When Boeing first considered extensive use of structural composites on the 787 Dreamliner, its engineers knew intuitively the epoxy/carbon fiber matrices would reduce weight significantly, allowing fuel savings and extended flying range. But after an intensive early look at composites, they realized fundamental design changes were possible that would allow functional systems integration, as well as changes in lamellar flow that would improve aerodynamics.
From a materials’ point of view, the 787 Dreamliner is one of the most revolutionary leaps in the history of manufacturing.
But in order to meet an ambitious delivery schedule – the first delivery is scheduled for May 2008 – there were tremendous hurdles to jump:
No one ever attempted to mass produce very large carbon-reinforced plastic structures, which are thermoset materials with significantly slower processing times than thermoplastics,
The critical tooling for such large sections was still very much in the development stage and, in fact, represented one of the few, small stumbles in the development program,
New coatings had to be developed to deal with the crack propagation issues, which are not a factor with aluminum. Engineers had to devise different systems to deal with electrical shorts because composites are not electrically conductive.
One area that was not new was the materials’ technology. “When we made the decision on composites’ use in the wings, fuselage, floor beams and so on, we went down a path based on a material that we had already had a significant amount of production experience with on the Triple 7,” says Dr. Alan G. Miller, director of technology integration on the 787 and former chief engineer for all materials technology at Boeing. “We knew the things like dimensional stability. We knew how composites impacted the manufacturing flows. We had a lot of design allowables databases. We had a lot of confidence from our customers.”
The Boeing 777 is 9 percent composites by weight, compared to 50 percent for the Boeing 787. Throughout the life of the 777, the Carbon-reinforced plastic materials (composites) were enhanced in terms of their properties, manufacturing and cost structure.
There are several different types of composites used on the 787, including bismaleimide, depending on specific applications requirements. There are several smaller parts made from discontinuous fibers that can be molded into odd shapes. There is also extensive use of thermoplastics in the interior of the aircraft, but that’s not a departure from previous designs.
All of the composites are supplied by Toray Industries, the world’s largest producer of carbon fiber. Since 2004, Boeing has placed composites orders with Toray estimated at more than $6 billion, creating pressure on prices and supplies for other users. The estimate was based on projected production as of 2006, numbers which are already out-of-date because of the spectacular success of the 787.
At the end of 2003, Toray had capacity to produce 7,300 metric tons of carbon fiber products. By this summer, capacity will reach 13,900 metric tons. Its investment in that time period for carbon fiber products are approaching a billion dollars. Expansions are coming in the U.S. (Tacoma, WA and Decatur, AL), as well as in France and Japan. Demand for PAN (polyacrylonitrile)-based carbon fiber is growing 15 percent a year, driven now by the 787. Later growth will come from full-scale penetration of the auto market and energy-related applications such as CNG tanks.
Mass Production Issues
Production processes for composites are significantly slower than they are for thermoplastics. “We have had no applications (before the Dreamliner) with high volumes,” says Miller. “The composites industry has never had to deal with this before. That was a mountain we had to climb.”
Traditional machine tool producers provided the most important solutions, in Miller’s view. One example is Cincinnati Machine Co. which is supplying new tape layers and fiber placement systems for the Dreamliner 787. “High-speed precision lay-up helps accelerate work flow to shorten build schedules and reduce work in process costs,” says Randy Kappesser, general manager of composites for Cincinnati Machines. The newest equipment can lay up tape over complex geometric shapes using proprietary software programs translated from CAD and laminate data. The software allows for offline part programming and simulating, including collision interference.
Reinforcing fibers are oriented in specific directions in the resin prepreg to deliver maximum strength only in the direction that is needed. A prepreg refers to fiber matrices already infused with resin.
Tools presented a bigger problem because of their size (as large as the airplane) and the evolution of the technology. Previously, composite tools had primarily been provided for boatbuilding, which is not a mass production industry.
“The technology area still playing out is tooling,” says Miller. “Left to its own devices, composite tooling can be fairly elegant or – if you’re not paying attention to it – it can be very clumsy and heavy … We had to meet with our technology partners up front to make sure the technology was mature enough to meet our production schedules.”
The gold standard in materials of construction for large composites tools has been an iron-nickel alloy called Invar, which is well-suited because of its controlled coefficient of thermal expansion (CTE). One problem with Invar is it does not lend itself well to lean manufacturing systems that have been a major goal at Boeing for several years. The main alternative is “soft” tooling, which is also made from composite materials. Aluminum is a mismatch because of its CTE. Soft tooling developed a bad reputation because of problems with the B2 “stealth” bomber program.
The tooling technology for the Dreamliner is extremely proprietary, but a few details have leaked about a company called Janicki Industries of Seedro-Wooley, WA. Janicki, a long-time yacht builder, has developed new soft tooling for the Dreamliner that has been a success except for one well-publicized mishap, when an error in a resin formulation led to leaking and a damaged mandrel.
“The innovation is not just in the materials, but in how they build the tooling,” says Miller. Janicki, for example, custom built three 5-axis CNC mills that have envelope sizes as large as 88 x 19 x 8 ft and accuracies to ±003 inches.
More Than Just Weight
Composites are worth the effort and are clearly on a fast track in the aircraft industry. The advantages go beyond weight savings. “We’re seeing increasingly practical ways to integrate functions into a single system,” says Miller. “Your structural system can also be a part of your acoustic damping system. It can also be a part of your thermal transfer system and your electrical system. The use of composites will grow because it’s not just about the structure any more.
