Aerospace materials: past, present and future
The development of new materials and better utilisation of existing materials has been central to the advancement of aerospace engineering. Advances in the structural performance, safety, fuel economy, speed, range and operating life of aircraft has been reliant on improvements to the airframe and engine materials. Aircraft materials have changed greatly in terms of mechanical performance, durability, functionality and quality since the first powered flight by the Wright Brothers in 1903. Furthermore, the criteria which are used to select materials for aircraft have also changed over the past 100 years. Figure 2.1 presents a timeline for the approximate years when new criteria were introduced into the selection of aircraft materials.
The main criteria for materials selection for the earliest aircraft (c. 1903–1920) was minimum weight and maximum strength. The earliest aircraft were designed to be light and strong; other design criteria such as cost, toughness and durability were given less importance in the quest for high strength-to-weight. Many of the criteria which are now critical in the choice of materials were not recognised as important by the first generation of aircraft designers, and their goal was simply to use materials that provided high strength for little weight. At the time the best material to achieve the strength-to-weight requirement was wood.
The situation changed during the 1920s/1930s when the criteria for materials selection widened to consider a greater number of factors affecting aircraft performance and capability. The design of aircraft changed considerably as commercial and commuter aviation became more popular and the military began to recognise the tactical advantages of fast fighters and heavy bombers. Improved performance from the 1930s led to aircraft capable of flying at fast speeds over long distances while carrying heavy payloads. The requirement for high strength-to-weight remained central to the choice of material, as it had with earlier aircraft, but other criteria such as high stiffness and durability also became important. Higher stiffness allowed sleeker and more compact designs, and hence improved performance. These new criteria not only required new materials but also the development of new production methods for transforming these materials into aircraft components. Aluminium alloys processed using new heat treatments and shaped using new metal-forming processes were developed to meet the expanding number of selection criteria. The importance of availability emerged as a critical issue in the selection of materials during the Second World War. For example, the supply of aluminium to Japan was cut off in the late 1930s/early 1940s, which forced their military to use magnesium in the construction of many fighter aircraft.
Major advances in aerospace technology, particularly jet aircraft, firstgeneration helicopters and rockets/missiles, occurred shortly after World War II. These advances placed greater demands on the performance requirements of the the airframe and engine materials. Another significant milestone was the introduction of pressurised cabin aircraft for high altitude flight during the 1940s. The increased pressure loads exerted on the fuselage led to the development of stressed skin panels made using high-strength material.
Around the same time, the need for materials with fatigue and fracture properties emerged as a critical safety issue, and represents the introduction of the damage tolerance criterion. Damage tolerance is the capability of an aircraft structure to contain cracks and other damage below a critical size without catastrophic failure. The unexpected failure of aircraft structures was common before and, in some instances, during World War II. Aviation was considered a high risk industry and aircraft crashes caused by catastrophic structural failures were common. Designers attempted to minimise the risk by building bulky structures which made the aircraft heavy, but structural failures continued leading to many crashes. The fatigue of metals became more widely recognised as an important issue in the mid-1950s when two Comet airliners, the first of a new generation of civil jet airliners, crashed owing to fatigue-induced cracks in the fuselage. The Comet accidents occurred in the post-war era when civil aviation was starting to boom, and the crashes threatened public confidence in aviation safety. Fracture toughness and fatigue resistance joined other important properties such as weight, stiffness and strength as essential properties in the choice of aircraft materials.
The development of supersonic aircraft together with advances in rocket technology during the 1960s prompted the need for high-temperature materials. The aerospace industry invested heavily in the development of new materials for supersonic airliners such as Concorde, high-speed fighters and surveillance aircraft for the Cold War, and spacecraft and satellites for the Space Race. The investments led to the development of heat-resistant airframe materials such as titanium alloys and special aluminium alloys that were capable of withstanding frictional heating effects during supersonic flight without softening. The need for more powerful engines for aircraft and rockets also drove the development of high-temperature materials capable of operating above 800 °C. New types of nickel-based alloys and other heat-resistant materials were developed to survive within the hottest sections of jet engines.
The need for damage-tolerant materials became more intense in the late 1970s when unexpected failures occurred in ultra-high-strength steel components in United States Air Force (USAF) aircraft. It became clear that the failures involved manufacturing defects and fatigue cracks so small that they could not be found reliably. The USAF introduced a damage tolerant design philosophy which accepted the presence of cracks in aircraft and managed this by achieving an acceptable life by a combination of design and inspection. Achieiving this required the use of materials that were resistant to fatigue cracking and failure.
