The new method to detect deformation and damage has application to aircraft, automobiles, bridges, machinery and other carbon-fiber composite products.

The research will be presented on Feb. 27 in San Diego at a meeting on "Smart Structures and Materials" sponsored by the Society of Photo-optical Instrument Engineers.

According to Deborah D. L. Chung, professor of mechanical and aerospace engineering at UB and principal investigator, this new method of fatigue detection is much less expensive and more efficient than the one now used. The current method involves using optical fibers embedded in composite components as sensors. But it is not optimal because of the added cost the optical fibers represent and because the fibers can degrade the composite's mechanical properties.

"We are not adding anything to the composite," explained Chung, "so it is less expensive than using optical fibers. Instead, we are exploiting the intrinsic behavior of the composite itself."

The "smart composites" exploit the fact that carbon fibers are much more electrically conductive than the polymer matrix into which they are placed. When a tensile stress is applied to a composite component, such as an airplane wing or the hull of a ship, the carbon fibers in the composite reversibly straighten, causing electrical resistivity along the fibers to reversibly decrease. That decrease is easily and continuously detectable by small, electrical probes placed on or near areas of stress. Upon damage, some fibers break, thereby causing electrical resistivity to increase irreversibly.

"The most critical aspect of the smart composites is that they are capable of real-time sensing of both damage and reversible deformations, which is particularly difficult to monitor," explained Chung.

By sensing both phenomena, the composites allow for control of deformation, recording of deformation and damage histories and lifetime prediction all in real time.

Fatigue monitoring is important, she said, because many disastrous breakages of engineering parts, such as aircraft components, result not from static loading, but from the weakening associated with cyclic loading, which can cause fatigue failure even when the stress is not very large.

Chung noted that beginning with the point at which a composite's fatigue life has been reduced by 50 percent, its electrical resistance keeps increasing, providing a continuous indication of the severity of the damage.

According to the tests the researchers conducted, resistivity started to increase in spurts when the composite had reached 55 percent of its fatigue life. At 89 percent of its fatigue life, resistivity began to increase gradually but continuously. At 99.9 percent, resistivity increased rapidly, both continuously and in spurts.

"This last figure shows that it will still be possible to avoid a disaster even when a part is so close to failing," said Chung.

Chung conducted the research with UB master's-degree candidate Xiaojun Wang.