Composite Materials: Building the Next Generation of Passenger Aircraft
Moving Beyond Metals
The Japan Aerospace Exploration Agency (JAXA), Japan’s equivalent of NASA, has made a name for itself internationally in recent years with such high-profile successes as the asteroid explorerHayabusa and the Kibo module that has been making important contributions to experiments on the International Space Station. But the agency’s activities are not restricted to space exploration. JAXA is also home to Japan’s only aircraft research center, working on everything from environmentally friendly engines and supersonic aircraft to carrying out safety tests with state-of-the-art simulators.
JAXA is Japan’s leading research institute in the fields of space and aviation technology. The cutting-edge technology developed for use in aircraft application promises to unlock new potential in a wide variety of fields.
One development that has made the news in Japan recently is the Mitsubishi Regional Jet, or MRJ, developed by the Mitsubishi Aircraft Corporation. This is the first passenger airplane to be designed and produced in Japan for 50 years. The fuselage of the plane uses a new type of composite material developed by the Civil Transport Team that is part of JAXA’s Aviation Program Group.
When the Wright brothers made the first successful manned flight in 1903, their pioneering plane was built of wood, steel wire, and cloth. Later aircraft used aluminum alloys and titanium. In recent years composite materials, and carbon fiber reinforced plastic (CFRP) in particular, have come to play an increasingly important role.
Providing the Materials for Boeing’s Latest Planes
CFRP is made by impregnating a resin (plastic) with carbon fibers that act as a reinforcing material. Carbon fiber is light, with a specific weight one-fourth that of iron, and approximately ten times stronger per unit weight. It is extremely durable and, unlike metal, presents no problems with rust. Boeing’s latest passenger aircraft, the B787, uses Japan-developed CFRP for about half of its structural material. This makes it the world’s first passenger airplane to contain a greater proportion of CFRP than aluminum by structural weight. CFRP is also used in the Mitsubishi Regional Jet and the Airbus A350, scheduled to debut in 2013.
Dr. Ishikawa Takashi, Executive Director of the Aviation Program Group at JAXA, explains the advantages of using CFRP in aircraft.
“First of all, the lighter weight means that the plane consumes less fuel. CFRP also has excellent specific strength and specific stiffness, and it doesn’t rust—so you don’t have to carry out maintenance as often as with conventional materials. This minimizes the toll it takes both on the environment and on the finances of the airline company. The material has advantages for passengers as well. If you use aluminum as the main structural material, the humidity inside the plane needs to be kept at 10% or lower. But thanks to the new material, cabin humidity in the B787 can be increased to nearly 50%. This makes for a much more comfortable environment for passengers.”
New Methods Reduce Costs
The increasing use of carbon fiber in aircraft manufacturing has seen production increase dramatically over the past 20 years, to 27,000 tons in 2010. Japan is a world leader in carbon fiber production, accounting for a full 70% of the global total. The Ministry of Economy, Trade and Industry expects the size of the world market to reach 125,000 tons in 2020 (roughly 4.5 times the figure for 2010). By 2030, it is hoped that Japan will earn some ¥3 trillion a year from the aircraft industry.
But there are still issues to be overcome—chiefly cost and safety. Carbon fiber has always been more expensive than aluminum alloys. On top of this, thermoplastic particles are added to make an intermediate material known as prepreg (from “pre-impregnanted,” meaning sheets of carbon fibers that have been hardened in advance with a partly cured resin), whose strength makes it ideal for shipment and processing. The prepregs are then placed in a mold and baked (or “cured”) in huge ovens called autoclaves. This fixes them into their final shape. In the case of a plane, the molded piece needs to be large enough to accommodate the main wings. The cost per plane comes in at tens of billions of yen.
To reduce costs, JAXA collaborated with the private sector to develop a new method of manufacture known as vacuum assisted resin transfer molding, or “VaRTM.” Ishikawa describes the new method: “Carbon fiber is placed between sheets of film and the air is removed to create a vacuum. Low viscosity plastic is injected and the fibers are impregnanted with resin. It’s a little bit like the vacuum storage bags used for bedding.”
The main advantage of the VaRTM method is that it does not require the use of an autoclave or similar curing equipment to heat the materials. But this results in a material that is not as strong as that made with the conventional method, using prepregs. Conventional manufacturing methods are therefore more suitable for making airplane fuselages and similar parts.
JAXA therefore came up with a hybrid approach that makes the most of both methods: using prepregs for flat parts with large surface areas and VaRTM materials for the framework and other components with complex shapes, in an attempt to achieve both high quality and low cost.
Further research is still underway to make absolutely sure of the safety of CFRP. One known issue is the risk of delamination following high impact. This causes the layers of the composite material to separate, leading to a significant loss of mechanical toughness. Because it is still not possible to simulate these conditions satisfactorily with computers, tests to prove that the model meets the safety requirements for certification have to be carried out using actual parts. Making it possible to run these tests on computers would reduce the time and cost of certification, and JAXA is therefore working to create models of failure characteristics.
Models for collisions with foreign objects, on the other hand, have already been established. Comparisons of test results and computer analyses have shown that almost completely accurate simulations are possible. “These results put us at the very forefront of international research,” Ishikawa says proudly.
Recent research has focused on prepreg materials in which cup-stacked type nanofibers, a type of carbon nanotube, are dispersed in resin. Researchers say that as nanotechnology is used in a wider range of applications, the possibilities for adding new functions onto CFRP will only continue to increase.
Possible uses of the remarkable characteristics of CFRP go far beyond aircraft. In the future, it is hoped that the material will also prove useful in fields such as automobiles, wind turbines, industrial machinery, and shipping. In all these areas, CFRP combines the twin attractions of low environmental impact and economic affordability. Carbon, widely seen as a villain in the problem of global warming, may yet bring ecological as well as economic benefits. It will be fascinating to watch this story unfold.
(Originally written in Japanese by Hayashi Aiko. Photographs by Hans Sautter.)