Plasma surface modification of advanced organic fibres

The interlaminar shear strength, interlaminar fracture energy, flexural strength and modulus of extended-chain polyethylene/epoxy composites are improved substantially when the fibres are pretreated in an ammonia plasma to introduce amine groups on to the fibre surface. These property changes are examined in terms of the microscopic properties of the fibre/matrix interface. Fracture surface micrographs show clean interfacial tensile and shear fracture in composites made from untreated fibres, indicative of a weak interfacial bond. In contrast, fracture surfaces of composites made from ammonia plasma-treated fibres exhibit fibre fibrillation and internal shear failure as well as matrix cracking, suggesting stronger fibre/matrix bonding, in accord with the observed increase in interlaminar fracture energy and shear strength. Failure of flexural test specimens occurs exclusively in compression, and the enhanced flexural strength and modulus of composites containing plasma-treated fibres result mainly from reduced compressive fibre buckling and debonding due to stronger interfacial bonding. Fibre treatment by ammonia plasma also causes an appreciable loss in the transverse ballistic impact properties of the composite, in accord with a higher fibre/matrix interfacial bond strength.     

Plasma surface modification of advanced organic fibres

A marked improvement in the interlaminar shear strength and flexural strength of aramid/ epoxy composites is observed when the fibres are pretreated in an ammonia or ammonia/ nitrogen gaseous discharge (plasma) to introduce amine groups on to the fibre surface. Scanning electron and optical microscopic observations are used to examine the microscopic basis for these results. Scanning electron micrographs of shear fracture surfaces show clean fibre/matrix separation in composites made from untreated fibres, indicative of weak interfacial bonding. In contrast, shear fracture surfaces of composites containing plasma-treated fibres exhibit clear evidence of fibre fibrillation and matrix cracking, suggesting stronger interfacial bonding. Optical microscopic examination of flexure specimens shows that enhanced strength results mainly from reduced compressive fibre buckling and debonding, due to an increase in fibre/matrix interfacial bond strength. This increase is not accompanied by any significant change in the interlaminar fracture energy or flexural modulus of the composites, but there is an appreciable loss in transverse ballistic impact properties. These results are also examined in terms of the observed increase in fibre/matrix interfacial strength.     

Determination of fibre/matrix interfacial shear strength by an acoustic emission technique

The application of acoustic emission (AE) measurements to locate the sources of fracture of a single high-strength fibre embedded in an epoxy matrix which is loaded in tension is described. From the micromechanical model and the fragment length distribution, interfacial shear strength values were calculated. The technique is demonstrated for small-diameter glass and graphite fibres as well as for fibres which exhibit fibrillar fracture, such as Kevlar and PBZT. Good agreement is found between the mean fragment length values obtained by optical and AE measurements for glass and graphite fibres. Values obtained for interfacial shear strength by the AE technique are comparable with those obtained using other techniques.     

Shear strength of polymers and fibre composites: 2. carbon/epoxy pultrusions

Pultrusions were made with carbon fibres and an epoxy resin. Three different curing agents were used, so that the matrices were resins with different glass transition temperatures. The composites were tested for shear strength at different temperatures, so that the effect of the resin shear strength on composite shear strength could be observed, with a fixed fibre architecture. It was found that the composite was always much stronger than the resin both for the 0 and 90° fracture modes. The 90° fracture surfaces contained many broken fibres, and shear hackles were observed in the resin-rich regions. These suggested that shear failure (rather than tensile failure) took place in the Iosipescu test for the 90° specimens. It was concluded that the fibre architecture played a dominant role in the composite shear strength, with interphase effects being involved also.     

