Introduction to aerospace materials
The importance of materials science and technology in aerospace engineering cannot be overstated. The materials used in airframe structures and in jet engine components are critical to the successful design, construction, certification, operation and maintenance of aircraft. Materials have an impact through the entire life cycle of aircraft, from the initial design phase through to manufacture and certification of the aircraft, to flight operations and maintenance and, finally, to disposal at the end-of-life.
Aerospace materials are defined in this book as structural materials that carry the loads exerted on the airframe during flight operations (including taxiing, take-off, cruising and landing). Structural materials are used in safety-critical airframe components such as the wings, fuselage, empennage and landing gear of aircraft; the fuselage, tail boom and rotor blades of helicopters; and the airframe, skins and thermal insulation tiles of spacecraft such as the space shuttle. Aerospace materials are also defined as jet engine structural materials that carry forces in order to generate thrust to propel the aircraft. The materials used in the main components of jet engines, such as the turbine blades, are important to the safety and performance of aircraft and therefore are considered as structural materials in this book.
An understanding of the science and technology of aerospace materials is critical to the success of aircraft, helicopters and spacecraft. This book provides the key information about aerospace materials used in airframe structures and jet engines needed by engineers working in aircraft design, aircraft manufacturing and aircraft operations.
Advanced materials have an important role in improving the structural efficiency of aircraft and the propulsion efficiency of jet engines. The properties of materials that are important to aircraft include their physical properties (e.g. density), mechanical properties (e.g. stiffness, strength and toughness), chemical properties (e.g. corrosion and oxidation), thermal properties (e.g. heat capacity, thermal conductivity) and electrical properties (e.g. electrical conductivity). Understanding these properties and why they are important has been essential for the advancement of aircraft technology over the past century.
Understanding the properties of materials is reliant on understanding the relationship between the science and technology of materials, as shown in Fig. 1.1. Materials science and technology is an interdisciplinary field that involves chemistry, solid-state physics, metallurgy, polymer science, fibre technology, mechanical engineering, and other fields of science and engineering.
Materials science involves understanding the composition and structure of materials, and how they control the properties. The term composition means the chemical make-up of the material, such as the types and concentrations of alloying elements in metals or the chemical composition of polymers. The structure of materials must be understood from the atomic to final component levels, which covers a length scale of many orders of magnitude (more than 1012). The important structural details at the different length scales from the atomic to macrostructure for metals and fibre-polymer composites, which are the two most important groups of structural materials used in aircraft, are shown in Fig. 1.2. At the smallest scale the atomic and molecular structure of materials, which includes the bonding between atoms, has a large influence on properties such as stiffness and strength. The crystal structure and nanoscopic-sized crystal defects in metals and the molecular structures of the fibres and polymer in composites also affect the properties. The microstructure of materials typically covers the length scale from around 1 to 1000 μm, and microstructural features in metals such as the grain size, grain structure, precipitates and defects (e.g. voids, brittle inclusions) affect the properties. Microstructural features such as the fibre arrangement and defects (e.g. voids, delaminations) affect the properties of composites. The macrostructural features of materials, such as its shape and dimensions, may also influence the properties. The aim of materials science is to understand how the physical, mechanical and other properties are controlled over the different length scales. From this knowledge it is then possible to manipulate the composition and structure of materials in order to improve their properties.
Materials technology (also called materials engineering) involves the application of the material properties to achieve the service performance of a component. Put another way, materials technology aims to transform materials into useful structures or components, such as converting soft aluminium into a high strength metal alloy for use in an aircraft wing or making a ceramic composite with high thermal insulation properties needed for the heat shields of a spacecraft. The properties needed by materials are dependent on the type of the component, such as its ability to carry stress without deforming excessively or breaking; to resist corrosion or oxidation; to operate at high temperature without softening; to provide high structural performance at low weight or low cost; and so on. Materials technology involves selecting materials with the properties that best meet the service requirements of a component as well as maintaining the performance of the materials over the operating life of the component by resisting corrosion, fatigue, temperature and other damaging events.
Most aerospace engineering work occurs in the field of materials technology, but it is essential to understand the science of materials. This book examines the interplay between materials science and materials technology in the application of materials for aircraft structures and jet engines.
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 of this book. 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.
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 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 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 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 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 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.
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 (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).
Selecting the best material for an aircraft structure or engine component is an important task for the aerospace engineer. The success or failure of any new aircraft is partly dependent on using the most suitable materials. The cost, flight performance, safety, operating life and environmental impact from engine emissions of aircraft is dependent on the types of materials that aerospace engineers choose to use in the airframe and engines. It is essential that aerospace engineers understand the science and technology of materials in order to select the best materials. The selection of materials for aircraft is not guesswork, but is a systematic and quantitative approach that considers a multitude of diverse (and in some instances conflicting) requirements. The selection of materials is performed during the early design phase of aircraft, and has a lasting influence which remains until the aircraft is retired from service.
