Surface characterization of poly(acrylonitrile) based intermediate modulus carbon fibres

The interface between the fibre and the matrix is a very important factor influencing the mechanical behaviour of composite materials. For superior composite performance, one must not only select optimal fibres and matrices, but also optimize the interface between them. However, the control of the interface properties is not an easy task. This work is an interdisciplinary and integrated approach to the problem. The effect of different degrees of a wet oxidative surface treatment on the surface of poly(acrylonitrile) based intermediate modulus carbon fibres (Courtaulds IM CG43–750) has been studied using classical thermodynamic as well as spectroscopic techniques, aimed at obtaining a complete physical and chemical characterization of the fibre surface. The results show that all aspects of the fibre surface are influenced by the surface treatments, which are specially designed to improve the adhesion between fibre and matrix. The study outlines the most important surface features improving this adhesion. The results concerning the characterization of the fibre surface contribute, when combined with micromechanical tests, to clarifying the adhesion mechanisms, revealing, at the same time, a mechanical interlocking and a chemical interaction.     

Development of durable cementitious composites using sisal and flax fabrics for reinforcement of masonry structures

Natural fibres are one of the most studied materials. However, the use of these fibres as reinforcements in composite materials for structural applications, especially for existing or historical masonry structures, remains a challenge. In this study, efforts were made to develop sustainable composites using cementitious matrices reinforced with untreated bi-directional fabrics of natural fibres, namely, flax and sisal fibres. The fibres were mechanically characterised by tensile tests performed on both single yarns and fabric strips. Ageing effects due to fibre mineralisation in alkaline cement paste environments may cause a reduction in the tensile strength of natural fibres. The matrices used to study fibre durability were a natural hydraulic lime-based mortar (NLM) mix with a low content of water-soluble salts and a lime-based grouting (NLG) mix containing natural pozzolans and carbonated filler. Tensile tests on impregnated single yarns subjected to wetting and drying cycles by exposure to external weathering were conducted at different ages to quantify these problems. Composite specimens were manufactured by the hand lay-up moulding technique using untreated fibre strips and an NLG matrix. The mechanical response of natural fibre reinforced cementitious (NFRC) composites was measured under tension, and the effect of the matrix thickness was also addressed. Both sisal and flax fibres showed good adhesion with the NLG matrix, making them capable of producing composites with ductile behaviour and suitable mechanical performance for strengthening applications in masonry structures.     

Single fibre pullout tests on auxetic polymeric fibres

A novel route for production of auxetic fibres has been adapted from conventional melt extrusion techniques. These fibres were reproduced, characterised and tested, for the first time, to assess the potential of auxetic fibres as reinforcements in composite materials. Initial experimental work has included the embedding of single fibres in modified epoxy resin. Auxetic fibre specimens were then compared with conventional fibre specimens through a specially designed fibre pullout testing procedure. The auxetic specimens displayed superior anchoring properties. The average maximum force at de-bonding of the auxetic fibres (0.95 N) was observed to be over 100% higher than that for conventional ones (0.44 N) and the average energy required to fully extract the auxetic fibre from the modified resin was 8.3 mJ while the conventional fibre required only 2.5 mJ on average. The results indicate that composites employing auxetic fibres as the reinforcement phase will exhibit enhanced resistance to failure due to fibre pullout.     

An experimental investigation of modified and unmodified flax fibres with XPS, ToF-SIMS and ATR-FTIR

Natural fibres are envisaged today as potential candidates for replacing glass fibres in composite materials. Although natural fibres have a number of advantages over glass fibres, the strong polar character of their surface is a limiting factor as, compatibility with strongly apolar thermoplastic matrices is very low. Such problems of incompatibility may be overcome with fibre pre-treatments, which can enhance compatibility although having a negative impact on the economics of using such materials. In this study two fibre pre-treatment methods, acetylation and stearic acid treatments, have been applied on flax fibres. The effect of these two pre-treatments has been examined by use of XPS, ToF-SIMS and FTIR spectroscopic methods. It was found that the fibre surface before treatment is very different to what may have been expected for cellulose materials. There is an appreciable coverage of the flax fibre surface with hydrocarbon compounds, possibly waxy substances, but no aromatic compounds were detected. All three spectroscopic methods revealed that the fibre surface chemistry has been altered after the treatments, and especially for acetylation it was found that ester bonds are present on the fibre surface after treatment. For the stearic acid treatment the situation still remains less conclusive. Finally, ToF-SIMS experiments revealed that the coverage of the fibre surface with acetyl groups and stearic acid is highly heterogeneous.     

