Conventionally, strain is measured using gauges that are attached to the structure to be monitored, but a new carbon nanotube-infused paint enables a noncontact method of strain detection. Rice University scientists developed the strain paint mixture to detect deformations in structures like airplane wings, buildings and bridges before such effects become visible to the naked eye, and without touching the structure. The composite coating could be read by a handheld infrared spectrometer. The new system provides an advantage over conventional strain gauges, which must be physically connected to their readout devices. The nanotube-based system can measure strain at any location and along any direction. An illustration shows how polarized light from a laser and a near-infrared spectrometer could read levels of strain in a material coated with nanotube-infused painted invented at Rice University. (Image: Bruce Weisman/Rice University) “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, a Rice professor of civil and environmental engineering and of mechanical engineering and materials science. “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.” Rice chemistry professor Bruce Weisman headed the discovery and analysis of near-infrared fluorescence from semiconducting carbon nanotubes in 2002. Since then, Weisman has devised and used novel optical tools to study the physical and chemical properties of nanotubes. In 2004, Nagarajaiah and his colleagues headed the development of strain sensing for monitoring structural integrity at the macro level using carbon nanofilms’ electrical properties. Since then, Nagarajaiah has been working to advance strain sensing methods using different nanomaterials. It was a stroke of luck that the two researchers attended the same NASA workshop in 2010. There Weisman presented findings on nanotube fluorescence, concluding his talk with an illustration of a hypothetical system that would use lasers to reveal strain in the nanocoated wing of a space shuttle. Nagarajaiah approached him after the presentation to put the hypothetical system to the test. Rice University professor Bruce Weisman introduced the idea of strain paint for finding weaknesses in materials with this slide from a presentation to NASA in 2010. (Image: Bruce Weisman/Rice University) Nanotube fluorescence demonstrates large, predictable wavelength changes when the tubes undergo deformation from compression or tension. Therefore, the nanotube-infused paint would experience the same strain as the surface it coats, enabling scientists to detect changes occurring underneath. Nagarajaiah is confident that the paint could be designed with multifunctional properties for specific applications. “It can also have other benefits,” he said. “It can be a protective film that impedes corrosion or could enhance the strength of the underlying material.” Further research is needed before the paint is commercialized, according to Weisman. “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 said. “These fabrication/engineering issues should be addressed to ensure proper performance, even before we start working on portable readout instruments. Researchers at Rice University invented a nanotube-infused strain paint that can help detect stresses in materials, including bridges, buildings and aircraft. From left: Rice professors Bruce Weisman and Satish Nagarajaiah, research scientist Sergei Bachilo and graduate student Venkata Srivishnu Vemuru; and Paul Withey, an associate professor of physics at the University of Houston-Clear Lake. (Image: Tommy LaVergne/Rice University) “There are also subtleties about how interactions among the nanotubes, the polymeric host and the substrate affect the reproducibility and long-term stability of the spectral shifts. For real-world measurements, these are important considerations.” Construction of a handheld optical strain reader should be relatively straightforward, Weisman said. It would involve combining components that already exist. “I’m confident that if there were a market, the readout equipment could be miniaturized and packaged,” he said. “It’s not science fiction.” The study was published online this month in Nano Letters. For more information, visit: www.rice.edu