Sensor Changes Color When Bridge Cracks Form
An Iowa State University researcher has recently created a multifunctional stretchable strain sensor that changes colors when covering a crack in a steel component.
Simon Laflamme, a professor in the Department of Civil, Construction and Environmental Engineering, worked with Harvard University to develop the stretchable polymer that changes color from white to blue when a crack in a structure is identified.
According to Iowa State, the polymer was also modified into a piece of flexible electronics that changes electrical properties upon a change in geometry, therefore augmenting the optical color feedback with an electrical feedback.
“Most of the research I’ve done on sensors was trying to create some sort of biological inspired skin. So like humans, if you hurt yourself, you know where the pain is and know how much you feel it,” said Laflamme.
“The sensors will be able to get better information, and therefore help us to conduct real time condition assessment to know what’s wrong, how wrong it is, and how to fix it.”
Laflamme’s goal during development was to create a sensor that was “revolutionary” and had never been seen before. However, one issue involved creating a mixture that was stable under normal environmental conditions.
“It took some time to create a stable mix of something that would not decay when exposed to the environment,” said Laflamme.
“A colleague from Harvard came here for a summer and we ended up refining the sensor and characteriz[ing] it. We tried to see how well it could detect a fatigue crack, what kind of colors we have for the resolution and compare against the state of the art.”
Last year, in July, researchers from Rice University announced that their strain-sensing smart skin system, capable of monitoring and detecting damage in large structures, was moving towards the implementation stage.
First revealed in 2012, the “strain paint” uses the fluorescent properties of carbon nanotubes to show when a surface has been deformed by stress. The multilayered coating, now developed as part of a non-contact optical monitor system known as S4, can be applied to structures such as bridges, buildings, ships and airplanes.
The new study provided details of the next-generation, non-contact system. “Next-generation 2D optical strain mapping with strain-sensing smart skin compared to digital image correlation” was published in the journal Scientific Reports.
The project was led by Rice chemist Bruce Weisman, structural engineer Satish Nagarajaiah and lead author and graduate student Wei Meng. Rice research scientist Sergei Bachilo and graduate student Ashish Pal are co-authors of the study.
Rice reports that the skin has three layers, with each layer configured to the surface they cover. The first layer is an opaque primer containing the digital image correlation (DIC) speckles, followed by a second layer of clear polyurethane that isolates the base from the nanotubes. A final sensing layer of individually coated nanotubes, suspended in toluene, is sprayed on top.
After the toluene evaporates, a sub-micron-thick sensing layer of nanotubes is bonded to the structure member. Then, an additional protective layer can reportedly be applied on top to keep the skin active for years. The system also uses a reader, such as a small visible laser, to excite the nanotubes, in addition to a portable spectrometer to see how they’re strained.