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

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.

This project will potentially benefit many users of composite materials as structural load carrying elements, or as blast-protection products. In the future, the research will directly benefit UK aerospace companies; with the proposed enhanced out-of-plane performance they could potentially design a composite lug arrangement, stringer to skin interface and run-out, buckling critical locations, high impact locations, etc. The enhanced properties mean that the knock-down factors applied to the design such as barley viable impact damage (BVID) will be reduced. Also, initial research has shown that notch sensitivity and stress concentration caused by open-holes is reduced. Currently, the overall structural weight of an aircraft is increased by over 10% to allow for these factors. By developing new materials and understanding, the designers will be able to reduce this conservatism, reduce weight of the airframe and therefore, fuel burn. This will have a greater environmental impact with reduction in greenhouse gas. Furthermore, new ultra-efficient aircraft designs such as the Boeing X-48C Blended Wing Body Research Aircraft (Environmentally Responsible Aviation project) might not be feasible with existing composite laminates due to the large out-of-plane stresses the composite pressurised blended wing would experience. This research therefore, is vital for the UK at this time. These new aircraft will reduce community noise, fuel burn and nitrogen oxides (NOx) emissions. These aircraft might enter service by 2025.
The UK is currently well positioned to benefit economically from these new technologies through the supply chain. A UKTI (UK Trade & Investment) and BIS market report completed in April 2010 and published in 2011 entitled 'UK Composites Supply Chain Scoping Study - Key Findings' estimated that there were 1,500 companies in the UK composites sector producing added value of around £1.1 billion. The UK aerospace industry represents 17% of global market and is the 2nd largest globally. The first generation laminated composite primary structures are now flying, so there is an urgent need to develop new performance and a move away from the current 2D design strategies. This could be realised in the medium term during a re-engineering of the existing aircraft in search of fuel efficiency (5 years) or on the next generation aircraft (10 years). This project however will aid development of composites structure capability in sectors other than aerospace. With the proposed new through thickness yarn and preforming methods, the automotive industry could increase deposition rates of composite materials, therefore reducing manufacturing costs. Through the proposed Pathways to impact, the information generated in this project will also be disseminated to the marine industry through an existing KTP, the renewables sector through a Marie Curie programme, and could influence existing DSTL research undertaken at University of Ulster.