Since composites consist mostly of fibres stuck together with epoxy resin, it is quite straightforward to glue components together with epoxy adhe-sives. Most glass-fibre gliders and light aircraft are almost entirely glued together, bolts being used only where particularly heavy loads are applied or at disassembly points. Strong glued joints can only be achieved by the correct selection of the adhesive and very careful preparation of the sur-faces to be joined.
Aircraft Structures165
Fig. 10.2 Composite wing spar construction (one of several possible methods).
glueing operations by forming several of the components at the same time, this is called co-curing. For example, a wing skin can be laid uncured into its mould and then the ribs and spar webs, also uncured, are added, with tooling to support and position them. The whole lot is then cured together in an autoclave. This leaves only the closing skin to be attached by other methods. The tooling needed for co-curing large aircraft parts is extremely complex and very expensive.
It is quite possible to use mechanical fasteners, such as bolts and rivets, to join composite components together or to attach them to metal com-ponents. Specially designed rivets and high-strength fasteners are avail-able for use in composites. Using rivets designed for metal may damage the composite. Ordinary bolts may be used and often some local rein-forcing feature may be added, such as a sleeve bushing in a hole or a glued-on metal face plate. Galvanic corrosiglued-on of the bolts or rivets must be prevented when carbon fibre is present (see Section 8.1). The main pre-caution is to prevent the ingress of water by applying jointing compounds or adhesive and ‘wet assembling’ the joint. Specially coated or plated fas-teners are available and it is common to use titanium rather than steel bolts.
Drilling holes in the composite for fasteners will greatly reduce its strength. To get around this it is necessary to reinforce it. Where a fitting is bolted to the composite structure it is often adequate to just glue metal facing plates either side of the composite and then bolt through the whole lot. Another way is to build up the thickness of the composite around the joint by adding more layers and placing them with the fibres lying in the direction that gives the best joint strength.
Bolting things to a foam or honeycomb sandwich structure requires something to stop the lightweight core being crushed when the bolts are tightened. A reinforcing block of dense foam, plastic or even wood can be imbedded in the core during manufacture. Alternatively, the cells of a hon-eycomb could be filled with a strong adhesive filler paste. Metal inserts can be bought or made, then glued into large holes cut into one side of the sandwich.
The face skins of a sandwich panel are usually very thin and need to be reinforced to carry the bearing loads around a bolt. The metal inserts often used have large diameter heads or flanges that spread the load over a larger area of skin. Large-diameter washers or metal face plates can be glued on to serve the same purpose. The idea is to spread the load over the face skin, rather than take it all on the thin edge of the hole.
A large number of specialist items are available for making attachments to composite. Adhesively bonded screw studs, anchor nuts and cable tie bases can be used to deal with many attachment problems without having to drill holes in the composite.
10.5 Fibres
10.5.1 Glass fibre
There are two main types of glass fibre: E-glass, which is the most common type, and S-glass, which is available mainly in the USA. Both types are similar but S-glass is slightly stronger. Woven glass cloths and tapes are available in a wide variety of styles and thicknesses. Because the bare glass fibres are very delicate, they are coated with a substance called ‘size’ to protect them during manufacture and weaving. There are several differ-ent types of size and epoxy resin does not stick properly to all of them. A glass cloth with a finish that is suitable for use with the chosen resin system must be used. Silane is the most popular for use with epoxy resin but there are others and some ‘universal’ finishes; a check should be made with the technical department of the supplier before using them.
10.5.2 DuPont’s ‘Kevlar’ aramid fibre
Aramid fibres look like thin yellow string and are very soft to the touch.
DuPont’s ‘Kevlar’ is the best known but there is also ‘Twaron’ by Akzo.
It is very strong in tension and very light. Stiffness is much better than glass fibre but not as good as carbon. A good range of cloths is available from many different suppliers. It is very easy to wet-out with resin and bonds well with epoxy to give a tough, lightweight composite.
Aramids are excellent for making stiff, lightweight shell structures but there are a few drawbacks: the material is weak in compression and is therefore not very good for carrying bending loads or for use in com-pression struts; it does not cut very easily, so special drills, saws and shears are needed for both the dry cloth and finished composite; the fibres tend to go fluffy around cut edges and holes and sanding down the surface can be a disaster. The material is also prone to soaking up moisture through exposed fibre ends (called wicking), which can damage the structure and therefore the components must be well sealed with paint.
One big advantage of Kevlar is that laminates made from it are very resistant to impact damage. Kevlar is sometimes used in ‘hybrid’ posites, mixed with carbon to get the strength and stiffness of carbon com-bined with the toughness and lightness of Kevlar.
