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.     

Effect of material and process parameters on the mechanical properties of unidirectional and multidirectional flax/polypropylene composites

Unidirectional (UD) and multidirectional (MD) flax/polypropylene composites were studied. Flax with varying retting degree and boiled flax was used as reinforcement for the UD composites and unmodified and maleic acid anhydride modified polypropylene (MAA-PP) was used as matrix. Multidirectional flax/polypropylene composites were manufactured on laboratory scale and on pilot scale. They were made from needle-punched hybrid flax/PP non-wovens. Normally retted flax as well as boiled flax was used. For the specimens made on pilot scale, a third kind of flax, namely bleached flax was also studied. The influence of different process times and temperatures on the mechanical properties of the composites was analysed. Generally, the composites have adequate good mechanical properties. The unidirectional composites of boiled flax combined with MAA-PP show the best mechanical properties. Contrary to the UD composites, flax treatment did not lead to the expected property improvements for MD composites.     

The effect of mechanical defects on the strength distribution of elementary flax fibres

Flax fibres are finding non-traditional applications as reinforcement of composite materials. The mechanical properties of fibres are affected by the natural variability in plant as well as the damage accumulated during processing, and thus have considerable variability that necessitates statistical treatment of fibre characteristics. The strength distribution of elementary flax fibres has been determined at several fibre lengths by standard tensile tests, and the amount of kink bands in the fibres evaluated by optical microscopy. Strength distribution function, based on the assumption that the presence of kink bands limits fibre strength, is derived and found to provide reasonable agreement with test results.     

Optimization of the mechanical performance of UD flax/epoxy composites by selection of fibres along the stem

In this study, we measure and compare the mass distribution, morphology and tensile properties of elementary flax fibres (Eden variety) along the stem. A significant improvement is found in the tensile properties of elementary fibres from the base to the middle of the stem, with the Young’s modulus and breaking stress being increased by 19.3% and 56.6%, respectively. UD thermoset composites were manufactured with fibres from the bottom, middle and top of the stem. The efficiency of the reinforcement is lowest with bottom fibres, which appear to be poorly individualized and have many cortical residues on their surface. Several scenarios are proposed to select the most suitable fibres according to the desired mechanical properties of the composite.     

Hybridisation effect on diffusion kinetic and tensile mechanical behaviour of epoxy based flax–glass composites

This paper aims at investigating the hybridisation effect on the diffusion kinetic and the tensile mechanical behaviour of flax–glass fibres reinforced epoxy composites. For this purpose, hybrid composites composed of flax and glass fibre laminates with different stacking sequences were consolidated by compression moulding and subjected to environment ageing. The obtained results show that the water uptake and the diffusion coefficient are clearly reduced by the addition of glass fibre layers in flax laminate. The ageing conditions performed show that the flax–glass hybridisation presents a positive effect in a wet environment at low temperatures (∼20 °C) in the Young’s modulus and the tensile strength. For example, the Young’s modulus fell by 50% and 41% for hybrid laminates with 6% and 11% of glass fibres, and by 67% for the Flax laminate. However, the flax–glass hybridisation was not necessarily a relevant choice when the hybrid laminates were exposed in a wet environment at high temperatures. Indeed, at 55 °C, this hybridisation had a negative effect on the tensile strength and on the specific tensile strength.     

Thermomechanical properties of bio-based composites made from a lactic acid thermoset resin and flax and flax/basalt fibre reinforcements

Low viscosity thermoset bio-based resin was synthesised from lactic acid, allyl alcohol and pentaerythritol. The resin was impregnated into cellulosic fibre reinforcement from flax and basalt and then compression moulded at elevated temperature to produce thermoset composites. The mechanical properties of composites were characterised by flexural, tensile and Charpy impact testing whereas the thermal properties were analysed by dynamic mechanical thermal analysis (DMTA) and thermogravimetric analysis (TGA). The results showed a decrease in mechanical properties with increase in fibre load after 40 wt.% for the neat flax composite due to insufficient fibre wetting and an increase in mechanical properties with increase fibre load up to 60 wt.% for the flax/basalt composite. The results of the ageing test showed that the mechanical properties of the composites deteriorate with ageing; however, the flax/basalt composite had better mechanical properties after ageing than the flax composite before ageing.     

