The main groups of materials used in aircraft construction nowadays are steel, aluminum alloys, titanium alloys, and fibre-reinforced composites.
Titanium alloys possess high specific properties, have a good fatigue strength/tensile strength ratio with a high fatigue limit, and some retain considerable strength at temperatures up to 400-500◦C. Generally, there is also a good resistance to corrosion and corrosion fatigue although properties are adversely affected by exposure to temperature and stress in a salty environment. The latter poses particular problems in the engines of carrier operated aircraft. Further disadvantages are a relatively high density so that weight penalties are imposed if the alloy is extensively used, coupled with high costs (of the material itself and due to its fabrication), approximately seven times those of aluminum and steel. Therefore, due its very particular characteristics (good fatigue strength/tensile strength at very high temperatures), titanium alloys are typically used in the most demanding elements of jet engines, e.g., the turbine blades.
Steels result of alloying Iron (Fe) with Carbon (C). Steels were the materials of the primary and secondary structural elements in the 30s. However, they were substituted by aluminum alloys as it will be described later on. Its high specific density prevents its widespread use in aircraft construction, but it has retained some value as a material for castings of small components demanding high tensile strengths, high stiffness, and high resistance to damage. Such components include landing gear pivot brackets, wing-root attachments, and fasteners.
If one thinks in pure aluminum, the first thought is that it has virtually no structural application. It has a relatively low strength and it is extremely flexible. Nevertheless, when alloyed with other metals its mechanical properties are improved significantly, preserving its low specific weight (a key factor for the aviation industry). The typical alloying elements are copper, magnesium, manganese, silicon, zinc, and lithium. Aluminum alloys substituted steel as primary and secondary structural elements of the aircraft after World War II and thereafter. Four groups of aluminum alloy have been used in the aircraft industry for many years and still play a major role in aircraft construction: Al-Cu (2000 series); Al-Mg (5000 series); Al-Mg-Si (6000 series); Al-Zn-Mg (7000 series)1. The latest aluminum alloys to find general use in the aerospace industry are the aluminum-lithium (Al-Li, 8000 series) alloys.
Figure 4.10: Sketch of a fibre-reinforced composite materials. © PerOX / Wikimedia Commons / Public Domain.
Alloys from each of the above groups have been used extensively for airframes, skins, and other stressed components. Fundamentally, because all of them have a very low specific weight. Regarding the mechanical properties of the different alloys, the choice has been influenced by factors such as strength (proof and ultimate stress), ductility, easy of manufacture (e.g. in extrusion and forging), resistance to corrosion and suitability for protective treatments (e.g., anodizing), fatigue strength, freedom from liability to sudden cracking due to internal stresses, and resistance to fast crack propagation under load.
Unfortunately, as one particular property of aluminum alloys is improved, other desirable properties are sacrificed. Since the alloying mechanisms/process are complicated (basically micro-structural/chemical processes), finding the best trade-off is a challenging engineering problem. In the last 10 years, aluminum alloys are being systematically substituted by fibre-reinforced composite materials, first in the secondary structures, and very recently also in the primary structural elements (as it is the case of A350 or B787 Dreamliner).
Fibre-reinforced composite materials
Composite materials are materials made from two or more constituent materials with significantly different physical or chemical properties, that when combined produce a material with characteristics different from the individual components. In particular, the aircraft manufacturing industry uses the so-called fibre-reinforced composite materials, which consist of strong fibers such as glass or carbon set in a matrix of plastic or epoxy resin, which is mechanically and chemically protective.
A sheet of fibre-reinforced material is anisotropic, i.e. its properties depend on the direction of the fibers working at traction-compression. Therefore, in structural form two or more sheets are sandwiched together to form a lay-up so that the fibre directions match those of the major loads. This lay-up is embedded into a matrix of plastic or epoxy resin that fits things together and provides structural integrity to support both bending and shear stresses.
In the early stages of the development of fibre-reinforced composite materials, glass fibers were used in a matrix of epoxy resin. This glass-reinforced plastic (GRP) was used for helicopter blades but with limited use in components of fixed wing aircraft due to its low stiffness. In the 1960s, new fibre-reinforcements were introduced; Kevlar, for example, is an aramid material with the same strength as glass but is stiffer. Kevlar composites are tough but poor in compression and difficult to manufacture, so they were used in secondary structures. Another composite, using boron fibre, was the first to possess sufficient strength and stiffness for primary structures. These composites have now been replaced by carbon- fibre-reinforced plastics (CFRP), which have similar properties to boron composites but are very much inexpensive.
Typically, CFRP has a Young modulus of the order of three times that of GRP, one and a half times that of a Kevlar composite and twice that of aluminum alloy. Its strength is three times that of aluminum alloy, approximately the same as that of GRP, and slightly less than that of Kevlar composites. Nevertheless, CFRP does suffer from some disadvantages. It is a brittle material and therefore does not yield plastically in regions of high stress concentration. Its strength is reduced by impact damage which may not be visible and the epoxy resin matrices can absorb moisture over a long period which reduces its matrix- dependent properties, such as its compressive strength; this effect increases with increase of temperature. On the contrary, the stiffness of CFRP is much less affected than its strength by the absorption of moisture and it is less likely to fatigue damage than metals.
Replacing 40% of an aluminum alloy structure by CFRP results, roughly, in a 12% saving in total structural weight. Indeed, nowadays the use of composites has been extended up to 50% of the total weight of the aircraft, covering most of the secondary structures of the aircraft and also some primary structures. For instance, in the case of the Airbus A350XWB, the empennage and the wing are manufactured essentially based on CRPF. Also, some parts of the nose and the fuselage are manufactured on CRPF. The A350XWB material breakdown is as follows (in percentage of its structural weight) according to Airbus:
- 52% fiber-reinforced composites.
- 20% aluminum alloys.
- 14% titanium.
- 7% steel.
- 7% miscellaneous.
1. The following aluminum alloys are commonly used in aircraft and other aerospace structures: 7075 aluminum; 6061 aluminum; 6063 aluminum; 2024 aluminum; 5052 aluminum.