Lightening-based design is a widely used approach in the majority of the transportation sectors and in particular in the automotive and aerospace industries. This design practice is favoured as it not only reduces fuel consumption, but it also reduces the amount of contaminants that are emitted into the environment. In the case of airplanes, by using lightweight materials it is possible to achieve greater acceleration, improved structural resistance and greater rigidity, and it is also possible to improve their safety features. Lighter aero structures can only be developed by using low-density materials that boast optimum structural behaviour. That is why nowadays carbon-fibre composite materials are widely used for manufacturing airplanes, after a gradual and direct substitution of the metal components.
The use of composites in the aeronautics industry
Composites are lightweight and offer high mechanical resistance, high rigidity and good fatigue resistance. These days thermostable composites (principally epoxy-resin based) are the most commonly used, nonetheless, thermoplastics are gaining ground thanks to the advantages that they are able to offer, for example, the fact that they do not need to be processed in autoclave and the fact that they are highly recyclable. Composites account for 50% of the weight of the latest AIRBUS A350 XWB and BOEING 787 models. A good example of a large-scale carbon component are the A350’s wings which are 32 metres in length and 6 metres in width.
Despite the advantages that these composite materials are able to offer, there are various factors that affect their more widespread use in the aeronautic sector. These include the cost (both of the starting material and of the forming process) and the process time (rolling, curing), as although automated processes such as AFP and ATL do exist, these are neither widely used, nor can they be used for all of the components and this is why manual processes continue to be used extensively. On the other hand, many in the sector are yet to be convinced of the durability that composite materials can offer.
Multi-material structures and scope
A strong trend that has emerged in aircraft design in recent years is hybridisation or the multi-material concept; this concept is based on the idea of combining metal materials with composites in order to provide an optimum solution in terms of durability, cost, processability and behaviour. In general terms, this consists of placing the right material in the right place, therefore making it possible to produce structures that weigh less than metal structures and that cost less than those made solely from composite materials. By combining these materials it is possible to attain a better structural performance than when using either of the components on their own.
There are two different approaches: the first of these is based on the idea of developing materials that combine metal and composites to produce semi-finished products such as GLARE. GLARE are FML (fibre metal laminate) plates that are produced by binding several very thin layers of metal (usually aluminium) and placing these between several layers of fibre-glass that has been pre-impregnated with epoxy resin. Although this system was patented by the multinational company, AzkoNobel in 1987, its use in the production of civil aircrafts was only certified by the Federal Aviation Administration (FAA) and the European Aviation Safety Agency (EASA) in recent times. This system was used for commercial Airbus A380, making it possible for its weight to be reduced by 30% compared to the AI panels.
The second approach is based on the idea of developing mixed aerostructures that use different methods to combine metal and composite substructures. These methods include adhesive bonding (e.g. composite skin and aluminium strengthening elements) and “one-shot processes” (co-consolidation or direct union).
The major advances have been made in the manufacturing of composite/titanium or composite/aluminium structures that integrate metal elements during the composite processing. For example, in the case of carbon fibre and epoxi prepregs, metal inserts or pieces are inserted during the composite lamination process and these are subsequently bonded during the autoclave curing stage (for example the metal anchors in composite traps). Another more innovative example is the bonding of Ti structures to composite materials during the LRI (Liquid Resin Infusion) process (for example bonding a composite wing to a Ti leading edge).
A final example is the in-situ co-consolidation of the composite on metal structures, which could be applicable to thermoplastic or thermostable materials if automated processes such as AFP or ATL are used.
This approach is used in the European ComMUnion project, led by AIMEN, which looks to validate this technology in aeronautic pieces.
In any of the previously mentioned cases, the critical phase that the success of the hybrid or multi-material structure depends on is the bonding of different types of material and as such the preparation of the metal surface is crucial.