While wireless sensors for detecting the strain placed on bridges and buildings, such as the SenSpot, are easier and cheaper to install than embedded wired networks of sensors, they still need to be in physical contact with the structure being monitored. Researchers at Rice University have now developed a new type of paint, infused with carbon nanotubes, that could make strain detection of materials in buildings, bridges and aircraft possible without actually touching the material.

The nanotube-infused paint, which the Rice scientists call “strain paint,” comes on the back of previous work by Rice chemistry professor Bruce Weisman, who in 2002 led the discovery that carbon nanotubes are fluorescent and their physical and chemical properties could be revealed using optical instrumentation.

Because nanotube fluorescence shows large, predictable wavelength shifts when the carbon nanotubes are deformed by tension or compression, the strain paint can provide a clear picture of the strain being exerted on a material on which it has been coated. Unlike the similarly nanotube-infused smart paint developed at the University of Strathclyde in Glasgow that relies on connecting electrodes, the strain paint allows strain to be measured at any location and along any direction with the use of a handheld infrared spectrometer.

“For an airplane, technicians typically apply conventional strain gauges at specific locations on the wing and subject it to force vibration testing to see how it behaves,” said Satish Nagarajaiah, who collaborated with Weisman on developing the paint. “They can only do this on the ground and can only measure part of a wing in specific directions and locations where the strain gauges are wired. But with our non-contact technique, they could aim the laser at any point on the wing and get a strain map along any direction.”

Nagarajaiah says the strain paint could also be customized with multifunctional properties for specific applications – as a protective film to prevent corrosion of the underlying material, for example. It is also clear, meaning it won’t affect the appearance of the material.

The researchers say the strain paint still needs further development before it can be brought to market, both in terms of optimizing its composition and preparation, and finding the best way to apply it to surfaces. And these fabrication and engineering issues must be addressed before moving onto the development of portable read-out instruments. However, they believe all of these problems can be solved and construction of a handheld optical strain reader should be relatively straightforward.

“There are already quite compact infrared spectrometers that could be battery-operated,” Weisman said. “Miniature lasers and optics are also readily available. So it wouldn’t require the invention of new technologies, just combining components that already exist. I’m confident that if there were a market, the readout equipment could be miniaturized and packaged. It’s not science fiction.”

The Rice team’s study was published in the American Chemical Society journal Nano Letters. The researchers explain their work in the following video.

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