Carbon-fiber reinforced plastic (CFRP): A material with excellent characteristics
Carbon-fiber reinforced plastic is a composite material that is made by graphitizing highly strong and hard carbon fibers being bundled and piled up, with plastic such as epoxy resin. Carbon fibers are produced by smothering synthetic fiber at a high temperature.
Because this material utilizes plastic in which carbon fiber is used as reinforcement, it is called carbon-fiber reinforced plastic (CFRP).
A characteristic of CFRP is, first of all, lightness. The specific gravity of steel material is approx. 7.8, aluminum alloy is approx. 2.7, and CFRP is approx. 1.6. This means that CFRP makes it possible to produce a very light structure. Carbon fibers also have the characteristic of being ten or more times stronger than iron. By utilizing these characteristics, which are high in both strength and lightness, this material is used in various industrial products, including airplanes.
Next, CFRP can be said to be a material that has an extremely high level of flexibility in design. This is essentially because it is possible to design the strength and rigidity in different directions depending on the direction (orientation) of the carbon fibers.
For example, when carbon fibers are piled up in the same direction, it is possible to make CFRP exceedingly strong in that direction. When the same amount of carbon fiber is piled up equally in all directions, it is possible to make a CFRP plate that has the same degree of strength in all directions.
If carbon fibers are woven, it is possible to make a CFRP plate that is reinforced in multiple directions and to mold it into different shapes before hardening using the pliability and stability of fabric. This method allows complex product shapes to be made.
Using these characteristics, structural designers can set a required shape, size, lamination direction, or carbon fiber level (the number of lamination layers) to develop strength or rigidity. As a result, it is possible to design a structure with a degree of lightness, high strength and high rigidity that cannot be realized by metal materials.
Moreover, a big feature of CFRP is difficult to corrode because it is a composite material of carbon fibers and plastic. By using this characteristic, it is possible to simplify structure inspection and maintenance or increase the interval of the inspection and maintenance.
CFRP, which has these characteristics, also began to be used in the wings and fuselages of commercial aircraft in 1980s. Now, it is an indispensable material in aircraft structures.
Furthermore, CFRP is not only highly strong and rigid, but also excellent in dimensional stability in environments that feature extreme difference in temperature. Because of this, it is also used in space appliances such as in the solar panels and primary structures of artificial satellites.
Its uses will be further expanded into infrastructure facilities, construction, automobiles, medical equipment, etc.
A Global Research Dilemma: Challenges in expanding the practical application of CFRP
CFRP is a high-potential and useful material to us, but it also has challenges.
First, CFRP has a much shorter history of practical application than metals, etc., so there are not enough experimental data or examples to truly verify its superiority and diversity of use.
As described above, CFRP is a material that has a high flexibility of design, and its material characteristics vary depending on the combination of fibers, resin, and fiber orientation. Companies all over the world research and apply the best combination for their CFRP products. It takes a lot of time and cost to acquire CFRP characteristic data, so these research and development data are hardly disclosed.
Development of a public database on the material itself, as well as test standards such as a material database acquisition method (standard testing method), also lags compared to metal materials. Few published data and inadequate development of a data acquisition method are a factor that prevents CFRP from spreading as a common industrial material.
Therefore, the aviation structural material laboratory is conducting research aimed at standardizing the acquisition of various types of CFRP strength properties, and developing a testing method for strength acquisition according to ISO (International Standardization Organization) in cooperation with the JAXA Aeronautical Technology Directorate, universities concerned, external agencies, etc. Although Japanese companies are prominent in the production of carbon fibers, we’d like to promote research and development so that Japan can also lead in CFRP product design and manufacturing technology by using carbon fibers, as well as standard development.
Next, a challenge to the practical application of CFRP is the degree of attention that it must be given when designing a structure. How CFRP is broken differs greatly from metals.
For example, if a heavy load is applied to a single point, a metal plate would be highly deformed and dented, while a CFRP plate surface would simply be scratched without so much as a small dent. However, the CFRP plate inside may be broken as layers just beneath the scratch come unstuck.
This is because the strength of the resin supporting the carbon fiber layers is low even though the strength of the carbon fibers themselves is high. This internal fracture between layers (delamination) can significantly lower the strength of a CFRP plate.
This characteristic means that it is difficult to find a part where the degree of strength is diminished with damage suffered in flight through a simple visual check if CFRP is used in an aircraft. A sophisticated new commercial aircraft in which CFRP is used are designed so that they can fly safely even if there are damages to the airframe structure that cannot be found through visual checks. However, it is necessary to overcome this weak point of CFRP if further weight saving or safety improvement is required in future.
So research to improve the strength between CFRP layers and technology development to make an accurate estimate of the strength of CFRP structures and improvements in the design accuracy of aircraft structures, even if there is a delamination between layers or a defect inside the CFRP laminates, are being promoted. In such research and development, it is necessary to repeat steady research activities in which the question of how a CFRP plate is complexly broken is physically observed in detail with the resulting phenomena being examined one by one using numerical analysis.