Silicon carbide (NicalonTM) fibre-reinforced borosilicate glass composites: mechanical properties

The objective of this study was to assess the applicability of an extrinsic carbon coating to tailor the interface in a unidirectional NicalonTM–borosilicate glass composite for maximum strength. Three unidirectional NicalonTM fibre-reinforced borosilicate glass composites were fabricated with different interfaces by using (1) uncoated (2) 25 nm thick carbon-coated and (3) 140 nm thick carbon coated Nicalon fibres. The tensile behaviours of the three systems differed significantly. Damage developments during tensile loading were recorded by a replica technique. Fibre–matrix interfacial frictional stresses were measured. A shear lag model was used to quantitatively relate the interfacial properties, damage and elastic modulus. Tensile specimen design was varied to obtain desirable failure mode. Tensile strengths of NicalonTM fibres in all three types of composites were measured by the fracture mirror method. Weibull analysis of the fibre strength data was performed. Fibre strength data obtained from the fracture mirror method were compared with strength data obtained by single fibre tensile testing of as-received fibres and fibres extracted from the composites. The fibre strength data were used in various composite strength models to predict strengths. Nicalon–borosilicate glass composites with ultimate tensile strength values as high as 585 MPa were produced using extrinsic carbon coatings on the fibres. Fibre strength measurements indicated fibre strength degradation during processing. Fracture mirror analysis gave higher fibre strengths than extracted single fibre tensile testing for all three types of composites. The fibre bundle model gave reasonable composite ultimate tensile strength predictions using fracture mirror based fibre strength data. Characterization and analysis suggest that the full reinforcing potential of the fibres was not realized and the composite strength can be further increased by optimizing the fibre coating thickness and processing parameters. The use of microcrack density measurements, indentation–frictional stress measurements and shear lag modelling have been demonstrated for assessing whether the full reinforcing and toughening potential of the fibres has been realized. This revised version was published online in November 2006 with corrections to the Cover Date.     

Axial compression fracture in carbon fibres

Axial compression fracture of carbon fibres was studied by embedding single fibres in epoxy resin and compressing the specimens parallel to the fibre axis. By careful optical monitoring of the fibre surface the earliest stages of fracture were identified leading to estimates of the fibre axial compression failure strengths. Compression strength decreases markedly from about 2.2 GN m^?2 for moderately oriented fibres to <1 GN m^?2 for highest modulus filaments. The trend towards decreasing compression strength with increasing anisotropy is explained on the basis of an increasing fibre microfibrillar nature. However fracture morphology studies show that the unduly rapid strength decrease results from an increasing degree of fibre outer layer ordering which accompanies increasing axial anisotropy in carbon fibres since cracking occurs first on the more highly aligned filament surfaces. It is suggested that fibre compression fracture changes from a shear to a microbuckling or kinking mode with increasing fibre anisotropy, where the latter initiates in individual, well-aligned but uncoupled microfibrils. The similarity of fine axial compression fractures in oriented carbon fibres to those found in elastica loop experiments is noted as are the possible implications which the low strain-to-failure in compression of very high modulus fibres might have for practical composites.     

An evaluation of acoustic emission from fibre-reinforced composites

An analysis of acoustic emission(AE) from epoxy matrices of different amounts of hardener and model composites containing a glass bead, carbon and glass fibres has been carried out to identify the sources of emission. A few AE events generated by microcracking were observed for epoxy matrix near the final fracture strain. From microscopic and emission observations it was found that the emission was generated by interfacial debonding at the pole for the model composite containing a single particle of the glass bead, and that the source of AE bursts for a continuous single carbon fibre/epoxy composite was succeeding fibre fractures along fibre length. The high AE activity due to fibre fracture was observed for a composite consisting of a bundle of glass fibres. The total of AE events was in agreement with the number of fibre fracture counted with the aid of a microscope in a carbon/epoxy composite. The shear strength at the carbon/epoxy interface was evaluated by a critical length of the fractured fibres using the AE results.     

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.     

