Composites in the aerospace industry
The age of high-tech composite materials
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The aerospace industry’s unrelenting quest to enhance the performance of its products is constantly driving the development of improved structural materials. EADS companies have always been at the cutting edge of advanced materials development.
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Fibre-based composite materials play an ever-increasing role in the construction of aircraft, helicopters, missiles, rocket launchers and satellites. The most common composites used in aerospace are carbon fibre reinforced plastics (CFRP), a mix of 60 percent carbon fibre and 40 percent resin. Other types include sandwich and honeycomb structures, fibre/metal laminates and glass fibre materials, while ceramic carbon fibre composites are used in high-temperature applications such as rocket motors.
The advantages of composites in aircraft design are their high strength-to-weight ratio, excellent fatigue endurance, corrosion resistance and a malleability that allows tailoring them to meet design requirements. Composites can be more easily formed into complex shapes (such as spheres) than their metallic counterparts. This not only reduces the number of parts making up a given component, but also reduces the need for fasteners and joints – often the weakest points of a structure.
Composite structures do, however, have a few drawbacks. For example, they tend to have higher manufacturing costs than metallic structures. Composites have stiffness and strength properties that may vary with temperature and moisture content and which depend on the thickness of a component. In some circumstances they may also be more sensitive to damage such as hail or bird strikes, not to mention accidental impact by servicing vehicles at airports.
Several different techniques are used in the production of CFRP aircraft parts. The most common method uses rolls of carbon fibre that have been pre-impregnated with resin, which are then cut and shaped as required and built into a series of layers. Pressure is then applied and the part is cured at high temperatures in an autoclave. In another commonly used method, the carbon fibre is shaped in a mould or by using sewing or braiding techniques. The liquid resin is added to this textile pre-form in the next step. This can be done using several processes, some of which involve placing the part in a vacuum and then curing it at room temperature.
A switch from metal to composites can make parts up to 40% lighter. The weight savings, which have a positive effect on both fuel consumption and the aircraft’s payload, are particularly important in a market environment characterised by high fuel prices and the prospect of ever more stringent aircraft emission standards. Besides being lighter than traditional materials, composites offer significant advantages in terms of operational reliability, leading to lower in-service operating costs.
As already mentioned, however, composite structures have a major drawback in terms of their higher manufacturing costs compared to metallic structures. Consequently, much research continues to be directed at reducing these costs by developing methods that do not require the use of an autoclave and by increasing the automation of manufacturing processes such as automated tape lay-up, robotic fibre placement, resin transfer moulding and resin film infusion.
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Airbus’ evolutionary use of composite materials
Airbus introduced the use of composites in 1972, in secondary structures such as the fin leading edges on the A300B. In 1982, Airbus was the first to use carbon fibre composites to make the spoilers, airbrakes, elevators and rudder on the A310-200. It was also the first manufacturer to use composites in primary structures, namely for the vertical fin of the A310-300 and the horizontal tailplane and flaps on the A320 in 1987. The use of composites was extended to long-range aircraft when CFRP was used to build the vertical and horizontal stabiliser (used as a fuel tank), rudder, elevator, flaps, ailerons, spoilers, landing gear doors and several fairings for the A340. Several key innovations followed, including the development of a carbon- fibre keel beam for the A340-500/-600 and a composite rear pressure bulkhead on the same aircraft – the first composite part to be used within the pressurised area of an aircraft.
Airbus A380
Airbus’ development of composites has led to the mainstream acceptance of the material in civil aviation manufacturing worldwide. On the A380, Airbus has applied composites to fuselage areas, a feat considered impossible twenty years ago. About 40 percent of the A380’s structure is manufactured using the latest generation of carbon composites and advanced metallic materials. The aircraft incorporates the world’s largest-ever composite rear fuselage section for an airliner, as well as a centre wing box that is produced largely from composites. The weight saving for that part alone amounts to about one and a half metric tons compared to the most advanced aluminium alloys. A CFRP design has also been adopted for the fin box and rudder, as well as for the horizontal tailplane and elevators. Furthermore, the rear pressure bulkhead and upper-deck floor beams are made of carbon-fibre composites and the fixed wing leading edge is manufactured from thermoplastics, a material that softens when heated and hardens again when cooled.
