Using carbon nanotubes, a new strain-sensing smart skin monitors and detects damage in large structures. The coating detects surface deformation by stress by utilizing the fluorescent properties of nanotubes. As part of the S4 non-contact optical monitoring system, the multilayered coating can be applied to large surfaces — bridges, buildings, ships, and airplanes, to name a few — where high strain poses an invisible threat.
Strain measurements are part of safety inspections. Strain-sensing has traditionally relied on two technologies: physical gauges attached to structures and digital image correlation (DIC), which compares images of surfaces with embedded speckles taken over time.
S4 easily withstands DIC. Even better, the two strain-sensing methods can work in tandem. The skin comprises three layers, the configuration determined by the surface it covers. Typically, an opaque primer with DIC speckles is applied first. A transparent polyurethane layer separates the base from the nanotubes in the second layer. Finally, an individually coated nanotube sensing layer suspended in toluene is sprayed on top.
Toluene evaporates, leaving a sensing layer of nanotubes bonded to the structural member that is sub-micron thick. An additional protective layer on top keeps the skin active for years. The strain-sensing device requires a reader (a small visible laser) to excite the nanotubes and a portable spectrometer to see the strain.