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The use of metals in aircraft structures had to await modern material development processes such as alloy development. This processes produced stronger and better materials which allowed high speed flight. Materials produced were better as they allowed heavy loads and they showed better resistance to stress corrosion cracking. The introduction of computers has been of massive input in the field of aerospace. These seen engineers perform deep analysis of strain; stress and fatigue on new materials before there are introduced in aircraft structures. And as a result, the number of aircraft accidents reduced drastically. This review covers the use of composite materials and aluminium alloys in the design of modern aircrafts, both civil and military. It also discusses the aircraft structures in which these materials are used in.
Composites are the most important materials to have been adapted in the aviation sector since the use of aluminium in the 1920's. Wright et al (2003) defines composite materials as, "a combination of linear elements of one material in a matrix of one another material". This implies that composites are engineered materials made from two or more ingredients with significantly differing properties, either physical or chemical. The application of composite date back in the 1940's to the F-15 (US Air Force) fighters, which used boron/epoxy empennages. Initially the percentage by weight of composite materials used was 2%.Since then, the use of composites has rapidly accelerated. In 1981, the British aerospace- McDonnell Douglas AV-8B Harrier flew with over 25% of its structures made of composite materials. This shows that composite materials were introduced at a very high rate in the aviation industry (Schmitt, 2008).
Though composites have been introduced in aviation with such a fierce rate, it was proved they are expensive to produce. They are also hard to inspect for flaws and some easily absorb moisture. Despite the above mentioned disadvantages, composites still play a major role in modern day aircrafts. This is so because of their greater strength and lighter weights. Callus (2007) claims that regardless of the disadvantages of composites, they were introduced because they allowed a quantum leap in aircraft performance. Performance is in the form of light weight, ability, useful payload and super high speeds.
USES OF COMPOSITES IN AIRCRAFT STRUCTURES
Since each aircraft is unique, it is impossible to generalise where various materials are being used in current aircrafts, but reference to a specific example illustrates the trend. Figure 1 below shows composite materials used in Boeing 787.
Figure 1. Composite materials used in Boeing 787 structures. Adapted from:
The above figure clearly gives a clear indication of the introduction of composites in aircraft structures. It can be confidently stated that composites form about 50% of the weight of the materials used in modern day aircrafts. This can be seen from the pie chart in figure 1. The commonly used composites are fibreglass, carbon laminate composites and carbon sandwich composite. Some composites include the Titanium and polymer matrix composites.
COMPOSITION OF COMPOSITES
Composite materials are made of two materials, one acting as the matrix and the other as the reinforcement material. These constituent materials determine the mechanical properties of the composite. The matrix has a lower density, stiffness and strength than the reinforcement material, and as a result the reinforcement of the matrix, to provide the majority of the strength and stiffness of a composite is accomplished by the fibres. They can be metallic, organic, synthetic or mineral. American Composite Manufacturers Association (2004) considers epoxy resins as one of the well known matrix material to have been used in a wide range of composite parts and structures. It further states that a major advantage of using Epoxy resins over other matrix materials is their lower shrinkage.
Figure 2. Aircraft advanced composite application usage. Adapted from: http://navyaviation.tpub.com/14018/css/14018_593.htm
The table shows that the in early aircrafts composites were introduced in smaller quantities. This is the case with the F-14 aircraft which was first introduced in September 1974 (Hickman, 2012). Comparing the percentage of composite materials of the F-14 with the F/A-18, which was introduced in January 1983, one can notice that the F/A-18 contained a higher percentage of composites which is 20% as compared to the 0.04% of the F-14. This 19.96% difference may have been due to the introduction of modern material development processes.
The mostly used reinforcement fibres are: glass fibre, carbon pitch based, Boron chemical vapour deposition (CVD) fibres, Alumina, Aramid, Carbon Polyacrylonitrile (PAN) and Polyethylene. Baker, Dutton and Kelly (2004, pp. 57) claim glass fibres are used mostly in airframes of gliders and in secondary structures such as fairings. The trio further explain that this is the case because this is where their low specific stiffness is not a problem in the design process, and because of their low cost as compared to high performance fibres. Such high performance composites include carbon fibre reinforced carbon. This is a composite material made from carbon fibre reinforcement in a carbon matrix.
Diagram of carbon-reinforced carbon
Figure 3. Material properties of a carbon fibre reinforced carbon. Adapted from: http://www.materialsviews.com/understanding-carbon-reinforced-carbon
According to Grolms (2011), carbon fibre reinforced carbon is used mainly in high performance and high cost applications in aerospace technology. He further explains that this composite material is used widely in nose cones, wing leading edges in space shuttles and in aircraft brake systems.
Aluminium has been the main structural element since 1930. This was made possible by its lightness as compared to other metals which are referred to as heavy, steel for example. Also, aluminium has been selected because of its indomitable strength to weight ratio. Although aluminium is not the strongest of the pure materials, its alloys use other elements to bridge the gap and improve its strength. Starke and Staley (1995) claims that aluminium is still selected as a structural material for the fuselage, wings and supporting structure for commercial airliners because of its well known performance characteristics, known fabrication costs, design experience and established manufacturing methods and practices. The duo continue on saying low specific gravity of aluminium leads to high specific properties giving aluminium alloys an upper hand in weight critical applications.
Weight and strength
Figure 4. Weight and Strength- aluminium is approximately one third as dense as steel. Aluminium alloys have tensile strengths of between 70 and 700 MPa. Adapted from: http://www.powerofaluminium.com/page.asp?node=45&sec=Properties .
Aluminium alloys were mainly created to tackle the weight problems of aircraft structures, but due to modern research and studies they have been recently studied for use in liquid oxygen and hydrogen fuel tanks, application which Starke and Staley (1995,pp.167) referred to as cryogenic. The development of aluminium-lithium alloy replaced the conventional airframe alloys. Its lower density property was thought to reduce the weight and accelerate the performance of aircrafts. This development lead to the introduction of commercial alloys 8090, 2090 and 2091 in the mid 1980's (Davis, 1993). Weldalite 049 and CP276 were introduced shortly thereafter. Davis (1993) further says that aluminium alloys have a superior fatigue crack propagation resistance as compared to other alloys. This is due to high levels of crack tip shielding, meandering crack path and the resultant roughness induced crack closure.