Optimising fibre alignment in twisted yarns for natural fibre composites

Natural plant fibres are short fibres that must be spun into continuous length yarns for the production of structured composites. Fibres in a twisted singles yarn are poorly aligned. The fibre alignment can be improved without sacrificing the yarn strength by forming a two-ply yarn from two singles yarns. In this paper, we analysed the differential geometry of fibre trajectory using an idealised twisted yarn model and derived the optimum two-ply yarn structure that gives the maximum Krenchel fibre orientation factor. In the optimum two-ply yarn, the ply twist is in the opposite direction to the singles twist and the ply-to-singles twist ratio is 0.28. Such a two-ply yarn construction is beneficial for all twisted yarns aimed for structural composites applications, particularly for yarns made from low cost natural fibres which are usually of short length, low strength and poor uniformity and thus require high twist to achieve sufficient strength for yarn manufacture and further handlings in composite fabrication.