Designing composites

Bikes: Carbon loadin'

Bridges: look mom, no rust

Update: 9 Jan 2002

The vertical tail fin of American Airlines flight 587, being retrieved for examination. Six lugs that mounted fin to airframe failed. Investigators, not to mention people who fly the Airbus 300, want to know why...
AP photo

Layers of oriented fiber are stacked together to make an extremely strong structure.
Advanced Composites

The B-2 bomber features widespread use of carbon-fiber composite material.
Aeronautical Systems Center, Engineering Directorate.

When the Airbus's vertical tail fin was found in Jamaica Bay, more than half a mile from the crash site, suspicion shifted to material failure, a more innocent foe -- but still a dangerous one.

We can't reconstruct the crash, but evidence indicates that pilots complained about losing control when the big fin fell off, and the plane started moving erratically. Within seconds, it veered into the ground.

The National Transportation Safety Board (NTSB) won't issue its analysis of the crash for months at best, but the best bet now is that carbon-fiber composites in the fin simply failed.

The crash got us Why Filers wondering. Why are composites coveted as space-age materials? Do the high-tech composites have drawbacks? And can we find an excuse to watch the construction of some of the world's best carbon-fiber bikes?

Two plus two equals 10
Although the composites in airplanes and spaceships are high-tech wonders, their roots are literally in clay. The first composite material was probably straw-reinforced earth, which remains in wide use millennia after being invented as a cheap but effective building material.

To understand composite material, drop by our jargon stop:

Compressive strength resists squashing. It's what prevents football linemen from getting crushed in those lovely pile-ups.

Tensile strength resists pulling apart. It prevents running backs (oy, football again!) from getting pulled in two when beefy tackles pull in opposite directions.

Shear strength resists getting cut. Scissors cut by shearing.

The ideal material would combine each type of strength, but in the real world, it's usually easier to join several materials to get the needed strength. In the original composite material, dried earth gave compressive strength, and straw gave tensile strength.

The "strength in unity" theme reappears in steel-reinforced concrete: Concrete resists compression, and steel has tensile strength.

Matters change slightly in fiberglass, a more advanced composite that bonds glass fibers in plastic to make light boats and cars. The fibers have both compressive and tensile strength, but they can supply compressive strength only when held tightly in the plastic.

As these examples demonstrate, a good composite is like a happy marriage. Strengths here compensate for weaknesses there -- but only so long as everybody sticks together. Once things fall apart, each member is on its own, and disaster lurks.

The incredible lightness of flying
Modern composites owe their existence to the aerospace industry, where lightness rules. Because heavy airplanes can't carry much payload, you'll find more aluminum planes than lead ones. Carbon fiber, the ultimate high-tech composite, is sometimes called "black aluminum" because it works like aluminum -- but weighs about 25 percent less.

Compared to older materials, advanced composites, particularly the carbon-fiber used in the Airbus, are stronger, lighter, easier to engineer, and more resistant to fatigue. They do not expand or contract when temperature changes, which is important in airplanes that may sit on a tropical runway one minute, and fly in the subzero stratosphere 10 minutes later. Last -- and this can be crucial -- composites don't corrode.

It's not that composites are perfect. They're expensive. And they can be brittle, says Douglas Cusack, a composite engineer at Trek Bicycle in Waterloo, Wis. In other words, once fibers start coming apart, they can fail catastrophically -- as apparently happened last month in New York.

What do composites have to do with the human diet?

Terry Devitt, editor; Pamela Jackson, project assistant; S.V. Medaris, designer/illustrator; David Tenenbaum, feature writer; Amy Toburen, content development executive; Eric G.E. Zuelow, project assistant

©2001, University of Wisconsin, Board of Regents.