Whether they're in airplane wings, bridges or other critical structures, cracks can cause catastrophic failure before they're large enough to be noticed by the human eye. A strain-sensing "skin" applied to such objects could help, though, by lighting up when exposed to laser light.
Developed by a team led by Rice University's Bruce Weisman and Satish Nagarajaiah, the skin is actually a barely-visible very thin film. It consists of a bottom layer of carbon nanotubes dispersed within a polymer, and a top transparent protective layer composed of a different type of polymer (carbon nanotubes are basically microscopic rolled-up sheets of graphene, graphene being a one-atom-thick sheet of linked carbon atoms).
As is the case with carbon nanotubes in general, the ones in the skin fluoresce when subjected to laser light. Depending on how much mechanical strain they're under, however, they'll fluoresce at different wavelengths. Therefore, by analyzing the wavelength of the near-infrared light that the nanotubes are emitting, a handheld reader device can ascertain the amount of strain being exerted on any one area of the skin – and thus on the material underlying it.
The skin has been tested on aluminum bars, which were weakened in one spot with a hole or a notch. While those bars initially appeared uniform to the reader, the skin dramatically indicated where the weakened areas were once the bars were placed under tension.