10.5.3 Carbon fibre (or graphite fibre)
Carbon fibres are very strong, stiff and light. There are two main varieties, called ‘high-modulus’ and ‘high-strength’, caused by variation of the man-ufacturing technique. Data on the performance of specific types of fibre is normally readily available from the manufacturers and suppliers.
The actual strength of carbon fibre is not much greater than that of glass fibre but it is lighter and three times as stiff (i.e. its Young’s Modulus is much greater, see Section 5.5.3). Unfortunately, it is also much more
stitched fabrics and uni-directional tapes (narrow tapes with all the fibres running parallel in the same direction), both as a dry cloth or as a pre-preg with epoxy. Carbon fibre is easy to work into complex curved shapes, wets-out well as a wet lay-up and forms a strong bond to epoxy resin. It can be cut and drilled quite easily, although special drills may be needed with pre-pregs to prevent damage around the holes.
One problem with carbon fibre is that it is a high-resistance electrical conductor, which might explode when struck by lightning. Aeroplanes and yacht masts made of carbon fibre must be fitted with lightning conductors unless the cross-sections of the components are so big that they can absorb a lightning strike without overheating. For aircraft this usually means building copper conductors into the wing around carbon spars or adding a conductive layer, such as aluminium mesh, over the outside of the whole aircraft.
The use of fibres in aircraft structures is illustrated in Fig. 10.3. Figure 10.4 shows an aircraft with an entirely composite main structure.
Fig. 10.3 Application of composite materials to the A340.
10.6 Resins
10.6.1 Polyester resin
Polyester resin is widely used in commercial, industrial and marine appli-cations. It is not normally used in aircraft because epoxies offer better strength and durability. The resin is easy to work with, being supplied as a clear liquid resin to which a few drops of liquid catalyst (hardener) are added and stirred-in just before use. It is sometimes used for low-cost tooling and moulds when accuracy and durability are not vital.
10.6.2 Epoxy resin
Epoxy is the resin system used in most aircraft structural applications.
There is a wide range of different formulations available to suit different Fig. 10.4 CMC Leopard 002 – an aircraft with an all composite main structure.
advice on the most suitable system for any specific application. The main differences are between the different temperatures used to cure the resin, the working temperature it is expected to see in service and the manu-facturing method used to make the composite components.
The greatest strength is achieved with a resin system that is cured at high temperature, typically 175°C under pressure in an autoclave. To get the lightest weight components for aerospace use, all excess resin must be eliminated. This is normally done by running the fibre tapes or cloth through a bath of mixed epoxy resin and then between rollers that force resin into the cloth and squeeze away the excess. This is done by the mate-rial suppliers and the resulting matemate-rial is called a ‘pre-preg’. To stop the resin from hardening in the pre-preg cloth before it can be cut and laid-up in the mould it is kept at cold temperatures, this is typically -10°C.
Even when stored in a freezer, the epoxy will eventually go off, so these materials have a limited ‘shelf life’ of 6 or 12 months.
The high-temperature curing epoxy systems are expensive to use because not only is an autoclave needed but the mould tools must be strong enough to withstand the repeated heating and cooling cycles of component production; this means that they must be made from expen-sive materials. Epoxy resin systems are being continuously developed to improve strength and reduce the cost and difficulty of component manu-facture. Low-temperature curing pre-preg systems are now available that give excellent strength when cured at temperatures of 75–100°C and that need only the pressure of a portable vacuum bag system, rather than the higher pressures of an autoclave.
For light aircraft and gliders the epoxy resin is bought in a two-part pack and mixed just before use, where it is brushed, rolled and squeezed into the dry cloths laid in the mould. Unlike polyester resin, it is very impor-tant to get exactly the right quantity of resin and hardener into the mixture. This is done by using accurate scales to weigh it out or by using a metering pump that delivers the correct ratio. Different resin systems require different mixing ratios and this is explained on the data sheet sup-plied with the resin. The component is left in the mould to cure at room temperature for 24 hours but it can take up to 14 days before it has reached its full strength. To speed up the curing and to make the compo-nent more resistant to high temperatures, it can be cooked in an oven (post-cured) at 45–80°C for several hours.
10.6.3 Vinyl ester resin
Some kit planes use vinyl ester resin systems which can be considered to fall between polyester resin and wet lay-up epoxy resin in terms of both strength and cost. These resin systems are mixed and handled in a very similar way to polyester and result in good chemical resistance and mod-erate strength. In most aircraft applications, the greater strength of epoxy is considered to outweigh the cost saving of vinyl ester.
10.6.4 Phenolic resin
Interior wall panels and cabin furnishings are made using phenolic resin systems. This is because phenolic is the only resin system that is adequately fire resistant and can meet the smoke and toxic fume emission require-ments when burnt. It is not strong enough to be used for the structure of the aircraft and is generally more difficult to process than epoxy.