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Researchers Use Fluorescing Carbon Nanotubes to Expose Stress in Structures

As a result of one unusual feature of carbon nanotubes, engineers can soon measure the accumulated strain in anything from bridges and airplanes, to pipelines, over the whole surface or down to microscopic levels.

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. (Courtesy of the Satish Nagarajaiah Group/Weisman Research Group)

They will accomplish this by shining a light onto structures coated with a two-layer nanotube film and protective polymer. Strain in the surface will be exposed as changes in the wavelengths of near-infrared light discharged from the film and captured by a miniaturized hand-held reader. The results will reveal to maintenance crews and engineers whether structures like aircraft or bridges have been deformed by stress-inducing events or standard wear and tear.

Similar to a white shirt under a UV light, single-wall carbon nanotubes fluoresce, a property detected in 2002 in the lab of Rice chemist Bruce Weisman. In a simple research project a few years later, the team demonstrated that stretching a nanotube varies the color of its fluorescence.

When Weisman’s results were read by Rice civil and environmental engineer Satish Nagarajaiah—who had been working individually on related ideas using Raman spectroscopy, but at the macro scale, since 2003—he proposed collaborating to turn that scientific occurrence into a beneficial technology for strain sensing.

Currently, Nagarajaiah and Weisman have published a pair of crucial papers about their “smart skin” project. The first can be read in Structural Control & Health Monitoring, and introduces the latest iteration of the technology they first publicized in 2012.

It illustrates a technique of placing the microscopic nanotube-sensing film separately from a protective top layer. Color variations in the nanotube emission specify the amount of strain in the underlying structure. The scientists say it enables 2D mapping of accumulated strain that cannot be accomplished by any other non-contact technique.

The second paper, in the Journal of Structural Engineering, covers the results of testing smart skin on metal specimens with irregularities where stress and strain are repeatedly 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 preliminary report, the scientists have improved the composition and preparation of the film and its airbrush-style application, and also created scanner devices that automatically trap data from several programmed points. In contrast to conventional sensors that just measure strain at a single point along one axis; the smart film can be selectively probed to expose strain in any location and direction.

The two-layer film is just a few microns thick, a fraction of the width of a human hair, and hardly visible on a transparent surface. “In our initial films, the nanotube sensors were mixed into the polymer,” Nagarajaiah said. “Now that we’ve separated the sensing and the protective layers, the nanotube emission is clearer and we can scan at a much higher resolution. That lets us capture significant amounts of data rather quickly.”

The scientists tested smart skin on aluminum bars under tension with either a notch or a hole to denote the places where strain tends to accumulate. Measuring these probable weak spots in their unstressed state and then again after applying stress revealed significant changes in strain patterns pieced together from point-by-point surface mapping.

We know where the high-stress regions of the structure are, the potential points of failure. We can coat those regions with the film and scan them in the healthy state, and then after an event like an earthquake, go back and re-scan to see whether the strain distribution has changed and the structure is at risk.

Satish Nagarajaiah, Civil and Environmental Engineer, and Professor of Civil and Environmental Engineering, Mechanical Engineering, and Materials Science and Nanoengineering, Rice University.

In their tests, the scientists said the measured results were a close match to strain patterns gotten through cutting-edge computational simulations. Readings from the smart skin enabled them to swiftly detect distinctive patterns near the high-stress regions, Nagarajaiah said. They were also able to observe vivid boundaries between regions of tensile and compressive strain.

We measured points 1 millimeter apart, but we can go 20 times smaller when necessary without sacrificing strain sensitivity.

Bruce Weisman, Professor of Chemistry, and Materials Science and Nanoengineering, Rice University.

That’s a jump over regular strain sensors, which only deliver readings averaged over several millimeters, he said.

The scientists see their technology making preliminary inroads in niche applications, such as examining turbines in jet engines or structural elements in their formative stages. “It’s not going to replace all existing technologies for strain measurement right away,” Weisman said. “Technologies tend to be very entrenched and have a lot of inertia.”

“But it has advantages that will prove useful when other methods can’t do the job,” he said. “I expect it will find use in engineering research applications, and in the design and testing of structures before they are deployed in the field.”

With their smart skin improved, the scientists are aiming to develop the next generation of the strain reader, a camera-like device that is capable of capturing strain patterns over a large surface concurrently.

Co-authors of both papers are Rice predoctoral researchers Peng Sun and Ching-Wei Lin and research scientist Sergei Bachilo. Weisman is a professor of chemistry and of materials science and nanoengineering. Nagarajaiah is a professor of civil and environmental engineering, of mechanical engineering, and of materials science and nanoengineering.

The research was supported by the Office of Naval Research and the Welch Foundation.

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