The abrasive wear behaviour of continuous fibre polymer composites

The dry abrasive-dominant wear behaviour of several composite materials consisting of uni-directional continuous fibres and polymer matrices was investigated. Seven materials were examined: neat epoxy (3501-6), carbon fibre epoxy (AS4/3501-6), glass fibre/epoxy (E-glass/ 3501-6), aramid fibre/epoxy (K49/3501-6), neat polyetheretherketone (PEEK), carbon fibre/PEEK (APC2) and aramid fibre/PEEK (K49/PEEK). The wear behaviour of the materials was characterized by experimentally determining the friction coefficients and wear rates with a pin on-flat test apparatus. First, the effects of the operation variables apparent normal pressure, sliding velocity and apparent contact area were observed. The dimensionless wear rate increased linearly as the apparent normal pressure increased and decreased as the apparent contact area increased. Second, through microscopic observations of the worn surfaces and subsurface regions, basic wear mechanisms were identified as a function of fibre orientation. Observations of fibre-abrasive particle interactions allowed for the differentiation of the dominating wear mechanisms. Finally, a network of data was compiled on the wear behaviour in terms of the three material parameters: fibre orientation, fibre material and matrix material. This enabled the systematic selection of an ideal low wear composite material which would consist of a PEEK matrix reinforced with aramid fibres oriented normal to the contacting surface and carbon fibres oriented parallel to the contacting surface.     

On the experimental investigation of crash energy absorption in laminate splaying collapse mode of FRP tubular components

In this experimental work the crash energy absorption of fibre reinforced plastic (FRP) tubular components that collapse in laminate splaying mode is investigated by means of a new testing method, the “curling test”. This test method was used trying rectangular carbon, aramid and glass FRP strips—in which the reinforcing fibres were in the form of reinforcing woven fabric (carbon and aramid FRP specimens) and multi-axial fibre reinforcements (glass FRP specimens). Apart from the analysis of the system of bending and friction forces acting on the specimens during the curling tests in comparison with the forces acting in the case the laminate splaying collapse mode and the observations related to the deformation and crushing induced on the FRP specimens by this force combination, the analysis of the test results focused on the influence of the most important geometric and laminate material properties—such as thickness, flexural rigidity, number of reinforcing fibre layers, laminate stacking sequence and constituent material mechanical properties—on the specific energy absorption and the peak load.     

Entangled Cross-Linked Fibres for an Application as Core Material for Sandwich Structures - Part I: Experimental Investigation

Entangled cross-linked fibres were studied for an application as core material for sandwich structures. Specimens were produced from carbon, aramid and glass fibres, and cross-links were achieved using epoxy spraying. It was observed that this type of entangled cross-linked fibres could be fabricated without any major technical difficulties. The scope of this paper is to study the effect of some different parameters on the mechanical properties of these materials. Different effects were investigated: effect of fibres length, of fibres nature, of mixing fibres, of carbon skins and of the resin. The first part of this paper deals with the production of these entangled cross-linked fibres. The compression, tension and three point bending tests are detailed in the second part and the results are compared with usual core material currently used in industries.     

Mechanical properties of carbon fibre-reinforced glasses

Carbon fibre-reinforced glasses exhibit very high values of flexural strength but usually a much less controlled fracture behaviour than SiC fibre-reinforced glasses. Some carbon fibre/glass composite combinations show a well controlled fracture, others a brittle fracture behaviour. The former combinations occasionally exhibit an increase in strength after an abrupt breakdown from the maximum strength. No correlation exists between the strength of the composites and the stresses in the glass matrix due to the thermal expansion mismatch between carbon fibres and glasses in contrast to the SiC fibre composites. The reason for that is seen in the structure of the surface and mainly in the anisotropic properties of the fibres, such as the large differences in the Young's moduli and thermal expansion coefficients parallel and perpendicular to the fibre axis. In particular, no radial compressive stress on the fibres can be built up at the fibre/glass interface because the thermal expansion coefficient of the fibres in the radial direction is much larger than that of the glass matrices used. Thus, the mechanism of load transfer from the matrix to the fibres is a complicated one, and cannot easily be predicted as in the case of the isotropic SiC fibres. A possible mechanism is described in order to interpret the experimental results.     