Shear capacity of steel and polymer fibre reinforced concrete beams

This paper deals with the application of a plasticity model for shear strength estimation of fibre reinforced concrete beams without stirrups. When using plastic theory to shear problems in structural concrete, the so-called effective strengths are introduced, usually determined by calibrating the plastic solutions with tests. This approach is, however, problematic when dealing with fibre reinforced concrete (FRC), as the effective strengths depend also on the type and the amount of fibres. In this paper, it is suggested that the effective tensile strength of FRC can be determined on the basis of the tensile stress-crack opening relationship found from wedge splitting tests. To determine the effective compressive strength of FRC, it is proposed to adopt the formula used for conventional concrete and modify it by introducing a fibre enhancement factor to describe the effect of fibres on the compressive softening behaviour of FRC. The enhancement factor is determined as the ratio of the areas below the stress–strain curves for FRC and for conventional concrete. The outlined approach has been verified by shear testing of beams containing no fibres, 0.5% steel fibre volume and 0.5% polymer fibre volume. The tests results are compared with estimations and show satisfactory agreements, indicating that the proposed approach can be used.     

Fabrication and characterization of <Emphasis Type="Italic">S. cilliare</Emphasis> fibre reinforced polymer composites

In the recent times, there has been an ever-increasing interest in green composite materials for its applications in the field of industries, aerospace, sports, household etc and in many other fields. In this paper, fabrication of Saccharum cilliare fibre reinforced green polymer composites using resorcinol formaldehyde (RF) as a novel matrix has been reported. A systematic approach for processing of polymer is presented. Effect of fibre loading on mechanical properties like flexural, tensile, compressive and wear resistances has also been determined. Reinforcing of the RF resin with Saccharum cilliare (SC) fibre was done in the form of particle size (200 micron). Present work reveals that mechanical properties of the RF resin have been found to increase up to 30% fibre loading and then decreases. Morphological and thermal studies of the resin, fibre and particle reinforced (P-Rnf) green composites have also been studied.     

Optimization of CFRP laminate dynamic properties using combinations of short/continuous fibres and stiff/flexible resin matrices

High volume fraction carbon fibre-reinforced plastics (CFRPs) have a stiffness-to-weight ratio much greater than some conventional metallic materials but the material damping is low. Damping may be increased by the use of short fibres or a matrix material with high dissipation. Experimental and theoretical studies are described which show that a lamina can be made which has high damping; the effects of lamina properties on those of the laminate have been investigated. The effects of fibre length, matrix type and fibre orientation have been assessed with the objective of optimizing the dynamic properties of laminated CFRP plates using combinations of short/continuous fibres and conventional/highly dissipative resin matrices.     

Statistical fracture of E-glass fibres using a bundle tensile test and acoustic emission monitoring

Mechanical strength studies have been carried out on fibre bundles used in composite manufacturing. The variability in mechanical properties of glass fibres has been studied using bundles of about 2000 filaments. The fibre strength distributions were analysed using the survival probability-applied strain (S–ε) curve, in relation with various experimental conditions. We also examine the effect of lubricant’s viscosity on the fracture behaviour of E-glass fibre bundles. Acoustic emission (AE) was monitored during the bundle tensile tests in order to verify that individual filament failures are statistically independent. On tensile tests with lubricated bundles of E-glass fibres, it is shown that each individual fibre break can be detected using AE. Hence, AE monitoring of a lubricated bundle of E-glass fibres provides a convenient and relatively quick method to obtain the Weibull parameters of strength distribution.     

