How are airplane materials selected and developed? The properties of essential materials for airplane construction, the use of duralumin and carbon fiber composites, and research into the next generation of aircraft materials are explained.
The airplane is an iconic symbol of humanity’s progress in science and technology. In the hundred years since the Wright brothers patented the airplane in 1906, it has evolved, had a profound impact on economic development and cultural exchange, and is now an integral part of modern society. For the current generation, airplanes are no longer a novelty, but an everyday form of transportation that we often see whenever we look up, but many of us have probably been on a plane at some point and thought it was amazing to see such a huge aircraft soaring through the air. Still, researchers around the world are working in many different fields to develop faster and more comfortable airplanes. But from a materials science perspective, what materials are chosen and developed to create machines that can carry dozens or hundreds of people through the air?
Before we get into what airplanes are actually made of, it’s worth talking about what properties materials need to have in order to be used in airplanes. First, it needs to be strong enough to carry dozens of people without cracking or breaking, but using denser materials to increase strength would require larger engines and more fuel, making it economically and practically inefficient. Therefore, materials that have a high strength per unit weight, or “specific strength,” rather than just strength, are better suited for aircraft materials. In addition, when a material is repeatedly subjected to a force, even a small one, the effects of that force accumulate inside the material, a process known as fatigue. Materials need to have a long ‘fatigue life’ to ensure that the airplane remains stable over the years. Airplanes also require properties such as abrasion resistance to withstand the air friction they experience during flight, and corrosion resistance to prevent corrosion from humidity or salt in the air. Unlike conventional airplanes, supersonic airplanes, which fly at speeds of several Mach, generate significant heat on their surfaces, requiring high heat resistance. While the importance of these properties varies depending on the type of airplane and its components, a combination of materials can be used to create a safe airplane. In addition, materials with good machinability and formability can save money in the manufacturing process, making them economical.
Early airplanes, including the Wright brothers’ first airplane, were primarily made of wood. Wood’s low specific gravity allowed for lightweight airplanes, but its lack of strength and other properties led to its replacement by duralumin, which was developed by German metallurgist Alfred Wilm (1896-1937) in 1903. Duralumin is a type of aluminum alloy that has similar properties to steel, such as strength, but is about one-third the weight of steel and easy to process, so it was mass-produced in the mid-20th century and used to make various parts of airplanes. ‘Super duralumin’ and ‘ultra super duralumin’, which have been strengthened through various improvement processes, are still widely used as airplane materials today. Examples include the Boeing 747-400, the standard airplane for long-haul international flights with more than 1,000 in service worldwide, and the Boeing 737 series, the largest airplane in Korean Air’s fleet. However, duralumin does not have excellent corrosion resistance and its strength is significantly reduced at high temperatures above about 200℃, so titanium alloys are used for supersonic airplanes that need to withstand high temperatures. Titanium alloys have a very high specific strength, fatigue resistance, and better corrosion resistance than duralumin, but the material itself is expensive and uneconomical to process due to its poor formability. Therefore, they are used only in the parts of the supersonic airplane where they are needed, such as the outer shell, firewall, and heat shield. In addition, special steel, a type of alloy, is used for bolts, nuts, and control surfaces that are subjected to large loads.
Recently, ‘carbon fiber reinforced composites’ have been actively researched as a substitute for structural materials in airplanes. A composite material is a material that combines two or more materials and has the best of each, and its properties can be adjusted by changing the type, ratio, and combination of materials to suit the application. In general, composite materials are made by adding reinforcements with high strength to the main material (matrix) with high ductility such as plastic or metal, and composite materials that use fibers with a thickness of several micrometers (μm) as reinforcement are called fiber-reinforced composites. Carbon fiber reinforced composites, which use carbon fiber as a reinforcing material, have high specific strength and inelasticity while having a specific gravity of only one-sixth that of iron, making them popular as aircraft materials. When a crack is formed inside a material due to a load, the load is concentrated around the crack and the crack advances, causing the material to fracture, but in the case of carbon fiber reinforced composites, the carbon fiber plays a role in stopping the advancement of the crack. Since they are not metallic, they are also highly resistant to corrosion. The use of these composites as structural materials can reduce the overall weight of an aircraft, which improves engine and fuel efficiency, reduces emissions of carbon dioxide and nitrogen oxides that contribute to global warming, and reduces aircraft noise. A prime example is the Boeing-787, which uses 50% of its fuselage in composites, with approximately 43% of that being carbon fiber reinforced composites, reducing overall weight by nearly 5 tons and increasing fuel efficiency by approximately 20%. Traditional duralumin-bodied aircraft are also gradually being replaced by carbon fiber aircraft, such as the B787 and Airbus’ A380. In addition, composite materials with improved properties by adding silica nanoparticles, materials coated with conductive materials to prevent lightning strikes, and composite materials using ceramic fibers are being actively researched as next-generation aircraft materials.
The demand for civilian airliners, cargo planes, and military jets will continue to grow, and the types of aircraft will continue to diversify, including small personal aircraft and unmanned aerial vehicles. The evolution of aircraft materials from wood to carbon fiber reinforced composites has contributed to the expansion of humanity’s living space. Currently, carbon dioxide emissions from aircraft account for about 2% of the world’s total emissions, but the development of lighter airplanes using next-generation materials could significantly improve this problem by allowing them to run on less fuel and smaller engines. Samsung used duralumin in its laptops in 2011 to develop a high-performance, ultra-lightweight laptop, and global automakers such as BMW and Toyota are currently focusing on research into carbon fiber-reinforced composites to lighten vehicle bodies and achieve higher fuel efficiency. Continued research into high-strength, low-weight aircraft materials will enable advances in a variety of areas.
