Designing composites

Carbon loadin'

Bridges: look mom, no rust

Update: 9 Jan 2002

Three of the six fittings that mounted the tail fin on American Airlines flight 587. A fragment of fin is still attached to fitting at top left.
NTSB

Six lugs like this, all made of composite material, broke during the Airbus crash. Did their failure doom the plane?
NTSB

Applying composite material to a form. By winding fiber around the form in several directions, the enormously strong carbon fibers can work together. The space shuttle's external fuel tank is made this way.
Advanced Composites.

Like a good human diet, composite materials benefit from plenty of fiber. Since composites have been around for eons, what makes advanced composites better than simple straw and mud? According to Lawrence Bank, a composite engineer at University of Wisconsin-Madison, "The key issue ...is a fiber content of 40 percent to 60 percent by volume." (For comparison, the steel reinforcing in concrete occupies two to four percent.)

Fibers are also used differently in advanced composites, he says. "We orient the fibers very specifically, in the direction that carries the load." And the fibers are slender -- generally 15 to 20 microns in diameter.

Fibers are chosen for tensile strength, resistance to bending, cost, damage tolerance, durability and weight. There are tradeoffs with every choice: For example, fiberglass makes canoes that are heavy, but rather cheap, while lighter canoes can be made of the more costly aramid, AKA Kevlar.

Carbon fiber makes the strongest composites in common use. These materials are expensive, but extremely light -- always a plus in aircraft design. The strength emanates from the carbon fibers, which have about 10 times the tensile strength of steel. One square inch of carbon fiber can take a 500,000-pound pull.

Togetherness is bliss
Whether we're talking carbon or the more exotic boron, the goal of composite design and processing is to force the fibers and matrix to work together. Think of it as exploiting strengths while masking weaknesses:

The matrix, generally a polymer or epoxy, acts like an adhesive to hold the fibers together. Matrix must also resist degradation and protect fibers from the environment.

The design orients strands of fiber in a way that can carry the load. Like the plies in plywood, the strands in each layer of fiber are strong in one direction but weak in another. Thus the space shuttle's huge external fuel tank was made by winding carbon fiber at various angles to the tank's long dimension. Designers also try to cram the maximum fiber into a design -- but if they go overboard, they get "dry fiber," an invitation to separation (delamination) and disaster.

During processing, thin sheets of fiber and resin are heated and pressed together, bonding the polymers to the fibers. One key goal is to reduce voids, the source of trouble. Trek Bicycle, for example aims for less than 2 percent voids in the lugs it uses to join bike tubes, says composite engineer Douglas Cusack.

Fiber fault-finding
For all the advantages of composites, the New York accident potentially demonstrates a nasty side: sudden failure. "It can take an impact, and look good, but have failed inside, and you can't see it," says Cusack. That's why Trek suggests, after a bad crash, that a carbon bike frames be junked.

Ouch! (These frames aren't cheap.)

The science of finding flaws in composites remains rather primitive. You can peer closely with a bright light, looking for delamination. You can drop a quarter on the surface and listen for the bright sound of a well-bonded structure or the dull sound that indicates delamination.

A higher-tech approach uses ultra-sound to listen for duplicate echoes signifying delamination. Ultrasound is rather straightforward on a flat surface. But at joints -- the critical areas -- the confusing layers of fiber give, logically enough, confusing echoes. Engineers may compare new ultrasound scans to previous ones, looking for changes in the echo that signal delamination.

A tale of a tail fin
Now that we're experts on composite materials, what can we make of the New York Airbus crash? The NTSB has yet to issue its report, but according to news reports, it encountered serious -- but apparently normal -- turbulence during the ascent.

Just before the crash, the black boxes revealed strange rudder movements, but it's not clear if they were ordered by a pilot trying to regain control or were caused by the separation of the tail fin.

The discovery of the vertical tail fin about half a mile from the crash site certainly implicates the fin in the crash. Investigators are focusing on six composite fittings that mounted fin to fuselage. NTSB photos show the remains of those fittings.

There are already disturbing signs that the attachments were primed to fail. The fin was repaired before the plane even left the factory, and in 1994, one mounting was reinforced to fix a delamination. According to Aviation Week and Space Technology, "The center lug failed in a nearly straight line parallel to bolts that Airbus added to try to stop a manufacturing delamination. A video made by NTSB ... shows what appear to be fastener holes along the break." (See "Composite Experts..." in the bibliography).

In other words, making the fin stronger may actually have weakened it. Composites are tough to repair because drilling cuts the fibers that give strength in the first place, and repairs make composite experts nervous. According to Lawrence Bank, "The fact that you would do a refix on this is of concern." While acknowledging that such repairs are sometimes necessary in the aerospace industry, he adds, "You'd like the part to fit, work, to be attached without changing the part."

Cusack, who worked in aerospace composites before shifting to bicycle design, says "It's always scary if you add material rather than have a good solid design up front."

Whole lotta composite in the air
Eventually, the mystery of flight 587 will be solved. And that's just as well, considering the soaring role of composites in civil aviation. The Airbus A340-600, introduced last March, is the first civil airliner to use glass-fiber thermoplastic composites in the wing. The upper fuselage of the giant A-380 will feature hectares (well, almost) of a composite-aluminum sandwich designed to marry composite's strength with aluminum's flame-resistance. (A key drawback of composites in planes is their susceptibility to heat and flame. See "Airbus on..." in the bibliography).

The story is much the same at Boeing. To achieve light weight and cost-effective design, Airbus's older rival plans to use composite by the yard in the fuselage of its new "sonic cruiser" plane (see "Boeing's Planned..." in the bibliography).

Lightweight is not just a mantra in aviation. Heard the one about the carbon-fiber bike?

©2001, University of Wisconsin, Board of Regents.