That’s not all. The superior strength of the composite fuselage will allow higher pressurization in the passenger cabin, making it easier to control temperature, humidity and ventilation. Composite materials are also more durable than aluminum, because of corrosion and fatigue benefits, as well as a dramatic reduction in fasteners. The structure of the 787 is essentially one giant macromolecule – everything is fastened through cross-linked chemical bonds reinforced with carbon fiber. “Early on, we made a decision to do a one-piece barrel,” says Miller. “We don’t have lap joints because we wrap them.”
The benefits of composites grow as Boeing engineers gain more experience.
“We discovered how to make better window frames, and when we did we changed the load transfer characteristics of the fuselage in that area,” says Miller. “That allowed us to go back to the one-piece barrel and take more weight out of that part of the design. We ended up saving three times what we were on the original design.”
Another benefit comes from extended lamellar flow (1.5 cubits) at the point where metal meets plastic: integration of the inlet of the Nacelle (engine housing) with the structure of the Nacelle. Boeing estimates the savings at 20,000 gal of fuel/year —just for that design enhancement.
“There is a giant coalescence of manufacturing technology, materials technology and forming that might actually allow us to make another jump in aerodynamic performance,” says Miller.
What are the Downsides to Composites?
They don’t corrode, but they do undergo a photooxidation process. As a result the composites must be painted. And therein lies another Dreamliner technology story. “We’ve come up with an intermediate barrier coating,” says Miller. “Our customers can strip or change the colors of the paint without having to go all the way down to the composite. That means they don’t have to sand.”
Another issue isn’t so much a downside as it is a difference. Composite structures require you to approach thermal and electrical management differently from aluminum, which has intrinsically higher electrical and thermal conductivity. How you move current from shorts, for example, is not something you have to think as much about in an aluminum structure. ”We’re a lot smarter about how we attack these issues than we were three years ago,” says Miller.
Who owns all of the technology developed for the Dreamliner, considering the huge global supply chain involved? “It depends on who pays for it,” says Miller. “If we paid for it, we probably own it and we may share it within the program.” If a supplier is contributing its own intellectual property, then the technology probably would be owned by the supplier. All of the major partners have their own extensive R&D programs, and obviously want to be able to leverage some of their know-how with other aircraft building programs.
That’s one reason why Boeing is very secretive about specific technology partners, such as tool makers. Any release of information by suppliers to media is carefully controlled by Boeing, as well.
There are also breakthroughs in titanium, aluminum, cool plastics
Composites aren’t the only materials’ news in the 787 Dreamliner. While composites represent 50 percent by weight (80 percent by volume) of the Dreamliner structure, other materials represented are aluminum, 20 percent; titanium, 15 percent; steel, 10 percent and 5 percent, other. Most notable among the “other” is the first-time widespread use in aircraft structures of plastic heat sinks. That’s right – plastic heat sinks.
Plastics that are highly loaded with heat-removing materials such as carbon or ceramics have been around for a while, but have not yet penetrated the aircraft market. Their great advantage is their ability to be molded into net shapes. The economics for plastics can be favorable depending on total tooling and finishing costs. They can be designed with additional surface areas as fins and ribs to improve convective heat transfer.
Costs and properties can be balanced depending on which engineering thermoplastics are used. For example, nylon can improve economics while liquid crystal polymer can improve properties. They are typically loaded 30 to 40 percent with thermally conductive materials.
Suppliers of thermally conductive thermoplastics include LNP Engineering Plastics (GE Plastics), RTP, Cool Plastics and DuPont. On the thermoset side, suppliers include Epoxies, Etc.
“Their thermal properties and other nonmetallic properties are really causing us to think differently about how we remove heat from airplanes,” says Dr. Alan G. Miller, director of technology integration on the Dreamliner.
Other new materials’ highlights on the ground-breaking aircraft are:
Titanium. The Dreamliner is the first big user of a new advanced alloy titanium in the aircraft industry. “You can really increase strength and get really high toughness,” says Miller. The new grade, designated 5553 (Ti-5Al-5V-5Mo-3Cr), supersedes another high-strength alloy, 1023 (Ti-10V-2Fe-3Al). Miller says Boeing is doing a lot of work collaboratively with the Russian titanium industry. According to the Moscow Times, Boeing is committed to purchase $18 billion of Russian titanium over 30 years. Last August Boeing announced a 50/50 joint venture with VSMPO-AVISMA to produce rough machined titanium forgings for the Dreamliner. Of course, politics are involved. Boeing wants to win more business in Russia and Russia would like Boeing to invest in its aircraft industry. Typically, titanium has been used in engine applications for rotors, compressor blades, hydraulic system components and nacelles. Boeing will also use Alcoa’s high-pressure titanium hydraulic adapters, which are said to provide up to a 49 percent weight savings over the previous design for this application. Each aircraft is expected to use more than 120 of these parts.
Aluminum. New technologies are emerging for extrusions in plates in aluminum-lithium alloys that intrigue Boeing’s Miller. It’s well known that aluminum-lithium alloys have lower density, good and often higher strength than conventional aluminum alloys, and provide higher modulus, and therefore, enable weight savings.” The trick is that you have to be very cautious in design so that you are using them in a way that make economic sense,” comments Miller. “This is not a technology push; it’s a business proposition. It has to buy its way on to the aircraft.”
Thermally conductive plastics offer significant improvements over conventional plastics:
NOTE: W/mK stands for watts per meter Kelvin.
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