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This article evaluates the mechanical properties of vacuum-infused unidirectional plant fiber composites (PFRPs), composing of bast fiber yarns and thermoset matrices. PFRPs are found to have lower fiber volume fractions than E-glass composites (GFRPs). Apart from the expected (30–40%) lower density of PFRPs, they have 60–80% lower tensile strength, 30–60% lower tensile stiffness, 5–10 times lower impact strength, and 20–30% lower interlaminar shear strength than GFRPs. Importantly, critical fiber lengths are of the same order (0.2–0.5 mm). Composites reinforced with flax rovings exhibit exceptional fiber tensile modulus of 65–75 GPa and fiber tensile strength of about 800 MPa. [ABSTRACT FROM PUBLISHER]

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]