An extraordinarily large number and wide variety of materials are available to aerospace engineers to construct aircraft. It is estimated that there are more than 120 000 materials from which an aerospace engineer can choose the materials for the airframe and engine. This includes many types of metals (over 65 000), plastics (over 15 000), ceramics (over 10 000), composites, and natural substances such as wood. The number is growing at a fast pace as new materials are developed with unique or improved properties.
The great majority of materials, however, lack one or more of the essential properties required for aerospace structural or engine applications. Most materials are too expensive, heavy or soft or they lack sufficient corrosion resistance, fracture toughness or some other important property. Materials used in aerospace structures and engines must have a combination of essential properties that few materials possess. Aerospace materials must be light, stiff, strong, damage tolerant and durable; and most materials lack one or more of the essential properties needed to meet the demanding requirements of aircraft. Only a tiny percentage of materials, less than 0.05%, are suitable to use in the airframe and engine components of aircraft, helicopters and spacecraft.
It is estimated that less than about one hundred types of metal alloys, composites, polymers and ceramics have the combination of essential properties needed for aerospace applications. The demand on materials to be lightweight, structurally efficient, damage tolerant, and durable while being cost-effective and easy to manufacture rules out the great majority for aerospace applications. Other demands on aerospace materials are emerging as important future issues. These demands include the use of renewable materials produced with environmentally friendly processes and materials that can be fully recycled at the end of the aircraft life. Sustainable materials that have little or no impact on the environment when produced, and also reduce the environmental impact of the aircraft by lowering fuel burn (usually through reduced weight), will become more important in the future.
The main groups of materials used in aerospace structures are aluminium alloys, titanium alloys, steels and composites. In addition to these materials, nickel-based alloys are important structural materials for jet engines. These materials are the main focus. Other materials have specific applications for certain types of aircraft, but are not mainstream materials used in large quantities. Examples include magnesium alloys, fibre–metal laminates, metal matrix composites, woods, ceramics for heat insulation tiles for rockets and spacecraft, and radar absorbing materials for stealth military aircraft.
Many other materials are also used in aircraft: copper for electrical wiring; semiconductors for electronic devices; synthetic fabrics for seating and other furnishing. However, none of these materials are required to carry structural loads. In this book, the focus is on the materials used in aircraft structures and jet engines, and not the nonstructural materials which, although important to aircraft operations, are not required to support loads.
Seldom is a single material able to provide all the properties needed by an aircraft structure and engine. Instead, combinations of materials are used to achieve the best balance between cost, performance and safety. Table 1.1 gives an approximate grading of the common aerospace materials for several key factors and properties for airframes and engines. There are large differences between the performance properties and cost of materials. For example, aluminium and steel are the least expensive; composites are the lightest; steels have the highest stiffness and strength; and nickel alloys have the best mechanical properties at high temperature. As a result, aircraft are constructed using a variety of materials which are best suited for the specific structure or engine component.
Table 1.1
Grading of aerospace materials on key design factors
Figure 1.3 shows the types and amounts of structural materials in various types of modern civil and military aircraft. A common feature of the different aircraft types is the use of the same materials: aluminium, titanium, steel and composites. Although the weight percentages of these materials differ between aircraft types, the same four materials are common to the different aircraft and their combined weight is usually more than 80–90% of the structural mass. The small percentage of ‘other materials’ that are used may include magnesium, plastics, ceramics or some other material.
1.3 Structural materials and their weight percentage used in the airframes of civilian and military aircraft. (a) Boeing 737, (b) Airbus 340–330, (c) Airbus A380, (d) Boeing 787, (e) F-18 Hornet (C/D), (f) F-22 Raptor. Photographs supplied courtesy of (a) K. Boydston, (b) S. Brimley, (c) F. Olivares, (d) C. Weyer, (e) J. Seppela and (f) J. Amann.
Aluminium
Aluminium is the material of choice for most aircraft structures, and has been since it superseded wood as the common airframe material in the 1920s/1930s. High-strength aluminium alloy is the most used material for the fuselage, wing and supporting structures of many commercial airliners and military aircraft, particularly those built before the year 2000. Aluminium accounts for 70–80% of the structural weight of most airliners and over 50% of many military aircraft and helicopters, although in recent years the use of aluminium has fallen owing to the growing use of fibre–polymer composite materials. The competition between the use of aluminium and composite is intense, although aluminium will remain an important aerospace structural material.
Aluminium is used extensively for several reasons, including its moderately low cost; ease of fabrication which allows it to be shaped and machined into structural components with complex shapes; light weight; and good stiffness, strength and fracture toughness. Similarly to any other aerospace material, there are several problems with using aluminium alloys, and these include susceptibility to damage by corrosion and fatigue.
There are many types of aluminium used in aircraft whose properties are controlled by their alloy composition and heat treatment. The properties of aluminium are tailored for specific structural applications; for example, high-strength aluminium alloys are used in the upper wing skins to support high bending loads during flight whereas other types of aluminium are used on the lower wing skins to provide high fatigue resistance.