The certification of new commercial aircraft required manufacturers to demonstrate that fatigue cracks could be detected before reaching the critical length associated with catastrophic failure. Aviation safety authorities such as the FAA introduced stringent regulations on the damage tolerance of safety-critical structures. New commercial aircraft would not be certified and permitted to fly unless new criteria on damage tolerance were met. This change in the certification requirements further increased the need for damage-tolerant materials with excellent fracture toughness and fatigue properties for both airframe and engine applications.
Although always important, weight reduction of civil aircraft became critical during the 1970s owing to rising fuel costs and the revenue opportunities associated with increased range and heavier payload. The OPEC fuel crisis of the 1970s, when the price of Avgas jumped by more than 500%, threatened the financial viability of the global aviation industry and sent many airline companies broke. The aerospace industry implemented new measures to minimise weight and maximise structural performance, and this included the greater use of higher-strength aluminium alloys and the introduction of carbon-epoxy fibre composite materials into secondary structures such as engine cowlings and undercarriage doors.
The realisation of the financial benefits of extending the life of ageing aircraft during the 1980s and 1990s provided greater focus on improved damage tolerance and corrosion resistance. With the rising cost of new aircraft and greater competition among airline companies, including the introduction of low-cost carriers into the aviation market, the need to prolong the operating life of aircraft became critical. New aluminium alloys with improved corrosion resistance and composites materials which are completely resistant to corrosion were used in greater quantities. The 1990s was an era when factors such as the costs of manufacturing and maintenance became increasingly important in the choice of materials. The 1990s was also an era when novel structural materials with radar absorbing properties and low thermal emissions were used in large quantities on stealth military aircraft. Although aircraft with limited stealth capability had been in operation since the 1970s, the need for extremely low radar visibility became a critical requirement that drove the development of radar-absorbing materials.
All of the factors for materials selection outlined above apply today in the choice of materials for modern aircraft: weight, stiffness, strength, damage tolerance, fracture toughness, fatigue, corrosion resistance, heat resistance and so on. The first decade of the 21st century is characterised by an emphasis on materials that reduce the manufacturing cost (by cheaper processing and assembly using fewer parts) and lower through-life operating cost (through longer life with fewer inspections and less maintenance). Reductions in greenhouse gas emissions by reducing aircraft weight and improving engine fuel efficiency are also contemporary issues in materials selection. There is also growing interest in producing materials with environmentally friendly manufacturing processes and using sustainable materials that are easily recycled.
The evolution of aircraft technology and the associated drivers in materials selection has meant that the airframe and engine materials are constantly changing. The approximate year of introduction of the main aerospace materials is shown in Fig. 2.2. Many materials have been introduced, with most being developed specifically for aerospace but later finding applications in other sectors such as rail, automotive or engineering infrastructure. It is important to recognise that continuous improvements have occurred with each type of material since their introduction into aircraft. For example, on-going developments in aluminium alloys have occurred since the 1920s to improve properties such as strength, toughness and corrosion resistance. Similarly, advances with composite materials since the 1970s have reduced costs while increasing mechanical properties and impact toughness. Aircraft designers now have the choice of dozens of aluminium alloys with properties tailored to specific applications and operating conditions.
In this chapter, we study the historical development of the major types of aerospace materials: wood, aluminium, magnesium, titanium, nickel superalloys and composites. The introduction of these materials into aircraft structures or engines, and how their usage and properties have changed over time is discussed. Also, the current status and future growth in aircraft production and how this may impact on the use of materials is examined. The on-going advances in materials technology for next-generation aircraft, helicopters and space-craft are also reviewed.
The era of aerospace materials arguably started with the first powered flight of Kitty Hawk by Orville and Wilbur Wright. The principal criterion used in the selection of materials for the first generation of aircraft (1903–1930) was maximum strength for minimum weight. Every other consideration in materials selection, including stiffness, toughness and durability, were secondary compared with the main consideration of high strength-to-weight. Weight had to be kept to an absolute minimum because of the low power (below 150 hp) of early aircraft engines. The airframes in the earliest aircraft were constructed almost entirely of wood because there were no other suitable materials that combined strength and lightness. The high-strength materials of the early 1900s, such as steel and cast iron, were about 10 times denser than wood and, therefore, too heavy for the airframe.
