Although the numbers vary according to the source, it’s estimated that the chances of a plane crashing are 1 in 11 million. Of those, around 17 percent are due to some kind of mechanical failure. Sometimes a tiny, undetectable crack can cause significant damage. A team of scientists has turned years of studying nanotubes into research that could help engineers discover and measure strain on aircraft, or other large structures, at the microscopic level.
The research team is developing a two-layer nanotube film and protective polymer that is applied to a surface and then lit up with ultraviolet light. During this process, surface strains appear as changes in the near-infrared light wavelengths emitted from the film and captured by a miniaturized handheld reader.
The unique ability for single-wall carbon nanotubes to fluoresce was discovered in the lab of Bruce Weisman, professor of chemistry at Rice University, in 2002. As work continued on the research over time, researchers demonstrated that stretching the nanotubes made the fluorescence change colors. Weisman soon began collaborating with Satish Nagarajaiah, a Rice civil and environmental engineer, to focus on using the technology for strain sensing.
The two researchers recently published a second paper on their “smart skin.” The first, published in 2012, described their process for depositing the microscopic nanotube-sensing film separately from a protective top layer. They have since refined the process by changing the composition and preparation of the two-layer film, which is only a few microns thick, and created a new scanner device. Instead of measuring strain just at one point along a single axis, it automatically measures multiple program points and can reveal strain in any direction and at any location.
The latest paper discusses testing results. The researchers tested their smart skin on aluminum bars under tension and created weak spots—either a hold or notch—to demonstrate where strain builds. These spots were measured in an unstressed state and again after applying stress.
“We know where the high-stress regions of the structure are, the potential points of failure,” said Nagarajaiah. “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.”
The measured results closely matched strain patterns obtained from advanced computational simulations. The smart skin’s readings made it possible to quickly identify distinctive patterns near the high-stress regions. Nagarajaiah noted that they were also able to see clear boundaries between regions of tensile and compressive strain.
“We measured points 1mm 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.”
The researchers believe the technology could be useful for testing turbines in jet engines or structural elements in their development stages. Their next research step is to develop a camera-like device that can capture an entire large surface at one time.
Interested in learning more about nanotubes and how they are inspiring innovation? Check out Carbon Nanotubes Transistors – The Future of Electronic Devices and Manufacturing a Carbon Revolution.