Titanium
Titanium alloys are used in both airframe structures and jet engine components because of their moderate weight, high structural properties (e.g. stiffness, strength, toughness, fatigue), excellent corrosion resistance, and the ability to retain their mechanical properties at high temperature. Various types of titanium alloys with different compositions are used, although the most common is Ti–6Al–4 V which is used in both aircraft structures and engines.
The structural properties of titanium are better than aluminium, although it is also more expensive and heavier. Titanium is generally used in the most heavily-loaded structures that must occupy minimum space, such as the landing gear and wing-fuselage connections. The structural weight of titanium in most commercial airliners is typically under 10%, with slightly higher amounts used in modern aircraft such as the Boeing 787 and Airbus A350. The use of titanium is greater in fighter aircraft owing to their need for higher strength materials than airliners. For instance, titanium accounts for 25% of the structural mass of the F-15 Eagle and F-16 Fighting Falcon and about 35% of the F-35 Lightning II. Titanium alloys account for 25–30% of the weight of modern jet engines, and are used in components required to operate to 400–500 °C. Engine components made of titanium include fan blades, low-pressure compressor parts, and plug and nozzle assemblies in the exhaust section.
Magnesium
Magnesium is one of the lightest metals, and for this reason was a popular material for lightweight aircraft structures. Magnesium was used extensively in aircraft built during the 1940s and 1950s to reduce weight, but since then the usage has declined as it has been replaced by aluminium alloys and composites. The use of magnesium in modern aircraft and helicopters is typically less than 2% of the total structural weight. The demise of magnesium as an important structural material has been caused by several factors, most notably higher cost and lower stiffness and strength compared with aluminium alloys. Magnesium is highly susceptible to corrosion which leads to increased requirements for maintenance and repair. The use of magnesium alloys is now largely confined to non-gas turbine engine parts, and applications include gearboxes and gearbox housings of piston-engine aircraft and the main transmission housing of helicopters.
Steel
Steel is the most commonly used metal in structural engineering, however its use as a structural material in aircraft is small (under 5–10% by weight). The steels used in aircraft are alloyed and heat-treated for very high strength, and are about three times stronger than aluminium and twice as strong as titanium. Steels also have high elastic modulus (three times stiffer than aluminium) together with good fatigue resistance and fracture toughness. This combination of properties makes steel a material of choice for safety-critical structural components that require very high strength and where space is limited, such as the landing gear and wing box components. However, steel is not used in large quantities for several reasons, with the most important being its high density, nearly three times as dense as aluminium and over 50% denser than titanium. Other problems include the susceptibility of some grades of high-strength steel to corrosion and embrittlement which can cause cracking.
Superalloys
Superalloys are a group of nickel, iron–nickel and cobalt alloys used in jet engines. These metals have excellent heat resistant properties and retain their stiffness, strength, toughness and dimensional stability at temperatures much higher than the other aerospace structural materials. Superalloys also have good resistance against corrosion and oxidation when used at high temperatures in jet engines. The most important type of superalloy is the nickel-based material that contains a high concentration of chromium, iron, titanium, cobalt and other alloying elements. Nickel superalloys can operate for long periods of time at temperatures of 800–1000 °C, which makes them suitable for the hottest sections of gas turbine engines. Superalloys are used in engine components such as the high-pressure turbine blades, discs, combustion chamber, afterburners and thrust reversers.
Fibre–polymer composites
Composites are lightweight materials with high stiffness, strength and fatigue performance that are made of continuous fibres (usually carbon) in a polymer matrix (usually epoxy). Along with aluminium, carbon fibre composite is the most commonly used structural material for the airframe of aircraft and helicopters. Composites are lighter and stronger than aluminium alloys, but they are also more expensive and susceptible to impact damage.
Carbon fibre composites are used in the major structures of aircraft, including the wings, fuselage, empennage and control surfaces (e.g. rudder, elevators, ailerons). Composites are also used in the cooler sections of jet engines, such as the inlet fan blades, to reduce weight. In addition to carbon fibre composites, composites containing glass fibres are used in radomes and semistructural components such as fairings and composites containing aramid fibres are used in components requiring high impact resistance.
Fibre-metal laminates
Fibre–metal laminates (FML) are lightweight structural materials consisting of thin bonded sheets of metal and fibre–polymer composite. This combination creates a material which is lighter, higher in strength, and more fatigue resistant than the monolithic metal and has better impact strength and damage tolerance than the composite on its own. The most common FML is GLARE® (a name derived from glass reinforced aluminium) which consists of thin layers of aluminium alloy bonded to thin layers of fibreglass composite. FMLs are not widely used structural materials for aircraft; the only aircraft at present that use GLARE® are the Airbus 380 (in the fuselage) and C17 GlobeMaster III (in the cargo doors).
Leave a Reply