Transverse properties of unidirectional glass/epoxy composites: influence of fibre surface treatments

An investigation has been made on effects of fibre surface treatments on transverse mechanical behaviour of unidirectional glass/epoxy composites. Model composite plates were processed by filament winding using glass fibres coated with different sizings changing by their epoxy functionality and their reactivity towards the matrix.In the first part of the study, transverse tension and microindentation characterisations were performed in order to correlate the ultimate behaviour of the composite with interfacial properties. Experiments revealed that the most reactive sizings promote the highest interfacial strength and also increase ultimate properties of laminates in transverse tension. This feature was attributed to the high crosslink density of the polymer network in interfacial areas.In the second part, finite element calculations were used to evaluate local strain and stress concentration in a composite submitted to transverse tension conditions. The general trend for the evolution of composite failure strain as a function of interfacial strength has been established. The modelling showed that a transition of the composite failure mode occurs at a global strain of 1.15%, from an adhesive rupture at the fibre/matrix interface to a cohesive rupture in the matrix. In the domain of adhesive rupture, the value of the composite failure strain appeared to be directly governed by the interfacial strength.Therefore, improving interfacial strength by use of fibre sizings with high epoxy functionality could constitute an interesting way to reduce transverse brittleness of composite structures.     

Interlaminar fracture toughness and associated fracture behaviour of bead-filled epoxy/glass fibre hybrid composites

To investigate enhancement of matrix-dominated properties (such as interlaminar fracture toughness) of a composite laminate, two different bead-filled epoxies were used as matrices for the bead-filled epoxy/glass fibre hybrid composites. The plane strain fracture toughness of two different bead-filled epoxies have been measured using compact tension specimens. Significant increases in toughness were observed. Based on these results the interlaminar fracture toughness and fracture behaviour of hybrid composites, fabricated using bead-filled epoxy matrices, have been investigated using double cantilever beam and end notch flexure specimens for Mode I and Mode II tests, respectively. The hybrid composites based on carbon bead-filled matrix shows an increase in both G _IC initiation and G _IIC values as compared to a glass fibre reinforced plastic laminate with unmodified epoxy matrix. The optimum bead volume fraction for the hybrid composite is between 15% and 20%. However, the unmodified epoxy glass-fibre composite shows a higher G _IC propagation value than that of hybrid composites, due to fibre bridging, which is less pronounced in the hybrids as the presence of the beads results in a matrix-rich interply region.     

Stacking sequence effects on fracture of notched carbon fibre/epoxy composites

The purpose of this study was to investigate the ability of the so-called damage zone model (DZM) to predict the influence of stacking sequence on the strength of notched carbon fibre/epoxy composites. The DZM is in essence based on the unnotched tensile strength, σ₀, and the apparent fracture energy, G_c^*, and the damage zone is modelled as a crack with cohesive forces acting on the crack surfaces. The DZM predicts fracture loads for three-point bend (TPB) specimens and specimens with circular holes quite accurately. As an attempt to explain the difference in strengths, the damage zone extension in the TPB specimens with different stacking sequence was examined.     

Torsion fatigue behavior of unidirectional carbon/epoxy and glass/epoxy composites

This paper presents results of the feasibility of carbon/epoxy composites (CFRP) as a future helicopter flexbeam material. Torsional behaviors of unidirectional CFRP and glass/epoxy composites (GFRP) with the same resin matrix were investigated. The initial torsional rigidity of CFRP was almost identical to that of GFRP. The torsional rigidities calculated using finite element analyses (FEA) agreed with the experimental results: the torsional rigidities are governed mainly by the material’s shear stiffness. Torsion fatigue tests were also conducted by controlling the angle of twist of the sinusoidal wave under a constant tensile axial load. No catastrophic failure occurred with either GFRP or CFRP, although decreased amplitudes of torque and torsional rigidities were observed according to the number of cycles. Results of X-ray CT inspections and numerical calculation by FEA revealed that degradation of a torsional rigidity is caused mainly by splitting crack propagation along the fiber direction. The torsion fatigue life of CFRP was superior to that of GFRP. Consequently, results confirmed that CFRP exhibits excellent properties as a torsional element of a helicopter flexbeam in terms of torsional rigidity and tension–torsion fatigue behaviors.     

