Material selection

Citation

Ford, T. (2003), "Material selection", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 4. https://doi.org/10.1108/aeat.2003.12775daf.003

Material selection

Material selection

Keywords: Aerospace, Materials

In the early days of aerospace applications for composite materials, the anticipated widespread use did not occur and their contributions were limited to secondary structures. Over the years, this state of affairs has changed considerably and the expertise now available allows polymeric composites to challenge many of the uses previously thought of as exclusively the province of metal structures. Use of advanced composites has reached a stage in which, for example, the forthcoming Airbus A380 will employ 16 per cent of polymeric composites in its construction. Carbon fibre reinforced plastic (CFRP) is used for the rear pressure bulkhead, upper deck floor beams, outer flaps, spoilers and ailerons, empennage and unpressurized fuselage, centre wing box, and engine cowlings. Thermoplastics are used for the fixed wing leading edge, and Glare for the upper fuselage panels, the material of which consists of alternate layers of aluminium and glass fibre reinforced adhesive.

At the same time as these developments have been taking place, improved metallic materials and processes have also appeared including high performance aluminium alloys, high strength steels and advanced titanium alloys. Manufacturing advances have been perfected, such as superplastic forming/ diffusion bonding (SPF/DB), and improved casting and forging. An example of SPF is aluminium alloy 2004 which is used in many aircraft including the Airbus A340 and Boeing 777. The latter aircraft incidentally, which entered service in the 1990s, has composite materials making up 10 per cent of the structural weight which for this company, is a significantly higher percentage than on any previous aircraft. Improvements in both metallics and composites mean that a specific application has to take into account all the advantages and potential problems that may occur with a particular structure or component, and a choice made.

Composite properties

The first application of composites on primary structure of a production aircraft was on the Airbus A320 which entered service in 1988. Improved matrix materials and higher strength reinforcing fibres led to applications on aircraft empennages to widen the primary structural uses. Although not subject to metallic corrosion, polymeric composites have a number of factors that have to be considered. One of the basic problems, particularly with early thermosets, is related to low velocity impact damage. Resistance to delamination is a most important property and over the last 15-20 years, the damage resistance and damage tolerance of toughened polymer structures has been investigated and improved.

A typical widely used thermoplastic is Cetex, which was originally developed in the 1980s and was at first used in simple parts such as brackets, etc. Over the years, confidence has grown and applications widened to include galleys, closets, divider panels, stowage bins, etc. Its damage tolerance characteristic also makes it suitable for cargo floor panels and the high service temperature is utilised in rear wing box fairings near the engines. Combined environmental and mechanical resistances are demonstrated in structural applications such as ribs. Low cost manufacturing methods include thermofolding in which preconsolidated laminates may be compared to metal sheet. Compression moulding is another method in which a laminate is heated beyond the softening temperature and the material pressed into complex shapes. Flow moulding, deep drawing and transition moulding may also be used. In numerous applications, sandwich panels are used where the use of thermoplastic face sheets offer advantages including the elimination of filling and sanding; the properties of metal are generally better than phenolic skinned panels, edges can easily be reinforced, and the panels can easily be curved.

Use of metals

High strength aluminium alloys are a primary structural material used traditionally in aircraft, 2024 and 7075 much in evidence. The goals of improved durability and weight saving have encouraged development of new aluminium alloys with improved combination of properties. For example, the upper and lower wing structures of the Boeing 757 and 767 are made with improved alloys relative to the older Boeing 747. These include 7150-T6 plate and extrusions (upper wing) and 2324 and 2224 extrusions (lower wing).

The Airbus A380 which represents the most up to date utilisation of materials, has 66 per cent aluminium, which includes advanced alloys on the mid and inner wing panels and inner flaps, metal bonded panels on the outer wing, and laser-beam-welded Al 6013 alloy for the lower fuselage panels, with improved corrosion resistance and fatigue behaviour. Titanium and steel account for 10 per cent of the structure.

Aluminium lithium alloys are yet to achieve large scale replacement of conventional alloys although development of improved Al-Li alloys, to improve fracture toughness, etc., has made progress. Newer materials have been used on the leading edges and outer lower wing skin panels of the Airbus A330 and A340 and other aircraft. Improvements in aluminium casting alloys and processes have resulted an increase in their use, particularly by Airbus. Cast titanium components have been used for some time in engine and airframe applications, with more complex shapes becoming practical by hot isostatic pressing (HIP).

One type of titanium alloys offers superior strength and is more easily processed and is sufficient for temperatures up to 400°C. Alloys such as Ti-6-4, are highly weldable. These alloys can provide weight savings as well as superior corrosion resistance compared with low alloy steels and aluminium alloys. A large proportion of the titanium used in all sections of the airframe is Ti-6-4, with all of the product forms utilised. Alloy Ti-6-6-2 has been extensively used in the landing gear structure of the Boeing 747. Another type of titanium alloy such as Ti-10-2-3 has good temperature and creep properties and is used on the landing gear of the Boeing 777 which results in weight savings compared with high strength steel and is not subject to stress corrosion cracking.

Terry Ford