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Laser technology used to detect and isolate heat damage in aircraft composites

Aircraft are frequently struck by lightning in flight or damaged in service by extreme heat exposure. How can potential structural damage from these incidents be detected?.

Researchers at the Department of Energy's (DOE) Oak Ridge National Laboratory (ORNL) have developed a method of imaging heat-induced damage in epoxy-resin composites. These fiber-reinforced plastics, called polymer matrix composites, are widely used in high-performance aircraft because they can be easily fabricated into parts of high strength and low weight. However, such materials may degrade when exposed to elevated temperatures from sources such as lightning, misdirected jet exhaust, and burning of spilled fuel.

Now, for the first time, early signs of thermal damage in advanced aircraft materials can be pinpointed quickly using ORNL's Composites Damage Imager (CDI). This system consists of a video camera, laser, computer, and special optics.

"The system can quickly and easily detect and display thermal damage to polymer matrix composites before measurable physical defects appear," says Eric A. Wachter, a scientist in ORNL's Health Sciences Research Division. "It offers a fast, flexible, accurate, and easy-to-use means for imaging damage to aircraft wings, flaps, and doors."

"These composites," Wachter explains, "can appear undamaged to the naked eye and even under a microscope. Yet, invisible thermal damage may have caused a composite material to lose more than half of its strength." Tests show that CDI can image critical thermal damage in composites that cannot be detected by traditional nondestructive evaluation techniques, such as ultrasonic and x-ray imaging.

When laser light is shone on an undamaged region of a composite, the resin emits light in a specific pattern of intensities and wavelengths. The pattern of this laser-induced fluorescence changes in a recognizable way when the laser light illuminates a damaged region of the composite. A camera that detects these "spectral shifts" can be used to image damaged areas.

"The fluorescence from a suspect region of a composite can be imaged instantly," Wachter says. "The high-resolution image is automatically recorded using a video camera containing a charge-coupled device and special optical filters. The device enables the production of a false-color image that highlights the damaged areas of the material."

The technology was developed to support work being conducted by the Oak Ridge Centers for Manufacturing Technology (ORCMT) for the Air Force and Navy to detect heat damaged composites in aircraft.

You can learn more about this research and many other exciting projects by visiting ORNL on Oct. 21, 1995, during its Community Day event. Many of our facilities will be open to the public that day. For additional information, call ORNL Public Affairs, 865-574-4160.

ORNL, one of the Department of Energy's multiprogram research laboratories, is managed by Lockheed Martin Energy Systems, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.