Thermal residual microstress generation during the processing of unidirectional carbon fibre/epoxy resin composites: regular fibre arrays

The thermal residual microstresses generated during the processing of unidirectional carbon fibre/epoxy resin composites are predicted, assuming regular fibre arrays. Stresses are determined in unit cells covering a range of fibre coordinations, fibre diameters, minimum interfibre distances and fibre volume fractions. The method of calculation involves the finite element method. Elastic material behaviour with temperature-dependent epoxy resin properties are assumed, together with transversely isotropic carbon fibres. Of the parameters studied, the greatest effect on the maximum principal stress was produced by the minimum thickness of epoxy resin between the fibres and the ratio of this thickness to the fibre radius. Values of the maximum principal stress were found in some cases to exceed the tensile strength of the epoxy resin. However, there was little experimental evidence to support this prediction. Cracks occurred only to a limited extent and occurred around the fibre/epoxy interface, rather than between the fibres, as predicted by the model. Reasons suggested for this discrepancy include relatively weak fibre/epoxy resin bonds and limitations on the accuracy of the stress generation model. Methods by which the model may be improved are discussed.     

Effects of interfacial coating and temperature on the fracture behaviours of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites

The enhancement of transverse fracture toughness of unidirectional Kevlar and carbon fibre reinforced epoxy resin composites (KFRP and CFRP) has been studied using polymer coatings on the fibres. The results obtained show a substantial improvement in the impact fracture toughness of both KFRP and CFRP with polyvinyl alcohol (PVAL) coating without any loss of flexural strength; but there is only a moderate increase in impact toughness with other types of coating (i.e. carboxyl-terminated butadiene acrylonitrile (CTBN) copolymer and polyvinyl acetate (PVA)) with some reduction in flexural strength. The dependence of impact fracture toughness of these composites (with and without PVAL coating) on temperature was analysed on the basis of existing theories of toughening mechanisms from measurements of fibre-matrix interfacial properties, debond and fibre pull-out lengths and microscopic observations. The beneficial effect of fibre coating with PVAL on transverse fracture toughness is shown to sacrifice little damage tolerance of the composites against delamination fracture.     

Dynamic mixed-mode I/II delamination fracture and energy release rate of unidirectional graphite/epoxy composites

Mixed-mode open-notch flexure (MONF), anti-symmetric loaded end-notched flexure (MENF) and center-notched flexure (MCNF) specimens were used to investigate dynamic mixed I/II mode delamination fracture using a fracturing split Hopkinson pressure bar (F-SHPB). An expression for dynamic energy release rate G_d is formulated and evaluated. The experimental results show that dynamic delamination increases linearly with mode mixing. At low input energy E_i ? 4.0 J, the dynamic (G_d) and total (G_T) energy rates are independent of mixed-mode ratio. At higher impact energy of 4.0 ? E_i ? 9.3 J, G_d decreases slowly with mixed I/II mode ratio while G_T is observed to increase more rapidly. In general, G_d increases more rapidly with increasing delamination than with increasing energy absorbed. The results show that for the impact energy of 9.3 J before fragmentation of the plate, the effect of kinetic energy is not significant and should be neglected. For the same energy-absorption level, the delamination is greatest at low mixed-mode ratios corresponding to highest Mode II contribution. The results of energy release rates from MONF were compared with mixed-mode bending (MMB) formulation and show some agreement in Mode II but differences in prediction for Mode I. Hackle (Mode II) features on SEM photographs decrease as the impact energy is increased but increase as the Mode I/II ratio decreases. For the same loading conditions, more pure Mode II features are generated on the MCNF specimen fractured surfaces than the MENF and MONF specimens.     

Tensile properties of real unidirectional Kevlar/epoxy composites

Mechanical behaviour, tensile strength and failure modes in real unidirectional Kevlar/epoxy composites, loaded parallel to the fibres, at volume fraction (V_f) range 0.26–0.73, were investigated. It was found that the measured tensile strengths deviated from the expected values calculated from the Rule of Mixture. The deviation, which was minimal at V_f of about 0.5, was mainly due to geometrical deficiencies typical of real composites. At V_f<0.5 it could be explained by non-homogeneous fibre spread and distribution of fibres. At V_f>0.5 the deviation was explained by the increasing lack of matrix between some adjacent fibres and by squeezing of fibres. The initial part of loading was typified by straightening out of non-axial fibres, accompanied by fibre/matrix debonding. The straightening process was completed at a stress level of about 0.6–0.7 of the composite strength. Matrix damage began at this stress level and continued to develop up to final failure. Failure of Kevlar fibres was noted to occur only at an extremely short loading interval coinciding with the catastrophic final failure. This was due to the small scatter of Kevlar fibre strength.     

