With grants totaling more than $700,000 from the Missile Defense Agency through the Office of Naval Research over the past three years, UMaine researchers are developing a sensor that can monitor the integrity of these materials in structures such as a missile or an aircraft wing.

The Columbia Accident Investigation Board is looking at the possibility that pinholes could have led to a weakening of leading edge wing panels made of carbon-carbon composite. The panels could have been susceptible to damage by foam debris that hit the shuttle's left wing after liftoff.

"NASA has taken some criticism for not monitoring the integrity of the shuttle wing structures, but it's not fair. Non-destructive testing techniques that can be applied to these types of materials are just being developed," says Mick Peterson, associate professor of mechanical engineering who leads the UMaine research effort.

In laboratory tests, Peterson's team has been able to use ultrasound to monitor the degradation of carbon-carbon composite material at temperatures of more than 1,000 degrees Centigrade.

"The degradation that carbon-carbon composites are susceptible to are not detectable at an early stage by traditional non-destructive testing techniques. Once significant damage has occurred to the material, fracture toughness may have decreased dramatically. We are now beginning to develop methods that can help us to understand the degradation mechanisms by using in-situ sensors. No one yet knows how to do the kind of detailed material monitoring that is required," says Peterson.

Carbon-carbon composites are constructed of carbon fibers embedded in a carbon matrix. They were developed in the 1960s for the space program because they retain their strength under high temperatures.

The theoretical useful life of carbon-carbon material is calculated by knowing how quickly it oxidizes, says Peterson. In the presence of oxygen and heat, the carbon slowly burns away. "But that useful life can change. If something comes in and gets hotter than it's supposed to or if there's some contamination, we don't know what the impact is on its lifetime. We could have accelerated these oxidation processes in the carbon, and those accelerated oxidation processes can lead to premature failure," says Peterson.

"The oxidation can be localized because of contamination. Then because of density variations in the carbon, it can essentially tunnel through these areas, and once that tunneling has occurred, you've got a porous interior and a hard exterior. Ultrasound is able to see through that top layer and identify the characteristics of that material under the surface. Our technique enables us to quantify the degradation mechanisms and the effect of contamination without requiring that an unreasonable number of tests be performed."

The high temperature sensor under development in Peterson's lab uses ultrasound to indicate the integrity of a carbon-carbon material in placve in a structure. In Peterson's laboratory, special precautions are taken to protect parts of the monitoring system that are sensitive to heat. The sensor generates an ultrasound signal at room temperature and uses fused quartz wave guides -- clear tubes about a half inch thick -- to transmit that signal to the heated material in a furnace.

Also working on the sensor project are Amala Mamilla, a master's student from Narasaraopet, A. P., India; Anthony Puckett, a Ph.D. student from Los Alamos, New Mexico; and Anish Senan, a master's student from Trivandrum, Kerala, India. Mamilla is running tests of carbon-carbon samples in a furnace with a controlled atmosphere that can be filled with an inert gas so that the carbon composites do not oxidize. By gradually introducing air containing oxygen into the furnace or contaminating the surface of the composite samples, she has characterized changes in the material as a function of oxidation.

Senan is perhaps working on what may be the best indicator of damage that may have actually occurred to the space shuttle materials. He is developing methods to obtain damping characteristics of carbon-carbon. Damping refers to the change experienced by the ultrasonic signal as it passes through material that has been degraded.

Puckett has developed a model of the ultrasonic signals in a way that allows researchers to evaluate the velocity and amplitude of the signal during oxidation. Signal processing, says Peterson, has been a major stumbling block to the project, but Puckett's work has solved that problem.

The research team has recently published results of the work in Acoustics Research Letters On-Line and previously presented the work at the Review of Progress in Quantitative Non-Destructive Evaluation. It has also submitted a paper, "In-Situ High-Temperature Monitoring if Carbon-Carbon Oxidation Using Time Reversal Mirrors," to the International Conference on Composite Materials scheduled for July 2003.

Peterson expects to have a prototype sensor system completed in 2003.