Composite Decisions

If you've struggled to take weight out of your designs without sacrificing strength, you could likely learn a thing or two from the engineers who created the world's biggest commercial airliner, the Airbus A380.

Measuring nearly 73m long, 24.1m high, 7.14m around the fuselage, the A380 gives new meaning to the word jumbo. It could carry more than 800 passengers on its two full-length decks, though the actual seating configurations from the airlines will likely have space for 550 passengers and lots of on-board lounges. The plane had its first test flight last April and is scheduled to go into commercial service by the end of this year with the first plane going to Singapore Airlines.

Plenty of planes experience weight issues as they move from design to production, and the A380 certainly had its share. Yet Airbus engineers ultimately managed to keep the big plane as light on its landing gear as possible by incorporating more composite materials than any plane in the company's history. Airbus claims its use of lightweight materials throughout the plane ultimately bought it a 15 metric ton payload increase.

Advances in composites technologies meant that the A380's designers had no shortage of materials from which to chose. So how did they match the requirements of each section of the aircraft to the optimal material? In a word, competition. Says Roland Thevenin, the company's senior composites expert and certification specialist, "We forced the best materials technologies to compete against one another."

This ruthless competitive evaluation pitted metal against composite, as well as composite against composite. It took not just weight and key mechanical properties into account, but also mixed in cost, manufacturability and repairability once the plane goes into service. The results of the competition can be seen throughout the A380's airframe, whose structure consists of what A380 design and analysis manager Serge Rabois describes as an "optimized hybrid" of disparate materials.

COMPOSITES UPSWING

Reinforced plastic composites make up a remarkable amount of that hybrid. Thevenin reports that about 22 percent of the primary structures, by weight, are made from various reinforced plastic composites, mostly carbon-fiber-reinforced epoxies supplied by Hexcel and Cytec. The company also uses a smattering of glass-reinforced epoxies on the vertical tail plane, as well as quartz-reinforced epoxy on the nose cone. While the structural applications mostly relied on thermoset composites, the plane does have a j-nose, part of the wing's leading edge, made from a glass-reinforced PPS. Thevenin notes that the thermoplastic, which replaces a riveted aluminum structure on earlier planes, was picked in part because it allowed the j-nose's skin and stiffeners to be resistance welded together.

For Airbus engineers, composites aren't really rocket science anymore. The company has more than 20 years experience with composite primary structures, starting with the A310 in 1985. Since then the company has gradually incorporated additional structural composite applications into each new plane. "We've always taken a step-by-step approach to minimize our risk," says Thevenin.

The A380 represents just another step — except that it's a very, very big step. It includes a host of composite structures that have been proven on previous planes, some of which have now chalked up between 12 and 35 million of flight hours. These structures include fins, flaps and even the huge rear pressure bulkhead. "Size alone was our only risk with these proven designs," Thevenin says.

For example, one consequence of super sizing proven composites designs is that Airbus engineers had to tighten their design margins on loads. With the smaller, less highly loaded composite structures of the past, Airbus engineers had the luxury of working with larger margins. The size of the A380 components took that luxury away. "Now we have no unnecessary margins," says Thevenin.

The A380 does break some new ground. It includes a handful of brand new composites applications, such as cross beams, upper frame work, wing ribs and the flap track. The plane also contains some more dramatic CFRP structures. The empennage and rear fuselage, which has a maximum diameter of more than 6 meters, are made from CFRP. So is the 2.4 × 7 × 7.8m, 11 metric ton center wing box that serves as the structural heart of the plane. Made mostly from carbon fiber composites, plus some aluminum ribs, the wing box represents a first for commercial aircraft of more than 100 seats.

In all these applications, the performance attributes of composites shouldn't come as a surprise to any engineer. "When we say, 'performance,' we primarily mean weight and strength," says Thevenin. He estimates that for a given strength, a composites structures weighs 15 to 25 percent less than its metal counterpart. The center wing box alone, weighs 1.5 metric tons less than a comparable aluminum wing box.

For all the use of composites on highly-loaded sections of the A380, the design avoids using them in areas where these impact-sensitive materials can be damaged by ground equipment. "Composites present opportunities, but they also have their limits," says Thevenin.

LOOK OUT FOR GLARE

A very different type of composite, a laminate of aluminum and glass-fiber, makes up another 3 percent of the A380. Called GLARE, this metal-fiber laminate consists of alternating layers of aluminum and sheets of glass-and-epoxy prepreg. Airbus uses about 5,000 square feet of it on the A380, mainly on upper fuselage skins and for the leading edges of the fin. GLARE weighs anywhere from 15 to 30 percent less than a standard aluminum sheet (2024 T3), with the exact savings depending on the type and thickness of GLARE's layers.