Wood was the material of choice in early aircraft because of its light weight, stiffness and strength (Fig. 2.3). Wood was also used because it was plentiful, inexpensive, and its properties were well understood through use in other structural applications such as buildings and bridges. Another important reason for using wood was the craftsmen who handbuilt the earliest aircraft were able to easily shape and carve timber into lightweight frames, beams and other structural components. However, wood is not the ideal material and has many inherent problems. The mechanical properties are variable and anisotropic which meant aircraft had to be over-designed to avoid structural damage. Many early aircraft experienced structural failures owing to inconsistent strength properties as the result of ‘soft’ or ‘weak’ spots in the wood. Furthermore, wood absorbs moisture, warps and decays over time, which meant that aircraft required continuous maintenance and on-going repairs.
The first generation of aircraft builders evaluated many types of timber, and found that fir, spruce and several other softwoods were best suited for making structural components with a high ratio of strength-to-weight. The aircraft industry later discovered that laminated plywood construction provided greater strength and toughness than single-piece wood. Laminated plywood consists of thin bonded sheets of timber orientated with the wood grain at different angles. The use of plywood reduced the weight penalty experienced with one-piece timber construction that had to be over-designed. As a result plywood rapidly gained popularity as a structural material in the period between the two world wars. Even during the World War II some fighters and light bombers were constructed from wood and plywood. Probably the most famous wooden aircraft during the war was the de Havilland Mosquito, which, for its time, was a highly advanced fighter/bomber capable of flying at 650 km h− 1.
The large-scale production of fighters, bombers and heavy load transport aircraft during World War II led to the demise of wood as an important structural material. Abundant supplies of high quality timber were not available to many countries during the war, which forced the greater use of alternative materials such as aluminium. Also, wood lacks the stiffness and strength required for many military aircraft, particularly bombers, cargo transporters and other heavy lift aircraft that have high loading on their wings and airframe. The use of wood continued to decline in the post-war era with the development of pressurised cabins for high-altitude flying. Today, few aircraft are constructed using wood, except for some gliders, ultra-lights and piston-driven aircraft, because cheaper, lighter and more structurally efficient materials are available.
The development of aircraft with greater engine power during the 1920s placed increased demands on wood construction that it struggled to meet. The loads on the wings and airframe increased as aircraft became larger and heavier. The wing loading on aircraft built during the 1910s was 30–40 kg m− 2, which could be supported using wooden frames. However, the construction of larger, heavier aircraft in the following decades increased pressure loading on the wings to 500–1000 kg m− 2. Figure 2.4 shows the general trend towards higher wing loads for military aircraft and passenger airliners over the past century. Loads on other parts of the aircraft, particularly the fuselage and tailplane, have also increased. Wood lacks the stiffness, strength and toughness to withstand high loads, and aircraft builders sought other lightweight materials with better structural properties.
With steel being too heavy, the aircraft industry in the 1920s turned to aluminium alloys as a replacement for wood. Aluminium is one of the lightest metals; being about 2.5 times lighter than steel. It is stiffer, stronger, tougher and more durable than timber. Also, aluminium can be easily fabricated into thin skin panels and readily machined into spars, stiffeners and beams for the fuselage and wings.
Aluminium had been available in commercial quantities to aircraft manufacturers since the early 1900s, but it was too soft. Aluminium was first used in the airframe of Zeppelin airships during World War I, but it lacked the strength to be used in fixed-wing aircraft that are more heavily loaded. Metallurgists during the early decades of the twentieth century improved the strength properties of aluminium by the addition of alloying elements and development of heat-treatment processes. Various types and amounts of alloying elements were added to aluminium using a trial and error approach to assess the effect on strength. The metals industry experimented with many alloying elements to increase strength and hardness. The industry also tested different heat treatments and metal-forming processes to improve the mechanical properties. A major breakthrough occurred when the addition of a few percent of copper and other alloying elements was found to increase the strength by several hundred percent. The development of the aluminium alloy ‘Duralumin’ in 1906 was largely responsible for the uptake of aluminium by the aircraft industry from the 1920s. Duralumin is an aluminium alloy containing copper (4.4%), magnesium (1.5%) and manganese (0.6%) which is strengthened by heat treatment. Duralumin sparked an explosion in the use of aluminium in highly-stressed aircraft structures, such as the skins, ribs and stiffeners in the wings and fuselage. The use of aluminium also provided the capability to increase the speed, range and size (payload) of aircraft over that possible with wood.
