Metals work with composites for aerospace

Tom Shelley reports on why both metals and composites are crucial in aerospace constructions.

Carbon fibre composites save weight but they do not conduct electricity very well and damage assessment poses problems. This means that, while carbon fibre composite is forming a much higher proportion of aircraft and rotorcraft construction than hitherto, it appears to be levelling off now and metals continue to have their place.

Using the two types of material in combination means that constructions are more complicated than they have ever been before and the end results are far from cheap. But with energy costs on the rise, and airlines succeeding or failing according to whether their incomes exceed fuel and other operating costs, combined complexity has to be the way forward, and even more advanced materials, such as carbon nanotubes, are likely to be found in the fabric of future designs.

"Don't underestimate metals", suggests Michael Overd, head of structure design and development at Agusta Westland when speaking to a UK Composites Supply Chain Programme Workshop, he commented that while use of composites in aerospace construction had been increasing, "It is now levelling off slightly", with the trend being towards metallic frames with carbon skins, which he called, "Hybrid architecture."

A similar view is held by Airbus, which calls its version of the concept the "Intelligent Airframe". Examples include the metallic strip network within the carbon fibre composite fuselage of the A350 in order to ground electric circuits and electronics, dissipate lightning strikes and reduce EMC problems. Another solution commonly applied to composites is to incorporate a layer of copper mesh.

In addition, the A350 cross beams, which are purely structural, are also found to be best made of aluminium lithium alloy. Nonetheless, carbon fibre composites will account for more than 50% of the total weight of the aircraft, as compared with about a quarter of the total weight of the A380.
Panels in Agusta Westland's newer helicopters are generally sandwich constructions, according to Overd. He said that monolithic composite constructions are not widely used, citing problems with guaranteeing the quality of bonds, which often leads aviation authorities to demand use of mechanical fasteners.

Apart from improved strength per unit weight, composite constructions improve damping and their stiffnesses can be tailored to suit their applications, but, Overd says: "One has to allow for manufacturing variability." Furthermore, if suppliers make even small changes such as new resin development, the modified design has to be re-qualified.

In helicopters, Overd said that, "The number one technology challenge" is high frequency vibration, which is difficult to computer model, and when this is done, "You have probably got it wrong." Another challenge is improving crashworthiness. Overd commented that air crashes usually result either from pilot error or maintenance problems. This means that no matter what the technology, there will always be occasional accidents.

To accommodate this, helicopter and aircraft structures need to collapse progressively, as in cars, to absorb impact energy and give occupants an improved chance of survival. This, he said is "Difficult" to accomplish with carbon fibre composites alone, but is easier to achieve if composite skins are attached to metal frames. Composites offer clear advantages over metal construction as regards, corrosion, fretting and fatigue and research and development continues.

Overd mentioned research work in Italy on composite manufacture using RTM – Resin Transfer Moulding, and also the growing interest in thermoplastic as opposed to cured composites. Thermoplastic composites have high damage tolerance, because they are not made from bonded layers of pre-preg, and offer high fracture toughness and impact resistance, good fatigue resistance, low storage cost and infinite shelf life.

They are hard to bond, but on the other hand, they can be fusion welded. One approach is to use resistance welding, which involves placing a layer of conductive material, such as a metal mesh or carbon strip between two surfaces and applying sufficient electric current to melt the thermoplastic polymer at the weld interface. A major research project in Europe is TAPAS – the Thermoplastic Affordable Primary Aircraft Structure consortium, which consists of eight Dutch companies and research institutes and Airbus.

Airbus already uses ultrasonically spot welded fixed wing leading edges made from Ten Cate's glass/PPS (Polyphenylene Sulphide) for the A380. The flight deck floor of the A400M is also made of thermoplastic composite as are the rudders and elevators of the Gulfstream 650.

TAPAS projects include the development by Kok and Van Engelen of induction welding technology for components made of large double curved structures made from high performance, unidirectional tape. Another project involves fusing together multiple preforms in a single step to create integrated structures.

A Fokker patent pending development improves stiffener to panel joints by incorporating radiused plastic fillets. In order to be able to produce larger skin panels and achieve high build rates, it will be necessary to automate laying down of thermoplastic unidirectional composite. In a full scale demonstrator, automated layup of a skin panel and stiffener preforms is being developed by Fokker Aerostructures and Airborne Composites, using ultrasonics as the heat source.

Press forming is being developed at Dutch Thermoplastic Components. This has led onto a European Union FP7 project, COALESCE2, the letters standing for Cost Efficient Advanced Leading Edge Structure. The project description notes that, "Leading edges of modern commercial aircraft are found to create an overly large part of the total wing costs."

The project aims to reduce these costs by more than 30%. In order to do so, it seeks, "New design principles, arrangement of stiffeners, geometric arrangement of assessment holes for maintenance and assembly methods." Both metallic and composite structural solutions are being explored, and the project is set to conclude at the end of March 2012.

Particpants are: Airbus Operations in the UK, Alenia Aeronautica, Delfoi in Finland, EADS, the Eidgenössische Technische Hochschule in Zurich, Fokker, Societe Nationale de Construction Aerospatiale Sonaca in Belgium and Stichting Nationaal Lucht – en Ruimtevaartlaboratorium in The Netherlands.

As well as improving the design of aircraft leading edges, Airbus is also very interested in possibilities offered by composites reinforced with carbon nano tubes, which are now becoming commercially available in quite large quantities. Apart from their immense strength, their electrical conductivity is a couple of orders of magnitude higher than that of carbon fibre.

However, it is not just a matter of mixing them into resin. They either have to be spun into nanotube fibres, or processed in such a way that they are fully dispersed, oriented in the same direction and well bonded to the matrix. One idea is to use them to stitch conventional carbon fibre pre-preg layers together to reduce the risk of delamination.

Other types of nano composites being studied include polymers reinforced with nano metre sized particulates. Nano particles can be silica, silicon carbide, silicon nitride, titanium oxide, zinc oxide, calcium carbonate, barium sulphate and nano clays. Metal matrix nanocomposites also show great promise, as we mentioned in our March 2011 edition.

Design Pointers
• Optimal fixed wing and rotorcraft constructions make use of a combination of carbon fibre composite, metals and sandwich constructions. There are advantages to be obtained by using light alloy frames and carbon fibre composite skins
• Carbon fibre composite used for outer skins of aircraft needs to made electrically conducting
• Thermoplastic composites are already in service in airframes and likely to be more widely used in future
• The next steps forward look likely to be the incorporation of carbon nanotubes in composites and the introduction of nano metal matrix composites.

Tom Shelley

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