The world's strongest carbon fiber is Toray Industries' T100G, says the Japanese manufacturer. Now China, denied access even to lower grades from foreign suppliers, is working on matching it.
China makes a limited volume of T800—a lower-grade but still very strong fiber that it cannot buy from Western countries and Japan—and plans are afoot to build a plant with 20 times the current capacity. These advances could improve the performance of Chinese military aircraft and, eventually, bring China into the global market for aerospace materials, in which it has so far had negligible presence.
The Chinese T800 grade will cost only 1,600 yuan ($262) per kilogram, compared with the 4,200 -yuan-per-kilogram cost of the established and much inferior local product, T300, the plant operator told the China Aeronautical Materials and Manufacturing Equipment Summit, organized by Galleon, in Beijing last week. It is unclear whether it is referring to output from its present facilities or the larger plant.
National aeronautics group Avic has meanwhile described its development work on composite aircraft structures in preparation for the proposed Comac C929 widebody airliner. Technical challenges forced the abandonment of plans to build a composite center wingbox for itsnarrowbody, Comac said at the summit.
The new carbon-fiber plant of Jiangsu Hangke Composite Materials Technology has a capacity of 50 metric tons of T800 carbon fiber a year, plus 100 tons of T700, the company says. A plant with a capacity of 1,000 tons of T800 a year is coming online, with capacity for 50 tons a year of M55J carbon fiber. The country will also work on the T1000 and M60J, which is another higher grade, Jiangsu Hangke says, giving no dates.
The “T” numbers approximately indicate the strength of the fibers, which are impregnated with resin and then baked to create carbon-fiber-reinforced plastic—composite. Toray describes its T1000G as “the world's highest-tensile-strength carbon fiber,” with a rating of 6,370 megapascals (9,239 psi). Jiangsu Hangke says its T800 has a tensile strength of 5,709 megapascals, while Toray rates its T800S fiber at 5,880 megapascals. The advantages to China in using its T800 in military aircraft are obvious from the relatively modest 3,530-megapascal strength of Toray's T300, which has been made for about three decades.
The U.S. and its allies do not export T800 carbon fiber to China, in order to prevent its use in military aircraft there. In May, a Chinese man pleaded guilty in a U.S. court to trying to take Toray T800 from the U.S. to China.
China is surely considering more than military applications for its improved carbon fibers, however. Some of the strongest carbon fiber is also used in civil aircraft. The Chinese industry is presumably looking to the proposed C929 widebody aircraft that Comac wants to build as a follow-on to the C919. And beyond that, it likely has ambitions to supply the global market, as evidenced by Jiangsu Hangke's willingness to discuss the properties of its materials at an open conference. Jiangsu Hangke appears to be associated with the government's Chinese Academy of Sciences.
Comac has chosen only foreign carbon fiber for the C919. But the Chinese industry has not missed out on large sales in that program, because the aircraft will use carbon fiber only in its tail and secondary structures. Until this year, the C919 was supposed to have a composite center wingbox, but the manufacturer switched to aluminum.
The reasons for the switch were partly economic but mostly technical, Comac says. “Mainly, it was related to the level of difficulty of certain technologies and, in the end, the problems were in two areas,” says a company official. “One was thermal conduction; some places [in the structure] were hot, and the composite material, which we chose initially, could not cope with that.” The other problem was dealing with electricity in the structure, whether static or from lightning.
“We have not stopped researching composite center wings,” says the official, referring to the C929. “We will continue this effort.”
Avic has made engineering samples of 12-meter (40-ft.) wing panels, says an official of that group's First Aircraft Institute, a design bureau. One piece, apparently an upper panel, was cured at only 120C (248F) under “vacuum pressure,” reducing costs, says an official. Another piece, apparently a lower panel, was formed with integral stringers that were cured separately before attachment.
The conference also saw photos of a composite aileron made with vacuum-assisted resin injection and a rudder component made using resin-transfer molding. Process improvements have reduced the designing time for large parts to 2-3 months, from 6-8.
Shanghai-based Comac says its largest autoclave has dimensions of 21 X 5.5 meters (70 X 18 ft.) but in Beijing, Avic has one of 30 X 7 meters.makes outer wings in 40-meter autoclaves.
Composite manufacturing is probably profitable to the Chinese state firms, because civil aircraft are not their only market, says an industry official. Chinese military orders are contracted at cost plus an agreed profit, explaining why the suppliers can make money. There is little manufacturing of composites for Chinese civil aircraft: Comac's first one, theregional jet, has only 1.5% composite content. Jiangsu Hangke expects the C929 to reach 25% composite content. Another official makes a similar prediction for the widebody and adds that its wing will be composite. There does not seem to be a plan to build a composite fuselage for the C929, development of which has not been launched.
China's state companies are not yet supplying aerospace aluminum to the global market, although they make the material for Chinese military aircraft and civil aircraft whose certification is not recognized by developed countries. That, too, is about to change, however.is working to qualify metal from the government's Southwest Aluminum. The material will be conventional aerospace-grade aluminum, not the more recent advanced aluminums or aluminum-lithium, says Antoine Gaugler, Airbus's purchasing manager for Asia. Aleris, a U.S. company, has set up a plant to make conventional aerospace aluminum in eastern China at Zhenjiang (AW&ST May 27, p. 37).
China is also working on ceramic matrix composite (CMC) but lacks practical applications of such material, which remains stable at temperatures that defeat even the best metals used in aircraft engines. Apart from work on parts for turbine engines, Chinese engineers have been applying CMC to ramjets and telemetry systems, says a leading researcher in the field. More than 4,000 articles have been made for 360 types of parts.
“China began work on ceramic matrix composites in the 1990s, several decades after other countries,” the Chinese researcher tells Aviation Week. “We are still behind North America and Europe, and in some areas we have been unable to catch up. In [development of] materials, we have been catching up faster.
“But we are far behind in applying and using the technology,” she says. “Without thorough testing and verification, we cannot believe the characteristics” determined in the laboratory. “In some areas, we have surpassed [foreign researchers], but in applications we are backward.” International experience shows that the cooperation of industry, universities and research institutes is key to success, she adds.
A CMC manufacturing technology national engineering center was approved this year as a base for promoting the industry. Two projects, one applicable to aero-engines and one for brakes, have been given the go-ahead. Among the work discussed at the conference, a CMC afterburner inner cone was tested for 24 hr. A problem in the connection structure was found but fixed. This effort appears to have been judged a success, since the researcher says it “laid a foundation” for engine development.
The Northwestern Polytechnical University at Xian has tested engine nozzle parts between ambient temperature and 820C. In one test, the nozzle of a Klimov RD-33 engine was tested at pressures of 0.28 megapascals, a speed of Mach 1.5, and 1,047C. This test proved the that higher operational temperature was possible, along with a saving in cooling air.