Influence of specimen type and reinforcement on measured tension–tension fatigue life of unidirectional GFRP laminates

It is well known that standardised tension–tension fatigue test specimens of unidirectional (UD) glass-fibre-reinforced plastics (GFRP) laminates tend to fail at end tabs. The true fatigue life is then underestimated. The first objective of this study was to find for UD GFRP laminates a test specimen that fails in the gauge section. The second objective was to compare fatigue performance of two laminates, one having a newly developed UD powder-bound fabric as a reinforcement and the other having a quasi-UD stitched non-crimp fabric as a reinforcement. In the first phase, a rectangular specimen in accordance with the ISO 527-5 standard and two slightly different dog-bone shaped specimens were evaluated by means of finite element modelling. Subsequent comparative fatigue tests were performed for the laminates with the three specimen types. The results showed that the test specimen type has a significant effect on the failure mode and measured fatigue life of the laminates. A significantly higher fatigue life was measured for the laminate with the powder-bound fabric reinforcement when compared to the laminate with the stitched reinforcement.     

Analysis of flax fibres viscoelastic behaviour at micro and nano scales

In glass or carbon fibres reinforced plastics, creep or stress relaxation, arise from the polymeric nature of the matrix. Plant fibres, used in bio-composites, are also polymers. Therefore, the issue of their service life requires studying the viscoelastic behaviour of both the matrix and the fibres. In this study, we investigate, at different length scales, the response of elementary flax fibres to tensile tests, as well as to nano-indentation tests on their secondary cell walls. The results of these experiments are then analysed via linear viscoelastic rheological models and identification procedures. The values of the identified parameters (relaxation time, viscosity and elastic stiffness) are discussed in relation to the microstructure of the flax fibre (cellulose microfibrils, hemicelluloses and pectins). The nano-indentation technique provides much more deterministic results than tension tests on an entire fibre. The scale of the secondary wall cell is then relevant to assess the viscoelastic behaviour of the fibres.     

Growth of carbon nanotubes on carbon fibre substrates to produce hybrid/phenolic composites with improved mechanical properties

Carbon nanotubes were grown by chemical vapor deposition (CVD) on different carbon fibre substrates namely, unidirectional (UD) carbon fibre tows, bi-directional (2D) carbon fibre cloth and three dimensional (3D) carbon fibre felt. These substrates were used as the reinforcement in phenolic resin matrix to develop hybrid CF–CNT composites. The growth morphology and other characteristics of the as grown tubes were analyzed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermal gravimetry (TGA) which confirmed a copious growth of multiwalled carbon nanotubes (MWNTs) on these substrates. The mechanical properties of the hybrid composites was found to increase with the increasing amount of deposited carbon nanotubes. The flexural strength (FS) improved by 20% for UD, 75% for 2D and 66% for 3D hybrid composites as compared to that prepared by neat reinforcements (without CNT growth) under identical conditions. Flexural modulus (FM) of these composites also improved by 28%, 54% and 46%, respectively.     

Mechanical properties of flax fibre/polypropylene composites. Influence of fibre/matrix modification and glass fibre hybridization

The effect of fibre treatments and matrix modification on mechanical properties of flax fibre bundle/polypropylene composites was investigated. Treatments using chemicals such as maleic anhydride, vinyltrimethoxy silane, maleic anhydride-polypropylene copolymer and also fibre alkalization were carried out in order to modify the interfacial bonding between fibre bundles and polymeric matrix. Composites were produced by employing two compounding ways: internal mixing and extrusion. Mechanical behaviour of both flax fibre bundle and hybrid glass/flax fibre bundle composites was studied. Fracture surfaces were investigated by scanning electron microscopy. Results suggest that matrix modification led to better mechanical performance than fibre surface modification. A relevant fact is that silanes or MA grafted onto PP matrix lead to mechanical properties of composites even better than those for MAPP modification, and close to those for glass fibre/PP.     

