(Invited) Progress in Using Carbon Nanotube Spectra for Mechanical Strain Sensing

A characteristic property of semiconducting single-wall carbon nanotubes (SWCNTs) is distinct near-infrared photoluminescence following excitation by visible light. Theory and experiment show that these optical emission peaks shift predictably in wavelength as nanotubes are compressed or stretched along their axis. We are exploiting this effect in a powerful new method intended for measuring mechanical strain in critical infrastructure components such as airframes, pressurized vessels, pipelines, support beams, etc. The method involves applying a dilute dispersion of nanotubes in a polymeric host onto the surface of the specimen to form a sub-micron thick film in which SWCNTs act as strain sensors. This layer is overcoated with a transparent protective top coat such as a polyurethane varnish. Subsequent strains in the substrate are transmitted to the nanotubes by load transfer. The substrate strain magnitude and direction are then measured by illuminating the surface at any point of interest with a small visible laser beam and spectrally analyzing the resulting near-IR nanotube emission. Single-point measurements currently provide strain magnitude resolution of ca. 100 microstrain, strain angle resolution of ca. 5 degrees, and spatial resolution of ca. 50 mm. Each reading takes less than one second, allowing compilation of strain maps from scanned data. In contrast to digital image correlation, which is currently the only commercial non-contact strain technology, the new method can measure strains induced when the specimen is not under observation. It thus has the potential for routine use in industrial structural health monitoring as well as in testing and development laboratories. We will also describe recent progress in adapting the technology to camera-based measurements using spectrally resolved fluorescence imaging.