‘Smart Skin’ Simplifies Spotting Strain in Structures

Estimated reading time: 5 minutes

Thanks to one peculiar characteristic of carbon nanotubes, engineers will soon be able to measure the accumulated strain in an airplane, a bridge or a pipeline—or just about anything—over the entire surface or down to microscopic levels.

They’ll do so by shining a light onto structures coated with a two-layer nanotube film and protective polymer. Strain in the surface will show up as changes in the wavelengths of near-infrared light emitted from the film and captured by a miniaturized hand-held reader. The results will show engineers and maintenance crews whether structures like bridges or aircraft have been deformed by stress-inducing events or regular wear and tear.

Image: Experimental (left) and simulated (right) strain maps around a hole through an aluminum bar show that nanotube-infused "smart skin" developed at Rice University can effectively assess strain in materials. The technique can be used for aircraft, spacecraft and critical infrastructures in which mechanical strain needs to be monitored. (Credit: Satish Nagarajaiah Group/Weisman Research Group/Rice University)

Like a white shirt under an ultraviolet light, single-wall carbon nanotubes fluoresce, a property discovered in 2002 in the lab of Rice chemist Bruce Weisman. In a basic research project a few years later, the group showed that stretching a nanotube changes the color of its fluorescence.

When Weisman’s results came to the attention of Rice civil and environmental engineer Satish Nagarajaiah—who had been working independently on similar ideas using Raman spectroscopy, but at the macro scale, since 2003—he suggested collaborating to turn that scientific phenomenon into a useful technology for strain sensing.

Now, Nagarajaiah and Weisman and have published a pair of important papers about their “smart skin” project. The first appears in Structural Control & Health Monitoring, and introduces the latest iteration of the technology they first revealed in 2012.

It describes a method of depositing the microscopic nanotube-sensing film separately from a protective top layer. Color changes in the nanotube emission indicate the amount of strain in the underlying structure. The researchers say it enables two-dimensional mapping of accumulated strain that can’t be achieved by any other non-contact method.

The second paper, in the Journal of Structural Engineering, details the results of testing smart skin on metal specimens with irregularities where stress and strain are often concentrated.

“The project started out as pure science about nanotube spectroscopy, and led to the proof-of-principle collaborative work that showed we could measure the strain of the underlying substrate by checking the spectrum of the film in one place,” Weisman said. “That suggested the method could be expanded to measure whole surfaces. What we’ve shown now is a lot closer to that practical application.”

Since the initial report, the researchers have refined the composition and preparation of the film and its airbrush-style application, and also developed scanner devices that automatically capture data from multiple programmed points. Unlike conventional sensors that only measure strain at one point along one axis, the smart film can be selectively probed to reveal strain in any direction and location.

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