The effect of fibre length on fracture toughness and notched strength of short carbon fibre/epoxy composites

The effect of radius of curvature on the tensile notched strength of random short carbon fibre/epoxy composites containing 1, 5 and 15 mm length fibres is studied. The strength of all laminates showed a sensitivity to the radius of curvature, with the tensile strength decreasing at smaller radii of curvature. A model is developed to predict notched strength based on assumed evolution and propagation of damage from the tip of the notch. The predictions of the model depend principally on two material properties: the unnotched tensile strength and fracture toughness. Reasonable agreement is achieved between the predicted notched strength and experimental data.     

Experimental evaluation of the elastic limit of carbon‐fibre reinforced epoxy composites under a large range of strain rate and temperature conditions

The mechanical behaviour of organic matrix composite materials such as T700GC/M21 carbon fibre reinforced polymer (CFRP) is generally considered by the industry as being orthotropic elastic for the sizing of aeronautical structures under normal isothermal “static” flight loads. During the aircraft lifetime, it may be exposed to severe loading conditions at various temperatures. However, the mechanical behaviour of CFRP is known to exhibit a linear behaviour or a non‐linear behaviour according to the types of loads that are considered creep or extreme conditions. The observed non‐linearity can be commonly attributed to several physical phenomena such as non‐linear viscosity, plasticity, or damage. In the literature, different models can be found that are based on three components: a first elastic reversible behaviour, a second non‐linear behaviour, and a failure criterion. An important issue is to understand and characterize the transition between the elastic reversible behaviour and the non‐linear behaviour. To answer this question, the present paper describes an experimental methodology that permits to evaluate this transition thanks to raw experimental data, and its application to a range of constant but different strain rate and temperature tests performed on the T700GC/M21 CFRP material.     

Interfacial micromechanics of technora fibre/epoxy composites

The application of Raman spectroscopy using a near-infrared laser (IR) for the study of the deformation micromechanics of Technora fibres and single Technoras fibre embedded in epoxy composites is reported. The shift with strain of the Raman band approximately at 1613 cm⁻¹ corresponding to p-phenylene ring deformation is studied. It is shown that the shift of the Raman band can be used to monitor the deformation micromechanics of the Technora/epoxy composites allowing the interfacial shear stress (ISS) to be determined. It was found that the maximum ISS was close to the shear yield stress of the resin indicating that there is good adhesion between the Technora fibres and the epoxy resin matrix.     

Effect of fibre angle on fracture properties of unidirectional flyash filled FRP composites

The fracture properties of unidirectional flyash filled and unfilled glass fibre and carbon fibre reinforced epoxy resin composites are studied in relation to the variation of width ratio (a/W) and fibre angle. The results indicate that the fracture toughness, fracture surface energy and change in elastic strain energy are dependent on the width ratio but the effect of fibre angle between 30 and 60° is not very dependent on fracture properties due to the arrest of the crack path in fibre composites by flyash particles.     

Piezoresistivity of unidirectional carbon/epoxy composites for multiaxial loading

The applied strain of carbon fibre reinforced plastics (CFRPs) is measurable by their electrical resistance changes. For damage monitoring of laminated CFRPs, piezoresistivity strongly affects the measured electrical resistance change through residual strain relief attributable to delamination cracks. Although several studies of CFRP laminates’ piezoresistivity have been published, this study uses single-ply CFRP for specific piezoresistivity measurements in four directions. A review of the theory of in-plane piezoresistivity reveals orthotropic properties of CFRP piezoresistivity. In the present study, piezoresistivity of multiaxial loading is derived, and the unsymmetrical piezoresistivity matrix is calculated using the measured piezoresistivity here. Effects of multiaxial loading in a misaligned unidirectional laminate are also discussed here. The misaligned laminate causes large shrink in the transverse direction during tensile tests; poor electrical contacts at electrodes increases the electric current in the transverse direction; these two effects cause decrease of electrical resistance for the poor electrical contact specimen with large fibre misalignment.     

