‘Smart Skin’ Simplifies Spotting Strain in Structures

Estimated reading time: 5 minutes

“Smart skin” able to detect strain in materials, invented at Rice University, starts with carbon nanotubes and their unique ability to change their fluorescence under stress. When attached to a surface, they can be used to monitor stress over time through spectroscopy. “Smart skin” able to detect strain in materials, invented at Rice University, starts with carbon nanotubes and their unique ability to change their fluorescence under stress. When attached to a surface, they can be used to monitor stress over time through spectroscopy. Courtesy of the Satish Nagarajaiah Group/Weisman Research Group.

The two-layer film is only a few microns thick, a fraction of the width of a human hair, and barely 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 researchers tested smart skin on aluminum bars under tension with either a hole or a notch to represent the places where strain tends to build. Measuring these potential weak spots in their unstressed state and then again after applying stress showed dramatic 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,” Nagarajaiah said. “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.”

In their tests, the researchers said the measured results were a close match to strain patterns obtained through advanced computational simulations. Readings from the smart skin allowed them to quickly spot distinctive patterns near the high-stress regions, Nagarajaiah said. They were also able to see clear 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,” Weisman said. That’s a leap over standard strain sensors, which only provide readings averaged over several millimeters, he said.

The researchers see their technology making initial inroads in niche applications, like testing turbines in jet engines or structural elements in their development 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 refined, the researchers are working toward developing the next generation of the strain reader, a camera-like device that can capture strain patterns over a large surface all at once.

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 Office of Naval Research and the Welch Foundation supported the research.

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