Researchers' Strain-Sensing "Smart Skin," for Surveying Building Stress, "Ready for Implementation"

A team of scientists at Rice University has unveiled a strain-sensing "smart skin," featuring embedded nanotubes," which it claims is ready to deploy as a sensor for wear and tear in large structures.

"Strain measurements are often made as part of safety-related inspections," explains chemist Bruce Weisman of the work. "That technical community is rightfully conservative, because their measurements must be reliable. So we need to overcome skepticism about new methods by proving that ours is as valid as the established ones. This paper presents our method's credentials as a serious strain measurement technology."

That method: a non-contact optical monitoring system called S4, based on a technology dubbed "strain paint" unveiled by Rice University almost a decade ago. Featuring embedded nanotubes, the paint-like coating uses fluorescence to detect excess strain and damage — providing a means of monitoring the entire structure for dangerous stress.

Interestingly, the team has opted to use the coating alongside existing digital image correlation (DIC) technology — where surfaces are coated in "speckles" visible to a computer imaging system and a series of images compared over time for changes. "We wanted to make a direct comparison to DIC, which is the only commercialized mapping method for strain out there," Weisman explains.

"It's used in a number of industries, and people have a fairly high level of confidence in it. To demonstrate that our method can stand side by side with it and get results that are similar or better, [graduate student and first author] Wei [Ming] devised a method to incorporate S4 and DIC so both techniques can be used simultaneously and even complement each other."

The resulting material is applied in three layers: An opaque primer with DIC speckles; a polyurethane protective coat; and the nanotube layer, sprayed in a toluene suspension which evaporates to leave a nanotube layer less than a micron thick. Once treated, the surface's strain levels are read on-site using a laser and portable spectrometer to build up a detailed strain map.

"I have no doubt that this is a state-of-the-art strain-mapping method," says structural engineer and co-author Satish Nagarajaiah. "We've tested it on structural members made of metals, plastics, and concrete with complex micro-cracks and subsurface damage, and it works in all cases. I believe we've reached the stage where it's ready for implementation, and we are engaging with industry to learn how it can help them."

The team's work is published in the journal Scientific Reports under open-access terms.