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In this study, the impregnated fibre bundle test, a common method used by carbon and glass fibre manufacturers to determine the properties of fibres used in composites, was adapted for natural fibres and validated by a round robin test on one type of natural fibres, namely flax fibres. Five European laboratories have carried out in parallel the impregnated fibre bundle test, on the same batch of hackled flax (long fibres), to check the applicability and reliability of this modified method on natural fibres. The results were compared to the more traditional single fibre test on elementary fibres. The back-calculated fibre stiffness shows a very low scatter between the five laboratories of less than ±5% (59.8 ± 2.4 GPa, as measured between 0 and 0.1% strain). The fibre ultimate tensile strength of 527 ± 138 MPa has a higher scatter, compared to stiffness values, as this property is highly sensitive to imperfections and flaws.

Flax fibers were used to process unidirectional composites by two different methods. Their mechanical properties obtained by tensile testing are discussed with respect to the properties of the fibers and those of the matrix (unsatured polyester). The similarity of the tensile curves of the composites and of the elementary fibers is attributed to the good adhesion of the fibers with the matrix. Moreover, as there is almost a linear evolution of the composite properties with the fiber volume fraction, these properties are used to estimate those of the real reinforcement material, that is, the flax bundles: the calculations lead to a fiber strength of 500800 MPa and a fiber modulus of roughly 30 GPa, which is half the values obtained by tensile testing elementary fibers. These data may be helpful when trying to model the deformation behavior of flax fiber-reinforced composites.

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Flax fibres are a promising reinforcement in the development of biocomposites and are finding new applications in transport structures. However, there is a perceived problem with plant fibres related to the variability of the properties of these natural materials. This paper describes the factors which affect variability, from plant growth conditions to fibre sampling and testing. A large number of test results are presented (characterization of elementary fibres, bundles, assemblies of bundles, and unidirectional composites), and it is shown that provided fibre supply is carefully controlled, characterization procedures are appropriate, and manufacturing processes are optimal then excellent composite properties can be achieved with low variability. Highlights ► This paper provides a unique review of a large quantity of data from hundreds of tests on flax fibers ► Data are discussed in terms of testing procedures and sample geometry ► Variability of composite properties is discussed in terms of fibre variability

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