Post-impact damage characterization of hybrid configurations of jute/glass polyester laminates using acoustic emission and IR thermography

In this work, residual post-impact properties of two configurations of E-glass/jute hybrid laminates are characterized, both manufactured using a total fibre volume of 50 ± 2% (14 glass fibre layers + 4 jute fibre layers). T-laminates included a core obtained by multiple layers of jute between two E-glass fibre reinforced skins, whilst in Q-laminates single layers of jute fibres were intercalated at different levels between E-glass fibre reinforced layers. All laminates were impacted at five levels of energy, from 5 to 15 J, and then subjected to post-impact flexural tests.The results suggest that T hybrids perform better at low impact energies (up to 10 J), which do not damage significantly the laminate core. In contrast, Q hybrids are better suited to withstand extensive damage produced by higher impact energies (12.5 and 15 J), in that they allow a more effective redistribution of impact damage in the structure. This was confirmed by acoustic emission (AE) monitoring during flexural loading, which offered indications on the maximum stress laminates can undergo after impact damage. Pulse IR thermography yielded information on their mode of failure by visualizing impact-damaged areas.     

An experimental study of synthetic fibre reinforced cementitious composites

Fibre reinforcement is one of the effective ways of improving the properties of concrete. However, current studios on fibre -reinforced concrete (FRC) have focused mainly on reinforcements with steel and glass fibres. Thin paper reports on an experimental programme on the properties of various synthetic fibre reinforced cementitious composites and the properties of the reinforcing fibres. Acrylic, polyester, and aramid fibres were tested in uniaxial tension, both in their original state as we!! as after ageing in nerO*nL Samples of these fibres were found to lose varying amounts of strength with time, depending on the ageing temperature. Two different test methods were used to measure the fibre-cement interfacial bond strength. The tensile properties of concrete reinforced with acrylic, nylon, and aramid fibres, in the form of random distribution or unioxial alignment, were studied by means of three different tests: compact tension, flexural, and splitting tensile tests. The properties of concrete, particularly that of apparent ductility, were found to be greatly improved by the inclusion of such fibre reinforcement.     

Impact and post-impact damage characterisation of hybrid composite laminates based on basalt fibres in combination with flax,hemp and glass fibres manufactured by vacuum infusion

The impact and flexural post-impact behaviour of ternary hybrid composites based on epoxy resin reinforced with different types of fibres, basalt (B), flax (F), hemp (H) and glass (G) in textile form, namely FHB, GHB and GFB, has been investigated. The reinforcement volume employed was in the order of 21–23% throughout. Laminates based exclusively on basalt, hemp and flax fibres were also fabricated for comparison. Hybrid laminates showed an intermediate performance between basalt fibre reinforced laminates on the high side, and flax and hemp fibre reinforced laminates on the low side. As for impact performance, GHB appears to be the worst performing hybrid laminate and FHB slightly overperforms GFB. In general, an increased rigidity can be attributed to all hybrids with respect to flax and hemp fibre composites. The morphological study of fracture by SEM indicated the variability of mode of fracture of flax and hemp fibre laminates and of the hybrid configuration (FHB) containing both of them. Acoustic emission monitoring during post-impact flexural tests confirmed the proneness to delamination of FHB hybrids, whilst they were able to better withstand impact damage than the other hybrids.     

