Article Novel thermoplastic yarn for the through-thickness reinforcement of fibre- reinforced polymer composites Thomas Dooher 1 , Cormac McGarrigle 1 , Dorian Dixon 2 , Alistair McIlhagger 1 , Eileen Harkin-Jones 1 and Edward Archer 1 Abstract Sewing has attracted a great deal of attention as a method to improve the delamination performance of carbon fibre laminate composites. A critical factor in the commercial exploitation of the technology is the development of a suitable sewing yarn, with other researchers looking at a variety of commercial fibres such as Kevlar. It would appear from the literature that fundamental research into what properties a suitable yarn should have has not been carried out. In this work, a commercial fibre designed for sewing applications was sourced from Toho Tenax (Germany) and used as a control. Unlike in published works, rather than relying on yarns which could be purchased commercially, selection criteria were drawn up and promising polymers identified and then extruded as a yarn. Based on the selection criteria, thermoplastic yarns were extruded using polysulfone, polyethersulfone, polyphenylsulfone and poly- etheretherketone. It was found that despite the fact that the commercial fibre had much better mechanical properties as a straight fibre, when it was knotted or looped (to try and simulate the effects of sewing), there was a dramatic decrease in the mechanical properties (the ultimate tensile strength dropped by 88% due to a single knot). There was no significant change in the mechanical properties of the thermo- plastic yarns. As a result, it is concluded the thermoplastic fibres created could potentially be better suited for sewing applications compared to commercial fibres such 1 Jordanstown Campus, University of Ulster, Northern Ireland, UK 2 Nanotechnology and Integrated Bioengineering Centre, University of Ulster, Northern Ireland, UK Corresponding author: Edward Archer, University of Ulster, Newtownabbey BT37 0QB, UK. Email: e.archer@ulster.ac.uk Journal of Thermoplastic Composite Materials 1–15 ª The Author(s) 2017 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0892705717743290 journals.sagepub.com/home/jtc