On the other hand, for expanding practical application of CFRP and its applications to products, technology development for cost reduction and mass production of CFRP products is necessary. Currently, the work of laying-up carbon fiber and resin sheets into product shapes (lamination process) and the work of hardening CFRP into product shapes (forming and hardening process) takes effort and time.
CFRP is finished into CFRP products by laminating a raw material and hardening resin, instead of being finished into products by processing a material that has fixed mechanical characteristics like metals by using a press or machine. So the lamination process and the forming and the hardening process take time.
Therefore, the development of technology that can complete the hardening of CFRP at the same time as lamination by using automatic laminating equipment (lamination-hardening simultaneous forming technology) is being promoted. This means a combination of the lamination process and the forming and hardening process. If practical application of this manufacturing method succeeds, the production of CFRP is expected to greatly increase in speed and lead to cost reduction.
Our laboratory is also promoting basic research and development into lamination-hardening simultaneous forming technology (In-situ consolidation) for CFRP by using automatic lay-up equipment.
Another challenge is how CFRP products should be treated after use.
For example, although a commercial aircraft is typically used for about 20 years, when it is retired, the airframe structure (to be disposed of) is crushed into pieces using heavy machinery, etc., and the metals are sorted according to material quality to be reused. However, no system for the recycling of CFRP has yet been established.
The uses of CFRP are expected to be greatly expanded not only into aircraft but also automobiles, daily necessities, etc. in the future. In order to spread the use of CFRP however, it will also be an important challenge to establish a circulation cycle as an industrial material. The JAXA Aeronautical Technology Directorate, along with a lot of universities and companies, are conducting this kind of research and development.
The Possibilities of CFRP: Expanding through trial and error
We consumers can expect that CFRP will develop as a common material that can make our lives more comfortable and enjoyable.
For example, the material evolution as expansion of CFRP application has greatly contributed to advancing the performance of golf clubs and tennis rackets. In addition, CFRP bicycles, which are light enough to be lifted with one hand, are popular, as well as lighter and longer CFRP fishing rods and artificial legs which are used by athletes who are active in track and field. By using the characteristics of lightness and flexibility of design, along with the superior degree of rigidity and strength, CFRP has a wealth of possibilities of application for various products.
Speaking about passenger planes, CFRP also contributes to improving passenger’s comfort. In conventional airframes in which aluminum metals are mostly used, it is necessary to prevent rust by drying out the inside, while airframes in which CFRP is used (B787, A350, etc.) have made it possible to humidify the inside because the anxiety of metal corrosion is reduced.
Moreover, even if a passenger plane flies approx. 10 km above ground level, it has become possible to pressurize the inside at the level of the fifth station of Mt. Fuji, that is 1.8 km in height, by using CFRP. This is because CFRP, being high in fatigue strength, could bear the stress of repetitive pressurization and decompression to a degree that would break aluminum. Thus, the airframe material change has led to improvements in the comfort of passengers.
CFRP can be expected to be used in various fields including automobiles, construction and disaster measures in future.
Now auto companies all over the world are vigorously promoting technology development to apply CFRP. If this application is realized, the body will become significantly lighter while the fuel efficiency will be greatly improved. This could in fact be realized in the not so distant future.
In addition, CFRP is attracting attention as a material for construction or a material to be used in structures for disaster measures. Currently as a method for the reinforcement of deteriorated concrete poles such as in highway construction, the technology of wrapping them with iron plates can be listed. If CFRP, which is rustless and incorruptible, is used instead of the iron plates, it is possible to reinforce such poles more effectively.
Research and development for constructing transportable light bridges by using CFRP are also being promoted. When a bridge is damaged by a disaster, etc., rapid transportation and installation of a bridge can be expected to accelerate recovery of the disaster area.
As described above, CFRP has various possibilities, but its history of practical application is short as stated above, so a variety of trial and error testing has been continuing. This means that CFRP is a material that still requires continual investigation with the gradual accumulation of technology.
Even the B787, which has already entered service and has been flying around the world, caused a delay in development because the CFRP structural part was broken in a development test. Even though CFRP is a wonderful material, it still requires use with a good understanding of its characteristics.
Our laboratory will offer research results and data to the world so that CFRP can continue to be much used in industrial products.
By the way, under the influence of the COVID-19 infection, which has spread throughout the world since the spring of 2020, the development of SpaceJet, the first domestically-produced passenger jet, has been suspended. SpaceJet also partly uses CFRP. We hope that COVID-19 will be controlled and people’s demand for travel will increase, and are looking forward to seeing the restart of the development of SpaceJet!
* Updated in November 2021 based on the article made public in February 2020.
* The contents of articles on Meiji.net are based on the personal ideas and opinions of the author and do not indicate the official opinion of Meiji University.
* I work to achieve SDGs related to the educational and research themes that I am currently engaged in.
Information noted in the articles and videos, such as positions and affiliations, are current at the time of production.