Following the initial success of Duralumin, the mechanical properties of aluminium alloys improved dramatically in the era between the two world wars owing to on-going research and development. From the 1950s, there was a much better understanding of the effects of alloy composition, impurity control, processing conditions and heat treatment on the properties of aluminium. Figure 2.5 shows the sustained improvement in the strength of aluminium alloys since the 1920s. Similar improvements have been achieved with other important properties, including longer fatigue life, greater fracture toughness and damage tolerance, and better corrosion resistance. These developments have been driven largely by the demands of the aerospace industry for more structurally efficient materials. Other major advances in aluminium technology occurred in the 1960s/70s when Al–Li alloys, which have higher stiffness and lower weight than conventional alloys, were developed. New heat-treatment processes developed in the 1970s/80s resulted in better toughness, damage tolerance and corrosion resistance.
Aluminium is the material of choice for most aircraft structures, and typically accounts for 70–80% of the structural weight of most commercial airliners and over 50% of military aircraft and helicopters. In recent years, however, the percentage of airframe weight consisting of aluminium has declined owing to greater use of carbon fibre-polymer composites. Figure 2.6 shows the recent decline in the use of aluminium in airliners owing to greater usage of composite materials in the fuselage, wings and other major structures. Despite this drop, aluminium will remain an important structural material for both aircraft and helicopters.
Like aluminium, magnesium has been used for many years as an airframe material because of its low weight. Magnesium is lighter (by nearly 40%) than aluminium, although it has never been a serious challenger to aluminium because of its higher cost and inferior structural properties. Magnesium alloys have lower stiffness, strength, fatigue resistance and toughness than the types of aluminium used in aircraft. The greatest problem with magnesium is poor corrosion resistance. Magnesium is highly susceptible to various forms of corrosion, and when used in aircraft requires corrosion protective coatings and regular inspections for corrosion damage.
Magnesium was first used in German military aircraft during World War I and used extensively in German and Japanese aircraft during World War II owing to limited supplies of aluminium. The use of magnesium reached its peak in the 1950s and 60s, and, since the early 1970s, usage has declined and now it is used sparingly (under 1–2% by weight) in modern aerostructures owing to corrosion problems and low mechanical properties. For example, Fig. 2.7 shows the fall in the use of magnesium in Russian-built Tupolev aircraft over the past fifty years, and this reflects the general reduction in magnesium usage in many aircraft types. Magnesium remains a useful material in aircraft and helicopters even though its usage is low, and it is unlikely to be completely eliminated from aircraft.
Titanium was first used in military and commercial aircraft during the 1950s. The original need for titanium arose from the development of supersonic aircraft capable of speeds in excess of Mach 2. The skins of these aircraft require heat-resistant materials that do not soften owing to frictional heating effects at supersonic speeds. Conventional aluminium alloys soften when the aircraft speed exceeds about Mach 1.5 whereas titanium remains unaffected until Mach 4–5. The USAF developed the SR-71 Blackbird with an all-titanium skin construction in the mid-1960s (Fig. 2.8). The SR-71 was a high-altitude reconnaissance aircraft with a maximum speed in excess of Mach 3, and, at the time, was one of the world’s most sophisticated and fastest aircraft. The SR-71 demonstrated the application of titanium in airframe structures. Titanium is stiffer, stronger and more fatigue resistant than aluminium, and for these reasons it has been used increasingly in heavily loaded structures such as pressure bulkheads and landing gear components.
The use of titanium in commercial aircraft has increased over recent decades, albeit slowly owing to the high cost of titanium metal and the high costs of manufacturing and machining titanium components. Although the mechanical properties of titanium are better than those of aluminium, the material and manufacturing costs are much higher and it is uneconomical to use in structural components unless they need to be designed for high loads. For this reason, the structural weight of titanium in airliners is typically under 10%, although higher amounts are used in new aircraft types such as the Airbus 350 and Boeing 787 as shown in Fig. 2.9. Titanium usage is greater in fighter aircraft because the loads on the wings and fuselage are higher and the cost is less critical in materials selection. Titanium is also used in gas turbine engine components required to operate at temperatures of 450–500 °C. Titanium has high static, fatigue and creep strengths as well as excellent corrosion resistance at elevated temperatures, which makes it suitable for jet engines. Titanium engine components include fan blades, guide vanes, shafts and casings in the inlet region; low-pressure compressor; and plug and nozzle assemblies in the exhaust section. Titanium alloys account for 25–30% of the weight of many modern jet engines.
The development of aircraft, helicopters and rockets is reliant on the development of materials for gas turbine engines or rocket motors that can operate at high temperatures for a long time without softening or degrading. Superalloys are an important group of high-temperature materials used in the hottest sections of jet and rocket engines where temperatures reach 1200–1400 °C. Superalloys are based on nickel, cobalt or iron with large additions of alloying elements to provide strength, toughness and durability at high temperature.
