Weibull analysis suggested that the failure mode of unsized fibres could be described by unimodal Weibull distribution, whereas the strength distribution of sized fibres appeared to be controlled by two exclusive types of flaw population, types A and B. Comparison of the Weibull plots between unsized and sized fibres revealed that the strength of unsized fibres was likely to be dominated by type A flaws existing on the bare glass surface and type B flaws may be related to the defects on the glass surface coated with silane. This was partially supported by the observation of fractured cross-sectional area using SEM. It was, therefore, proposed that the strength difference between unsized and sized glass fibres may be more reasonably interpreted from the surface protection standpoint as opposed to the flaw healing effect. The results obtained from this study showed that silane coupling agent plays a critical role in the strength retention of commercially manufactured E-glass fibres and the silane effect on the fibre strength is also affected by the change in gauge length of the sample.     

High-strength graphite fibre/lithium aluminosilicate composites

Graphite fibre/lithium aluminosilicate composites of matrix composition Li₂O Al₂O₃·nSiO₂ wheren=3, 4, and 8, have been developed with a high volume fraction of undirectionally aligned graphite fibres. Graphite fibre/ceramic matrix tapes were produced on a drumwinding apparatus and hot-pressed to final dimensions. This composite system exhibits a combination of useful properties, including high strength, low density, excellent thermal shock resistance and impact strength. Shear strength, cyclic modulus of rupture and modulus of elasticity and optical and electron microscopic evaluation of microstructure are discussed. Modulus of rupture as a function of vol % fibres was studied. The high modulus of rupture and impact strength are discussed in terms of the relative properties of fibre and matrix.     

Mechanical behaviour of coir fibres under tensile load

Stress-strain curves of coir fibres have been determined. Mechanical properties including initial modulus, strength and percentage elongation of coir fibres have been evaluated as functions of retting treatment (during retting the coconut husks are soaked in saline water for a period of about six months to facilitate the extraction of fibres presumably due to a bacterial process), fibre diameter, gauge length and strain rate. No significant differences in mechanical properties were observed between retted and unretted fibres. The strength and percentage elongation seem to increase for both retted and unretted fibres up to a fibre diameter of 0.2×10^–3 m whereafter they remain almost constant. On the other hand, moduli seem to decrease with increase in diameter of the fibre. The observed modulus values and percentage elongation have been related to microfibrillar angle. Observed strength values have been explained on the basis of structural changes occurring with an increase in the diameter of the fibre. Scanning electron/microscope studies have indicated that the failure of the fibre is due to the fracture of the cells themselves accompanied by the uncoiling of microfibrills. There is no appreciable variation in strength and percentage elongation with strain rates for any one diameter of the fibre. On the other hand, with increase in gauge length, a decrease in both strength and percentage elongation at break has been observed. These have been attributed to an increase of probability of defects and localized deformation and gentle necking, respectively.     

Fracture statistics of a unidirectional composite

We propose a model dealing with the prediction of the failure stress of a unidirectional composite 0°; it is based on a probabilistic micro-macro approach. Experimental tests have been carried out on specimens (unidirectional composite 0° T300/914) with different gauge lengths in order to estimate the scale effect in the failure probability distribution.The distribution of defects along the fibres was estimated through the multifragmentation and the single fibre test. The image analysis technique was used to estimate the local volume fraction of the fibres in the bulk of the material. The above physical information is introduced in the model based on a finite element analysis. The scale effect and the influence of the involved parameters on the failure of the material were studied at two different scales and a good agreement was found between the numerical predictions and the experimental results.     