Relationship between structure and mechanical properties for aramid fibres

The relationship between structure and mechanical properties for a series of twelve wellcharacterized aramid fibres has been determined. The fibres were produced under a variety of processing conditions and the fibre structure has been characterized using transmission electron microscopy. In particular, both the overall degree of molecular orientation in the fibres and the difference in structure between the fibre skin and core regions have been investigated in detail. The mechanical properties of the fibres have been evaluated using conventional mechanical testing and molecular deformation followed using Raman microscopy to monitor strain-induced band shifts. It has been shown that the mechanical properties of the fibres are controlled by the fibre structure. In particular, it is shown that the fibre modulus is governed by the overall degree of molecular orientation. It is also demonstrated that the fibre strength is controlled principally by the overall molecular orientation but may also be reduced by the presence of a highly-oriented skin region. It has been found that the rate of shift of the Raman bands per unit strain is proportional to the fibre modulus except for fibres with large differences in molecular orientation between fibre skin and core regions. For these fibres the rate of shift reflects the higher orientation of the skin.     

The application of optical fibres as witness devices for the detection of plastic strain and cracking

Low cost optical fibres have recently become readily available for telecommunications purposes. Silica fibres are characterised by high elastic strains to failure. The feasibility of using these fibres for structural integrity monitoring particularly for offshore structures is investigated. The basis of the technique is that a fibre may be bonded to a critical part of a structure and provides an optical path which will be broken if the fibre fails due to plastic strain or crack opening in the critical area.
Groups of fibres which have been given predetermined fracture strains by surface etching were encapsulated in special packs. These packs were bonded to steel and concrete tensile specimens. Strain transfer occurred successfully between the specimens and individual fibres. The distribution of strain to fibre fracture appeared to be uniform along the fibre. The use of several fibres with a range of fracture strains caused fibres to break progressively with increasing strain. For applications to offshore structures it has been found possible to use water-repellent adhesives which can be applied and cured in sea water and suffer no deterioration.
The advantages of this system include versatility, relatively low cost, adaptability to continuous monitoring and the possibility of being fitted retrospectively and refitted after repair operations.     

Microstructure and mechanical characteristics of alpha-alumina-based fibres

The high-temperature mechanical behaviour of alumina-based ceramic fibres has been investigated by the comparison of a dense pure alumina fibre, a porous pure alumina fibre and a zirconia-reinforced dense fibre. Tensile and creep tests have been conducted up to 1300°C in air in parallel with microstructural investigations on the as-received and tested fibres. Room-temperature behaviour of the fibres is close to that of bulk materials having the same microstructure, but the fibre form allows higher failure stresses to be attained. High-temperature deformation of the three fibres is achieved by grain-boundary sliding ( ), and is accompanied by isotropic grain growth. The specific microstructures of each fibre induce differences in the creep threshold levels as a function of temperature and stress and also in creep rates and resistance to damage. Despite better resistance to creep and damage of the zirconia-reinforced fibre, alumina-based fibres are limited to applications below 1100°C. Grain boundaries are the principal cause of mechanical degradation at high temperature with these fibres.     

A novel method for the measurement of elastic moduli of fibres

The elastic modulus of fibres used in composite materials is a parameter of prime importance in the determination of overall mechanical behaviour. However, evaluation of Young’s modulus, E, of a fibre is a delicate operation given the small dimensions (diameter typically a few tens of microns), and therefore low forces involved in tensile testing. This article treats a novel method of modulus assessment involving the bending of fibres, clamped at one extremity, by forced vibrations. The fibre behaves as a beam, and when the forced oscillations approach (one of) the resonant frequency(ies) of the fibre, the bending amplitude increases. Classical beam theory allows evaluation of Young’s modulus from knowledge of resonant frequency, and fibre dimensions and density. Preliminary application of the technique using fibres of E-glass, having well known elastic characteristics, has given good results and shown its inherent potential. Subsequently, the technique developed was used on recycled fibres in order to obtain their Young’s modulus and to assess their loss of mechanical properties when compared to virgin fibres.     