Since the introduction of jet engines in the post-World War II period the aerospace industry has invested heavily in alloy development, metal-casting processes, and metal-forming technologies to raise the maximum operating temperature of superalloys. The need to improve the efficiency and thrust of engines has resulted in an enormous increase in the temperature at the entry of the high-pressure turbine section. This temperature has risen over the past sixty years from 800 to 1600 °C, and future engines will probably be required to operate at about 1800 °C. These increases are only possible with the development of materials capable of operating for long periods at extremely high temperatures. Figure 2.10 shows the general trend for improvement in the operating temperature limit (creep strength) of nickel-based superalloys used in high-pressure turbine blades since the late-1960s. Advances in alloy composition, impurity control and casting technology (including the development of directional solidification and single crystal casting methods) together with the development of thermal ceramic coatings for superalloys have increased greatly the maximum operating temperature, thus resulting in increased engine performance by raising the power-to-weight ratio and fuel economy. Fuel consumption is an important metric in evaluating the operational efficiency of an airliner, and Fig. 2.11 shows the improvement in the fuel economy since the late 1950s. Over the fifty-year period, fuel consumption of engines has dropped by 50% and the aircraft fuel burn per seat has fallen over 80%. Continuous advances in the main factors that affect fuel burn rate, viz, airframe design, engine design, flight control and navigation, and advanced materials, have led to large improvements in fuel economy. Not only does this cut the operating cost of aircraft, but it also reduces greenhouse gas emissions and other pollutants owing to lower fuel consumption.
2.10 Improvement in the temperature capability (creep strength) of nickel superalloys used in jet engines since the late 1960s. The alloy type and casting methods (CC = chill casting; DS = directional solidification casting; SC = single-crystal casting) are given.
The durability and operating life of engine components have also improved dramatically in recent decades owing to advances in their materials. For example, when the Boeing 707 entered service in 1958, the engines were removed for maintenance after about 500 h of operation. Most of the maintenance related to deterioration of the high-pressure turbine blades that were made of early versions of superalloys. Today, a Boeing 747 class engine can operate for 20 000 h without major maintenance. This remarkable improvement is in part the result of advances in the metallurgy of nickel-based superalloys and other high-temperature materials, including ceramic coatings.
Fibre-reinforced polymer composites are another important group of aerospace materials that have a long history of usage. Composites were first used in the 1940s for their high strength-to-weight ratio and corrosion resistance. The first generation of composite material consisted of glass fibres in a low-strength polymer matrix. The potential application of this material was demonstrated during the late 1940s and 1950s in various prototype aircraft components and filament-wound rocket motor cases. However, the aerospace industry was initially reluctant to use composites in large quantities because the original fibreglass materials were expensive to produce; difficult to manufacture with a high degree of quality control; their mechanical properties were inconsistent and variable owing to inadequate processing methods; and they were prone to delamination cracking when subjected to impact events such as bird strike. Furthermore, the elastic modulus of fibreglass composite is low and therefore it is not suitable in structural applications that require high stiffness. Composites were gradually introduced into semistructural aircraft components during the 1950s and 1960s, such as engine cowlings and undercarriage landing gear doors, to reduce weight and avoid corrosion.
The evolution of primary aircraft structures from aluminium to composite has been slow owing to the commercial risk involved with making the change. The aerospace industry, particularly those companies producing civil aircraft, is conservative owing to the financial and safety risk of changing structural materials. The industry recognised that the changeover from aluminium to composite may possibly provide benefits such as increased airframe life and reduced production costs and weight, but none of these were guaranteed. Furthermore, the transition from metals to composites requires a complete change of production facilities, which comes at huge expense. Aluminium remains a satisfactory material, despite problems with fatigue and corrosion, and the strong incentive to replace this material with composite was lacking for many years. Furthermore, the large weight savings attributed to composites are not always achieved, and often the reduction in mass achieved by replacing aluminium with composite has been modest.
A major change in the use of composite material occurred in the 1960s with the commercial production of carbon fibres. Carbon-fibre composites are lightweight, stiff, strong, fatigue resistant and corrosion resistant, and for these reasons their potential application in both airframes and engines was immediately recognised by the aerospace industry. However, the high cost of carbon fibres, poor understanding of the design rules, structural properties and durability together with technical challenges in certification meant that the initial use of carbon-fibre composites was small. Until the 1970s, the use of carbon-fibre composites was limited to semistructural components which accounted for less than 5% of the airframe weight. Corrosion problems with aluminium and the OPEC energy crisis in the 1970s were incentives for the aerospace industry to expand the use of carbon-fibre composites in both fighter aircraft and commercial airliners. As design methods and manufacturing processes improved and the cost of carbon fibre dropped the amount of composite material used in aircraft increased during the 1980s and 1990s, as shown in Fig. 2.12.
