2. Composites

In this chapter a technical and historical introduction on composites is presented. An overview of the most common production processes is given. At the end an integrated design process, typical for composites, is discussed. At the end the relevance of composites for industrial and socio-economic development in developing countries is discussed.

Composite composition


2.1 Composites in general

Composites are hybrid materials made of a polymer resin reinforced by fibres, combining the high mechanical and physical performance of the fibres and the appearance, bonding and physical properties of polymers, see figure 2.1. The short and discontinuous fibre composites are responsible for the biggest share of successful applications, whether measured by number of parts or quantity of material used. Less visible however, but growing enormously since the last decade, are the applications of continuous fibre reinforced polymers. Instead of mass-manufactured polymer-based products, the domain for continuous fibre reinforced composite parts is in general with advanced capital-intensive materials and products. By changing the direction of the fibres in the resin, the material properties can be tailored to the external loads. To optimise the construction multiple adjusted layers (laminae) can be used to form a laminate, see figure 2.2. Typical fibres are glass, carbon, aramid and natural fibres. Epoxy, polyester and polypropylene form common resins.

Tailored composite

By this joining, the poor capabilities and drawbacks of the individual components disappear. For instance, composites combine a high stiffness and strength with a low weight and their corrosion resistance is often excellent. Composites have worked their way up amongst wood and metal due to their outstanding price performance ratio during a lifetime. A powerful approach in improving this ratio is to minimise the steps required from raw material to end product. Composites have the capability of materialising into a structure in a single step, see figure 2.3. Additional economic benefits are the inexpensive raw materials (e.g. when using reinforcements as glass fibre or natural fibre) and the little to none required maintenance during service. Composites are now a part of everyday life, and have entered nearly all major industrial sectors, including aerospace, ground transport, packaging, sports industry and civil engineering. Most current applications are modern; however, some are in fact quite ancient.

Single step manufacturing


2.2 Historical overview

Past

After making and controlling fire and inventing the wheel, spinning of continuous yarns is probably the most important development of mankind, enabling him to survive outside the tropical climate zones and spread across the surface of the Earth. Flexible fabrics made of locally grown and spun fibres as cotton, flax and jute were a big step forward compared to animal skins. More and more natural resources were used, soon resulting in the first composites; straw reinforced walls, and bows (figure 2.4) and chariots made of glued layers of wood, bone and horn. More durable materials as wood and metal soon replaced these antique composites.

Composite Korean bow

Present

Originating from early agricultural societies and being almost forgotten after centuries, a true revival started of using lightweight composite structures for many technical solutions during the second half of the 20th century. After being solely used for their electromagnetic properties (insulators and radar-domes), using composites to improve the structural performance of spacecraft and military aircraft became popular in the last two decades of the previous century. First at any costs, with development of improved materials with increasing costs, nowadays cost reduction during manufacturing and operation are the main technology drivers. Latest development is the use of composites to protect man against fire and impact (figure 2.5) and a tendency to a more environmental friendly design, leading to the reintroduction of natural fibres in the composite technology, see figure 2.6. Increasingly nowadays, the success of composites in applications, by volume and by numbers, can be ranked by accessibility and reproducibility of the applied manufacturing techniques, see section 2.3.

Left: Lightweight composite military helmet - Right: Interior parts of the Mercedes A-200

Future

In future, composites will be manufactured even more according to an integrated design process resulting in the optimum construction according to parameters such as shape, mass, strength, stiffness, durability, costs, etc. Newly developed design tools must be able to instantaneously show customers the influence of a design change on each one of these parameters.

2.3 Manufacturing processes

In this paragraph the most commonly used manufacturing processes are introduced. Although many variants on these techniques exist, this overview gives a good indication of production possibilities.

Left: Hand laminating - Right: Boat hull

Hand laminating

The fibres, usually mats, are cut and placed in a mould, see figure 2.7. The resin is applied by rollers. One option is to cure while using a vacuum bag, then it's called vacuum bagging. By applying vacuum, excess air is removed and the atmospheric pressure exerts pressure to compact the composite. A possible product is the boat hull shown in figure 2.8. The advantages are the high flexibility and the simplicity of the process and the cheap tooling. The long production time, the labour intensive character and poor possibilities for automation are considered to be disadvantages.

