The fibre pull-out energy of misaligned short fibre composites

A theoretical study on the fibre pull-out energy has been carried out for short fibre-reinforced composites. Two probability density functions were introduced for modelling the fibre-length distribution and the fibre-orientation distribution. By taking into account the effect of snubbing friction between fibres and matrix at the fibre exit point during fibre pull-out, and that of the fracture stress of fibres obliquely crossing the fracture plane (i.e. the inclined strength of fibres), the fibre pull-out energy of composites has been derived as a function of fibre-length distribution and fibre-orientation distribution, as well as interfacial properties. The previously existing fibre pull-out energy theories can be deduced from the present model. The effects of fibre-length distribution, fibre-orientation distribution, interfacial properties, snubbing-friction coefficient and parameter A for determining the inclined strength of fibres on the fibre pull-out energy, have been studied in detail. The present study provides the necessary information as to which fibre-length distribution, fibre-orientation distribution and interfacial property are required to achieve a desired fibre pull-out energy and hence a desired composite toughness. High-strength fibres, a large fibre-volume fraction and a large fibre diameter for a comparatively large mean fibre length, are shown to be favourable for achieving a high fibre pull-out energy. This revised version was published online in November 2006 with corrections to the Cover Date.     

电晕处理对高性能PBO纤维的表面性能及其界面粘接性能的影响

对高性能PBO纤维表面进行了电晕处理,优化了其处理工艺。用XPS,FT-IR和SEM研究了处理前后纤维表面化学结构及物理结构的变化,通过单丝拔出试验和短梁剪切试验评价了PBO纤维与树脂基体的微宏观界面粘接性能。结果表明:经电晕处理后,PBO纤维表面含氧量增多,表面浸润性得到改善,单丝拔出的PBO-环氧界面剪切强度(IFSS)提高了25.6 ％,但短梁剪切强度(ILSS)的提高不明显。     

Effect of surface treatment upon the pull-out behaviour of aramid fibres from epoxy resins

A detailed study has been undertaken of the pull-out behaviour of aramid fibres with different surface characteristics from blocks of an epoxy resin matrix. The fibres employed had either no surface treatment (HM), a standard surface finish (HMF) or had been treated with a special epoxy-based adhesion-activating finish (HMA). The point-to-point variation of axial fibre strain along the fibres both inside and outside of the resin matrix has been determined from stress-induced Raman band shifts. This has enabled the distribution of interfacial shear stress along the fibre/matrix interface to be determined and, in combination with scanning electron microscope analysis of the specimens following pull-out testing, the failure mechanisms to be elucidated. It is found that pull out of the HM fibre takes place by a debond propagating along the fibre/matrix interface at a low level of interfacial shear stress. The HMF fibre showed better adhesion to the epoxy matrix with pull out occurring in a complex manner through both separation of the fibre skin and failure at the fibre/finish interface. No evidence of debonding was found for the HMA fibre and failure occurred by fracture of the fibre at the point where it entered the resin block.     

Effect of heat treatment in air on the structure and properties of barium osumilite reinforced with Nicalon fibres

Microstructural studies have been carried out on glass-ceramic matrix composites, consisting of barium osumilite reinforced with Nicalon fibres, which have been subjected to heat treatment in air in the range 600–1100 °C. Parallel studies have involved the measurement of the friction stress between fibre and matrix and the flexural strength of the composite. The matrix was shown to consist of barium osumilite, hexacelsian, mullite and a silica-rich glass, the thermal mismatch of these different phases leading to the development of appreciable strains. Whilst high-temperature treatments caused the formation of voids due to flow of the glassy phase, the major factor controlling the mechanical properties of the composite was the fibre/matrix interface. A change in microstructure, from a weak carbon-rich interface to one where the fibre and matrix were strongly bonded together by a silica layer, was thus reflected in an increase in the interfacial friction stress and a change in fracture behaviour from one showing fibre pull-out and delamination to one with brittle characteristics.     

Fibre/matrix adhesion of cellulose fibres in PLA,PP and MAPP: A critical review of pull-out test,microbond test and single fibre fragmentation test results

The present work deals with the practical fibre/matrix adhesion of regenerated cellulose fibres (lyocell) and bast fibre bundles (flax, kenaf) in different matrices (polylactide-PLA, polypropylene-PP, maleic-anhydride-grafted polypropylene-MAPP). The influence of different testing procedures (pull-out test, microbond test, fragmentation test) on the fibre/matrix characteristics is discussed. The results of the different tests showed the same trends, but the absolute values differ. Clearly higher interfacial shear strength (IFSS) for cellulose fibres was found in PLA and MAPP in comparison to PP due to higher polarity. In addition, bast fibres displayed higher apparent IFSS values compared to lyocell because of their rougher surface and their chemical composition. The apparent IFSS of the pull-out test resulted in higher values compared to results obtained from the fragmentation test. This phenomenon is explained by different stress distributions due to variable specimen geometry, different behaviour of failure and the friction which occurs between fibre and matrix during fibre pull-out in the pull-out test.     