Three-dimensional finite element analysis of the stress concentration at a single fibre break

A linear elastic analysis is performed of a single broken fibre surrounded by six equally spaced fibres. These fibres and the surrounding epoxy matrix are modelled separately whilst the rest of the composite is treated as a homogeneous, orthotropic material. The distance of the adjacent fibres is fixed based on an assumed fibre volume fraction of 0·60. The analysis shows that the stress concentration in the adjacent fibres is 1·058, much lower than the value of 1·104 predicted by Hedgepeth and van Dyke (J. Comp. Mater., 1 (1967) 294–309). The positively affected length where there is an increase in stress is only about half the ineffective length of the broken fibre. Further away from the break the axial stress in the adjacent fibres actually drops below the nominal axial stress. This results in a very small enhanced probability of failure in the adjacent fibres. Very close local fibre spacing around the broken fibre increases the maximum stress in the adjacent fibres by less than 3%.     

Statistics of single‐fibre tensile strength incorporating spatial distribution of microcracks

The random distribution of single‐fibre tensile strength has been commonly characterized by the two‐parameter Weibull statistics. However, the calibrated Weibull model from one set of strength data at a given gauge length cannot accurately predicts the strength variation of the fibre at different gauge lengths. Instead of presuming the two‐parameter Weibull distribution or any other specific statistical distribution for the single‐fibre strength to begin with, this work proposes an approach to incorporating the appropriate spatial flaw distribution within a fibre and synchronizing multiple sets of tensile strength data to evaluate the single‐fibre strength distribution. The approach is examined and validated by published single‐fibre strength data sets of glass, ceramic and synthetic and natural carbon fibres. It is shown that the single‐fibre strength statistics does not necessarily always follow the two‐parameter Weibull distribution.     

The size effects on the mechanical behaviour of fibres

The size of a fibre affects its mechanical properties and thus is of theoretical and practical importance for studies of the rupturing process during loading of a fibrous structure. This paper investigates the overall effects of length on the mechanical behaviour of single fibres. Four types of fibres, ranging from brittle to highly extensible, were tested for their tensile properties at several different gauge lengths. Different from most previous studies where the focus has been on the gauge length effects on a single property such as fibre strength or breaking strain, this paper look comprehensively into the effects of length on all three of the most commonly studied mechanical properties, namely strength, breaking strain and initial modulus. Particular emphasis is placed on initial modulus and on the interactions between all three parameters. Influences of strain rate and fibre type on the size effects are also investigated. The effect of potential fibre slippage on experimental error is examined. An image analysis method is used to measure the real fibre elongation in comparison to the same fibre elongation obtained directly from an Instron tester. Finally, a statistical analysis is carried out using the experimental data to test the fitness of the Weibull theory to polymeric fibres. This was done as the Weibull model has been extensively utilized in examining fibre strength and breaking strain, although it is supposed to be valid only for the so-called classic fibres to which more extensible polymeric fibres do not belong. This revised version was published online in November 2006 with corrections to the Cover Date.     

Thermal damage effects on delamination toughness of a graphite/epoxy composite

The effects of overheating AS/3501-6 graphite/epoxy composite material have been examined after exposures in air up to 350°C. Degradation was assessed from measurements of the interlaminar fracture toughness in both Mode I tension and Mode II shear as well as the interlaminar shear strength and hardness. Exposures up to 30 minutes at 225°C and up to 15 minutes at 300°C did not significantly reduce the interlaminar toughness. However, longer exposure at 300°C and short exposures at 350°C produced catastrophic decreases in toughness values, shear strength and hardness. The primary origin of embrittlement was attributed to deterioration of the epoxy as opposed to any fibre or fibre-matrix interfacial weakening.     

Photoelastic study of the elastic stress distribution around discontinuous fibres

The mechanism by which load is transferred from a discontinuous fibre to surrounding unbroken fibres has been examined in some detail using two dimensional photoelastic models in which the applied load is aligned with the fibre axes. The gap in the broken fibre is assumed to have occurred during fabrication and is thus filled with the matrix material. Three different end gap configurations have been analysed and it is concluded that, for all practical purposes, the disturbance due to the broken fibre does not extend beyond the immediately adjacent fibres. Substantial shear stresses are developed in the matrix for some distance along the fibre from the discontinuity with a consequent early transfer of load. Paradoxically, this effect is not accompanied by the development of reduced maximum stresses at the broken fibre tip for any of the configurations included in these tests.