The aerospace industry is an environment in which the research and use of new materials plays a priority role. There is still much scope for optimising their use and, above all, for the introduction of nanomaterials in aircraft construction.
Since its inception, the Aviation industry ha avanzado en sus investigaciones hacia la obtención de materiales con determinadas propiedades, las cuales permiten el correcto funcionamiento de las aeronaves en todo su rango operativo.
There are several characteristics that are sought after in a avionicsThe main characteristics of the steel are its mechanical and fatigue strength, elasticity, density and corrosion resistance, as well as its price.
A good indicator of the suitability of an aeronautical material is the mechanical strength to density ratio, conocida como resistencia específica, siendo de interés que presente altos valores de la misma.
It should be noted that these properties must be maintained in all working conditions of an aircraft. Both on the runway of an airport, where aircraft can encounter 40°C in summer and high humidity conditions, and in cruising conditions, at an altitude of about 11 000 m, where the temperature drops to -50°C.
Another factor to consider is that during operation the structural elements are subjected to drastic changes in load distribution. For example, the wing spars work by "supporting" the wings while the aircraft is on the ground. Subsequently, in flight, they "support" the fuselage from the lift generated by the wings so that the tensile and compressive zones are reversed.
To this must be added the vibrations suffered by the aircraft in the air. This was a problem after the first scheduled flights began, as the materials used had not been selected for their fatigue resistance, which is why some aircraft began to collapse after years of good performance.
As the industry has evolved, in terms of materials, two of the focuses of research have been the increase of specific resistance of them and the improved manufacturability of aircraft and aircraft components.
Aviation took its first steps with models built in wood and textile material. The wood provided the structural function, while the fabric provided the supporting medium. Despite its low density, there are quite resistant woods; however, this material is affected by biological action, as well as reacting negatively to humidity.
It quickly gave way to the use of metal for the aircraft's structure, namely of steels. However, its high density, low corrosion resistance and the galvanic couple it forms with aluminium ruled it out as a good aircraft material. Today, its use in aeronautics has been relegated to very specific parts, such as the landing gear and certain hardware.
This leads to the aluminium. It is true that in its pure state it does not have good mechanical properties. Moreover, until a few decades ago it was a very expensive metal. However, advances in its production process and the use of its alloys led to its rise as an aeronautical material.
Currently the predominant alloys in the industry are known as duraluminium. This type of alloy offers a higher specific strength than steel and improve other properties. Some of these alloys for aeronautical use belong to the 2XXX series of aluminium-copper alloys and the 7XXX series of aluminium-zinc alloys. Today, new alloys, such as aluminium-lithium alloys, are making inroads.
Another advantage of aluminium is its corrosion behaviour thanks to its rust, the aluminawhich, when formed, completely covers the base metal, leaving its surface protected in a process called "passivation".
In certain parts of the aircraft, such as the engine, it is necessary to use alloys which have a good thermal resistanceThe use of alloys of titanium. Its density is still below that of steels, although it is higher than that of aluminium. It has a high resistance to corrosion (it is also passive) and maintains good mechanical properties. The main disadvantages are its high cost and the difficulty of machining. It is therefore only applied to some engine parts or to coatings for hypersonic aircraft, where the interaction with shock waves generates high temperatures. The most commonly used alloy is Ti-6Al-4V.
In recent years, non-metallic materials have been gaining importance in aircraft construction, in particular the composite materials.
These materials arise from the union of two or more different materials, insoluble and separable by mechanical action. Of these, one acts as the matrix and the other as the reinforcement. The former is responsible for giving shape and cohesion to the composite material as well as transmitting compressive forces, while the reinforcement improves its behaviour in the face of the rest of the mechanical loads. The most commonly used in aeronautics are those that use carbon or glass fibre as reinforcement.
Composites have increased their presence considerably to 50 % in the two latest models of the major aircraft manufacturers: Airbus' A350 XWB and Boeing's B787 Dreamliner.
In addition, these materials allow a better integration of parts as soon as they are formed. An example of this is the one-piece construction of the A350 XWB tail cone. This strategy reduces cost by reducing the number of transports, the number of subsequent assemblies and the added weight due to joining.
Another factor to be considered is the ecological aspect, in an industry where the emphasis is increasingly on the commitment to the environment. In relation to this, the future forecasts of major manufacturers predict the use of environmentally friendly compounds and biopolymers, which would facilitate recycling.
But if there is one branch of research that should be highlighted for its novelty, it is the use of nanomaterials in aircraft construction. The aim is to achieve a structural and functional improvement, in addition to a reduction in weight.
In the coming years, companies wishing to achieve and maintain their position as leading firms will have to continue researching new alternatives. Some of the advances that the industry foresees in this area are based on the search for materials that are self-repairing or that adapt their shape to the passenger.
In any case, this is achieved at the cost of increased investment, both in the initial design of the product and in its production process. The motivation behind this investment must focus on the improvements that will be generated in all the above-mentioned production aspects, so that a more efficient, modern and cleaner industry is developed.