Analysis of the single-fibre pull-out test by means of Raman spectroscopy: Part II. Micromechanics of deformation for an aramid/epoxy system

The single-fibre pull-out test has been analysed for Kevlar-49 fibres in a cold-cured epoxy resin by using both a conventional pull-out experiment and Raman spectroscopy. The interfacial shear strength (ISS) has been estimated from the pull-out force for fibres with a range of embedded lengths. Raman spectroscopy has been used to analyse the distribution of fibre strain in the pull-out test by mapping the variation of strain along an aramid fibre undergoing pull-out from the epoxy resin matrix. At low strains the behaviour follows elastic shear-lag analysis but, as the fibre strain is increased, debonding takes place at the fibre/matrix interface. It is found that this debond propagates along the interface until the entire fibre is debonded. The fibre is then pulled out of the resin matrix by a frictional pull-out process. It is shown that the conventional pull-out experiment produces only an apparent value of ISS and that through a partial-debonding model it is possible to use the interfacial parameters obtained from the Raman analysis to predict the data from the conventional test.     

Microstructure of autoclaved refined wood-fibre cement mortars

Autoclaved cement mortars reinforced with refined wood-pulp fibres have been tested in flexure and the fracture surfaces examined by scanning electron microscopy. The observations indicate that failure occurs by the dual mechanism of fibre fracture and fibre pull-out, and that the interfacial bonds between the fibres and the cement matrix are stronger than had been previously considered.     

Effects of fibre volume fraction on the stress transfer in fibre pull-out tests

The elastic stress transfer taking place across the fibre/matrix interface is analysed for the fibre pull-out test by means of both micromechanics and finite element (FE) analyses. A special focus has been placed on how fibre volume fraction, V_f, affects the interface shear stress fields in the model composites containing both single and multiple fibres. In a so-called ‘three-cylinder model’, where a fibre, a matrix and a composite medium are coaxially located, the constraint imposed on the central fibre due to the surrounding fibres is properly evaluated. It is shown in the FE analysis that the differences in stress distributions between the composite models containing single and multiple fibres become increasingly prominent with increasing V_f. The principal effect of the presence of surrounding fibres in the multiple-fibre composite model is to suppress effectively the development of stress concentration near the embedded fibre end and thus eliminate the possibility of debond initiation from this region for all V_f considered. This is in sharp contrast to the single-fibre composite model, in which the interfacial debond can propagate from the embedded end if V_f is larger than a critical value. These findings are essentially consistent with the results from micromechanics analysis on the same specimen geometry. The implications of the results for the practical fibre pull-out test as a means of measuring the interface properties are discussed.     

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.     

RTM Hemp Fibre-Reinforced Polyester Composites

Hemp fibre-reinforced polyester composites were prepared using a Resin Transfer Moulding (RTM) technique and the flexural and impact behaviour investigated. Flexural stress at break and flexural modulus showed an increasing trend with fibre content. Impact strength was found to decrease at low fibre content, then gradually increase with further addition of fibres.A strong interfacial adhesion between hemp and polyester was obtained using chemically modified hemp. This modification consisted in introducing reactive vinylic groups at the surface of the fibres, via esterification of hemp hydroxyl groups, using methacrylic anhydride. Increased bonding between fibres and matrix did not affect the flexural stress at break of the composite but was detrimental to toughness. This behaviour was ascribed to a change in the mode of failure, from fibre pull-out to fibre fracture, resulting in a marked reduction in the energy involved in the failure of the composite, leading to a more brittle material.     

Matrix morphology and fibre pull-out strength of T700/PPS and T700/PET thermoplastic composites

The main objective of this fundamental study was to investigate effects of processing conditions and resulting matrix morphology on interfacial bond strength of fibre reinforced thermoplastic composites. Using a hot stage microscope, single fibre pull-out samples were produced with T700S high strength carbon fibre and two semicrystalline thermoplastic matrices, polyphenylene sulphide (PPS) and polyethylene terephthalate (PET), respectively. Processing temperatures and cooling histories were the major variables in sample preparation. The T700S fibre had no clear effect on the surrounding PPS and PET matrix morphology, as long as direct cooling at constant rates was selected. A transcrystalline phase around the fibres could be induced in the T700S/PPS system, if isothermal crystallization was carried out at 227°C. Fibre pull-out tests were conducted at room temperature and two basic failure paths were observed, i.e. debonding at the fibre-matrix interface and cohesive failure of the matrix close to the fibre surface. The results indicate that slow cooling rate and a resulting coarse spherulitic morphology around the fibres correlate with high interfacial shear strength. In fact somewhat higher strength values were obtained for samples with transcrystalline layers around the fibres.     

