Novel thermoplastic yarn for the through-thickness reinforcement of fibre-reinforced polymer composites

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 polyetheretherketone. 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 thermoplastic yarns. As a result, it is concluded the thermoplastic fibres created could potentially be better suited for sewing applications compared to commercial fibres such as Kevlar and further work is planned to sew the yarns and test the delamination performance.