Major milestones in the use of carbon-fibre composites were applications in primary structures of fighter aircraft such as the Harrier (AV-8B) and Hornet (F-18) and in the tail section of the Boeing 777 in the 1990s. The use of composites in the fuselage and wings of modern airliners such as the A380, A350 and B787 are recent major events. The use of carbon-fibre composites in helicopter components, such as the body, tail boom and rotor blades, has also increased dramatically since the 1990s. Composites are the first material since the 1930s to seriously challenge the long-held dominant position of aluminium in airframe construction, and the competition between these two materials is likely to be intense in coming years.
Composite materials are increasingly being used in low-temperature components in jet engines because of their light weight. Carbon-fibre composites were first used in gas turbine engines in the 1960s for their light weight and high mechanical properties. Because of their low softening temperature, however, composites can only be used in low-temperature engine components such as the air inlet fan where the temperature remains below 180–200 °C. An early engine application for carbon-fibre composites was in fan blades in the Rolls-Royce RB-211 high-bypass turbofan, which was designed in the late 1960s for aircraft such as the Lockheed L-1011 (TriStar). Unfortunately, Rolls-Royce pushed the state-of-the-art with carbon-fibre composites too far because the blades were vulnerable to damage from bird impact and there were manufacturing problems. This combined with other technical problems and major cost blow-outs in the development of the RB-211 caused Rolls-Royce to become insolvent in 1971 and the company was nationalised by the UK Government. This incident demonstrates the serious problems that can occur when new materials are introduced into critical aircraft components before their capabilities are fully characterised and certified. Despite this initial set-back, major aircraft engine manufacturers continued to develop composite components during the 1970s/1980s, and in recent years carbon-fibre materials have been used reliably in fan blades and inlet casings. The use of composites is expected to increase further in coming years with the development of higher-temperature polymers and improvements in impact damage tolerance.
Other types of composites have been developed for airframe and engine applications, although their usage is much less than that of polymer matrix materials. Metal matrix composites (MMC) were first developed in the 1950s/60s to improve the structural efficiency of monolithic metals such as aluminium. MMCs are composed of a hard reinforcing phase dispersed in a continuous metal matrix phase. The reinforcement is often a ceramic or man-made fibre (boron, carbon) in the form of small particles, whiskers or continuous filaments. The development of MMCs resulted in materials that are stiffer, harder, stronger and, in some cases, lighter and more fatigue resistant than the base metal. Early applications of MMCs included fuselage struts in the space shuttle orbiter, ventral fins and fuel access doors in the F-16 Fighting Falcon, and main rotor blade sleeves in some helicopters. The use of MMCs in jet engines has been evaluated, but, to date, the applications are limited to components such as fan exit guide vanes in specific engine types and they are not widely used in large-scale commercial production of engine parts. MMCs have largely failed to make a major impact in structural or engine applications. During the development of MMCs it became obvious that these materials are expensive to produce; difficult to forge, machine and join; and have low ductility and toughness; and for these reasons they are not often used.
Ceramic matrix composites (CMCs) are another class of composite material introduced into aircraft and spacecraft in the 1970s. CMCs consist of ceramic reinforcement embedded in a ceramic matrix. CMCs were developed for high-temperature applications which require materials with higher strength and toughness than conventional monolithic ceramics. The most famous CMC is reinforced carbon–carbon which gained fame through its use in heat shields on the space shuttle, brake discs for aircraft, and engine nozzle liners for rockets and missiles.
Fibre-metal laminates (FML) were developed as damage-tolerant composite materials for aircraft structures during the 1980s. The original FML was called ARALL, which consists of thin layers of aramid fibre composite sandwiched between layers of aluminium alloy. Difficulties with manufacturing and problems with moisture absorption lead to the development of an alternative FML known as GLARE, which comprises alternating layers of fibreglass composite and aluminium. GLARE has higher strength, fatigue resistance, damage tolerance and corrosion durability than monolithic aluminium, and was first used widely in the upper fuselage of the Airbus 380 and later in cargo doors for the C-17 Globemaster III heavy-lift transporter. The future of FMLs in other large aircraft is uncertain owing to high production and manufacturing costs.