Ammonia plasma treatment of ultra-high strength polyethylene fibres for improved adhesion to epoxy resin

A study of the effect of plasma treatments on the mechanical properties and adhesion of ultra-high strength polyethylene fibres to epoxy resin is reported. Fibres were treated with ammonia plasma under various time and power conditions. The fibre/matrix interfacial shear strength was measured using load and fibre pull-out data obtained in a single-fibre pull-out test. The debonding was optically as well as acoustically monitored. Optical birefringence patterns were visible at the fibre debond region. Acoustic emission signals generated from debonding and stick-slip processes were also detected. A more than four-fold increase in the interfacial shear strength was achieved by plasma treating the fibres at the discharging condition of 30 W and 0.5 torr for 1 min. The birefringence patterns showed, qualitatively, that the shear in the matrix around the fibres increased for treated fibres and extended further into the matrix material. Surface topography of the pulled out fibres showed that the failure mode was unchanged by the treatment.     

Crack bridging and fibre pull-out in polyethylene fibre reinforced epoxy resins

An investigation has been undertaken of the stress distributions in high-performance polyethylene fibres bridging cracks in model epoxy composites. The axial fibre stress has been determined from stress-induced Raman band shifts and the effect of fibre surface treatment has been followed using untreated and plasma-treated polyethylene fibres. It is found that when the specimen is cracked, the fibres do not break and stress is transmitted from the matrix to the fibre across the fibre/matrix interface. A debond propagates along the fibre/matrix interface accompanied by friction along the debonded interface. The axial stress distributions in the fibres can be analysed using a partial-debonding model based upon shear-lag theory and it is found that the maximum interfacial shear stress at the bond/debond transition is a function of the debond length. The debonding process has been modelled successfully in terms of the interfacial fracture energy-based criterion developed by Hsueh for the propagation of a debond along a fibre/matrix interface accompanied by constant friction along the interface.     

Treated polyethylene fibres as reinforcement for epoxy resins

Ultra-high-modulus polyethylene (UHMPE) fibres were treated in order to develop favourable surface and, possibly, microstructure characteristics. The main aim was to eliminate the microfibrillar morphology of the fibre and improve interfacial bonding between fibre/matrix so that better compressive properties can be achieved in reinforced resins. Calendering at 130 °C was performed, and the surface treatment used oxidative solutions. Adhesive bonding to epoxy matrices was highly improved in chromosulphate-treated material exceeding that of a commercial, corona-treated product, but the mechanical properties of these fibres deteriorated. Calendering did not significantly affect fibre strength and only improved adhesive bonding slightly. The use of these treated reinforcements is expected to improve the performance of composite materials, especially at low fibre volume fractions, because of their improved interfacial characteristics.     

Interfacial shear stress in SiC fibre-reinforced cordierite

An analytical model based on a consistent shear-lag theory was developed to predict the interfacial shear stress in single fibre pull-out tests. The calculations show that the stress is highly dependent on the specimen thickness and the method of testing. Data for the debond stress and the interfacial shear stress were measured for single SiC fibres embedded in a magnesium aluminium silicate (cordierite) matrix. The effect of fibre embedded length, processing schedule, and matrix toughening were investigated. For a fixed sample support configuration during testing, good agreement was obtained between the model predictions and experimental data.     

Fracture mechanics of single-fibre pull-out test

Pull-out of an elastic fibre from an elastic matrix was investigated. A simple pull-out mechanics has been developed, based on the fracture energy criterion, to describe the debonding process, including friction. Experiments were carried out using polytetrafluoroethylene (PTFE) fibres embedded in a polypropylene (PP) matrix. It was found that growth of an interfacial crack was stable after the initiation of a debond at the loaded fibre end. At first, the debonding force increased linearly with the crack length due to friction in the debonded region. However, the crack accelerated after reaching a critical length, dependent on the embedded fibre length. At this point, the force required to propagate the debond levelled off. Thus, further increase in the debonding force was not necessary to further complete the debonding process. The debonding force was found to be in good agreement with that predicted by the present theory. Techniques for determining the interfacial properties, including adhesive fracture energy, compressive residual stress and coefficient of friction, were considered. In addition, a simple criterion has been derived to predict which fibre end, either embedded end or loaded end, will debond first when the specimen is subjected to an axial load.     

