COMPOSITE MATERIALS
REVOLUTIONISE AEROSPACE ENGINEERING
PT.1
Aerospace engineering is changing. Aeroplanes have traditionally been made out of metal – usually alloys of aluminium; now however, engineers are increasingly working with carbon fibre composites. Tim Edwards, a structural engineer at Atkins, describes the making of composite wings and the ztake-up of them across the aerospace industry.
 |
Airbus A400M, the next generation of military airlifter expected to make its first flight later this year, has wings made from carbon fibre composite. By using composites rather than metalsthe overall strengthto weightratio of materialsused in aircraft design can improve by up to 20% |
Fibrous composite materials were originally used in small quantities in military aircraft in the 1960s, and within civil aviation from the 1970s. Bythe 1980s, composites were being used by civil aircraft manufacturers for a variety of secondary wing and tail components such as rudder and wing trailing edge panels. However, it is with the advent of the latest generation of airliners, such as the Airbus A380, the world’s largest passenger aircraft, that these materials have been deployed extensively in primary load-carrying structure. The A380 uses composite materials in its wings, which helps enable a 17% lower fuel use per passenger than comparable aircraft.
LIGHTER, STRONGER
Composite materials (for aerospace uses, this is usually a carbon/epoxy mix) can provide a much better strength-to-weight ratio than metals: sometimes by as much as 20% better. The lower weight results in lower fuel consumption and emissions and, because plastic structures need fewer riveted joints, enhanced aerodynamic efficiencies and lower manufacturing costs. The aviation industry was, naturally, attracted by such benefits when composites first made an appearance, but it was the manufacturers of military aircraft who initially seized the opportunity to exploit their use to improve the speed and manoeuvrability of their products. Civil aircraft manufacturers have been slower to implement them in their airframes for two reasons: stringent civil airworthiness requirements deterred the wholesale adoption of relatively unproven materials and the flat price of fuel in the late 1980s reduced the need for increased fuel efficiency in emerging airliner designs.Now, however, with extensive experience in the use of composites within the industry, and against the backdrop of European-wide targets to reduce emissions from aircraft, the value of realising the full potential of this important technology is clear.
FATIGUE FREE
Carbon fibre reinforced plastic (CFRP) – carbon fibres embedded in an epoxy matrix – derives its high structural performance from the prodigious strength of the individual strands of carbon. By way of comparison, the ultimate strength of aerospace grade aluminium alloys is typically 450 megapascals (MPa – a unit of stress or pressure, one MPa being about 10 times atmospheric pressure), whilst that of a carbon fibre would be five times that value. As carbon composites are, additionally, only 60% of the density of aluminium, the potential for weight reduction in an airframe application is also apparent. Glass, aramid and boron fibres are also used, but for primary load-bearing structure, carbon fibres have the best combinationof strength and cost.In addition to strength and weight, fibrous composites are thought to be virtually immune from ‘fatigue’. Relatively small cracks in metal continue to growand it was this phenomenon of progressive cracking that saw the demise of the first de Havilland Comet design during the early jet age in the 1950s. However, because of thestructure of composites – they are non-homogeneous – cracks will not be able to spread. This means that structural engineers can perform design and analysis assuming much higher resistance to stress, and concernthemselves less with the long term durability of the structures they design.
MANUFACTURING COMPOSITES
When applied to aircraft structures, carbon compositesare generally supplied in uni-directional (UD) form: thin (~0.125 – 0.25 mm thick) sheeor tapes of parallel fibres that have been pre-impregnated with resin that has yet to set. This form of the material is ideal for the manufacture of thin plates that are used so extensively in airframe structures. Manufacturers use tape-laying machines to lay down layers, or plies, of this material, one on top of the other, to form single piece sub-components. By laying successive plies in different directions, the strength and stiffness of the component can be tailored to match the demands of the engineer, allowing adequate structural properties to be attained for minimum weight. Modern tape-laying machines can fabricate an entire wing skin in one piece, eliminating the fasteners that are routinely used in metallic designs and thus saving manufacturing cost and further reducing overall weight. To complete the manufacturing process, the component is cured within an autoclave, which subjects the component to pressure at an elevated temperature to consolidate and harden the layers of plies into a single monolith of carbon/epoxy laminate.
Source: Ingenia Magazine
