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Abstract: In directly addressing the question, ‘can flax replace E-glass as a reinforcement for structural composites?’, this manuscript adopts a novel comparative case study approach to investigate the manufacture and mechanical testing of full-scale 3.5-m composite rotor blades (suitable for 11kW turbines) built from flax/polyester and E-glass/polyester. The resin transfer moulded flax blade is 10% lighter (fibre mass saving of 45%) than the identical construction E-glass blade. Static flap-bending tests, conducted in accordance to certification standards, confirm that like the E-glass blade, the flax blade satisfies the structural integrity requirements under ‘normal operation’ and ‘worst case’ loading. It is consequently claimed that flax is a potential structural replacement to E-glass for similar composite small wind turbine blade applications. The failure root bending moment and corresponding tip displacement of the flax blade are 11.6kNm and 2300mm, respectively. The blades exhibit distinctly different load–deflection curves and failure modes. The mean flexural rigidity of the flax and E-glass blades are estimated to be 24.6kNm2 and 43.4kNm2, respectively. It is interesting to find that although flax fibres and their composites are generally recognized for their stiffness, a flax blade cannot compete against an E-glass blade in terms of stiffness. [Copyright &y& Elsevier]

The structural potential of plant fibres as reinforcing agents can only be realized when the highest reinforcement efficiency is employed. Hence, aligned plant fibre composites are of interest. However, due to the short length of technical plant fibres, the reinforcement needs to be in the form of staple fibre yarns, which have a twisted structure. Although twist facilitates yarn processability, it has several detrimental effects on the composites produced from such twisted yarn reinforcements; one of which is fibre obliquity and misalignment. This results in a drastic drop in composite mechanical properties. No analytical model currently exists to accurately predict the effect of yarn twist on aligned plant fibre composite tensile strength. In this paper, a novel mathematical model is developed. The model is based on (i) a modified rule of mixtures for plant fibre composites, (ii) well-defined structure-property relationships in an idealised twisted staple fibre yarn and (iii) the Krenchel orientation efficiency factor. The developed model includes a corrected orientation efficiency factor of cos2(2α), where α is the yarn surface twist angle. The model is validated with extensive experimental data from Goutianos and Peijs (Goutianos S, Peijs T (2003) Adv Compos Lett 12(6):237) and is found to be a near-perfect fit (R2 = 0.960). Experimental data from other studies are also used for further verification. [ABSTRACT FROM AUTHOR]

Abstract: There is a noticeable lack of fatigue data on plant fibre composites (PFCs) which seriously limits their prospective use in fatigue critical components. The objective of this manuscript is to provide a complete set of fatigue data on vacuum infused aligned PFCs. S–N lifetime diagrams have been constructed to investigate the effect of (i) plant fibre type/quality, (ii) fibre content, (iii) textile architecture, and (iv) stress ratio, on PFC cyclic loading behaviour. At each stage, the fatigue performance of PFCs is compared to that of E-glass/polyester (GFRP) composites. A comprehensive constant-life diagram has been generated to facilitate life prediction of PFC components. While plant fibre type and quality, textile architecture and composite fibre content have a significant impact on the static properties, they have little impact on the material fatigue strength coefficient b (which dictates the slope of the S–N curve). In essence, higher static properties are a sign of superior fatigue loading capacities throughout their lifetime. Increasing stress ratios lead to improved fatigue performance in PFCs. Fracture mechanisms and modes are the same for composites made from three different bast fibres, but depend on fibre content, textile architecture and stress ratios. Although the absolute fatigue performance of GFRPs is far superior to PFCs, it is a revelation to find that fatigue strength degradation rates are lower in PFCs than in GFRP. [Copyright &y& Elsevier]

Abstract: The effect of fibre volume fraction on the physical and tensile properties of aligned plant fibre composites (PFCs) produced via vacuum infusion has been investigated. There is no clear correlation between fibre volume fraction and porosity. However, low fibre content PFCs are prone to intra-yarn voids, while high fibre content PFCs are prone to inter-yarn voids. This is due to changing resin flow dynamics with increasing fibre content. The tensile behaviour of PFCs with increasing fibre content is similar to that of conventional fibre reinforced plastics (FRPs). Interestingly, the non-linear stress–strain response of plant fibres has been transferred to the composites. Fibre content and tensile properties are found to be linearly related, as per the rule of mixtures. A void content of up to 4% is found to have minimal effect on the tensile properties of PFCs. The minimum and critical (vf , crit ) fibre volume fractions for aligned flax and jute–polyester composites are found to be substantially higher than conventional aligned FRPs; vf , crit for jute–polyester and carbon–polyester is 8.1% and 2.4%, respectively. A simple model has also been developed to approximate the theoretical maximum obtainable fibre volume fraction of PFCs reinforced with staple fibre yarns. The absolute theoretical maximum fibre content is found to be 58.9%, which agrees with experimental values in literature. A high vf , crit (∼10%) and low vf , max (∼45%) implies that the range of useful fibre volume fractions for PFCs containing staple fibre twisted yarns is only 35%. [Copyright &y& Elsevier]