For example, Airbus uses two kinds of GLARE — a standard version and a high-static-strength version. Each kind of GLARE also offers the ability to vary the orientation of the fibers. "We tailored the fiber orientation to specific loading conditions," says Rabois. Thus, the company uses different orientations to accommodate hoop stresses, axial loads, biaxial loads and shear loads. "GLARE looks like a metal but behaves like a composite," Rabois says of the ability to use fiber orientation to optimize mechanical properties.

GLARE has other things going for it from a properties standpoint. According to Rabois, the layers of fiber fight both crack propagation and corrosion. Fiber layers also enhance fire resistance, since the fibers act as an insulator. Finally, the fibers increase impact performance "by creating a secondary load path," Rabois notes.

The material scores when it comes to design freedom too. "With aluminum joints, rivet hole placement and sizes are limited by fatigue performance," Rabois points out. "GLARE has no limits on joint placement." What's more, the material allows large panels to be created from spliced sheets of the material. Airbus interleaves the plies of two smaller panels and permanently joins them. According to Rabois, this method eliminates many rivet holes and the related stress concentrations. "The size of the panels is limited only by the size of your autoclave and transportation equipment," he says.

All these design advantages don't come at the expense of repairability. Rabois says that GLARE repairs almost the same way as straight aluminum, except for a couple of minor procedural changes — such as not dressing out dents and using carbide tools.

As with any material, however, there are always trade-offs. And in GLARE's case, it involves a loss of stiffness. The material has a flexural modulus that's 5 to 6 percent lower than a comparable aluminum sheet. That stiffness hit isn't a big deal within an individual structure, but it can shift loads to other parts of the airframe. "When you reduce the modulus of one structure, you often put a burden on other structures," Rabois explains. On the A380, the lower modulus of the upper fuselage structures transfers some of the overall loading to the lower fuselage, he continues.

Then there's cost. Rabois acknowledges that GLARE has a "slightly higher price" than standard aluminum. But he says it costs about the same as advanced aluminum alloys when looked at from a cost per kilogram of weight saved. It may also have some in-service cost benefits, such as reduced inspection for cracks. So it may be a bargain at the end of the day.

THE RIGHT COMPOSITE

The competition among reinforced plastic, metal laminate components and metal alloys may seem pretty easy to handicap. Airbus engineers had only to match the mechanical and physical properties of each material to the loading conditions experienced by the different parts of the airframe. This aspect of Airbus's materials selection process is straightforward enough, though certainly difficult given the size and complexity of the A380.

Yet the competition between different materials often remained close for structures that initially made sense in more than one type of material. "We knew there was no one perfect material for every structure," says Rabois. Take the fuselage, for example. Many of the upper skins and the rear fuselage made sense in metals, GLARE or CFRP when considered from a mechanical property standpoint. Likewise, all three lend themselves to later repairs.

The real tie-breaker often came down to manufacturability. The use of advanced welding techniques, for example, favored unexpected metal choices and designs in some structures. And Airbus could only consider so many composites structures for the A380 because it can make them in the first place. "Our designs are driven by manufacturing and vice versa," says Thevenin.

He adds that composites in particular pose some unique challenges because their properties depend on the manufacturing process. "With composites, you make the material as you make the part," he says.

Airbus uses subcontractors for many of its composite parts, but it keeps the really challenging jobs in house. "We concentrate on the most difficult parts," says Christian Valade, head of manufacturing at the company's composites center in Nantes, France. "Difficult" in the world of composites means big parts, thick parts or those with complex geometries. The Nantes plant handles all of these, including the A380's huge center wing box, as well as the huge (17 meter), highly loaded (500-plus tons) keel beam for the A340-600.

During a recent tour of the center, Valade pointed out a collection of proprietary manufacturing techniques used to produce Airbus composites. Among them are automated lay-up machinery with a patented twin-head design that increases productivity and a patented system for making round nacelle acoustic panels without the splices that in the past reduced acoustic performance. The company also makes extensive use of "co-curing" systems that cure stringer and panels in a single autoclave cycle.

Thevenin reports that composites design will only increase in future planes, like the forthcoming A350 (see graphic). "We can take composites even further," he says, noting that the workload at the Nantes plant will triple by 2007.

But don't go thinking all future aircraft will automatically continue to bump up the amount of composites. Rabois says aluminum alloys and titanium continue to grow more capable and cost effective. And with each new plane, the materials competition starts all over again.

Reach Senior Editor Joe Ogando at [email protected] .