Behaviour of glued fibre composite sandwich structure in flexure: Experiment and Fibre Model Analysis

The behaviour of glued composite sandwich beams in flexure was investigated with a view of using this material for structural and civil engineering applications. The building block of this glue-laminated beam is a new generation composite sandwich structure made up of glass fibre reinforced polymer skins and a high strength phenolic core material. A simplified Fibre Model Analysis (FMA) usually used to analyse a concrete beam section is adopted to theoretically describe the flexural behaviour of the innovative sandwich beam structure. The analysis included the flexural behaviour of the glued sandwich beams in the flatwise and the edgewise positions. The FMA accounted for the non-linear behaviour of the phenolic core in compression, the cracking of the core in tension and the linear elastic behaviour of the fibre composite skin. The results of the FMA showed a good agreement with the experimental data showing the efficiency and practical applications of the simplified FMA in analysing and designing sandwich structures with high strength core material.     

Suppression of crack growth in hybrid fibre composites

A theoretical analysis based on the assumed form of the strain field surrounding a crack bridged by reinforcing elements has been used to examine the growth of a crack propagating transversely to the fibres in hybrid fibre composites. An intermingled carbon fibre/glass fibre polymer matrix system has been considered. Two situations have been investigated. In the first of these the effect of the addition of carbon fibres on the development of cracks resulting from the failure of the glass fibres by stress corrosion has been studied. The analysis indicates that crack growth can be severely inhibited by a 5% volume fraction of type III carbon fibres. The analysis has been used also to investigate the process by which strong high failing strain glass fibres inhibit the growth of cracks caused by the fracture of localized clusters of low failing strain carbon fibres. The predictions of this analysis agree with existing experimental data on glass fibre/carbon fibre hybrids.     

Plasma surface modification of advanced organic fibres: Part V Effects on the mechanical properties of aramid/phenolic composites

Aramid fibres have been treated in ammonia and oxygen plasma to enhance adhesion to resole phenolic resins. The plasma treatments resulted in significant improvements in interlaminar shear strength (ILSS) and flexural strength of composites made from these materials. Composites containing aramid fibres with epoxide groups reacted on to the ammonia plasma-treated fibre surface also showed further improvements in ILSS and flexural strength. Scanning electron and optical microscopic observations were used to examine the microscopic basis for these results, which have been compared with those obtained previously for aramid/epoxy and aramid/vinyl ester composites. For composites containing oxygen and ammonia plasma-treated fibres, the enhanced ILSS and flexural strength are attributed to improved wetting of the surface-treated aramid fibres by the phenolic resin. However, for those containing fibres with reacted epoxide groups on the ammonia plasma-treated fibre surfaces, the enhanced composite properties may be due to covalent chemical interfacial bonding between the epoxide groups and the phenolic resin. Effects of catalyst levels and cure cycle on the ILSS of composites laminated with untreated fabric has also been examined and optimum values have been determined. The catalyst concentration has an influence on the phase-separated water domain density in the matrix which in turn, affects the available fibre/matrix bonding area and hence the composite ILSS and flexural strength. This revised version was published online in November 2006 with corrections to the Cover Date.     

Bioinspired self-healing of advanced composite structures using hollow glass fibres

Self-healing is receiving an increasing amount of worldwide interest as a method to autonomously address damage in materials. The incorporation of a self-healing capability within fibre-reinforced polymers has been investigated by a number of workers previously. The use of functional repair components stored inside hollow glass fibres (HGF) is one such bioinspired approach being considered. This paper considers the placement of self-healing HGF plies within both glass fibre/epoxy and carbon fibre/epoxy laminates to mitigate damage occurrence and restore mechanical strength. The study investigates the effect of embedded HGF on the host laminates mechanical properties and also the healing efficiency of the laminates after they were subjected to quasi-static impact damage. The results of flexural testing have shown that a significant fraction of flexural strength can be restored by the self-repairing effect of a healing resin stored within hollow fibres.     