The key requirements and factors that aerospace engineers must consider in the selection of materials are listed below and in Table 1.2.
|Processing costs, including machining, forming, shaping and heat treatment costs.|
|In-service maintenance costs, including inspection and repair costs.|
|Recycling and disposal costs.|
|Availability||Plentiful, consistent and long-term supply of materials|
|Manufacturing||Ease of manufacturing.|
|Low-cost and rapid manufacturing processes.|
|Density||Low specific gravity for lightweight structures.|
|Static mechanical properties||Stiffness (elastic modulus).|
|Strength (yield and ultimate strength).|
|Fatigue durability||Resistance against initiation and growth of cracks from various sources of fatigue (e.g. stress, stress-corrosion, thermal, acoustic).|
|Damage tolerance||Fracture toughness and ductility to resist crack growth and failure under load.|
|Notch sensitivity owing to cut-outs (e.g. windows), holes (e.g. fasteners) and changes in structural shape. Damage resistance against bird strike, maintenance accidents (e.g. dropped tools on aircraft), impact from runway debris, hail impact.|
|Environmental durability||Corrosion resistance.|
|Moisture absorption resistance.|
|Wear and erosion resistance.|
|Space environment (e.g. micrometeoroid impact, ionizing radiation).|
|Thermal properties||Thermally stable at high temperatures.|
|High softening temperatures.|
|Low thermal expansion properties.|
|Electrical and magnetic properties||High electrical conductivity for lightning strikes.|
|High radar (electromagnetic) transparency for radar domes.|
|Radar absorbing properties for stealth military aircraft.|
Cost. The whole-of-life cost of aerospace materials must be acceptable to the aircraft operator, and obviously should be kept as low as possible. Whole-of-life costs include the cost of the raw material; cost of processing and assembling the material into a structural or engine component; cost of in-service maintenance and repair; and cost of disposal and recycling at the end of the aircraft life.
Mechanical properties. Aerospace materials must have high stiffness, strength and fracture toughness to ensure that structures can withstand the aircraft loads without deforming excessively (changing shape) or breaking.
Damage tolerance. Aerospace materials must support the ultimate design load without breaking after being damaged (cracks, delaminations, corrosion) from bird strike, lightning strike, hail impact, dropped tools, and the many other damaging events experienced during routine operations.
Thermal properties. Aerospace materials must have thermal, dimensional and mechanical stability for high temperature applications, such as jet engines and heat shields. Materials must also have low flammability in the event of aircraft fire.
Environmental durability. Aerospace materials must be durable and resistant to degradation in the aviation environment. This includes resistance against corrosion, oxidation, wear, moisture absorption and other types of damage caused by the environment which can degrade the performance, functionality and safety of the material.
The materials used in aircraft have a major influence on the design, manufacture, in-service performance and maintainability. Materials impact on virtually every aspect of the aircraft, including cost, design options, weight, flight performance, engine power and fuel efficiency, in-service maintenance and repair, and recycling and disposal at the end-of-life.
Understanding the materials used in aircraft relies on understanding both the science and technology of materials. Materials science involves studying the effects of structure and composition on the properties. Materials technology involves understanding how the material properties can be used to achieve the in-service performance requirements of a component.
Although there are over 120 000 materials, less than about 100 different materials are used in the airframe and engines of aircraft. The four major types of structural materials are aluminium alloys, fibre–polymer composites (particularly carbon fibre–epoxy), titanium alloys and high-strength steels; these materials account for more than 80% of the airframe mass in most commercial and military aircraft. An important high temperature material for jet engines is nickel-based superalloy. Other materials are used in the airframe or engines in small amounts, and include fibre–metal laminates, ceramic matrix composites, magnesium alloys and, in older and light aircraft, wood.
Selection of the best material to meet the property requirements of an aircraft component is critical in aerospace engineering. Many factors are considered in materials selection, including whole-of-life cost; ease of manufacturing; weight; structural efficiency; fatigue and damage tolerance; thermal, electrical, electromagnetic and radar absorption properties; and durability against corrosion, oxidation and other damaging processes.
Barrington, N., Black, M. Aerospace materials and manufacturing processes at the millennium. In: Cantor B., Assender H., Grant P., eds. Aerospace materials. Bristol: Institute of Physics Publishing; 2001:3–14.