Analysis of the flax fibres tensile behaviour and analysis of the tensile stiffness increase

The knowledge of the behaviour of flax fibres is of crucial importance for their use as a reinforcement for composites materials. Flax fibres were tested under tensile loading and in repeated loading–unloading experiments. We have shown that fibre stiffness increases with the strain.This phenomenon is attributed to the orientation of the fibrils with the axis of the fibre when a strain occurs. By using micro-mechanical equations, the Young's modulus of a flax fibre is estimated by taking into account the composition of the fibre and the evolution of the orientation of the fibrils during a tensile test. A good agreement is found between experimental and calculated results. The origin of the large spread observed in the mechanical characteristics is analysed here.     

Regenerating the strength of thermally recycled glass fibres using hot sodium hydroxide

Results are presented from the ReCoVeR project on the regeneration of the strength of thermally conditioned glass fibres. Thermal recycling of end-of-life glass fibre reinforced composites or composite manufacturing waste delivers fibres with virtually no residual strength or value. Composites produced from such fibres also have extremely poor mechanical performance. Data is presented showing that a short hot sodium hydroxide solution treatment of such recycled fibres can more than triple their strength and restore their ability to act as an effective reinforcement in second life composite materials. The implications of these results for real materials reuse of recycled glass fibres as replacement for pristine reinforcement fibres are discussed.     

Effect of steel fibres on mechanical properties of high-strength concrete

Steel fibre reinforced concrete (SFRC) became in the recent decades a very popular and attractive material in structural engineering because of its good mechanical performance. The most important advantages are hindrance of macrocracks’ development, delay in microcracks’ propagation to macroscopic level and the improved ductility after microcracks’ formation. SFRC is also tough and demonstrates high residual strengths after appearing of the first crack. This paper deals with a role of steel fibres having different configuration in combination with steel bar reinforcement. It reports on results of an experimental research program that was focused on the influence of steel fibre types and amounts on flexural tensile strength, fracture behaviour and workability of steel bar reinforced high-strength concrete beams. In the frame of the research different bar reinforcements (2∅6 mm and 2∅12 mm) and three types of fibres’ configurations (two straight with end hooks with different ultimate tensile strength and one corrugated) were used. Three different fibre contents were applied. Experiments show that for all selected fibre contents a more ductile behaviour and higher load levels in the post-cracking range were obtained. The study forms a basis for selection of suitable fibre types and contents for their most efficient combination with regular steel bar reinforcement.     

Influence of water ageing on mechanical properties and damage events of two reinforced composite materials: Flax–fibres and glass–fibres

Moisture absorption and durability in water environment are major concerns for natural fibres as reinforcement in composites. This paper presents a study on the influence of water ageing on mechanical properties and damage events of flax–fibre composites, compared with glass–fibre composites. The effects of the immersion treatment on the tensile characteristics, water absorption and acoustic emission (AE) recording were investigated. The water absorption results for the flax–fibre composites show that the evolution appears to be Fickian and the saturated weight gain is 12 times as high that the glass–fibre composites. Decreasing continuously with increasing water immersion time, the tensile modulus and the failure strain of flax–fibre composites are hardly affected by water ageing whereas only the tensile stress is reduced regarding the glass–fibre composites. AE indicate that matrix–fibres interface weakening is the main damage mechanism induced by water ageing for both composites.     

Influence of an Agatha flax fibre location in a stem on its mechanical,chemical and morphological properties

The mechanical properties of flax fibres are analysed as a function of their biochemical and morphological characteristics. The fibres, from the Agatha variety, have been selected from either the top, the middle or the bottom of the stems. The results of each analysis are discussed according to the position of the fibre in the stem and compared among themselves. Considering a flax fibre as a natural composite, this study underlines the complexity of its structure and shows that many parameters intervene in its deformation behaviour.     

Flax fibre and its composites – A review

In recent years, the use of flax fibres as reinforcement in composites has gained popularity due to an increasing requirement for developing sustainable materials. Flax fibres are cost-effective and offer specific mechanical properties comparable to those of glass fibres. Composites made of flax fibres with thermoplastic, thermoset, and biodegradable matrices have exhibited good mechanical properties. This review presents a summary of recent developments of flax fibre and its composites. Firstly, the fibre structure, mechanical properties, cost, the effect of various parameters (i.e. relative humidity, various physical/chemical treatments, gauge length, fibre diameter, fibre location in a stem, oleaginous, mechanical defects such as kink bands) on tensile properties of flax fibre have been reviewed. Secondly, the effect of fibre configuration (i.e. in forms of fabric, mat, yarn, roving and monofilament), manufacturing processes, fibre volume, and fibre/matrix interface parameters on the mechanical properties of flax fibre reinforced composites have been reviewed. Next, the studies of life cycle assessment and durability investigation of flax fibre reinforced composites have been reviewed.     