Resin injection techniques



Left: RTM principle - Right: Vacuum injection of a boat hull

The fibre mats are placed inside a mould. In case of Resin Transfer Moulding (RTM), this mould consists of two solid parts, see figure 2.9, whereas with vacuum injection a single solid mould and a foil are used, see figure 2.10. A tube connects the mould with a supply of liquid resin, which is pumped or transferred through the mould, impregnating the fibres. After curing the mould is opened and the product is removed. The big advantage is the capability of rapid manufacture of large, complex, high-performance structures, such as the F22 frames presented in figure 2.11.

Composite structural parts of F-22 Raptor

Hot press methods

Composite pre-forms (already mixed resin and fibres) are inserted in a mould and are cured using pressure and heat. Various methods exist. With injection moulding, resin granules and short fibres are mixed and transported to the mould by a spindle, see figure 2.12. In this way very complex products, like housings of telephones, TV's, cameras and keyboard parts can be made. With sheet moulding reinforced mats are placed in a press, see figure 2.13. Various other hot press methods exist.

Injection moulding of a telephone cover
Left: Sheet moulding of an Alfa Romeo car hood - Right (top) Filament winding - Right (bottom) Composite LPG container vessel

Filament winding

Filament winding is a process in which continuous fibres or tows are wound over a rotating mandrel, see figure 2.14. They can be resin-impregnated before, during or after placement. The advantages of filament winding are the repetitiveness of an accurate fibre placement, the use of continuous fibres resulting in high strength and the possibility to construct rather large structures. The disadvantages are the requirement of a removable mandrel (not with all applications) and the hard to define outer surface. A typical product is a LPG container, see figure 2.15.

Pultrusion

Pultrusion is a continuous process to manufacture composite profiles at any length. The impregnated fibres are pulled through a hole (the heated mandrel), which is shaped according to the desired cross-section of the product, see figure 2.16. The resulting profile is shaped until the resin is dry. The advantages are the manufacturing of thin-wall shapes of "endless" length (figure 2.17), large variety in cross-sectional shape and the possibility for a high degree of automation. Disadvantage is the restriction to one cross-section, shape variation in transverse direction is not possible.

Left: Pultrusion principle - Right: Pultruded profiles

2.4 Design philosophy

As mentioned, the big advantages of composite products are that they can be tailored according to the external loads and that single step manufacturing is possible. To fully utilise these two properties, product design is an integrated process involving concepts, materials and manufacturing processes; the trinity essence, see figure 2.18. Due to integrated design, the product can be optimised in terms of costs, manufacturing time, weight, etc. according to market demand.

Trinity essence

For instance, using this integrated design philosophy, a composite chassis-less trailer (figure 2.19) is manufactured with a 30 % weight reduction compared to a conventional trailer provided with a steel chassis. Due to the lower weight and an aerodynamic shape, fuel consumption is significantly reduced, has a positive effect on costs and the environment. An example of cost-reduction can be found in composite bridges. Although manufacturing of such a bridge is more expensive than of a traditional bridge out of steel and concrete, the lower weight saves a lot of money on the costs of the foundation and on transport costs. In addition, the higher resistance against corrosion requires less maintenance whereas in general the lifetime is much longer. All benefits mentioned lead to a significant long term cost reduction. Manufacturing of pressure vessels with filament winding, see figure 2.15, is a very rapid process. Significant reduction in production time is achieved compared to the welding of a similar metal pressure vessel. Secondly, the weight of the wound vessel is significantly reduced compared to a steel one.

Composite trailer

2.5 Relevance for developing countries

In this chapter an introduction was given on composites and various products were mentioned. Highly advanced products in the military and transport areas were discussed. The question rises, what aircraft parts and lightweight trailers can contribute in developing a country? The purpose of this chapter is not in the first place to show how advanced the products and manufacturing processes are at present, but more to show the capabilities of the material and the wide variety of production processes resulting in a large range of applications. By choosing from the wide range of available resins, fibres, fibre placement and production processes a very specified product can be manufactured. All applications shown in this chapter fulfil a need, a need existing in the modern western society. By designing according to the "trinity essence", composite products can be tailor made to fulfil a specific need.

In developing countries, different needs exist, often of a more basic character. Applications that come to mind are bridges, construction materials for the building industry and water tanks. For these products less sophisticated resins and fibres are required than for aeroplanes and spaceships and domestically grown natural fibres (see chapter 3) can be applied, significantly reducing cost. In addition, the accuracy of the manufacturing processes can be lower as well, allowing a simplification in machine lay-out and operation. So, by designing according to the "trinity essence", products can be manufactured (using less sophisticated and cheaper materials and processes) that fulfil a specific need, hence contributing to the development of a country.