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.

I feel the need for some research!

Well, its been a habit of mine to write exactly what I understood. In my previous posts, I've just given some (vague) ideas of how certain things work and nothing more. Blogging for more than a couple of years now, I feel that its time for me to take my writing to the next level. Yeah, from now my posts would have not just simple understandings of various phenomena or gadgets, but also some theory supporting it!

And yeah, as I promised, my next post will be about the composite used in the construction of the Airbus A380.

Landing Gear of the Airbus A380

Before writing this article, I just want to say - "I'M BACK!"

I switched to the National Geographic channel at 10.30 AM and found that the program was to end at 11 AM. The program was about the making of the world's largest airliner- The Airbus A380. At that time of the program, the material used for making its body was being shown. It was actually a composite - Glass and Aluminium together. I'll post a separate article about this. For now I just want to write about what impressed me the most - the landing gear!

The landing gear is one of the most important components of an aircraft. For an aircraft like the A380, it should be strong enough to bear the immense weight ( The Airbus A380 weighs about 276.8 tonnes when empty) as well as absorb the shock off landing. For such a massive component, the people at Airbus came up with a very simple yet effective design for the shock absorber. Its the 'cycle pump' concept. The concept is simple- the shock absorber is a piston that moves inside a cylinder just similar to a cycle pump. As the air inside the cylinder is compressed, the shock is absorbed. In the A380's landing gear, instead of just air, oil is used As greater energy is required to compress the highly viscous oil into a chamber, greater shock is absorbed and the landing is smoother. In the NGC program, the engineer who designed the shock absorber demonstrated its capabilities using ordinary cycle pumps filled with water. It was amazing to see how those pumps absorbed the shock when a big piano was dropped(the array of pumps were fitted under the piano) and kept it intact!

And they even showed a footage of a Korean Airlines' Boeing plane landing safely in a crosswind with ONLY ONE wheel touching down first. That very well described how strong they ought to be! Hail engineering! :-)