Tuesday, June 9, 2009

The power of Composites

Can a sheet of aluminium withstand an impact of a 2kg bird at 700 kmph? No! It can't. Can the same impact be withstood by a sheet of fibre glass? I'm not sure or may be not. But a sheet of Aluminium reinforced with glass fibres? Yeah, it can. And thats the material used for making the body of the Airbus A380.

Lets see how it works. There are two essential properties that most engineering materials are expected to possess. One is that it should not deform considerably and the other is that the material should not fail or fracture. Now, for the body of the aircraft, a strong material that isn't heavy is required. Naturally we go for aluminium based alloys for this purpose. But is that strong enough to survive a a bird's impact at a relative velocity of about 700 kmph? Obviously not! So, there is need for reinforcement of the alloy with some stronger material. Surprisingly, the engineers have chosen glass! Glass is a highly brittle material and the possible failure of this material is through a brittle fracture. The mechanism of brittle fracture is given by Griffith's theory.

According to this theory, there are microscopic flaws(cracks) which propagate to form bigger cracks on the application of excessive stress. A part of the energy from the load is used to create the new surfaces as the new cracks are formed. When these cracks propagate to the surface, fracture occurs and the material fails. This is the mechanism by which glass fractures. When glass is heated and drawn into a fibre, these microscopic cracks are eliminated. Thus, when glass is in the form of fibres, fracture is prevented to a greater extent. These glass fibres are used to reinforce the Aluminum alloys and this composite is used to build the body of the A380.

A typical fiberglass sheet.