Impact of electrochemical cycling on the tensile properties of carbon fibres for structural lithium-ion composite batteries

Carbon fibres are particularly well suited for use in a multifunctional lightweight design of a structural composite material able to store energy as a lithium-ion battery. The fibres will in this case act as both a high performance structural reinforcement and one of the battery electrodes. However, the electrochemical cycling consists of insertions and extractions of lithium ions in the microstructure of carbon fibres and its impact on the mechanical performance is unknown. This study investigates the changes in the tensile properties of carbon fibres after they have been subjected to a number of electrochemical cycles. Consistent carbon fibre specimens were manufactured with polyacrylonitrile-based carbon fibres. Sized T800H and desized IMS65 were selected for their mechanical properties and electrochemical capacities. At the first lithiation the ultimate tensile strength of the fibres was reduced of about 20% but after the first delithiation some strength was recovered. The losses and recoveries of strength remained unchanged with the number of cycles as long as the cell capacity remained reversible. Losses in the cell capacity after 1000 cycles were measured together with smaller losses in the tensile strength of the lithiated fibres. These results show that electrochemical cycling does not degrade the tensile properties which seem to depend on the amount of lithium ions inserted and extracted. Both fibre grades exhibited the same trends of results. The tensile stiffness was not affected by the cycling. Field emission scanning electron microscope images taken after electrochemical cycling did not show any obvious damage of the outer surface of the fibres.     

Why are designers fascinated by flax and hemp fibre composites?

Flax and hemp fibres have been increasingly used as reinforcement in polymer composites. First, an overview is given of the technical arguments which convinced designers of consumer goods to use these fibres. In particular, their composites show higher specific stiffness than glass fibre composites in both tension and plate bending and only slightly lower values than carbon fibre composites in plate bending. Moreover, flax and hemp fibres possess a much higher vibration damping capacity, making them excellent candidates for applications in sporting goods or musical instruments. Secondly, the paper describes how designers relate to the non-technical characteristics of these natural fibres. Many concrete examples are given from different application domains in consumer goods: sports, mobility, music and sound, furniture and interior design. The fascination of designers for these bio-based materials combined with the recent introduction in the market of new, composites-oriented preforms of flax and hemp fibres, is rapidly increasing the number and variety of composite products using flax and hemp fibres as reinforcement.     

Shear characterisation of uni-directional fibre reinforced thermoplastic melts by means of torsion

Intra-ply shear appears during the forming process of hot thermoplastic laminates with a uni-directional fibre reinforcement. This paper proposes a torsion bar test to characterise the longitudinal shear mechanism, which can be performed with a standard rheometer. Sensitivity analyses showed that most reliable shear property measurements can be obtained by using torsion bar specimens with a close to square cross section. The method is implemented in practise and critically evaluated. Storage and loss moduli were determined for carbon UD/PEEK specimens at high temperatures. Non-linear material behaviour was found for relatively small shear strains. The linear regime was focussed on subsequently, where the characteristics were found to be similar to that of a visco-elastic solid or weak gel, confirmed by a dominant storage modulus and a weak frequency dependency. Future work is recommended to be focussed on the large strain regime, for which this paper provides a found basis.     

Commingled natural fibre/polypropylene wrap spun yarns for structured thermoplastic composites

A major challenge for natural fibre composites is to achieve high mechanical performance at a competitive price. Composites constructed from yarns perform better than composites made from random nonwoven mats. However, the twist structure of conventional ring spun yarns prevents the full utilization of fibre mechanical properties in the final composites. We produced flax/polypropylene commingled wrap yarns in which all flax fibres were twistless. Composites made from the wrap yarn demonstrated significant improvement of flexural modulus. Most currently available low cost natural fibres, such as decorticated hemp, cannot be efficiently made into yarns because of their lack of cohesion. Adding polypropylene fibres to decorticated hemp improved textile processing performance. The polypropylene fibres served as a carrier for the natural fibres during processing and became the polymer matrix in the final composites.