By William Huang
When you watch an airplane fly overhead, you might wonder, “Hmm, how does that thing fly?” Most people think of the engine as the driving force of this amazing machine. Although this is true, the materials that make up the plane structures, such as airfoils, skin, nose, etc. are an essential part as well.
Since the beginning of flight in 1903, scientists and engineers have continually looked toward new and improved aerospace materials. These materials must be specially designed and implemented for various purposes. Researchers often consider weight, strength, stiffness, temperature and corrosive resistance, and cost efficiency. Arguably the most important factor to consider is weight. Reducing the weight of an aircraft is essential to improving fuel efficiency, increasing speed, and decreasing air pollution. This has led to the evolution from cloth and wood to complex alloys, carbon fibers, and more. Let’s take a look into the past, present, and potential future of aerospace materials.
What WAS used back then?
Let’s go back to December 17, 1903, the first ever successful powered, controlled flight by the Wright Brothers. The materials used in the airplane were fairly simple: wood for the airframe, muslin cloth for the skin, and aluminum for the engine. The airframe is the main body of the aircraft and the skin is the outer covering. As you can see, the materials that were used were light-weight, relatively inexpensive, and just strong enough to hold an airplane together. However, they were not very durable; a small tear could easily form in the cloth and a small fire could quickly burn it to the ground. Over the next decade, cloth was taken out of the picture and replaced with plywood for the skin, which is slightly heavier, but much stronger and has greater uniform strength along the grains than solid wood.
Eventually, with WWII spurring rapid progress in aerospace design, aluminum became the most popular material due to its lightweight, great strength, and cheap, easy production. In fact, forty years ago, almost everything from the main body to the engine was made of pure aluminum; that’s about 70% of the aircraft! Additionally, when exposed to air, the metal forms a thin aluminum oxide coating that acts as a natural anti-corrosion mechanism. Despite this, new, more complex materials were beginning to be used, such as titanium, graphite, fiberglass, and other composites and alloys. Although in relatively small quantities, these new materials began to take over the industry.
What’s used NOW?
Today, only about 20% of an aircraft is made of pure aluminum. However, the metal is still used in various alloys that have improved qualities, such as titanium and nickel alloys. These alloys, also called heat resistant super alloys, make it possible for the production of high-performing parts and are able to withstand extremely high pressures, temperatures, and corrosion. These enhanced properties are a result of included elements such as aluminum and chromium and the alloy’s complex crystalline lattice structure. For example, nickel super alloys, which are commonly used in military and commercial jet turbines, are able to withstand temperatures as high as 1600 °F (870 °C) as well as high levels of corrosion.
Besides alloys, composites are also widely used in the aerospace industry. They have less weight and thus increase fuel efficiency while still being easy to design and manufacture. Additionally, they can also be made into larger, more complex shapes and parts that metallic materials are not able to do as they require heavy fasteners and joints to put together. As a result, composites are allowing the development of fewer and larger aircraft parts and designs, decreasing potential failure points. Currently, carbon fiber reinforced polymers, which are fiber-reinforced materials that uses carbon as their primary structural component, dominate the composite materials portion of the industry. Honeycomb materials are also often used to provide sturdy internal structures. From the name, you can probably guess that these materials have the geometry of a honeycomb; as a result, they use a minimal amount of material, making them lightweight and cost effective but also structurally rigid.
Diagram of the different materials used in the Boeing 787 Dreamliner [9].
What WILL be used?
As in the past, researchers are still developing new materials to be used in aircraft. In the near future, heat resistant super alloys may be gradually replaced by ceramic matrix composites, which consist of ceramic (inorganic and non-metallic, often crystalline materials) fibers that are in a ceramic matrix. Ceramic matrix composites are on average, one third the weight of nickel super alloys and can withstand temperatures that are 500 °F (260 °C) higher.
Electron microscope image of a ceramic matrix composite [10].
An issue with composite materials is that they often require specialized methods of maintenance. Additionally, defects in the material are sometimes hard to detect and prevent the entire composite from performing properly. To account for this, self-healing technology is currently being researched and developed. One of the main methods being studied is the use of liquid epoxy resin, a type of polymer, to reach the damaged area and repair it by quickly hardening. The implementation of self-healing materials will greatly reduce repair and maintenance costs of aircraft and allow for greater possibilities.
Overall, aerospace materials have come a long way from cloth and wood to advanced super alloys and composites and eventually to self-healing technology. The consideration of aerospace materials is crucial to the future of the aerospace industry as engineers are constantly looking for ways to increase fuel efficiency and decrease air pollution in the midst of such climate conscious times.
References
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Mraz, S. (2013, March 31). A century of progress in aircraft materials - Part 1 Wood and Fabric. Retrieved from https://www.machinedesign.com/archive/century-progress-aircraft-materials-part-1-wood-and-fabric.
Mraz, S. (2017, March 13). Basics of Aerospace Materials: Aluminum and Composites. Retrieved from https://www.machinedesign.com/materials/basics-aerospace-materials-aluminum-and-composites.
Specialized Alloys Used for the Aerospace Industry. (2013, September 23). Retrieved from https://www.steelforge.com/literature/metal-tidbits/aerospace/.
Gardiner, G. (2017, April 11). The next generation of ceramic matrix composites. Retrieved from https://www.compositesworld.com/blog/post/the-next-generation-of-ceramic-matrix-composites.
Medibonu, S. (2017, April 27). Self Healing Materials for Aircraft Applications. Retrieved from https://contest.techbriefs.com/2017/entries/aerospace-and-defense/7555.
First Flight? (2017, March 22). Retrieved from https://airandspace.si.edu/stories/editorial/first-flight.
Aluminum Honeycomb Core-Aerospace Grade - Toray Advanced Composites. (n.d.). Retrieved from https://www.toraytac.com/product-explorer/products/UCD6/Aluminum-Honeycomb-CoreAerospace-Grade.
Protecting Aircraft Composites from Lightning Strike Damage. (n.d.). Retrieved from https://www.comsol.com/blogs/protecting-aircraft-composites-from-lightning-strike-damage/.
Wood, K. (2013, November 1). Ceramic-matrix composites heat up. Retrieved from https://www.compositesworld.com/articles/ceramic-matrix-composites-heat-up.
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