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 500-800 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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