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Researchers Develop New Method to Detect Damage in Failing Infrastructure
June 23, 2017

The American Society of Civil Engineers (ASCE) has estimated that more than $3.6 trillion in investment will be required by 2020 to rehabilitate and modernize the nation’s failing infrastructure. An important element in any modernization effort will be the development of new and improved methods for detecting damage in these structures before it becomes critical.

Fiberglass and aluminum test strips illuminated in UV light. The white areas show the polymer/quantum dot coating. Image courtesy of LASIR Lab, Vanderbilt University.

As part of this modernization effort, an interdisciplinary research team at Vanderbilt University’s Laboratory for Systems Integrity and Reliability (LASIR) is developing a new sensing system. According to Cole Brubaker, a doctoral student in civil engineering, just sprinkle a pixie dust of nanoparticles into a batch of clear polymer resin and you get “what I call a ‘mood ring material,’ a smart material that changes color when it is damaged or about to fail.”

Smart sensing technologies are one of the hot new fields in civil, mechanical and aerospace engineering. These efforts have generally focused on developing networks of physical sensors that are attached to structures of interest. However, this approach has been hindered by high cost as well as power and data processing requirements.

In research supported by a grant from the U.S. Office of Naval Research, the LASIR researchers are taking a different tack by incorporating fluorescent nanoparticles into the material itself that react to stress by changing their optical properties in order to create a new kind of detection system that can monitor these structures in an efficient and cost-effective fashion.

“Currently, there are two ways to keep everything from bridges to aircraft safe,” explained LASIR Director, Douglas Adams, Daniel F. Flowers Professor of civil and environmental engineering. “One is to send people out to look at them with a flashlight. The problem with this is that it is labor-intensive and the people can’t see very small cracks when they form. The other is to install elaborate sensor networks that constantly look for small cracks and detect them before they grow too large. The problem is that these networks are very expensive and, in the case of aircraft, add a lot of weight. “So we need to somehow change the materials we are using so they illuminate these tiny cracks.”

The team’s initial studies, published in the Proceedings of the SPIE Conference on Sensors and Smart Structures Technologies for Civil, Mechanical and Aerospace Systems, have determined that adding a tiny concentration of special nanoparticles (1 to 5 percent by weight) to an optically clear polymer matrix produces a distinctive light signature that changes as the material is subjected to a broad range of compressive and tensile loads.

The Vanderbilt group isn’t the only research team using nanoparticles to create smart materials, but they have a special advantage. They are using a particular type of nanoparticle called a white light quantum dot. These quantum dots are unique because they emit white light where other quantum dots only emit light at specific wavelengths.

These special quantum dots were discovered accidentally in 2005 in the laboratory of Sandra Rosenthal, Jack and Pamela Egan Professor of Chemistry at Vanderbilt. “We were trying to make the smallest cadmium selenide quantum dots possible and, when we did, we were astonished to discover that they emit in a broad spectrum,” she recalled.

“White light quantum dots have very unique optical properties compared with other nanoparticles,” explained Talitha Frecker, a chemistry graduate student who is participating in the study. “The white light fluorescence is a surface phenomenon.”

When Prof. Adams learned about Prof. Rosenthal’s discovery, he realized that her quantum dots were tailor made for creating smart materials. “When we put these nanoparticles into a material, they observe and react to what is going on around them,” Prof. Adams explained.

In a series of preliminary tests that Brubaker and his colleagues have conducted, Prof. Adam’s expectation has been confirmed. They have coated fiberglass and aluminum strips with a polymer coating containing white light quantum dots and subjected them to varying degrees of external load. They have determined that the intensity of the emission spectrum produced by the quantum dots decreases as the load increases. The drop-off is largest with the initial loading and gradually decreases at higher levels of load.

According to Kane Jennings, Professor of Chemical and Biomolecular Engineering who is participating in the project along with doctoral student Ian Njoroge, “The mechanism is still a bit unclear, but we have demonstrated that entrapping these quantum dots in ultra-thin polymer films on metal surfaces can provide advance warning when the underlying metal is about to sustain physical or chemical damage.”

The researchers theorize that the quantum dots emit light in a broad spectrum because more than 80 percent of the atoms lie on the surface. They also know that the bonds between the surface atoms and molecules surrounding them plays a critical role.

“The end result is that the strength of the quantum dot emissions gives us a permanent record of the level of stress that a material has experienced,” said Brubaker.

In this fashion, the researchers have verified that the material can act as a new kind of strain gauge that permanently records the cumulative amount of stress that the material to which it is applied experiences.

“There is a lot we have to learn before we can create a smart material that is ready for real world applications, but all the signs are positive,” said Prof. Adams. “Some of our commercial partners are very interested so there is a good chance that it will be adopted if it performs as well as we think it will.”

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