Compressive strength of unidirectional and crossply carbon fibre/PEEK composites

The commonly accepted production methods of composite systems generally result in departure of the plies properties from transverse isotropy due to stresses acting during fibre—matrix bond formation. This anisotropy coupled with the composite structure affects compressive loading; the ultimate stresses as well as the direction, in- or out-of-plane, of kink propagation. A unidirectional and a crossply carbon fibre/PEEK composites were compression tested at ambient and elevated temperature as well as exposed to various chemical environments. Significant disruptions in fibre—matrix interface in the crossply composite were indicated. The compression tests showed that failure occurred through in-plane and out-of-plane fibre bucking and kinking in the unidirectional and crossply composites, respectively. Failure of the longitudinal plies in the crossply laminate occurred at significantly higher compression stress than for the unidirectional composite. Compressive failure mechanisms in unidirectional and multi-directional laminates are considered.     

Interface chemistry in carbon fibre/epoxy matrix composites

Microcomposite test methods were used to measure the properties of the interphase of HTA/922 carbon/epoxy composites. The shear strength of the interphase resin is lower than that of the bulk resin. It is suggested that the discrepancy arises from changes in resin chemistry at the fibre/matrix interface. Bulk resin samples where the proportions of the constituents had been altered were tested. Resin with a reduced level of hardener matched the mechanical behaviour of the interphase resin. It is concluded that, for the system examined, the interphase resin had a lower hardener concentration than the bulk resin.     

Relationship between strength variability and size effect in unidirectional carbon fibre/epoxy

Unidirectional carbon fibre/epoxy exhibits a size effect which is not necessarily consistent with the variability of specimen strengths predicted on the basis of classical Weibull theory. In some cases the variability is higher than expected and this may be due to other sources of variability in the tests apart from the material. However, in bending tests consistently lower variability than expected has been measured. This is explained qualitatively in terms of the splitting that occurs during failure which means that the composite behaves between the extremes of a brittle solid and a loose fibre bundle. This also implies that the size effect should be more strongly dependent on specimen length than indicated by Weibull theory. Three-point bending tests on specimens of different lengths but the same cross-section have been carried out which support this.     

The effect of fibre misalignment on the compressive strength of unidirectional carbon fibre/epoxy

The effect of a misalignment angle between the fibres and loading axis of a unidirectional composite is analysed by considering the shear strains induced by the misalignment. It is shown that shear instability in the matrix drastically reduces the predicted compressive strength even for very small misalignments. The same trend is predicted for composites with initial fibre curvatures due to the misalignment angle associated with the curvature. The reduction in compressive strength often attributed to initial fibre curvature may therefore actually be due to fibre misalignment angles. Small misalignments are hard to avoid during the manufacture and testing of unidirectional composites and so these results cast serious doubts on the possibility of measuring a true ultimate compressive strength for this kind of material.     

Fatigue delamination behaviour of unidirectional carbon fibre/epoxy laminates reinforced by Z-Fiber® pinning

Z-Pin reinforced carbon-fibre epoxy laminates were tested under Mode I and Mode II conditions, both quasi-statically and in fatigue. Test procedures were adapted from existing standard or pre-standard tests. Samples containing 2% and 4% areal densities of carbon-fibre Z-pins (0.28 mm diameter) were compared with unpinned laminates. Quasi-static tests under displacement control yielded a dramatic increase of the apparent delamination resistance. Specimens with 2% pin density failed in Mode I at loads 170 N, equivalent to an apparent G_IC of 2 kJ/m². Fatigue testing under load control showed that the presence of the through-thickness reinforcement slowed down fatigue delamination propagation.     

Prediction of mode I fracture toughness of unidirectional fibre composites with arbitrary crack fibre orientation from its lowest or matrix fracture tougness

Corrigendum

Prediction of mode I fracture toughness of unidirectional fibre composites with arbitrary crack fibre orientation from its lowest or matrix fracture tougnessby P.K. Sarkar and S.K. Maiti     

Demonstration of pseudo-ductility in unidirectional hybrid composites made of discontinuous carbon/epoxy and continuous glass/epoxy plies

A new, partially discontinuous architecture is proposed to improve the mechanical performance of pseudo-ductile, unidirectional (UD) interlayer carbon/glass hybrid composites. The concept was successfully demonstrated in different laminates with high strength and high modulus carbon and S-glass epoxy UD prepregs. The novel hybrid architecture provided pseudo-ductile tensile stress–strain responses with a linear initial part followed by a wide plateau and a second linear part, all connected by smooth transitions. The best hybrid configuration showed 60% improvement in modulus compared to pure glass, 860 MPa plateau stress and 2% pseudo-ductile strain. The initial modulus, the plateau stress and the overall tensile stress–strain response of each specimen configuration were predicted accurately.