Recent EU directives (e.g., ELV and WEEE) have caused some rethinking of the life cycle implications of fibre reinforced polymer matrix composites. Man-made reinforcement fibres have significant ecological implications. One alternative is the use of natural fibres as reinforcements. The principal candidates are bast (plant stem) fibres with flax, hemp, and jute as the current front runners. The work presented here will consider the characterisation of jute fibres and their composites. A novel technique is proposed for the measurement of fibre density. The new rule of mixtures, extended for noncircular cross-section natural fibres, is shown to provide a sensible estimate for the experimentally measured elastic modulus of the composite. 1. Introduction There have been a number of recent reviews [1–9] of natural fibre reinforcements and their composites. Virk et al. [10] proposed an extension to the rule of mixtures (ROM) for the estimation of the Young’s modulus of a composite with reinforcements of noncircular cross-section: where is the Young’s modulus, is the volume fraction, is the fibre area correction factor (FACF) [10], is the fibre diameter distribution factor (FDDF) [11], is the fibre length distribution factor (FLDF) [12], and is the fibre orientation distribution factor (FODF) [13] with subscript , , , and being composite, fibre, isotropic matrix, and voids ( ), respectively. For the case of hollow fibres, the voidage should be classified as that internal to the fibre (i.e., the lumen in natural fibres), , and that external to the fibre, , such that . When the fibres are characterised taking into account the internal features, the internal voids are not expected to influence the mechanical properties of the composites. The separation of internal and external voids is equally applicable to man-made hollow fibres [14, 15]. For hollow fibres, it is then necessary to determine the density of the fibre material (primarily cellulose in natural fibres) and the volume fraction of the void within the fibre. Typical values for each of these parameters would be(i) = 14–87？GPa (bast natural fibres) [6], ~70？GPa (glass), ~140？GPa (aramid), or ~210？GPa (carbon),(ii) = 1–3？GPa (polymers),(iii) = 0.1–0.3 (random orientation), 0.3–0.6 (woven fabric), or 0.5–0.8 (unidirectional),(iv) = 1.42 (i.e., 2697？μm2/1896？μm2) for jute [10] as the factor compensates for the overestimate in the apparent cross-sectional area (CSA) when CSA is derived from an apparent diameter with the assumption of circular cross-section and is calculated as the ratio of apparent CSA/true
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