Using nanotubes in paint to remotely detect strain

Could detect hidden strain in airplane wings and bridges by just pointing a device
June 25, 2012

Spectrometer can detect minute deformation in a single-wall carbon nanotube substrate based on a shift in the spectrum (color) of its fluorescence (credit: Paul A. Withey et al./Nano Letters)

“Strain paint” made with carbon nanotubes can help detect strain in buildings, bridges and airplanes by its fluorescence, using a handheld laser-infrared spectrometer device, Rice University scientists have discovered.

In fact, this method could tell where a material is showing signs of deformation well before the effects become visible to the naked eye, and without touching the structure.

The researchers said this provides a big advantage over conventional strain gauges, which must be physically connected to external readout devices

In addition, the nanotube-based system could measure strain at any location and along any direction.

Rice chemistry professor Bruce Weisman led the discovery and interpretation of near-infrared fluorescence from semiconducting carbon nanotubes in 2002, and he has since developed and used novel optical instrumentation to explore nanotubes’ physical and chemical properties.

Satish Nagarajaiah, a Rice professor of civil and environmental engineering and of mechanical engineering and materials science, and his collaborators led the 2004 development of strain sensing for structural integrity monitoring at the macro level using the electrical properties of carbon nanofilms — dense networks/ensembles of nanotubes. Since then he has continued to investigate novel strain sensing methods using various nanomaterials.


Strain testing prototype apparatus (credit: Paul A. Withey et al./Nano Letters)

Nanotube fluorescence shows large, predictable wavelength shifts when the tubes are deformed by tension or compression. The paint — and therefore each nanotube, about 50,000 times thinner than a human hair — would have the same strain as the surface it’s painted on and give a clear picture of what’s happening underneath.

“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,” Nagarajaiah said.

“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.”

He said strain paint could be designed with multifunctional properties for specific applications. In addition, “it can be a protective film that impedes corrosion or could enhance the strength of the underlying material.”

Weisman said the project will require further development of the coating before such a product can go to market. “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.”

Support for the research came from the National Science Foundation, the Welch Foundation, the Air Force Research Laboratory and the Infrastructure-Center for Advanced Materials at Rice.

Paul A. Withey et al., Strain Paint: Noncontact Strain Measurement Using Single-Walled Carbon Nanotube Composite Coatings, Nano Letters, 2012, DOI: 10.1021/nl301008m