In this blog post, we will look at how the advancement of materials science has contributed to the design and performance improvement of aircraft, and examine the characteristics of various materials and examples of their application.
Airplanes are a symbol of the development of human science and technology. Since the Wright brothers patented the airplane in 1906, it has continued to evolve over the past century, greatly influencing economic development and cultural exchange, and has now become an indispensable part of modern society. To the current generation, airplanes are no longer something mysterious, but a common mode of transportation that can be seen every now and then. However, many people must have thought that it was amazing to see such a huge aircraft floating in the air and flying leisurely when they took a flight. Research in various fields is still being actively conducted around the world to develop faster and more comfortable airplanes. So, from the perspective of materials science, what materials were selected and developed to create a machine that could carry dozens or hundreds of people and fly in the sky?
Before discussing what an airplane is actually made of, it is necessary to talk about what properties materials must have in order to be used in an airplane. First, it is essential to have high strength that will not be dented or broken even when carrying dozens of people. However, if a material with a high density is used to increase strength, a larger engine and more fuel will be required, making it economically and practically inefficient. Therefore, materials with a high specific strength, or “specific strength,” rather than simple strength, are suitable for aircraft materials. In addition, if a small amount of force is repeatedly applied to a material, the effects of that force accumulate inside the material, which is called “fatigue.” To ensure the stability of an airplane even after it has been in operation for many years, the materials must have a long fatigue life. In addition, the airplane must have the physical properties of “wear resistance” to withstand the air friction it will encounter during flight, and “corrosion resistance” to prevent corrosion caused by humidity or salt in the air. Unlike conventional aircraft, supersonic aircraft, which fly at speeds of several Machs, generate a lot of heat on their surfaces, so they require high heat resistance. Although the importance of these properties varies depending on the type of aircraft and its parts, materials that have all of the above properties can be used to make safe aircraft. In addition, materials with good machinability and formability are economical because they can save costs in the manufacturing process.
The Wright brothers’ first aircraft and other early aircraft were mainly made of wood. Wood has a low specific gravity, which makes it possible to make lightweight aircraft, but it does not have outstanding strength or other physical properties, so it was replaced by “duralumin” developed by German metallurgist Alfred Wilm (1896-1937) in 1903. Duralumin is a type of aluminum alloy that has many advantages, including physical properties such as strength that are similar to steel, a specific gravity of about one-third that of steel, and easy processing. It was mass-produced in the mid-20th century and used to make various parts of airplanes. “Super-duralumin” and ‘super-super-duralumin,’ which have been strengthened through several improvement processes, are still widely used as airplane materials. This includes the Boeing 747-400, which is the standard model for long-haul international flights with more than 1,000 aircraft in operation worldwide, and the Boeing 737 series, which Korean Air has the largest number of. However, duralumin has the disadvantage of not having excellent corrosion resistance and significantly deteriorating in strength at high temperatures of about 200°C or higher, so titanium alloy is used in supersonic aircraft that must withstand high temperatures. Titanium alloys have a very high specific strength, are resistant to fatigue, and have better corrosion resistance than duralumin, but the material itself is expensive and has poor formability, making it uneconomical to use in the manufacturing process. Therefore, it is used only in limited areas, such as the outer shell of supersonic aircraft, firewalls, and heat-resistant walls. In addition, special steel, a type of alloy, is used for parts such as bolts, nuts, and control levers that are subjected to heavy loads.
Recently, “carbon fiber reinforced composite materials” have been actively studied as materials that can replace the structural materials of airplanes. Composite materials are materials that combine two or more materials to create a material with the best of each. The characteristics of the material can be adjusted by changing the type, ratio, and combination of materials according to the application. Generally, composite materials are made by adding a reinforcing material with high strength to a main material (matrix) with excellent ductility, such as plastic or metal. Composite materials with reinforcing materials made of fibers with a thickness of several micrometers (μm) are called fiber-reinforced composite materials. Carbon fiber reinforced composite materials, which use carbon fiber as a reinforcing material, have high specific strength and specific stiffness, but their specific gravity is only one-sixth that of steel, making them popular as aircraft materials. When a crack forms in a material due to internal stress, the stress is concentrated around the crack, causing the crack to propagate and the material to fracture. In the case of carbon fiber reinforced composite materials, the carbon fibers act to prevent the propagation of these cracks. Since it is not metal, it also has excellent corrosion resistance. Using this composite material as a structural material can reduce the overall weight of the aircraft, which will improve engine and fuel efficiency, and further reduce the amount of carbon dioxide and nitrogen oxides that cause global warming and the noise caused by the aircraft. A typical example is the Boeing 787, which uses 50% composite materials, of which about 43% are carbon fiber reinforced composites, reducing the overall weight by about 5 tons and increasing fuel efficiency by about 20%. Aircraft with conventional duralumin bodies are also gradually being replaced with aircraft using carbon fiber, such as the B787 and Airbus A380. In addition, composite materials with improved physical properties by adding silica nanoparticles, materials coated with conductive materials on the surface to prevent lightning strikes, and composite materials using ceramic fibers are being actively researched as next-generation aircraft materials.
In the future, the demand for private passenger and cargo planes and military jets will continue to increase, and the types of demand will continue to diversify, including small private aircraft and unmanned aerial vehicles. Aircraft materials, which have evolved from wood to carbon fiber reinforced composite materials, have greatly contributed to expanding the living space of humanity. Currently, carbon dioxide emissions from aircraft account for about 2% of the world’s total emissions, but if lighter planes are developed using next-generation materials, they can be operated with less fuel and smaller engines, which will significantly improve this problem. Samsung developed a high-performance, ultra-lightweight laptop by using duralumin in its 2011 laptop, and global automakers such as BMW and Toyota are currently focusing on research to reduce the weight of car bodies and achieve high fuel efficiency by applying carbon fiber reinforced composite materials. If research on high-strength, low-weight aircraft materials continues in the future, it will be possible to achieve progress in various areas.
