Use of composites set to grow
With a growing emphasis on fuel efficiency in virtually every walk of life, it is hardly surprising that aircraft manufacturers are striving to reduce the weight of both exterior and interior components.
While small gains can be made by specifying high-specification alloys that enable less material to be used in metallic components of comparable strength and stiffness, far greater benefits are available if components can be redesigned so as to be made from composites. For example, a carbon-fibre reinforced plastic may easily have a specific stiffness (stiffness per unit mass) and a specific strength at least four times greater than that of aluminium alloys.
State-of-the-art fighter aircraft already make extensive use of composites. On the Eurofighter Typhoon, for example, over 70percent of the aircraft shell is comprised of carbon-fibre composite (CFC), namely the outer fuselage, wings (including in-board flaperons) and rudder. In addition, a significant proportion of the structural members are also constructed from CFC. The Typhoon’s radome has a complex layered glass-fibre reinforced plastic (GFRP) structure that is manufactured using
high-accuracy automated processes.
Few people would predict that commercial airliners will make such extensive use of composites, though some believe that composites could well account for up to 50percent of the weight of future aircraft. Meanwhile, the AirbusA380 is said to be approximately 25percent composite by weight. CFC, GFRP and quartz-fibre reinforced plastic are used extensively in wings, fuselage sections, tail surfaces, and doors, and the A380 is the first commercial airliner with a central wing box made of CFC. In addition, much of the interior will be manufactured from composites.
There is clearly a trend towards the greater use of composites in aircraft structures, but the gestation period for new designs is comparatively long. Nevertheless, it should be remembered that many of the internal fittings have a life expectancy that is far shorter than the airframe. Composites have been the predominant class of material for aircraft interiors for decades, but there is still scope for more metallic components to be redesigned for manufacture from composites.
A third area where composites are now being used in the aerospace industry is in mould tools for composite components. Composite components almost always have to be heat-cured, which poses problems if the tooling material on which they are cured is made from a material with a different coefficient of thermal expansion. The answer, therefore, is to create tooling from materials that have a coefficient of thermal expansion that is virtually identical to that of the components – which means the tools are, themselves, now being made from composites (Fig.1).
So far we have discussed the use of composites in the aerospace industry in very broad terms, but the word ‘composite’ covers a very wide range of materials. At the lowest level, there are carbon, Kevlar and other fibres that are used in the form of continuous fibres, chopped fibres and in both non-woven and woven fabrics (Fig.2). If the fabric is pre-impregnated with an uncured epoxy matrix, it is referred to as prepreg (Fig.3).
Going a step further, thin sheets of composite (typically carbon fibre or Kevlar paper) can be bonded either side of a sheet of honeycomb aluminium or non-metallic material to create a bonded board that is remarkably strong, stiff and light. These boards can be used, for example, as flooring panels in passenger or large transport aircraft, though the development of a new type of ‘sandwich’ panel using non-hexagonal honeycomb creates further opportunities for non-flat components. Hexcel’s HexwebHRH-36 Flexcore honeycomb combines the company’s innovative non-hexagonal, flexible cell geometry with the latest high-performance core material. Constructed from Kevlar paper and reinforced with a high-temperature-resistant phenolic resin, HexwebHRH-36 Flexcore retains its strength at temperatures up to 175°C and maintains its mechanical properties even when it is curved. This special type of bonded board is also said to provide higher shear strengths than comparable hexagonal cores of equivalent density and can therefore give valuable weight savings for an equivalent performance.
Moving on from components formed from bonded boards or prepreg, Hexcel describes HexMC as a new kind of cost-effective carbon-fibre epoxy moulding concept that enables detailed three-dimensional shapes – which could not easily be made from high-performance composites – to be manufactured in series production (Fig.5). The properties obtained are said to be close to those found in quasi-isotropic laminates produced from the same fibres and resins as used in HexMC – which is due to the fact that HexMC is a compression moulding material consisting of chopped unidirectional prepreg tape.
It has already been mentioned that composites are starting to play a crucial role in the production of tooling for heat-cured components. Hextool is Hexcel’s new patent-pending composite tooling material, based on HexMC, that enables machinable composite moulds to be produced. This new concept for lightweight, efficient large-scale tools is said to be cost-effective compared with conventional composite tools and metal moulds, including Invar (an iron-nickel alloy that has a low coefficient of thermal expansion).
The lay-up of the tooling involves minimal labour and time (there is no debulking) on a master mould that does not have to be dimensionally accurate. Once cured, the tool is then machined to the final desired shape and has the major advantage that it has a coefficient of thermal expansion that matches that of the carbon/epoxy structural parts produced on it. Furthermore, the tool is also lighter and has faster heat-up and cool-down rates than metal tooling, and can be easily repaired or modified.
