The effect of transverse compressive stresses on tensile failure of glass fibre-epoxy

Four point bending tests were carried out on 32-ply glass fibre-epoxy specimens with 0 ° plies on the surface. Different combinations of 0 ° and ± 45 ° interior plies allowed the transverse stresses at the surface to be varied. All specimens failed by fibre tension, and the strength was relatively insensitive to transverse compressive strengths. The maximum stress failure criterion was found to provide a reasonable fit to the experimental data, and is recommended for design purposes.     

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.     

The effect of fibre length and interfacial bond in glass fibre-epoxy resin composites

The effect of fibre length on the strength of glass fibre-epoxy resin composites has been examined by beam bending experiments on uniaxially aligned material. The results agree well with theoretical predictions and the critical fibre length is found to be 12.7 mm (0.5 in.).A method of measuring the interfacial shear strength of the fibre-matrix interface has been developed and the measured value of the interfacial shear strength found to be 9.5 N mm^–2.The mechanism of shear failure is examined and discussed in detail.Formerly at Glasgow     

Hygrothermal ageing and fracture of glass fibre-epoxy composites

The effects of hygrothermal ageing upon the failure characteristics and work of fracture of glass fibres in epoxy are described. A collection of data based on the direct observation and measurement of fibre debond length and fibre pull-out length after fracture is displayed in empirical failure diagrams. Similarly, a vast number of experimental measurements of work of fracture is displayed in a three-dimensional diagram where the axes are work of fracture, humidity and ageing time. This information is combined with models of fracture in the construction of a fracture map which is used to interpret hygrothermal ageing phenomena.     

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.     

Fracture toughness and mechanical properties of glass fibre-epoxy composites

The present study investigated fracture properties and various mechanical properties of a set of unidirectional glass fibre-epoxy resin composites. This set was comprised of samples with volume fraction of fibres in the range 0.29 to 0.75. An identical set of composites was water-boil treated for 7 days, and the effect of this treatment on the above properties was examined. The work of fracture (γ _F) and the fracture surface energy of initiation (γ ₁) results were compared with existing theoretical models for the prediction of fracture toughness. It was discovered that theγ _F results agreed with the pull-out model [6], suggesting that this was the major contribution to the fracture energy of the complete process. Theγ ₁ values corresponded generally with the surfaces formation model [9], proposing that the creation of new fibre, matrix and fibre-matrix surfaces controls the stage of fracture initiation.     

The effect of fibre sizing on fibres and bundle strength in hybrid glass carbon fibre composites

Tests have been carried out on single carbon fibres supplied in the sized and unsized conditions, as well as impregnated tows and tows in a glass–carbon fibre hybrid composite of the same fibre. The results were analysed using a Weibull distribution for the strengths of the reinforcing fibres and composites. The tensile strength of the single fibres appeared to be unaffected by the sizing of the filaments. In the case of the impregnated tows, an increase in characteristic strength of 7% was observed for the unsized fibres. The strength of the impregnated tows in hybrid composites was seen to be 15% higher than those tested in air. This can be attributed to the “hybrid effect”. This revised version was published online in November 2006 with corrections to the Cover Date.     

Flexural properties and dynamic mechanical properties of glass fibre-epoxy composites

The viscoelastic behaviour of glass fibre (GF)-epoxy composites was studied by flexural tests and dynamic mechanical measurements. In relation, the influence of surface treatment of GF on viscoelastic behaviour was also examined. Using the results of flexural tests under a variety of constant temperature and strain rate, master curves of flexural strength () and flexural strain () were obtained for matrix epoxy and GF composites. The magnitudes of the master curves were different between matrix epoxy and GF composites. The fracture mode was influenced by temperature, strain rate, and G F surface treatment. The magnitude of storage modulus and effectiveness of adhesion at the GF-matrix interface were also influenced by GF surface treatment. Relationship between the results of flexural strain and loss modulus were considered for GF composites.     

The effect of oxygen partial pressure on the oxidation behaviour of carbon fibres and carbon fibre/glass matrix composites

Carbon fibres were examined for oxidative stability, both in the as-received condition and in a carbon fibre/glass matrix composite, to determine the effect of oxygen partial pressure on their oxidation behaviour. The oxidation data as a function of oxygen partial pressure (0.05, 0.21, and 1.0 atm) were then analysed to determine how the structure of the fibre surface impacts the oxidation behaviour. For the as-received material, it was determined that the data are best described by Temkin-type adsorption kinetics, while the fibres encased in the glass matrix exhibited Langmuir-type kinetics. This relevance of this distinction is discussed.     

