Maximum material performance

Three dimensional carbon fibre composite constructions made by ingenious processes including 3D weaving, are being demonstrated in applications from motor vehicle body frames to critical aircraft parts. Many of these build on technologies for 3D weaving that were originally revealed in Eureka’s first edition in December 1980. The technique was eventually used to make lightweight manhole covers capable of bearing the weight of tanker wheels in petrol station forecourts. While woven carbon fibre composites have since become commonplace in aircraft control surfaces and upmarket sports car body panels, it is only now that the technology has reached the point that 3D composites can be used in safety critical structural parts. The car frame technology has been developed by Axon Technologies, which has been spun out of Cranfield University by former associate professor Steve Cousins. “Our objective is to lower the cost of entry into car manufacturing and to reduce lifetime costs,” he says. “Our technology does not just produce a shell with a foam core, but incorporates a series of ‘I’ beams. This allows our structures to be used to make engine mounts, door supports and structural parts to which suspension parts can be attached.” His idea is to place carbon fibre braid around foam cores. Surrounded cores are then laid side by side by a robot in a tool of specific shape and infused with resin. After consolidation and curing, the final structure consists of a beam with linked, internal carbon fibre rectangular tubes with foam cores. Inserts, attachments, and services such as cables, optical fibre, hydraulics and pneumatics can all be included during the laying up phase. The tooling is made from low cost materials using automatic machining, so that time from design to production is reduced to a few days. Axon is as strong as steel and 60% lighter. For applications that require fuel to move a structure such as a car body, the savings in fuel costs average around 20%. Axon can also be used to create curved structures of much greater complexity than other light-weight materials such as aluminium alloys or wood. The company is currently developing space frames for Caterham 7 cars, looking at crash worthiness and repair issues. A modified Caterham 7 with some Axon parts took part in the Shell EcoMarathon at Rockingham recently. Biz-Karts has commissioned Axon to evaluate and design a carbon fibre go-kart and the UK National Composites Network wants Axon to help a piano manufacturer to lighten its instruments. Axon is also developing crash structures and components with Ford, Lola and Red Bull Racing. Up and away The first industry to embrace carbon fibre composites – apart from makers of sports equipment – was aerospace. Woven carbon fibre sheets and folded sheets have become commonplace, though a seminar organised by DTI Global Watch highlighted developments in structures that were woven in 3D. It reported the results of a recent mission to various US companies. 3TEX, based in North Carolina, and Diaphorm Technologies, based in New Hampshire produce 3D woven carbon fibre preforms up to 40mm thick in widths up to 3m. Fibre volumes of 58-65% are achievable. These pre-forms do not yet seem to be in production aircraft, but are mainly at the stage of manufacturing demonstrators. Marcel Buckley from Airbus UK revealed that his company has developed a carbon fibre composite landing gear rib. “It was fashioned on a resized A340 aluminium gear rib,” he said. “The object of the exercise was to prove that such a rib could be manufactured in a single shot from a complex 3D preformed part assembled using 2D and 3D woven fabrics.” The part was made using 3D woven core, 2D plies, 3D-Woven Pi sections and 2D shear web plies and weighed 285kg as opposed to the 335kg aluminium allow part actually used. “Which method is deployed in full scale manufacturing will depend on the ability to balance structural need with the ability to manufacture to fixed quality,” he said. The main application market for 3D composites in the US is armour for fighting vehicles and blast protection. One of the barriers to its wider use in aerospace is a lack of software tools to model its mechanical behaviour under realistic load conditions. Buckley said: “The key need is a robust design and analytical system that takes account of the 3D weave and part complexity, and can be integrated with existing CAD systems.” Professor Andrew Long from the University of Nottingham’s Polymer Composites Group is working on just such a system, with the University of Leuven. “We are developing techniques based on generic textile models to predict both mechanical and processing properties. TexGen models the geometry of textiles at the unit cell level, including 3D textiles. In the UK, this approach is being adopted by the DTI funded 3-D-Coms project, led by Rolls Royce, to predict residual stresses and failure for 3D woven textile composites. Composites in jet engines It is not just car bodies and airframes. Jet engines are increasingly using carbon fibre composites. After the problems caused to Rolls Royce by the original carbon fibre fans for the RB211 engines, these were for a time considered impractical. Nonetheless, GE introduced composite fan blades to commercial aviation on its GE90-77B engine in November 1995. Those blades have performed well, powering Boeing 777s without routine on-wing maintenance or in service issues for more than a decade. The company’s latest GEnx engine uses carbon fibre for its fan blades, fan case and variable-bleed valve ducts at the exit of the booster stage. The engine will power the 450-seat 747-8 challenger to the A380 and the 787. The blades are made with 400 plies of pre-pregged tape with the plies thinning out from the base to the tip. Since sharp-edged composite materials tend to fray, the blades use replaceable titanium cladding for the leading, tip and trailing edges. This edging also spreads the energy of foreign object damage into the composite material. The front fan containment case is developed and made by GKN Aerospace which has produced a long list of other parts that it believes that can be made out of carbon fibre composite, including: front fan frame, fan exit case, acoustic panels, fan spacer, spinner, fan inlet case, bypass ducts, core cowls and “advanced composite high temperature applications”. DTI Global Watch 3TEX Nottingham University Polymer Composites Group Eureka says: 3D composites offer ways to make lighter structures of high stiffness and strength for automotive and aerospace applications – but they seem to have taken a long time to get there Pointers * Axon is as strong as steel and 60% lighter. For applications that require fuel to move a structure such as a car body, savings in fuel costs average around 20% * A 3D carbon fibre composite landing gear rib to meet the specifications of the Airbus A380 weighs 285kg as opposed to the 335kg aluminium alloy part actually used * The main barrier to wider application of 3D composites in automotive and aerospace design is the inadequacy of computer modelling tools, which cannot accurately predict performances