An investigation into engineered thermoplastic polymer composite filament for through thickness reinforcement of laminated carbon fibre composites.

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Laminated composites from dry carbon fibre preforms are increasingly being used to produce primary structures in several industries. However, the poor performance in the out-of-plane (through the thickness) direction, and delamination has been a cause of concern, requiring the careful analysis of load paths to limit out-of plane stress. Furthermore, this has a limiting effect on the design freedom for composite components and could challenge the use of composites for future aerodynamic or structural concepts. Several 'first-generation' methods have been proposed to improve out-of-plane performance including z-pinning, selective interlayers and hybrids, protective layers or resin toughening; one method that is becoming increasingly successful is to reinforce composites with a fibre that connects the layers together running from the upper to lower surface of the laminate. This method shows potential but has been limited by the lack of suitable materials available for through-thickness reinforcement where we have hitherto been limited to carbon fibre, glass, basalt, Aramid or other polymeric fibres. Also, there is a limited understanding of the mechanisms involved in out-of-plane rate-dependent response of composite materials. This proposal aims to develop a new understanding of through-thickness reinforcement and to research a method to produce composites with a through-thickness response which is designer defined. This will be done by placing a 'second-generation' manufactured yarn with optimised properties in the through-thickness direction, thereby enabling the design of optimum Ez (laminate through-thickness Young's modulus) for a given loading scenario. The new yarn will be made by compounding extrusion of a thermoplastic monofilament reinforced with carbon fibres of specified length, optimising material and process parameters, and using these yarns as through-thickness reinforcement in carbon fibre/epoxy laminates. The performance will be characterised and a predictive analytical elastic stiffness model will be developed. Also, the visco-elastic properties of the new through thickness yarn will be related to the transverse impact performance of the laminated composite. These subjects have not been previously researched and if successful, the results could be transformative and generate global impact for UK composites research and industry. In the future the research will benefit aerospace companies; with the proposed enhanced out-of-plane performance they could potentially design a pressurised blended wing, composite lug arrangement, stringer to skin interface and run-out, buckling critical locations, high impact locations, etc.