On-going advances in materials technology are essential to the success of the aerospace industry in the design, construction and in-service operation of aircraft. The aerospace industry is broadly defined as an industry network that designs, builds and provides in-service support to aircraft, helicopters, guided missiles, space vehicles, aircraft engines, and related parts. The industry includes small to medium-sized enterprises that design, manufacture or service specific aerospace items for large global companies such as Boeing, EADS and Lockheed-Martin who design, assemble, sell and provide in-service support to the entire aircraft system.
Improvements in materials, whether by making them cheaper, lighter, stronger, tougher or more durable, have been crucial to the development of better and safer aircraft. It is worthwhile examining the current state and future growth of the aerospace industry to understand the need for on-going advances in aerospace materials. The global aerospace industry in 2008 had an annual turnover of about $275 billion. This makes aerospace one of the most valuable industries in the world. The largest national players are the USA (which has about 50% of the market), European Union, Japan, Canada and, increasingly, China. The aerospace industry employs world-wide about 1.5 million people, with many engaged in highly skilled professions. The aerospace industry is not only important because of the economic wealth and employment it generates, but also from the generation of knowledge. Globally, aerospace drives innovation and skills in the engineering and information technology sectors which are then used by other (non-aerospace) industries. This includes the development of new materials and their manufacturing processes.
Figure 2.13 shows the approximate value of the different market segments in the aerospace industry. About 50% (or ~$140 billion) of the annual turnover is derived from the manufacture and sales of aircraft, of which the greatest share (23%) is from large commercial airliner sales. In comparison, military aircraft (7%), business jets (6%) and helicopters (4%) account for a relatively small, but still valuable, share of the market. The other 50% of industry turnover is related to the in-service support and maintenance of aircraft.
Aerospace is not a stable, constant and predictable industry; but is a volatile industry subject to fluctuations in the growth and recession of the global economy. Not surprisingly, the growth of the aerospace industry closely tracks the demand for air travel, which over the past thirty years has grown at an average rate of 8%. Figure 2.14 shows the number of new large commercial aircraft sales in the period between the mid-1970s and mid-2000s. The number of aircraft sales grew during prolonged periods of global economic growth and dropped during economic recessions or major terrorist attacks that affected public confidence in aviation safety. This trend is expected to continue, with faster growth occurring during periods of economic prosperity and slower (or negative) growth during recessions and when the public lacks confidence in aviation safety.
Despite the fluctuations in new aircraft sales, the major aerospace companies predict an era of sustained growth. Several key statistics illustrate the projected growth in civil aviation between 2004 and 2024:
• The world-wide fleet of large passenger aircraft is expected to more than double by 2024, growing to over 35 000. About 57% of the fleet operating today (9600 airliners) is forecast to still be in operation in 2024 and the remainder (7200 airliners) to be retired. An additional 18 500 aircraft are needed to fill the capacity demand.
• New aircraft deliveries between 2004 and 2025 are forecast to include about 11 000 single-aisle and small jet freighters, 2000 small twin-aisle and regional freighters, and over 1600 large twin-aisle aircraft.
• Passenger traffic is forecast to grow at an average of 4.8% per year, resulting in a three-fold growth between 2004 and 2023. Airfreight is forecast to grow even faster, with freight tonnes kilometres increasing at an average of 6.2% per year.
In addition to a growing number of new aircraft, the age of existing aircraft has risen considerably over the past decade and is likely to continue rising. The average age of military aircraft is also increasing, as shown in Fig. 2.15.
Growth of the aviation industry must be supported by the construction of new aircraft and the life extension of existing aircraft. The materials used in new aircraft have a major economic impact on the profitability of manufacturers and airline companies. Similarly, the materials used in existing aircraft have a large influence on the airframe life and maintenance costs. The future of the aerospace industry is reliant on advances in materials technology. Development of new materials and research into the life extension and durability of existing materials is essential for the on-going success of the industry.
The future success of the aerospace industry both in terms of the cost-effective manufacture of new aircraft and the cost-effective extension of the operating life of existing aircraft is reliant on on-going improvements to existing materials and the development of new materials. Advances in materials technology is classified as evolutionary or revolutionary. Evolutionary advances mean that small, incremental improvements are made to existing materials, such as a new alloy composition, processing method or heat treatment. Examples include the addition of new alloying elements to nickel superalloys to increase the creep resistance and maximum operating temperature or the development of a new thermal ageing treatment for aluminium alloys to increase their resistance to stress corrosion. The evolutionary approach is often preferred by the aerospace industry since past experience has shown that almost every new material has some initial problems. The aerospace industry is more comfortable with incremental improvements on conventional materials, for which they have good knowledge of the design, manufacture, maintenance and repair issues.