HIGH TEMPERATURE HIGH PERFORMANCE COMPOSITES

A high performance composite material consists essentially of aligned, continuous carbon, glass, aramid or ceramic fibres in a thermoset or thermoplastic matrix. The types, advantages and disadvantages of the various matrices available which can operate at elevated temperatues are described, particular attention being paid to the work of fracture. The main fabrication methods for both types of material are outlined and Some fracture properties for composites together with illustrations of the effects of short term temperature exposure on the modulus, strength and interlaminar shear strength, are provided.     

Measurement of the anisotropic strength and toughness of Pitch-55 carbon ribbons

Composites with long reinforcing fibres can be effectively toughened by controlled crack deflection at interfaces of such reinforcements. An important element in achieving this end is the measurement of the strength and toughness of the reinforcing fibres. Here we present results of such experiments. Since measurements of these properties are difficult in fibres of micron dimensions, specially prepared Pitch-55 ribbons of 600 μm width and 35 μm thickness were obtained for doing the tests. As with carbon fibres, these ribbons show strong anisotropy in their elastic and inelastic properties. Hence, the work of fracture along and across the graphitic planes were determined. The average longitudinal tensile strength of these ribbons was found to be 0.6 GPa. Due to inhomogeneities and a possible associated size effect this value is considerably lower for ribbons than that for fibres. The average specific work of fracture of the ribbons across the graphitic planes was determined to be 3.5 J m⁻². Unusually high values of works of fracture of 166 J m⁻² were obtained for cracks propagatin along the longitudinal graphitic lamellae. These high values were attributed to a profuse kinking type of plasticity on the graphitic lamellae lying parallel to the crack plane. An asymptotic elastic-plastic analysis for single crystals due to Rice [12], was used to estimate the level of inelastic energy dissipation. These estimates were close to the experimentally observed values. The calculation of the dissipated inelastic energy requires knowledge of the interlaminar shear strength across the graphitic lamellae. A torsion experiment was designed to measure the interlaminar shear strength across the weak graphitic planes, giving a value of 83 MPa. An in-plane longitudinal shear modulus of 65 GPa was obtained from the linear portion of the quasi-static torque twist curve of the ribbon. Another measure of this modulus was determined independently from torsional vibration tests, giving an average value of 51 GPa.     

Electron irradiation effects on interlaminar shear strength of glass or carbon cloth reinforced epoxy composites

Interlaminar tensile shear tests are conducted to study the degradation mechanisms of electron irradiated glass or carbon cloth reinforced epoxy laminates. Interlaminar shear strength decreases significantly after the dose exceeds 3000 Mrad for glass/epoxy, but remains constant up to 12 000 Mrad for carbon/epoxy. SEM photos reveal that debonding of glass fibres and epoxy matrix (or degradation of silane coupling agents) plays an important role in the dose-dependent strength reduction of glass/epoxy laminates. The decrease in the interlaminar shear strength corresponds to that in the three-point bending strength. On the other hand, the SEM fracture appearance is almost dose-independent for carbon/epoxy laminates. In addition, some preliminary irradiation tests are conducted at –120° C to observe the effects of irradiation temperatures.     

Jute-reinforced polyester composites

Raw jute fibre has been incorporated in a polyester resin matrix to form uniaxially reinforced composites containing up to 60 vol% fibre. The tensile strength and Young's modulus, work of fracture determined by Charpy impact and inter-laminar shear strength have been measured as a function of fibre volume fraction. These properties all follow a Rule of Mixtures relationship with the volume fraction of jute. Derived fibre strength and Young's modulus were calculated as 442 MN m^–2 and 55.5 GN m^–2 respectively. Polyester resin forms an intimate bond with jute fibres up to a volume fraction of 0.6, above which the quantity of resin is insufficient to wet fibres completely. At this volume fraction the Young's modulus of the composite is approximately 35 GN m^–2, the tensile strength is 250 MN m^–2, the work of fracture is 22 kJ m^–2 and the inter-laminar shear strength is 24 MN m^–2. The properties of jute and glass fibres are compared, and on a weight and cost basis jute fibres are seen in many respects to be superior to glass fibres as a composite reinforcement.     