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After intensive trials, a new material specifically designed for aerospace applications is being used on a civil airliner for the first time. The upper fuselage shell of the A380 is fashioned from GLARE (GLAss-REinforced), a fibre-metal laminate made of alternating thin layers of aluminium and glass-fibre-reinforced prepreg. In addition to being some ten percent less dense than aluminium alloys – resulting in a weight saving of around 800 kilograms – GLARE has proved superior in terms of fatigue and resistance to fire and damage. The new material is also exceptionally corrosion-resistant, with the first glass fibre layer preventing any penetration beyond the superficial aluminium coating. GLARE uses a hot-bonded manufacturing process but is repaired in the same way as standard aluminium.
Airbus A400M
While commercial airliners have incorporated an increasing amount of composites, military transport aircraft, by contrast, have been largely metallic. This will change when the A400M transport built by Airbus Military enters service, as the overall composite content of its structure is equivalent to 35% of the aircraft’s empty weight.
The A400M is the first Airbus to make major use of composites throughout the wing’s primary structure, including the carbon-fibre spars and the top and bottom wing skins. One of the main criteria in any modern aircraft programme is designing to a tight cost target. In this case it meant creating a wing spar design that could be produced by an automated process, and then designing a manufacturing system able to make such a large, complex part to the exacting quality standards specified for such a heavily loaded part of the primary structure.
Airbus A350 XWB
To best satisfy market needs, Airbus is taking the “Intelligent Airframe” approach, a design philosophy that balances the optimum combination of new materials, advanced design and production methods and the step-by-step implementation of smart structure technologies. The choice of the best material for each specific structural component takes the local airframe requirements into account, while also allowing for the different material strengths and drawbacks.
As a result of this philosophy, the new A350 XWB will feature an airframe made of 52% composites and 20% ultra-light alloys (such as aluminium-lithium). The A350 XWB will have an all-new composite wing with a span of 64 metres. Composites will also be used for the fuselage skin panels and rear fuselage, the centre wing box, the belly fairing and the vertical and horizontal tailplanes. Aluminium and aluminium-lithium are featured in the fuselage frames, floor beams and gear bays, while titanium is being used in the landing gear, engine pylons and attachments.
Each of the three fuselage sections will have four long CFRP fuselage skin panels (top, bottom and two sides), which will be attached to metallic frames. The innovative use of all-new CFRP panelled fuselage skins on aluminium frames will make manufacturing easier than producing a single-barrel composite fuselage, Boeing’s approach for its 787. The hybrid design of the A350 XWB’s panels will save weight via optimum fibre lay-up and skin thickness, which can be tailored to the local requirements of each individual airframe part. Making the panels as long as possible reduces the number of circumferential joints, while the longitudinal joints contribute to the fuselage bending strength. The metal frames provide the necessary electrical conductivity in addition to their role as lightweight structural elements. The panel concept for the latest Airbus model clearly puts the right material in the right place.
Eurofighter Typhoon
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Fighter aircraft often drive new technologies because of their special requirements. For example, lightweight structures are necessary for a higher payload, improved agility, and shorter take-off and landing capabilities. Typical composite parts on military aircraft were the fins, landing-gear doors and fuselage components developed by EADS predecessor and partner companies as part of the Tornado and Alpha Jet programmes. A significant part of the current composites know-how in terms of design, damage tolerance, manufacturing technologies and in-service behaviour (e.g. moisture pick-up and crack sensitivity) was generated within these programmes. Today, modern fighter aircraft make extensive use of composite materials throughout the airframe structure. A typical example is the Eurofighter, where composites account for 40% of its structural weight and 70% of its outer skin area.
Composites research
Research into composites technologies currently being conducted in the EADS divisions and by the EADS Innovation Works includes cost-reducing manufacturing methods that do not require the use of an autoclave and which increase the automation of manufacturing processes such as automated tape lay-up, robotic fibre placement, resin transfer moulding and resin film infusion techniques. A considerable effort is being made to develop more advanced design methods, sophisticated structure analysis methods and digital simulation techniques and to advance the state-of-the-art in non-destructive testing and inspection, structural health monitoring, quick repair and lightning-strike protection technologies.