Revolutionary advances are the application of new materials to structures or engines that are different to previously used materials. An example was the first-time that carbon-fibre composite was used to fabricate a primary load-bearing structure (the tail section) on a commercial airliner (B777) in the mid-1990s. Another example was the use of GLARE in the A380 in 2005, which was the first application of a fibre-metal laminate in an aircraft fuselage. Revolutionary materials usually have had limited success in being directly incorporated into aircraft owing to the high costs of manufacturing, qualification and certification. The cost and time associated with developing a new material and then testing and certifying its use in safety-critical components can cost an aerospace company hundreds of millions of dollars and take 5–10 years or longer. The introduction of new material can require major changes to the production infrastructure of aircraft manufacturing plants as well as to the in-service maintenance and repair facilities. For example, some suppliers of structural components to the Boeing 787 had to make major changes to their production facilities from metal to composite manufacturing, which required new design methods, manufacturing processes and quality control procedures as well as reskilling and retraining of production staff. As another example, the introduction of new radar absorbing materials on stealth aircraft such as the F-35 Lightning II require new repair methods in the event of bird strike, hail impact, lightning strikes and other damaging events. Despite the challenges, revolutionary materials are introduced into new aircraft when the benefits outweigh the potential problems and risks.
Evolutionary and revolutionary advances across a broad range of materials technologies are on-going for next-generation aircraft. It is virtually impossible to give a complete account of these advances because they are too numerous. Some important examples are: development of high-temperature polymers for composites capable of operating at 400 °C or higher; new polymer composites that are strengthened and toughened by the addition of nano-sized clay particles or carbon nanotubes; new damage tolerant composites that are reinforced in the through-thickness direction by techniques such as stitching, orthogonal weaving or z-pinning; multifunctional materials that serve several purposes such as thermal management, load-bearing strength, self-assessment and health monitoring, and self-actuation functions; bio-inspired polymer materials with self-healing capabilities; sandwich materials that contain high-performance metal-foam cores, truss or periodic open-cell cores; new tough ceramic materials with improved structural capabilities; rapidly solidified amorphous metals with improved mechanical properties and corrosion resistance; and new welding and joining processes for dissimilar materials. The aerospace materials for this century are sure to be just as ground-breaking and innovative as the materials used in the past century. On-going improvement in structural and engine materials is essential for the advancement of aerospace engineering. After many years of commercial service it might be expected that structural and engine material technology would be approaching a plateau, and the pace of innovation would decline. However, customer demands for higher performance, lower operating costs and more ‘environmentally friendly’ propulsion systems continue to drive materials research to ever more challenging goals.
The materials used in aircraft structures and engines have changed dramatically over the past century or so to meet the advances in aircraft technology. As aircraft have become faster, larger and more technically advanced, the demands on the materials have become more intense. The evolution of aerospace materials has been controlled by the evolution of the factors used in their selection. Originally materials were selected based on their strength and weight, but as aircraft have become more advanced the selection has become based on a multitude of structural performance, durability, damage tolerance, economic, environmental and other factors. This has forced improvements to the properties of aerospace materials over the past 100 years, and these improvements will continue with on-going research and development of new materials and processing methods.
The most used aircraft structural material since the 1930s is aluminium alloy. Over the past 80 years or so there have been continuous improvements in the strength, corrosion resistance and other properties of aluminium through research that has led to better alloy compositions, control of impurities, superior heat treatments and forming processes. Aluminium will remain an important structural material for both civil and military aircraft despite the increasing use of composite materials.
The use of carbon-fibre composites has increased greatly since the mid-1990s, and this material is now competing ‘head-to-head’ with aluminium as the dominant aerospace structural material. The increased use of composites is the result of many factors, including improvements in the properties of the fibres and polymer matrix, better design techniques and manufacturing methods, and the requirements for greater structural efficiency, reduced operating costs, and better fatigue and corrosion resistance for aircraft.
The development of jet engine technology since the mid-1940s has relied heavily on advances in high-temperature materials, particularly nickel-based superalloys. Improvements to alloy composition and casting methods have resulted in large increases in the temperature limit of engines over the past 60 years or so, resulting in greater thrust and power. There have also been large improvements in the fuel economy and emissions for jet engines, resulting, in part, from advances in materials technology.
The general approach to implementing new materials into aircraft is through evolutionary advances, which simply means incremental improvements to the properties of the materials already in use. Revolutionary advances which involve the application of new material are less common, although they do occur when the benefits outweigh the cost and risk.