5-minute knowledge: Composite – whats so special? Why is it perfect for optimization?

Everybody knows whats composite: the race car, an airplane, ordinary goods which should look expensive, all of them use composite materials – sometimes because of the technical advantages, sometimes only out of design purposes. A composite basically is a new „material“ made of at least two different materials – a mixture. Composites normally have one material that provides strength (fibers) and one material that bonds everything/the fibers together (matrix). The result is one that provides new features, compared to the single materials alone. The composite will be a bond of two or more materials with new features when seen together but all materials will keep their characteristic when observed alone. Composites are used to increase stiffness, lower the weight, decrease density etc. Thats also mostly the purposes of using optimization in the design process: to increase the stiffness, lower the weight etc.

Using composites has got advantages and disadvantages. Main advantage is: there are a lot more variables to tweek the final features of the material, according to the purpose of use that is to be achieved. Disadvantages are mostly inherent in the combination of different materials. Due to high stiffness changes between the different materials there might occure failure between them. And these need to be detected – which is not so simple.

The common composite is prepared out of a fiber material (e.g. glass, carbon, aramid or also natural materials) and a „matrix“ material that bonds these together – mostly plastic.

There are composites that can be used within injection molding, they contain short fiber reinforced plastics. Typically the fibers used have a length between 0.01 to 5mm. The volume of fibers in the whole composite is around 5-30%. The main advantage is the increased stiffness of the composite in comparison to the pure plastic. The fibers used are mostly glass fiber. The typical density of these is between 1.8-2.7 t/m³. These composites are considered to be istotropic. A disadvantage of short fiber composites is, that they tend to orient themselves in direction of the flow during the moulding process – what could lead to non isotropic behaviour …. Matrix materials used are often thermoplastics, which are heated to be able to bond with the fibers. Thermoplastics will get liquid when heated. Thermoset plastics will get stiff by a chemical reaction and a heating process, while thermoplastic will get their stiffness by cooling down.

The most of the other compsites contain so called long fibers/endless fibers. Typically in form of sheet materials. There are woven or unidirectional fabrics/fibers, some are stiched together. For high performance purposes carbon or aramid fibers are used. Thermoset materials are used for the matrix. To increase stiffness and decrease matrix percentage they are heated within an oven (conclave) where they are also often pressed to increase bondage of the material. The lower the matrix volume fraction, the better the stiffness and lower the density, as carbon is lighter compared to the matrix.

Within industries the aim to use carbon fiber reinforced plastics (cfrp) is different.

Car manufactures use it for higher priced cars to increase performance. As manufacturing of composite parts is more complex and time consuming, they are normally not used in normal consumer cars. The cost/weight ratio from composite compared to metal is higher. Typically composites are used to be „shown“ beside their technical use – like in an engine bonnet. Or for their technical advantage of having a high stiffness/weight ratio for highly loaded parts. In general it is a replacement for steel.

Airplane oems use cfrp to replace aluminium. Loads within airplanes are lower compared to the size of the structure in comparison to cars. Mostly plate materials are used to be replaced by cfrp.

Problems within manufacturing are e.g. the high tolerance in thickness compared to steel or aluminium. Composites for themselves also do not conduct current – what is necessary to be used as an outer shell in an airplane due to risk of lightning strike. And finally: in case of a damage it is very difficult to find delamination spots (fibers seperate from the matrix inside of the composite), repairability is much worse compared to metal structures.

To come to the final question: why are they perfect to be used in optimization? There are so many variables within the material (which type of fiber, which direction of the fibers, even the layups can differ in number, direction and thickness etc). All this is difficult wihout the help of a computer to find the perfect material in each location of a part. Composite materials can be different at every location within themselves, this is the main advantage of composites – and the reason to use optimization to find this optimum.