Shear strength and fracture toughness of carbon fibre/epoxy interface: effect of surface treatment

Textile-reinforced composites have become increasingly attractive as protection materials for various applications, including sports. In such applications it is crucial to maintain both strong adhesion at fibre–matrix interface and high interfacial fracture toughness, which influence mechanical performance of composites as well as their energy-absorption capacity. Surface treatment of reinforcing fibres has been widely used to achieve satisfactory fibre–matrix adhesion. However, most studies till date focused on the overall composite performance rather than on the interface properties of a single fibre/epoxy system. In this study, carbon fibres were treated by mixed acids for different durations, and resulting adhesion strength at the interface between them and epoxy resin as well as their tensile strength were measured in a microbond and microtensile tests, respectively. The interfacial fracture toughness was also analysed. The results show that after an optimum 15–30 min surface treatment, both interfacial shear strength and fracture toughness of the interface were improved alongside with an increased tensile strength of single fibre. However, a prolonged surface treatment resulted in a reduction of both fibre tensile strength and fracture toughness of the interface due to induced surface damage.     

Evaluation of interface fracture energy for single-fibre composites

Raman and luminescence spectroscopy have been used for the first time to determine the interface fracture energy for single-fibre composites. By using the measured fibre stress distributions in single-fibre fragmentation composite specimens and a simple energy-balance scheme, the energy for the initiation of interfacial debonding has been estimated for carbon (T50) and α-alumina (PRD-166 and Nextel 610) fibres embedded in epoxy resins. It has been found that the interface fracture energy shows good sensitivity to changes in the level of fibre/matrix adhesion due to surface treatment and sizing of the fibres. It is also found that the values of interface fracture energy correlate well with measured values of interfacial shear strength determined for the same fibre/matrix systems.     

Surface modification of superdrawn polyoxymethylene fibres

The pull-out fracture of surface-modified superdrawn polyoxymethylene fibres embedded in rubber is discussed from a fractographical viewpoint. The morphologies of the pull-out fracture plane were very similar to those of the fracture surface in single lap-joint tests and the true pull-out stress coincided with the shear strength of a single lap-joint, indicating that the pull-out failure is strongly related to single lap-joint shear fracture.     

Strengthening in and fracture behaviour of CNT and carbon-fibre-reinforced epoxy–matrix hybrid composite

Advanced materials such as continuous fibre-reinforced polymer matrix composites offer significant enhancements in strength and fracture resistance properties as compared with their bulk, monolithic counterparts. In the present work, mode-I (tensile) fracture behaviour of the neat epoxy (without nano- or hybrid reinforcements), nanocomposite (with amino-functionalized multi-walled carbon nanotube (MWCNT) reinforcement to neat epoxy) and hybrid composite (with amino MWCNT and carbon fibre reinforcements to neat epoxy) along with their flexural strength and interlaminar shear strength has been reported and discussed. Limited topological studies have also been conducted to understand the nature of material fracture and its dependence on the notch orientation. The results thus obtained are analysed and discussed in detail to elucidate: (i) alignment of fibre and its influence on the anisotropy in strength and fracture resistance, (ii) dependence of notch root radii on the apparent fracture toughness and concurrence to strain-controlled fracture and (iii) finally, the nature of J–R curves. The results thus obtained have revealed that the resistance to fracture is significantly increased with the addition of amino-functionalized MWCNTs and carbon fibres. In the hybrid composite, fracture resistance is greater in the longitudinal orientation of fibres than in the transverse orientation and it exhibits a significantly higher strength–fracture toughness combination.     