Investigation on effect of fibre hybridization and orientation on mechanical behaviour of natural fibre epoxy composite

Nowadays bio fibre composites play a vital role by replacing conventional materials used in automotive and aerospace industries owing to their high strength to weight ratio, biodegradability and ease of production. This paper aims to find the effect of fibre hybridization and orientation on mechanical behaviour of composite fabricated with neem, abaca fibres and epoxy resin. Here, three varieties of composites are fabricated namely, composite 1 which consists of abaca fibre and glass fibre, composite 2, which consists of neem fibre and glass fibre, whereas composite 3 consists of abaca, neem fibres and glass fibres. In all the above three varieties, fibres are arranged in three types of orientations namely, horizontal (type I), vertical (type II) and 45\(^{\circ }\) inclination (type III). The result shows that composites made up of abaca and neem fibres with inclined orientation (45\(^{\circ }\)) have better mechanical properties when compared with other types of composites. In addition, morphological analysis is carried out using scanning electron microscope to know the fibre distribution, fibre pull out, fibre breakage and crack propagation on tested composites.     

Effect of chemical treatments of Alfa (Stipa tenacissima) fibres on water-sorption properties

Natural fibres have prooved to be an excellent reinforcers in composite materials. Since many years, for economic and environmental reasons, there has been an increasing interest in using plant fibres in composite systems. However, the main disadvantage of the natural fibres being used as reinforcers is their hydrophilic nature, therefore, the ageing of composite materials can be pronounced because of the diffusion of water molecules leading to a swelling effect. Moreover, the adhesion between natural fibres and the polymer matrix is insufficient. In this work, various chemical surface treatments have been performed on the Alfa (Stipa tenacissima) fibre. These different treatments involve acetylation (Ac), with the help of chemicals such as styrene (S), acrylic acid (AA) and maleic anhydride (MA). The treatment effects on the fibres have been characterized by means of infrared spectroscopy, surface energy, and microscopy analysis. A detailed investigation on the water sorption characteristics of Alfa fibres has been carried out. It was found that treatments reduced the overall water uptake of Alfa fibres. In particular styrene treatment allows to increase significantly moisture resistance of these fibres.     

Deformation micromechanics of a model cellulose/glass fibre hybrid composite

Interfacial stress transfer in a model hybrid composite has been investigated. An Sm³⁺ doped glass fibre and a high-modulus regenerated cellulose fibre were embedded in close proximity to each other in an epoxy resin matrix dumbbell-shaped model composite. This model composite was then deformed until the glass fibre fragmented. Shifts of the absolute positions of a Raman band from the cellulose fibre, located at 1095 cm⁻¹, and a luminescence band from a doped glass fibre, located at 648 nm, were recorded simultaneously. A calibration of these shifts, for both fibres deformed in air, was used to determine the point-to-point distribution of strain in the fibres around the breaks in the glass fibre. Each break that occurred in the glass fibre during fragmentation was shown to generate a local stress concentration in the cellulose fibre, which was quantified using Raman spectroscopy. Using theoretical model fits to the data it is shown that the interfacial shear stress between both fibres and the resin can be determined. A stress concentration factor (SCF) was also determined for the regenerated cellulose fibre, showing how the presence of debonding reduces this factor. This study offers a new approach for following the micromechanics of the interfaces within hybrid composite materials, in particular where plant fibres are used to replace glass fibres.     

Fundamental reproducibility of Raman band positions and strain measurements of high-modulus carbon fibre — the effect of laser-induced heating

We have shown that the E_2g, A_1g and second-order ( 2700 cm^–1) Raman band positions of Hercules HMS4 carbon fibres shift as a function of incident laser power. These shifts arise as a result of changes in the local fibre temperature. The sensitivity of band position to varying laser power is different from fibre to fibre within the same tow — thus equally strained fibres subjected to the same laser power can show widely different band positions and so different apparent strains. The same effect can be observed from point to point on individual fibres. If laser power is not carefully controlled (both in magnitude and stability), errors in the accuracy of the measured strain can be greater than the measured strain itself, and have been shown to approach 70% of the breaking strain. These results mean that strain measurements obtained from composite materials containing this fibre must be interpreted with caution unless the laser beam intensity at the fibre is precisely controlled. This effect will also be important with other types of carbon fibre, since in previous work we have observed that laser-induced sample heating occurs with a wide range of materials from cokes to pitch-based fibres.     