The effect of fibre dispersion on initial failure strain and cluster development in unidirectional carbon/glass hybrid composites

By adding glass fibres to carbon fibre composites, the apparent failure strain of the carbon fibres can be increased. A strength model for unidirectional hybrid composites was developed under very local load sharing assumptions to study this hybrid effect. Firstly, it was shown that adding more glass fibres leads to higher hybrid effects. The hybrid effect was up to 32% for a hybrid composite with a 10/90 ratio of carbon/glass fibres. The development of clusters of broken fibres helped to explain differences in the performance of these hybrid composites. For 50/50 carbon/glass hybrids, a fine bundle-by-bundle dispersion led to a slightly smaller hybrid effect than for randomly dispersed hybrids. The highest hybrid effect for a 50/50 ratio, however, was 16% and was achieved in a composite with alternating single fibre layers. The results demonstrate that thin ply hybrids may have more potential for improved mechanical properties than comingled hybrids.     

Behaviour of woven hybrid basalt-carbon/epoxy composites subjected to laser shock wave testing: Preliminary results

This study addresses the effect of basalt fibre hybridization on the damage tolerance of carbon/epoxy laminates subjected to laser shock wave tests. Interply hybrid specimens with two different stacking sequences (sandwich-like and intercalated) were tested at different laser intensities and residual post-shock properties of the different configurations have been characterized by quasi-static three point bending tests monitored by acoustic emission. Results indicate that the best compromise in terms of both quasi-static properties (2% reduction in flexural strength compared to all carbon laminates) and damage tolerance appears to be the sandwich-like structure with basalt fibre skins. In particular, this configuration exhibited the highest damage tolerance among the hybrids, with a percent decrease in flexural strength of about 5% compared to 15% in the case of all carbon laminates. Damage induced by laser shock testing in carbon-basalt woven fabric/epoxy composites is mainly inter-ply delamination. This study also highlights the tougher behaviour of basalt plies in response to a sudden application of load compared to carbon layers with a favourable hybridization effect.     

Transcrystallized interphase in thermoplastic composites

Hot-stage microscopy and differential scanning calorimetry were used to investigate the isothermal crystallization of polypropylene in the presence of a large variety of fibres. The occurrence of transcrystallization was found to depend on the type of fibre used and the crystallization temperature. The list of fibres which transcrystallize polypropylene is similar to that for other semicrystalline thermoplastics. In particular we found that aramid fibres and high-modulus carbon fibres do induce transcrystallization, whereas high-strength carbon fibres and glass fibres do not transcrystallize polypropylene. The radial growth rates of the polypropylene spherulites and the transcrystallization region were found to be identical over a range of isothermal crystallization temperatures. However, the ability of aramid fibres and high-modulus fibres to induce transcrystallization in polypropylene is dependent on the crystallization temperature. No transcrystallization was observed in quiescently crystallized polypropylene above 138° C.     

Hybrid composites based on aramid and basalt woven fabrics: Impact damage modes and residual flexural properties

The low-velocity impact behaviour of hybrid laminates reinforced with woven aramid and basalt fabrics and manufactured by resin transfer moulding was studied. Specimens with different stacking sequences were tested at three different energies, namely 5, 12.5 and 25 J. Residual post-impact properties of the different configurations of aramid/basalt hybrid laminates were characterized by quasi static four point bending tests. Post-impact flexural tests have been monitored using acoustic emission in order to get further information on failure mechanisms. Results indicate that hybrid laminates with intercalated configuration (alternating sequence of basalt and aramid fabrics) have better impact energy absorption capability and enhanced damage tolerance with respect to the all-aramid laminates, while basalt and hybrid laminates with sandwich-like configuration (seven basalt fabric layers at the centre of the laminate as core and three aramid fabric layers for each side of the composite as skins) present the most favourable flexural behaviour.     

A study of the mechanical behaviour on fibre reinforced hollow microspheres hybrid composites

This paper presents the results of an investigation into the effects of hollow glass microsphere fillers and of the addition of short fibre reinforcements on the mechanical behaviour of epoxy binding matrix composites. Properties like flexural stiffness, compressive strength, fracture toughness and absorbed impact energy, were studied. The specimens were cut from plates produced by vacuum resin transfer moulding having a microsphere contents of up to 50% and with fibre reinforcement up to 1.2% by volume. The tests performed with unreinforced composites show that flexural and compressive stiffness, maximum compressive stresses, fracture toughness and impact absorbed energy decrease significantly with increasing filler content. However, in terms of specific values, both flexural and compressive stiffness and impact absorbed energy increase with microsphere content. The addition of glass fibre produces only a slight improvement in the flexure stiffness and fracture toughness, while increasing significantly the absorbed impact energy. In contrast, the addition of a small percentage of carbon fibres produces an important improvement in both fracture toughness and flexure stiffness, when hybrid composites with 0.9% carbon fibre are compared to unreinforced foam, but did not improved absorbed impact energy.     

