Composites in aero engine components  

Composites are materials made up from two or more dissimilar components. In practice, almost all are fibre-reinforced. They can be regarded as either strong plastics reinforced by adding even stronger fibres, or as masses of fibres stuck together by adhesive.

Compared with metals, most are softer, and so are less able to withstand erosion (for example when taking off in a sandstorm), and they also cannot at present be used for the hottest parts of aero engines. Within these limitations, they can enable parts to be much lighter for equal rigidity, cheaper to make from fewer parts, structurally more efficient. and also better able to damp out noise.

Stronger and much more rigid composites have been introduced, notably CFRP (carbon fibre) composites, graphite fibre composites, and Kevlar, in which the filaments resemble spider-web material.

All use fine filaments ranging from one to three thousandths of an inch in diameter, which is generally finer than human hair. All far surpass the strongest metals in specific tensile strength (strength divided by density), but they cannot be used alone. They have to be bonded together by an adhesive, of which the most common are various epoxy resins. The fibres are first wetted with the resin and then made into the part by various methods.

Aerospace Prepregs.

One technique is first to form strips or sheets called prepregs. Pieces cut from these are then assembled in a stack and bonded together. Aerospace prepregs are manufactured from continuous fibres which have been infused with a thermosetting resin system, creating a pliable and tacky sheet of material. The precise specification of the fibres, their orientation and the resin matrix can be specified to achieve the optimum lamina performance. Prepregs can be manufactured with unidirectional or woven fibres and the volume of fibres per square metre can also be specified according to requirement.

Alternatively, a single filament, or rather a cable of some hundreds of filaments or a flat tape made from thousands, is then wound all over a mandrel - a former having the shape of the finished part - until the required thickness is built up.

Filament winding is always used for such items as high-pressure containers and rocket engine nozzles, and is increasingly used for drum-type structures such as bypass ducts. For example, the bypass duct of the GE F404, formerly a distinctive ribbed component made by chem-milling titanium, is now made lighter and more cheaply from CFRP composites.

One of the many advantages of such aerospace composites is their record specific strength and stiffness, which, compared with metal, can reduce a part's weight by 20-50 per cent. Another is that the axis of the fibres can be tailored to lie parallel to the direction of the applied load.

In prepregs all the fibres lie parallel, but when laying up the plies to form a finished part the various layers can have the fibres running in different directions. Another advantage is that manufacturing cost can be significantly lower than with metals. The chief limitation on the use of composites has been their inability to operate at very high temperatures, though in recent years several new adhesives, notably polyimides, have extended the upper temperature limit from some 140°C to at least 290°C.

In the 1950s, before CFRP composites were invented, aero engine firms were experimenting with composites based on aluminium. The reinforcement comprised either long filaments or small `whiskers' of Bo (boron), SiC (silicon carbide) or A1203 (alumina).

The Al matrix limits operating temperature to about 400°C, while glasses (which interface extremely well with SiC) can often go higher. Using titanium as the matrix extends the upper limit to about 800°C, with 1,000° in prospect.