JOURNAL OF COMPOSITE MATERIALS Article Fibre architecture modification to improve the tensile properties of flax-reinforced composites Rishad Rayyaan , William Richard Kennon, Prasad Potluri and Mahmudul Akonda Abstract As far as the tensile properties of natural fibres as reinforcements for composites are concerned, flax fibres will stay at the top-end. However, an efficient conversion of fibre properties into their corresponding composite properties has been a challenge, due to the fibre damages through the conventional textile methods utilised to process flax. These techniques impart disadvantageous features onto fibres at both micro- and meso-scale level, which in turn degrade the mechanical performances of flax fibre-reinforced composites (FFRC). Undulation of fibre is one of those detrimental features, which occurs during traditional fibre extraction from plant and fabric manufacturing routes. The undulation or waviness causes micro-compressive defects or ‘kink-bands’ in elementary flax fibres, which significantly undermines the performances of FFRC. Manufacturing flax fabric with minimal undulation could diminish the micro-compressive defects up to a substantial extent. In this research, nonwoven flax tapes of highly aligned flax fibres, blended with a small proportion of polylactic acid have been manufactured deploying a novel technique. Composites reinforced from those nonwoven tapes have been compared with composites reinforced with woven Hopsack fabrics and warp knitted uni- directional fabrics from flax, comprising undulating fibres. The composites reinforced with the highly aligned tapes have shown 33% higher fibre-bundle strength, and 57% higher fibre-bundle stiffness in comparison with the composites reinforced with Hopsack fabric. The results have been discussed in the light of fibre undulation, elementary fibre individualisation, homogeneity of fibre distribution, extent of resin rich areas and impregnation of the fibre lumens. Keywords Waviness, flax fibre composites, longitudinal tensile strength, elementary fibre individualisation, lumen impregnation, knee-point in the stress–stain curve Introduction Among the natural fibres that are currently being used as composites’ reinforcements, flax falls into the top- end of the list, in terms of tensile strength and modu- lus. 1 However, composites reinforced with mineral fibres such as glass or carbon inevitably endow longi- tudinal tensile and compressive properties beyond the capability of composites reinforced with natural fibres, such as flax. In an effort to reduce this gap, several researchers have succeeded in improving the mechan- ical performance of flax fibre-reinforced composites (FFRC) by modifying the flax fibre morphology through chemical treatment on fibres. 2–6 Chemical treatments, however, involve additional manufacturing steps, which increase manufacturing costs. As a substitute method, mechanical performance of flax fibres inside composites can be improved by modifying the fibre geometry such as: by the reduction of fibre waviness. The geometry of the fibres in composites dir- ectly influences the mechanical performance of the composites. 7–18 The research discussed in this paper is focused on how to reduce fibre waviness for improve- ment of mechanical performances of FFRC. In pursuit to that, nonwoven flax tapes blended with a small School of Materials, The University of Manchester, UK Corresponding author: Rishad Rayyaan, The University of Manchester, Oxford Road, Manchester, Lancashire M13 9PL, UK. Email: rishad.rayyaan@manchester.ac.uk Journal of Composite Materials 0(0) 1–17 ! The Author(s) 2019 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0021998319863156 journals.sagepub.com/home/jcm