Evaluation of mechanical and thermal properties of banana–flax based natural fibre composite

Hybrid materials of any kind are the keynote for today’s demands. This paper deals with one of such hybrid composite made of natural fibres namely, banana and flax fibres. The structural build-up is such that one layer of banana fibre is sandwiched between two layers of flax fibres by hand layup method with a volume fraction of 40% using Epoxy resin and HY951 hardener. Glass fibre reinforcement polymer (GFRP) is used for lamination on both sides. This lamination also increases the overall mechanical properties along with better surface properties. The properties of this hybrid composite are determined by testing its tensile, impact, and flexural loads using a Universal testing machine. Thermal properties are analysed and hybrid composites of flax and banana with GFRP have better thermal stability and flame resistance over flax, banana with GFRP single fibre hybrid composites. Morphological analysis is done using Scanning Electron Microscope (SEM). The result of test shows that hybrid composite has far better properties than single fibre glass reinforced composite under impact and flexural loads. However it is found that the hybrid composite have better strength as compared to single fibre composites.     

Carbon,polyethylene and PBO hybrid fibre composites for structural lightweight armour

The properties of hybrid composite materials (based on carbon and organic fibres) have been investigated, with a view to using these materials in polymer composites lightweight structural armour. Laminates were manufactured and their specific ballistic properties and specific compressive strength after impact were determined. The introduction of organic fibres in order to improve ballistic properties has been successful. The associated reduction of specific compressive strength can be more than compensated for by an increase of specific ballistic properties. To achieve this, fractions of each fibre type and their distribution must be optimised and matrices must be optimised to the function of the respective fibre.     

Influence of temperature on fracture initiation in PET and PA66 fibres under cyclic loading

The fatigue of single thermoplastic fibres has been well documented to occur in a reproducible manner when they are subjected to certain cyclic loading conditions. The fatigue fracture morphologies of these fibres are very distinctive and differ markedly from other types of failure. This type of behaviour, which is clearly seen with the unambiguous tests on single fibres, must reflect behaviour of fibres in more complex structures which are subjected to cyclic loading. Only limited numbers of reports have, however, shown similar fracture morphologies with fibres extracted from fibre bundles embedded in a matrix material such as rubber. Usually the fractured ends of fibres taken from structures are seen to be shorter than those obtained in single fibre tests and also they show more complex and confused crack growth. The present study reveals that the low thermal conductivity of the fibres, exacerbated when they are embedded in a rubber matrix, leads to very high temperature rises, which is not the case in single fibre tests and under these conditions, crack initiation occurs across the fibre section instead of being restricted to the near surface region. Tests on single fibres at temperatures up to and beyond the glass transition temperature have shown how the fracture morphologies become modified. The fatigue process has been seen to become generalised throughout the fibre and failure occurs due to the coalescence of several cracks, some of which are initiated in the core of the fibre. In all cases, the cracks can be seen to have been initiated by solid inclusions in the fibres.     

Failure mechanisms and fracture energy of hybrid materials

A shear-lag model of hybrid materials is developed. The model represents an alternating arrangement of two types of aligned linear elastic fibres, embedded in a linear elastic matrix. Fibre and matrix elements are taken to fail deterministically when the axial and shear stresses in them reach their respective strengths. An efficient solution procedure for determining the stress state for arbitrary configurations of broken fibre and matrix elements is developed. Starting with a single fibre break, this procedure is used to simulate progressive fibre and matrix failure, up to composite fracture. The effect of (1) the ratio of fibre stiffnesses, and (2) the ratio of the fibre tensile strength to matrix shear strength, on the composite failure mechanism, fracture energy, and failure strain is characterised. Experimental observations, reported in the literature, of the fracture behaviour of two hybrid materials, viz., hybrid unidirectional composites, and double network hydrogels, are discussed in the framework of the present model.