A Bridge Too Far Gone

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 have 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. The sensing skin 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, 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. The skin could be a permanent veneer over strain- and corrosion-prone hot spots including joints on bridges, buildings and airplanes. 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. Engineers have used sensors to check for damage on a point-to-point basis before. But they've never been able to get a complete picture of a large area.