Designing composites into industry

In the first of two articles, Engineering Materials talks to the UK's leading advocates of composite materials discussing the opportunites for innovation and find out why engineers should not be daunted. Justin Cunningham reports.

Composites are materials that everyone has heard about, but surprisingly few really work with and even fewer understand. Unless you work in aerospace or have a performance driver that outweighs cost, then you might think composite materials are too exotic for your applications.

Yet globally this is a rapidly-growing market with material development and its manufacture moving quickly. Applications are increasingly being found in numerous mainstream industries and as a result there is fierce international competition to get a foothold in this high added value activity.

The rhetoric from the likes of Vince Cable and others in government is that they are serious about composites and see it as a key technology to create growth and a marker for innovation on the world's industrial stage. As a result it has announced investment in a number of initiatives to support its further roll out and proliferation.

Dr Sue Halliwell, secretary of Composites UK, says: "We need to do more to innovate and turn our ideas into products and jobs so that we don't miss out on any opportunities to create economic growth through manufacturing.
Investing in the National Composites Centre and the High Value Manufacturing Technology and Innovation Centre, the Government believes the UK can build a competitive advantage, increase our market share of existing sectors and ensure the use of composites in new industries.

"The choice of which [fibre and resin system you use] will depend on the application. The UK is exporting both prepreg [pre-impregnated] and resins to the European wind industry and has domestic resin, fabric and fibre supply."

About 70-80% of Fibre Reinforced Polymers used fulfil the same need; removing weight from a structure while maintaining or improving, strength and stiffness. Wind turbine blades can be made lighter and longer to generate more power; a car chassis can be lighter to improve fuel efficiency; and similarly weight saving allows aircraft to fly further.

However, there are hundreds of different fibres and grades of fibre to choose from, as well as weave patterns and resin systems. There are also many different preparations and manufacturing methods, different cure cycles and numerous other considerations, all of which affect the material properties and yield different results. While selecting the most appropriate combination is a daunting task, it does allow the material to be tailored for specific applications.

As well as mechanical properties the aesthetics of FRP, particularly carbon fibre weave, are attracting significant interest. The material has come to represent performance and as a result, some production car parts, and many aftermarket parts are being made from it because of its distinctive appearence.

Peter Chivers, chief executive of the National Composites Centre, says: "There is an element of design and market appeal here. Carbon fibre door mirrors or the knob on the gear stick don't really add any performance, but they look good. And this is an opportunity to add value to a product."

As well as the difficulty of material and resin system selection, historically it has been cost that has been the biggest obstacle to the proliferation of composite materials. This, combined with the limited number of people with the knowledge to exploit the materials' value through clever design and manufacture, means it is still largely the performance-driven engineering markets that are exploiting its potential.

There is, however, an increasing degree of 'trickle down'. Though in most cases it is still at the premium and high end, more composites are being used on cars, sports equipment and the popular, and lucrative sector, of racing bicycle frames.

Extracting the value from composites is a very specific skill. Their production lends itself to making single large components, in unconventional shapes, with strength and stiffness in the requisite areas and direction. The aerospace industry exploits this versatility by making very large, single piece fuselage sections, as do wind turbine manufacturers making large, single-piece blades.

"You have lots of components on a typical car body and on a part-for-part basis it is much more expensive," says Chivers. "But if you could make the car body all from one piece, then suddenly the cost of what you're comparing with isn't the cost of the individual parts, it's all the parts, the assembly and all the overheads and indirect costs that go with that. You are much further down the value chain and have a lot more opportunity for price comparison. We are doing lots of work on trying to increase the value of the deliverable."

In addition, the materials have come down in cost with more manufacturing processes being developed. This allows increasing opportunity for innovation stemming from design.

Engineers and applications that have used composites over a period of time have evolved products and structural architecture to exploit the material to its fullest.

"With bike frames, originally composite tubes were replacing the metallic tubes," says Chivers. "But these often had metallic joint pieces. So they started making composite joint pieces which looked like metallic frames and worked in the same way. All the loads were in the corners and you don't want to put your joints where all the loads are. That is not good design."

The frames evolved and now resemble 'flowing' structures, with the material draped around the corners, directing strength where necessary. The shapes are now much less conformal, often quite exotic, with a big shift toward aerodynamics because of the ability to make these shapes. "It is just opening your eyes to what the technology can do for you," says Chivers.

FRP materials are by no means the answer to every problem, but there are increasing areas where they might be able to offer a competitive solution. The materials offer the opportunity for tailored properties for a specific applications and much more exotic structural shapes.

The aim is that they become another option in the design engineers' toolkit. Designers need to understand the merits and limitations of FRP materials and where they fit into the design spectrum.

Composites defined:
'Composites' is an all encompassing term that can include anything from complex metal matrices to mud and straw bricks.

The term is perhaps most commonly used to describe fibre reinforced polymer (FRP) composites. Fibres vary enormously and can include anything from carbon, glass and Kevlar to naturally-derived fibres such as flax or hemp. The woven fibres can be placed and overlapped in specific directions (called lay-up) to tailor strength and stiffness in a structure.

The fibres are then impregnated with a reinforcing plastic or resin system which is cured to produce a very strong, yet comparatively light material.

•See the next issue of Engineering Materials for part two of this article, which highlights specific design advice when using composites in your products.

Justin Cunningham

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