Composites are produced when two different materials are mixed together. This means the properties from both the materials can be enhanced. In the case of pedalos by adding fibres/strands of glass to polyester resins you create GRP which is a very tough, rigid and lightweight material.
Types of Composite Material
There are three types of composite material:
- fibre-reinforced composites
- particle based composites
- sheet based composites
The most commonly used composite is fibre-reinforced composite. GRP is a fibre-reinforced composite.
Sheet or moulded materials are called 'man-made boards'. This is when wood (fibres) are bonded together to create a new material.
FRC's have good strength to weight ratio. This means they can be very lightweight and have a low density but remain strong compared to their weight. They are resistant to corrosion, meaning they do not rust. They have good fatigue resistance. This means it has a good ability to repeatedly hold different weights/loads. They have low thermal expansion. This means the volume does not change in response to a change in temperature.
Kevlar is a mixture of aromatic and aramid molecules which are melten and spun into fibres. The long chain molecules are help together with strong hydrogen bonds. The chains are crossed over making the material high in stregth. Kevlar is five times stronger than steel and half the density of fibreglass. Kevlar fibres are woven into a cloth. This cloth can be made into protective equipment. It protects the wearer by acting as a net. Imagine a football net, when the ball is kicked into the net it absorbs the force, this is how the kevlar cloth works. Although the wearer is still affected it prevents serious injury. It is used as body protection, sports equipment, sails for windsurfing and gloves. Kevlar has a igh strength to weight ration, low electrical conductivity, high toughness and chemical resistance.
Concrete is a particle based composite. Concretes are very good when pushed together but very poor when pulled apart. In order to get rid of the failure uner tension (being pulled apart) reinforcing bars can be put it the concrete shuttering before the concrete is cast. The reinforcing bars are surrounded by the concrete. This design has allowed for longer spans in buildings and bridges although the concrete cuold still crack in places where the concrete is under tension therefore the reinforcing bar is put under tension before and during the casting of the concrete. Once it has been set the tension is released effectively putting the concrete uner compression (pushed together). This means the concrete can hold heavier loads or bridge longer spans. The span is the distance from the starting point to the ending point of the bridge.
Particle Based Composite Materials
Particle based composites all have similar properties.. they have high strength in compression and less so in tenstion (for meanings see previous card), they are very stable, they have consistent strength and they have no surface defects.
Concretes are made of materials called aggregates, as well as sand and cement. The properties of the material depend upon the ratio of the materials it consists of. It is thoroughly mixed when dry, water is added and mixing is continued until every particle is coated in cement paste. The cement psate is used as a bonding agent. When it is cast it is left to harden, the temperature will then rise because of the chemical reactions taking place.
Cermets are another group of particle based composite. They are mixtures of metal and ceramic particles. Because of the materials cermets consist of they cannot be processed in the same way as materials that melt at a lower temperature. Sintering is one of the few processes suitable.
Sheet Based Composites
Metal/polymer composites are a recent development. An example of one of these materials is ALU composite, it is a laminate of 0.3mm aluminium sheet with polyethylene inbetween it. The overall thickness is about 3mm, creating a lightweight material that is rigid and has good impact resistance.
Advantages of these kinds of materials include that is lessens sounds and vibrations, it is also a good thermal insulator and strong. It is used in the car industry as it reduces road noise.
So sheet based composites are simply a sandwich of different materials, in this example aluminium is the bread and polyethylene is the filling.