Bridge skin could reveal cracks and corrosion beneath

A new "skin" for bridges, buildings and airplanes could aid inspectors looking for cracks and corrosion that could lead to a catastrophic failure like the recent Minneapolis bridge collapse.

(Photo by Martin Vloet, U-M Photo Services)

Researchers at the 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, says Jerome Lynch, assistant professor in the College of Engineering and lead author of a paper on the research. The paper was published online in the journal Nanotechnology.

(Above) Multifunction carbon nanotube composite sensing skin for structures. (Below) Sensing skin spatial mapping of crack damage inside cementitious element (Images courtesy College of Engineering)

"Both corrosion and cracking are very serious issues for the more than 500,000 bridges in the United States," Lynch says. "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 says.

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.