The effect of transverse shear on the longitudinal compressive strength of fibre composites

Experimental work on glass/epoxy composites shows that the compressive strength is sensitive to the method of gripping, that the failure mode in compression varies with fibre volume fraction, and that bending of the specimen may occur as a result of misalignment. Some aspects of these observations are examined. The critical Euler buckling load is significantly reduced if transverse shear occurs. The buckling load depends on specimen dimensions and a good deal of scatter results from this. The predicted compressive strength taking into account the effect of transverse shear and specimen geometry includes the experimental results within a wide scatter band. The present analysis based upon the macro-buckling of the specimen, reproduces some predictions of compressive strength based upon the micro-buckling of fibres.     

Organoclay effect on transverse compressive strength of glass/epoxy nanocomposites

This research focuses on the fabrication of glass fiber/epoxy nanocomposites containing organoclay as well as understanding the organoclay effect on the transverse compressive strength of nanocomposites. To demonstrate the organoclay effect, three different loadings of organoclay were dispersed, respectively, in the epoxy resin using a mechanical mixer followed by sonication. The corresponding glass/epoxy nanocomposites were produced by impregnating dry glass fiber with organoclay epoxy compound through a vacuum hand lay-up procedure. Unidirectional block specimens were employed for transverse compression tests on a hydraulic MTS machine. Experimental observations indicate that glass/epoxy nanocomposites containing organoclay exhibit higher transverse compressive strength than conventional composites. Furthermore, the failure mechanisms for all tested specimens were found to be fiber and matrix debonding. Therefore, results indicate that the increasing characteristic in transverse failure stress may be ascribed to the enhanced fiber/matrix adhesion modified by the organoclay.     

The effect of hydrostatic pressure and loading rate on compressive failure of fiber-reinforced ceramic-matrix composites

The influence of hydrostatic confinement on compressive strength and corresponding failure mechanisms is explored for SiC-reinforced glass-ceramics tested at different strain rates. Two composite architectures (0° and 0°/90°) are studied, and their behavior is compared with that of monolithic glass-ceramic tested under similar conditions. Composite confined pressure results are interpreted in terms of fiber buckling under quasi-static conditions and fiber kinking at high pressures, and compared with monolithic (non-composite) microfracture coalescence at low pressures and shear band formation under more intense confinement. In particular, dilatational fracture within the matrix dominates composite failure at low pressures, while high pressures cause a transition to shear-dominated mechanisms based on fiber kinking.     

Finite element modelling of interfacial failure in model carbon fibre-epoxy composites

Finite element analysis has been used to model a single unsized carbon fibre embedded in an epoxy matrix subjected to tensile loading. The predicted fibre strain distribution is compared with experimental data, obtained using the technique of laser Raman spectroscopy, for a number of incremental applied strain levels. Good correlation is obtained on the assumption that the prevailing mode of interfacial failure in the composite involves a conical matrix crack initiating at the fibre end. The geometry of the matrix crack is estimated on the assumption that the crack propagates in a self-similar manner.     

Reduction in tensile and flexural strength of unidirectional glass fibre-epoxy with increasing specimen size

Size effects in tensile failure were investigated by means of tensile and four-point bending tests. Tapered tensile specimens with plies dropped off internally showed a reduction in strain at failure with increasing gauge length. Scaled bending tests also showed a reduction in strain with increasing specimen size. These two effects and the relationship between the tensile and flexural results could all be fitted satisfactorily with a Weibull strength model.     

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.     

The effect of low energy impact on the tensile strength of coated and uncoated glass particulate composites

The residual tensile strength of glass filled particulate composites has been determined after low energy impact for various energy values. The material systems constructed for the needs of this research consisted of epoxy resin filled with glass beads. The glass beads were either uncoated or alternatively coated with a reactive silane based bonding agent. Specimens with various filler volume fractions were available. The effect of silane coating as well as the filler volume fraction was analytically discussed. Finally, a model developed in previous work for continuous fibre reinforced composite laminates was adopted to describe the residual tensile strength after impact. In most of the cases the predicted curves fit the experimental results very well.     

Photoelastic study of the durability of interfacial bonding of carbon fibre-epoxy resin composites

The physical techniques of polarizing microscopy, including the quantitative measurements of small optical retardations, have been used to investigate elastic fields adjacent to short carbon fibres in epoxy resin composites. The elastic fields associated with shear stress distribution along the fibre-matrix interface have been employed to monitor the initiation of interface debonding during hot (100 °C) water uptake. By examining the development of stress birefringence during resin swelling in the resin adjacent to individual fibres, the differences in the durability of interfacial bonding and the fibre failure modes for differently coated fibres have been obtained. The results show that the state of self-stress in model composites, comprising a single carbon fibre in a film of epoxy resin, can, by immersion in hot distilled water, be enhanced to such an extent that the axial tension in the fibre can be sufficient to initiate fibre fracture. The results also show that, for fibres that have been given certain proprietary surface treatments, the fibre fractures by different failure modes.