Aug. 9, 2007 A new "skin" for bridges, buildings and airplanes could be a sixth sense for inspectors looking for cracks and corrosion that could lead to a catastrophic failure like the recent Minneapolis bridge collapse.
Researchers at the University of Michigan's College of Engineering developed a coating that could be painted or sprayed on structures to sense their stability over time. It would allow inspectors to check for damage without physically examining a structure.
Today, inspectors rely heavily on their eyes to find weak points. Bridges are scrutinized every two years and if experts see red flags, they do more tests. Aircraft are routinely examined too, but scheduled check-ups might not catch all potential problems. Fissures or rusting could be happening beneath the surface as well, said Jerome Lynch, assistant professor in the U-M College of Engineering and lead author of a paper on the research. The paper was published online in the journal Nanotechnology.
"Both corrosion and cracking are very serious issues for the more than 500,000 bridges in the United States," Lynch said. "The sensing skin would give bridge officials an unprecedented technology to track the evolution of corrosion and crack damage. It would revolutionize the way current bridge health assessment is conducted, resulting in dramatically safer structures and lower-cost inspection processes.
"This is really an automated technology requiring no human intervention to work," he said.
The sensing skin that Lynch and his colleagues created is an opaque, black material made of layers of polymers. Networks of carbon nanotubes run through the polymers. Carbon nanotubes are a fundamental building block of the nanotechnology revolution.
Each layer of the sensing skin can measure something different. One tests the pH level of the structure, which changes when corrosion is happening. Another layer registers cracks by actually cracking under the same conditions that the structure would.
The perimeter of the carbon nanotube skin is lined with electrodes that are connected to a microprocessor, or tiny computer. To read what's going on underneath the skin, scientists (or inspectors) send an electric current through the embedded carbon nanotubes. Corrosion and cracking cause changes in the electrical resistance in the nanotube skin.
The microprocessor then creates a two-dimensional visual map of that resistance. The map shows inspectors any corrosion or fracturing too small for human eyes to detect.
Lynch says the skin could be a permanent veneer over strain- and corrosion-prone hot spots including joints on bridges, buildings, airplanes and even the space shuttle. When it's time to examine the health of the structure or aircraft, an inspector could push a button and in minutes, the skin would generate an electrical resistance map and wirelessly send it to the inspector.
Lynch sees a use for this technology in space. Ever since the Columbia disaster, he explained, an astronaut must conduct a space walk to visually inspect the shuttle for impact damage that might have happened during launch. This new skin would eliminate the need for that. It could detect the location and degree of any impact damage.
The novelty of this skin is what Lynch calls "distributed sensing technology." Engineers have used sensors to check for damage on a point-to-point basis before. But they've never been able to get such a complete picture of a large area. "For the first time, this gives us a straightforward way to gain direct insight into the structure of the material," Lynch said.
Others contributing to this work are: U-M associate professor of chemical engineering Nick Kotov; U-M assistant professor of chemical engineering Nadine Wong Shi Kam, a Michigan Society Fellow in Kotov's lab; and civil and environmental engineering graduate students Ken Loh and Tsung-Chin Hou. The National Science Foundation funded the research.
U-M, though its Office of Technology Transfer, is seeking commercialization partners to help bring this technology to market.
The paper, "Spatial conductivity mapping of carbon nanotube composite thin films by electrical impedance tomography for sensing applications." is available online in the journal Nanotechnology.
Lynch has also done work on wireless sensors that could be embedded in bridges and buildings to sense cracks without any help from humans.
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