INTERLAMINAR FRACTURE ENERGY OF UHMPE/EPOXY COMPOSITES BY DOUBLE CANTILEVER BEAM AND PEEL TESTS

Abstract— Interlaminar mechanical properties of composite materials such as shear strength and fracture toughness depend on the level of fibre-matrix adhesion which has to be optimised according to the end-use of the composite. Various surface treatments of fibres are used for this purpose. Double cantilever beam (DCB) tests are commonly used for estimating the interlaminar fracture toughness in Mode I. It is shown in this work that this parameter can be conveniently determined using a simpler technique involving a 90° flexible-to-rigid substrate peel test. The values of G_Ic determined by DCB and 90° peel tests are comparable within acceptable experimental error margins. These two alternative techniques are used for assessing the effectiveness of a novel surface engineering process for enhanced adhesion of ultra-high modulus polyethylene (UHMPE) fibres to an epoxy matrix.     

Mechanical properties of 3D KD-I SiCf/SiC composites with engineered fibre-matrix interfaces

Three-dimensional (3D) silicon carbide (SiC) matrix composites reinforced with KD-I SiC fibres were fabricated by precursor impregnation and pyrolysis (PIP) process. The fibre-matrix interfaces were tailored by pre-coating the as-received KD-I SiC fibres with PyC layers of different thicknesses or a layer of SiC. Interfacial characteristics and their effects on the composite mechanical properties were evaluated. The results indicate that the composite reinforced with as-received fibre possessed an interfacial shear strength of 72.1 MPa while the composite reinforced with SiC layer coated fibres had a much higher interfacial shear strength of 135.2 MPa. However, both composites showed inferior flexural strength and fracture toughness. With optimised PyC coating thickness, the interface coating led to much improved mechanical properties, i.e. a flexural strength of 420.6 MPa was achieved when the interlayer thickness is 0.1 μm, and a fracture toughness of 23.1 MPa m^1/2 was obtained for the interlayer thickness of 0.53 μm. In addition, the composites prepared by the PIP process exhibited superior mechanical properties over the composites prepared by the chemical vapour infiltration and vapour silicon infiltration (CVI-VSI) process.     

Studies of the flexural properties of asbestos-reinforced phenolic composites

Factors which may influence the flexural strength of asbestos-reinforced phenolic composites were investigated. These are: fibre-to-resin adhesion, the degree of fibre dispersion, the relative fracture strains of the fibres and matrices, and voids. Fibre-to-resin adhesion was promoted by pre-coating asbestos fibres with phenol-formaldehyde in solution state throughin situ polymerization. Fibre dispersion was controlled by applying shear agitation to the mixture of fibres in solution duringin situ polymerization. The ratio of the fracture strain of the fibres to that of the matrix was varied by using resins having different fracture strains. Voids were found to be present in all cases. The size of voids was not significantly affected by different processing conditions. It is concluded that the flexural strength of the composites is largely controlled by voids in the moulded parts. Other factors have little effect on the flexural strength when they are varied over a range of practical importance.     

Sizing resin structure and interphase formation in carbon fibre composites

The single-embedded filament fragmentation test has been used to study the effect of fibre coatings on the adhesion of surface treated (oxidized) Type A and HS carbon fibres to an epoxy matrix. The presence of a sizing resin on the as-received fibres reduced the interfacial shear strength of the composite. For the unsized fibres, which were coated in the laboratory from commercial aqueous based sizing emulsions, a molecular weight dependence was observed. This suggests that compatibility of the deposited size with the matrix determines the adhesive bond between fibre and matrix and the formation of an interphasal region. On the other hand, deposition of a sizing resin from solution led to the differing conclusion that chemical interaction with the fibre surface had occurred. During composite fabrication these sizing resins will therefore have to act as ‘coupling agents’ to the matrix. Solvent extraction of emulsion-deposited sizing resins, particularly at elevated temperatures, appeared to promote their interaction with the fibre surface. The same trends in interfacial shear strength were observed in a second epoxy resin matrix of higher modulus, albeit at an increased magnitude. In this way, the plasticizing role of the ‘low’ molecular weight emulsion based size could be identified. Maximum likelihood statistics have been used to estimate the standard deviation on the value of interfacial shear strength.