Structural health monitoring: Carbon nano paint can detect strain

Researchers at Rice University have developed a new type of paint made with carbon nanotubes that can help detect strain in aircraft and other structures via a handheld infrared spectrometer.

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Scientists at Rice University (Houston, Texas) reported on June 21 that a new type of paint made with carbon nanotubes can help detect structural strain in buildings, bridges and airplanes. The Rice scientists call their mixture “strain paint” and are hopeful it can help detect deformations in composite structures. The study, published online this month by the American Chemical Society (Washington, D.C.) journal Nano Letters, details a method by which a material, coated with the paint, can show signs of deformation before the effects become visible to the naked eye, and that the deformation can be measured with equipment that does not physically contact the structure. The researchers said this provides a big advantage over conventional strain gauges, which must be physically connected to their readout devices. In addition, the nanotube-based system could measure strain at any location and in 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 mechanical engineering and materials science, and his collaborators led the 2004 development of strain sensing for structural integrity monitoring at the macro level. His team used the electrical properties of carbon nanofilms — dense networks or ensembles of nanotubes. Since then he has continued to investigate novel strain-sensing methods, using a variety of nanomaterials.

Through a stroke of luck, Weisman and Nagarajaiah attended the same NASA workshop in 2010. There, Weisman gave a talk on nanotube fluorescence. As a flight of fancy, he said, he included an illustration of a hypothetical system that would use lasers to reveal strains in the nanocoated wing of a Space Shuttle. “I went up to him afterward and said, ‘Bruce, do you know we can actually try to see if this works?’” recalls Nagarajaiah. Nanotube fluorescence shows large, predictable wavelength shifts when the tubes are deformed by either tension or compression. The paint — and therefore each nanotube — would suffer the same strain as the surface it is painted on and give a clear picture of what’s happening underneath the painted surface.

“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 says. “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 noncontact technique, they could aim the laser at any point on the wing and get a strain map along any direction.” He adds that the coating could be a protective film that impedes corrosion or could enhance the strength of the underlying material.
Weisman says the project will require further development of the coating before such a product can go to market. “We’ll need to optimize details of its composition and preparation, and find the best way to apply it to the surfaces that will be monitored,” he says. “These fabrication/engineering issues should be addressed to ensure proper performance, even before we start working on portable readout instruments.” He explains that there are also subtleties regarding interactions among the nanotubes, the polymeric host and the substrate that affect the reproducibility and long-term stability of the spectral shifts. But none of those problems seem insurmountable, he says, and construction of a handheld optical strain reader should be relatively straightforward.