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Snap judgements: aerospace composites offer many benefits, but a tendency to break without warning limits their uptake. Research projects aim to find out why this happens.

Carbon composites have made a big impact on the aerospace sector in recent years as plane makers hope to gain from the benefits these materials, made from carbon, glass and aramid fibres, offer. The primary advantage comes in weight savings, compared to metal structures of equivalent size, but composites also offer high strength and little susceptibility to corrosion which leads to savings on maintenance and servicing.

However, they are also brittle and can fail with little warning and so a pair of university research centres is collaborating on a programme to develop a new generation of composite materials that will deform without breaking.

The project begins on 1 July and is being led by a collaboration between the University of Bristol's Advanced Composites Centre for Innovation and Science (Accis) and the Composites Centre at Imperial College London, aided by industrial partners. The [pounds sterling]6 million project, funded by the Engineering and Physical Sciences Research Council, is scheduled to last for six years.

The director of Accis, and a professor of aerospace structures, Michael Wisnom says the aim is to get over the inherent brittleness of the current generation of composite materials. "We want to maintain the existing benefits of composites, of high strength and low weight, but tackle this issue of failing catastrophically and without any warning. With metal you usually get some deformation and effectively some warning before it fails. With composites they look fine and then they go," says Wisnom.

The project's aim is to introduce ductility into composites by pursuing a couple of avenues. "The first is by introducing ductile fibres into the composite. That's the holy grail and a very long-term objective," says Wisnom.

The other approach involves changing the architecture of composites in two distinct ways. The first is to examine how to incorporate excess material within the composite so that, like a piece of string being put under tension, it can stretch while absorbing force but doesn't break. The other way is to introduce elements within the material that can move around and slip over each other while still carrying loads.

This is extremely challenging - "That's why this is a long-term project with this budget. It will need significant work," says Wisnom - and a large element will be testing. With some of the specific ideas being pursued subject to commercial confidentiality agreements, Wisnom gives little away about plans for incorporating ductile fibres, although he mentions that carbon nanotubes show promise.

[ILLUSTRATION OMITTED]

But regarding the composites with altered architecture, Wisnom says these will be put through a comprehensive series of tests, run in tandem with developing computer modelling so that the latter can be validated by the former.

To a degree, this is going back to basics. "Composites have been around for years but there's still no full understanding of the key behaviours. People have got round this through a process of very thoroughly testing each component individually," says Wisnom.

Among the early work will be precise tension tests, applied to, for example, a carbon fibre tow, which is a bundle of about 10,000 fibres. By embedding the tow in transparent glass-epoxy it can have force applied to it and the effects can be visually tracked while the surrounding block helps carry the load.

Other work will include notched tests, to establish the detailed effect of putting a hole in a composite structure, and, much further down the line, testing centred on electro magnetic fields (EMF). "There are all sorts of issues for EMF interference and screening. There's a large amount of work been done on this but it's true to say there are still lots of things not well understood," says Wisnom.

RELATED ARTICLE: TWI works on inspection

Another project looking at composites has been launched recently, concentrating on speeding up production techniques and, specifically, the inspection part of the process. The four-year project, led by TWI Wales, part of technology developer TWI, aims to make inspection times four times shorter than at present, through use of non-destructive testing techniques.

The IntACoM (Improving the Inspectibility of Aerospace Composite Materials) project will see collaboration between industrial partners such as Rolls-Royce, GKN Aerospace and Bombardier Aerospace alongside academia, in the form of Swansea University, Swansea Metropolitan University and the University of Wales.

TWI Wales regional manager Philip Wallace says inspection techniques are not sufficiently developed to meet industry needs and take so long they are hampering the manufacturing process.

Wallace outlines IntACoM's targets: "Our aim is to speed up the process by 400% without losing reliability or sensitivity. Once we have developed the inspection technology at the manufacturing level we will further develop testing and inspection capabilities for in-service maintenance and repair."

The [pounds sterling]3.8 million project was launched in February and is funded by the Welsh Assembly Government.
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Title Annotation:Aerospace and Defence
Author:Wilks, Neil
Publication:Environmental Engineering
Geographic Code:1USA
Date:May 1, 2011
Words:798
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