Flexural behaviour of hybrid laminated composites

The present paper studies the flexural behaviour of hand manufactured hybrid laminated composites with a hemp natural fibre/polypropylene core and two glass fibres/polypropylene surface layers at each side of the specimen. When compared with full glass fibres reinforced polypropylene laminates, the hybrid composites have economical, ecological and recycling advantages and also specific fatigue strength benefits. Static and fatigue tests were performed in three point bending for both laminates to evaluate flexural strength properties and fatigue behaviour. Fatigue damage was measured in terms of the stiffness loss. Failure sites and mechanisms were evaluated through microscopy studies and a 3D numerical analysis using finite element method.     

Windsurf-Board Sandwich Panels Under Static Indentation

In recent years composite materials have found application in several fields as sport and sea transportation, where the incidence of the cost of materials is not significant compared with the required high mechanical performances. As a matter of fact, in some sports the whole equipment is nowadays realized in composite materials (i.e. windsurf boards, snowboards). The aim of the present work is to evaluate the mechanical performance of some sandwich structures produced by vacuum bagging technology for the windsurf boards production. The behaviour of the structures is tested under static indentation conditions; different fibres materials, for the skins, and different polystyrene foams, for the core, have been taken into account. In particular both the effect of the kind of fibre (glass, carbon and kevlar fibres) and the effect of the polystyrene cells size (and its density) have been investigated. The purpose was to obtain a stiff structure able to bear localized loads. Additionally, the effects induced on the indentation resistance by both the speed and the diameter of pin have been analysed.     

Hybrid effects in the bending stiffness of graphite/glass-reinforced composites

The flexural modulus of graphite/glass-reinforced hybrids exhibits deviations from the rule-of-mixtures base-line. The deviation increases as the segregation of the glass and graphite layers increases, and it reaches a maximum when the two fibre types are arranged in 3 layers. When the stiffer fibres (graphite) are in the outer layers the deviation is positive but it is negative with the less stiff fibres (glass) on the outside. The extent of deviation is shown to be predictable by the analysis.     

The environmental response of hybrid composites

Hybrid composite specimens containing a total of 60 or 75 vol % of unidirectional fibre were prepared from HT S-carbon fibre and E-glass fibre, HT S-carbon fibre and Kevlar 49 fibre, and E-glass fibre and Kevlar 49 fibre with a standard anhydride cured epoxide resin. The specimens were divided into four groups and subjected to the following environments: (A) room temperature and humidity; (B) soaked in water for 300 h at 95° C and then oven dried at 60° C to a constant weight; (C) thermally cycled 100 times between –196 and 95° C; (D) cycled 35 times between –196 and water at 95° C. The flexural properties of the samples were measured at room temperature after exposure. The modulus of the hybrid materials was not significantly affected by any of the treatments, although thermal cycling with or without water caused a large decrease in the modulus of all Kevlar fibre/resin and to a lesser extent all glass fibre specimens. The flexural strength of the unexposed carbon fibre/glass fibre and glass fibre/Kevlar fibre hybrids showed a positive deviation from the rule of mixtures behaviour at low volume loadings of the lower extension fibre. Wet thermal cycling or soaking in water caused a substantial reduction in the flexural strength of glass fibre/Kevlar fibre specimens. The interlaminar shear strength of all three fibre combinations was not affected by dry thermal cycling, but the effects of soaking in water and especially thermal cycling with water exposure were significant and irreversible.