The development and implementation of smart structure technologies holds the promise of further weight reductions and a less frequent need for maintenance. The research programmes are in full swing, so that future aircraft structures will see the step-by-step introduction of self-adaptive, self-monitoring and “self-healing” characteristics. Tomorrow’s smart structures are also likely to include nanotechnology, which may lead not only to significantly improved material properties in terms of thermal or electrical conductivity and fire resistance, but also enable the production of multifunctional materials with sensor functions and structural elements that are capable of changing their dimensions, shape or function on command (“morphing”).
Eurocopter: Record-holder for CFRP materials
NH90 final assembly line at the Eurocopter plant in Donauwörth. The high proportion of CFRP in the airframe structure can clearly be discerned during assembly before the final coat of paint is applied
Consistent lightweight construction is even more important for helicopters than for aircraft, for whilst the airflow bears part of an aircraft’s load, every gram of weight on a helicopter has to be lifted by the propulsive force. This is the reason why, for many years now, the cabin structures have been made almost entirely of composites – the higher cost in comparison to a metal-based construction is soon written off in day-to-day operations as a result of the reduced fuel consumption. Corrosion resistance is equally important, if not more so: many Eurocopter helicopters carry supplies to oil-rigs, and are thus constantly exposed to extremely salty air. CFRP materials are highly resistant to hazards of this kind, lasting easily for the entire service life of the helicopter.
High-performance plastics had already become an established tradition among the Eurocopter founding companies, for Eurocopter was the first company ever to manufacture rotor blades from this material over 30 years ago. Not only is a helicopter rotor blade exposed to immense centrifugal forces, but the approaching air impacts the leading edge of the blade at almost the speed of sound, causing the blade to oscillate in all directions. It has to withstand all this for thousands of hours, at the same time being as lightweight as possible. Nothing but fibre composite materials can perfectly fulfil these requirements. The service life of rotor blades made of aramid and glass fibre fabrics is 200 times longer than that of aluminium constructions.
The NH90 multipurpose helicopter built by the NHIndustries consortium (Eurocopter, Agusta Westland and Stork Fokker) has an airframe made almost entirely of carbon and glass fibres. Composites account for 85 percent of this helicopter’s structural weight, the highest proportion to be found in any aircraft manufactured by EADS companies. The four blades of the NH90’s main rotor are made of a honeycomb construction covered with an outer skin of mixed glass fibre and carbon fibre composite material.
Astronautics: Ceramic composites
Reusable propulsion systems in space travel will need to be capable of multiple launches in future. Ceramic composites are therefore gaining increasing importance in the construction of engines for rockets and satellites. They have the advantage of lower weight, less need for cooling and the ability to withstand increased operating temperatures as high as 2000 degrees Celsius. Moreover, ceramic materials feature a unique property: they become stronger as the temperature rises.
This characteristic predestines them for use in rocket engines, which have to withstand extreme temperatures. One such application is combustion chambers with expansion nozzles made of carbon-fibre reinforced silicon carbide (C/SiC), which are currently being developed under a collaborative venture between EADS Innovation Works and the EADS Astrium division to produce a new apogee engine that will manoeuvre large communication satellites into their final position in geostationary orbit. Until now it has been very expensive to manufacture components from these materials. This situation will soon change, chiefly as a result of new manufacturing methods that make use of textile processing techniques such as robot-assisted braiding and sewing.
Another technique of particular interest for economic reasons is liquid siliconizing, a process in which molten liquid silicon is infiltrated into the component woven from carbon fibres and reacts with the carbon from the matrix carbon previously introduced into the fibrous structure to form silicon carbide.
EADS Astrium has developed a material known as SICTEX, which is manufactured in a process that combines the efficient production techniques of textile technology with cost-effective resin and silicon infiltration methods. In addition to the production routes for carbon-fibre reinforced silicon carbide, EADS is developing even more fibre-reinforced ceramic materials.
The use of new manufacturing techniques brings costs down to a level that makes ceramic-based fibre composites an interesting proposition for other industries, such as the production of brake disks for cars.
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