'Smart Skin' simplifies the detection of tensions in structures

'Smart Skin' simplifies the detection of tensions in structures

Thanks to a special feature of carbon nanotubes, engineers will soon be able to measure the stress accumulated in an aircraft, a bridge or a pipeline – or just about anything – over the entire surface or even at the same level. microscopic.

To do this, they will illuminate the structures covered with a film of two-layer nanotubes and a protective polymer. The surface voltages will appear as changes in the wavelengths of the near infrared light emitted by the film and captured by a miniaturized portable reader. The results will show engineers and maintenance crews whether structures such as bridges or aircraft have been deformed by stressful events or normal wear and tear.

The experimental stress maps (left) and simulated (right) around a hole crossing an aluminum bar show that the "smart skin" developed by nanotubes developed at the University Rice can effectively badess the stress of materials. This technique can be used for aircraft, spacecraft and critical infrastructure in which mechanical stress is to be monitored. (Credit: Satish Nagarajaiah Group / Weisman Research Group / Rice University)

As a white shirt under ultraviolet light, fluorescent single-walled carbon nanotubes, a property discovered in 2002 in the laboratory of the rice chemist, Bruce Weisman. A few years later, in a basic research project, the group showed that stretching a nanotube changes the color of its fluorescence.

When Weisman's results were brought to the attention of Satish Nagarajaiah, civil engineer in civil engineering and environment at Rice, who worked independently on similar ideas using Raman spectroscopy, but on a macroeconomic scale since 2003, he suggested collaborating to turn this scientific phenomenon into a useful technology. stress detection.

Nagarajaiah and Weisman have published two important articles about their "Smart Skin" project. The first appears in Structural Control and Health Surveillance and presents the latest version of the technology first revealed in 2012.

It describes a method of depositing microscopic nanotube detection film separately from a protective upper layer. The color changes in the emission of nanotubes indicate the amount of stress in the underlying structure. The researchers say that this allows a two-dimensional mapping of the accumulated strain that can not be obtained by any other non-contact method.

The second article, published in the Journal of Structural Engineering, details the results of intelligent skin tests on metal samples with irregularities, in which stress and deformation are often concentrated.

"The project started as a pure science on nanotube spectroscopy and led to a collaborative proof-of-principle work that showed that we could measure the underlying substrate voltage by checking the spectrum of the film in one place" said Weisman. "This suggests that the method could be extended to measure whole areas. What we have shown now is much closer to this practical application. "

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

The "intelligent skin" capable of detecting tensions in materials, invented at Rice University, begins with carbon nanotubes and their unique ability to modify their fluorescence under stress. When they are attached to a surface, they can be used to monitor stresses over time by spectroscopy. The "intelligent skin" capable of detecting tensions in materials, invented at Rice University, begins with carbon nanotubes and their unique ability to modify their fluorescence under stress. When they are attached to a surface, they can be used to monitor stresses over time by spectroscopy. With the kind permission of 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 is barely visible on a transparent surface. "In our initial films, the nanotube sensors were mixed with the polymer," Nagarajaiah said. "Now that we have separated the layers of detection and protection, the emission of nanotubes is clearer and we can digitize at a much higher resolution. This allows us to capture large amounts of data quite quickly. "

The researchers tested smart skin on live aluminum bars with a hole or notch to represent the places where stress tends to accumulate. The measurement of these potential weak points in their unstressed state, and then again after applying a stress, showed dramatic changes in the deformation patterns reconstructed from point-by-point surface mapping.

"We know where the heavily stressed areas of the structure are, as well as the potential points of failure," Nagarajaiah said. "We can cover these areas of the film and scan them in a healthy state, then after an event like an earthquake, go back and check again if the stress distribution has changed and the structure is in danger. "

In their tests, the researchers stated that the measured results closely matched the strain patterns obtained through advanced computer simulations. Intelligent skin readings allowed them to quickly spot distinctive patterns near highly stressed areas, said Nagarajaiah. They also could see clear boundaries between the tensile and compressive stress regions.

"We measured points 1 millimeter apart, but we can go 20 times smaller if necessary without sacrificing stress sensitivity," said Weisman. This is a leap forward from standard stress sensors, which only provide average readings of several millimeters, he said.

Researchers see their technology making initial breakthroughs in niche applications, such as turbine testing in jet engines or structural elements under development. "It's not going to replace all existing technologies for measuring stress immediately," Weisman said. "The technologies tend to be very rooted and have a lot of inertia.

"But it has advantages that will be useful when other methods can not do the job," he said. "I hope it will be used in engineering research applications, as well as in the design and testing of structures prior to field deployment."

With intelligent skin being refined, researchers are working to develop the next-generation constraint reader, a camera-type device that can simultaneously capture patterns of stress over a large area.

The two authors are co-authors: Peng Sun and Ching-Wei Lin, undergraduate researcher in Rice, and Sergei Bachilo, research scientist. Weisman is a professor of chemistry, materials science and nanoengineering. Nagarajaiah is a professor in civil and environmental engineering, mechanical engineering, materials science and nanoengineering.

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