Visualization of debonding of fully and partially embedded glass fibres in epoxy resins

Very short glass fibres have been embedded in bars of epoxy resin and the debonding process was observed under the microscope as the polymer was stressed. In addition, fibre pull-out specimens have been similarly watched while the fibre was pulled. The interfaces of the fully embedded fibres failed across the fibre ends at strains of 0·2–0·3%, and circumferential failure started at about 0·6% strain. The low values for the end failures are compatible with models involving stress concentrations at the fibre ends. The circumferential failure value is in agreement with the results of earlier pull-out studies. In the case of the pull-out specimen, the behaviour was complex. Fibres with short embedded lengths debonded first across the embedded end. Failure of the cylindrical surface was too fast for the direction of crack propagation to be determined. Longer fibres first debonded at the fibre entry point and then arrested while debonding occurred across the embedded end. Final failure was again very fast. Long fibres debonded continuously, starting at the entry point. A slow fracture process appeared to be involved at least initially, so that the average shear stress on the region still bonded increased continuously throughout the process. Fast fracture occurred only in the very last stages of the process. These observations are compatible with the traditional theory, but reverse bonding is not ruled out for the shortest embedded lengths.     

Micromechanical characterisation of fibre/matrix interfaces

Theoretical analyses for the single fibre pull-out and push-out models under monotonic loading are given which are based on a shear-lag analysis in a fracture mechanics approach considering non-constant friction at the debonded interface as a result of fibre Poisson contraction (or expansion). The solutions allow the determination of typical fibre/matrix interfacial properties such as the interfacial fracture toughness, G_ic, the coefficient of friction, μ, and the residual clamping stress, q₀. Under cyclic loading the interfacial properties are expected to degrade as a result of repetitive abrasion, and a power law function is assumed between μ and the number of elapsed cycles, N. However, G_ic is assumed to be unaffected and a fracture mechanics based debond criterion is derived for the relationship between the external applied stress, the debond length and the reduced friction coefficient for both fibre pull-out and fibre push-out. In addition, the relative displacements between the free fibre end and the matrix top are obtained for cyclic fatigue when the fibre is loaded and unloaded. A relationship obtained for the protrusion (or intrusion) length in fibre pull-out (or push-out) experiments allows the severity of the interface frictional degradation to be evaluated and characterised. Similarities and differences in the frictional degradation behaviour between fibre pull-out and push-out are also identified.     

Deformation micromechanics in model carbon fibre-reinforced composites

Raman spectroscopy has been used to study the deformation micromechanics of the single-fibre pull-out test for a carbon fibre/epoxy resin system using surface-treated and untreated versions of the same type of PAN-based fibre. It has been possible to determine the detailed strain distribution along embedded fibres and it has been found that it varies with the level of strain in the fibre outside the resin block. The variation of interfacial shear stress along the fibre/matrix interface has been determined using the balance of forces equilibrium and this has been compared with the single values of interfacial shear strength determined from conventional pull-out analyses. It has been demonstrated that it is possible to identify situations where the interface is well-bonded, partially debonded or fully debonded and also to follow the failure mechanisms in detail. It has been found that the level of interfacial adhesion is better for the surface-treated fibre and that, for the untreated fibre, interfacial failure takes place by the cohesive failure of a weakly-bonded surface skin that appears to be removed by the surface pretreatment process.     

A MODEL TO PREDICT THE FATIGUE LIFE OF FIBRE-REINFORCED TITANIUM MATRIX COMPOSITES UNDER CONSTANT AMPLITUDE LOADING

A model based on micro-mechanical concepts has been developed for predicting fatigue crack growth in titanium alloy matrix composites. In terms of the model, the crack system is composed of three zones: the crack, the plastic zone and the fibre. Crack tip plasticity is constrained by the fibres and remains so until certain conditions are met. The condition for crack propagation is that fibre constraint is overcome when the stress at the location of the fibre ahead of the crack tip attains a critical level required for debonding. Crack tip plasticity then increases and the crack is able to propagate round the fibre. The debonding stress is calculated using the shear lag model from values of interfacial shear strength and embedded fibre length published in the literature. If the fibres in the crack wake remain unbroken, friction stresses on the crack flanks are generated, as a result of the matrix sliding along the fibres. The friction stresses (known as the bridging effect) shield the crack tip from the remote stress, reducing the crack growth relative to that of the matrix alone. The bridging stress is calculated by adding together the friction stresses, at each fibre row bridging the crack, which are assumed to be a function of crack opening displacement and sliding distance at each row. The friction stresses at each fibre row will increase as the crack propagates further until a critical level for fibre failure is reached. Fibre failure is modelled through Weibull statistics and published experimental results. Fibre failure will reduce the bridging effect and increase the crack propagation rate. Calculated fatigue lives and crack propagation rates are compared with experimental results for three different materials (32% SCS6/Ti-15-3, 32% and 38% SCS6/Ti-6-4) subjected to mode I fatigue loading. The good agreement shown by these comparisons demonstrates the applicability of the model to predict the fatigue damage in Ti-based MMCs.