Materials used in Aircraft
Any brief history of aircraft materials must mention timber as one of the first materials used to make a powered, manned flight. The Wright Flyer consisted mainly of Sitka spruce and bamboo glued and screwed together to form a canvas-covered frame. Wooden aircraft were very successful in the early years of flight but by the end of World War I their days were numbered. Today, timber is only suitable for comparatively small aircraft. As aircraft became larger, materials with better specific strength (strength to weight ratios) became necessary. Today aircraft consist largely of aluminium alloys with steel, titanium alloys and polymer composites forming the smaller proportion. The balance of materials does depend on the type of aircraft as military fighter planes have much higher proportions of composites and titanium alloys.
Aircraft need to be made of lightweight materials to increase payload and save in fuel consumption. The more passengers a plane can carry, the more profit an airline company can make.
Whilst pure aluminium has low specific gravity, good corrosion resistance and excellent thermal and electrical conductivity it is too weak and ductile to be used on its own. In 1906 Dr Alfred Wilma, a German metallurgist, discovered that aluminium alloyed with copper and heat treated correctly could be made far stronger. The alloy of aluminium with 4% copper is called Duralumin and the heat treatment process is called precipitation hardening. These alloys have typically low specific gravity (around 2.7) and high strength (450 MPa). They are limited by a maximum service temperature of about 660°C. Since then, other heat treatable aluminium alloys have been developed for aircraft use. These include a range of complex aluminium-zinc alloys which develop the highest strength of any aluminium alloy. These alloys have led to modern aircraft design where the skin of the fuselage and wings are stressed aluminium alloy members which reduces the overall weight.
The aluminium alloys mentioned above have the disadvantage of not being as corrosion resistant as pure aluminium so a thin layer of pure aluminium is often pressure welded to both sides of the alloy. This material is called Alclad.
Titanium, though very expensive, is used where high strength is needed in load bearing applications such as landing gear and engine mounting brackets.
Steel is used where strength is needed in restricted spaces, for example in the carriageways. Alloy steels can be heat treated to give very high mechanical properties and take up less volume, which is very important, as there is not very much “free space”. It is used sparingly though as it is heavy and suffers from increased brittleness (low energy to cause fracture) at the low temperatures found at very high altitudes.
Lithium is also used as an alloying element for improved properties.
One property that is appropriate for selecting materials for aircraft use is their specific strength. The following list indicates the approximate specific strength for some materials:
- Aluminium 30 Mpa
- Mild Steel 51 MPa
- Duralumin (heat treated) 150 MPa
- Al-Zn (heat treated) 220 Mpa
- Titanium alloy (heat treated) 270 MPa
Modern aircraft are extremely efficient and fast flying machines. The advances in materials technology accounts for the economy and reduction in weight while speed comes from development of the aircraft’s shape and its engines.
Advances in polymer composites are making them increasingly more popular. They have very attractive low density and high mechanical properties. Composites consist of fibres of glass, carbon, Kevlar or boron reinforced in an epoxy resin matrix. They are replacing some of the aluminium alloys in commercial aircraft and find even greater applications in military aircraft such as the Eurofighter.