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Sensing Unseen Structural Damage

Thursday, August 25, 2016

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Bridges and roadways are subject to wear and deterioration from a number of factors, ranging from age to events like earthquakes and hurricanes. Damage and defects are sometimes obvious, but even those invisible to the naked eye have the potential to lead to catastrophic failure.

Recognizing the importance of keeping on top of the health of these and other structures, engineers Erik Thostenson and Thomas Schumacher, are striving to put a “smart skin” of carbon nanotube composites to work as an early detection system, the University of Delaware (UD) reported.

Thomas Schumacher, Erik Thostenson
Evan Krape / University of Delaware

Thomas Schumacher (left) and Erik Thostenson are leading research on a new technique to monitor the health of structures including roads and bridges.

When in place, the nanotube composite is intended to help monitor the condition of structures and alert their owners to potential problems.

Creating the Composite Sensor

Thostenson and Schumacher, both affiliated faculty members in the University of Delaware’s Center for Composite Materials, recently published their latest research efforts in the Journal of Nondestructive Evaluation.

According to their paper, the sensor is made of a nonwoven synthetic polymer fabric that is coated with nanotubes and then infused with epoxy resin through a vacuum assisted resin transfer molding technique.

The nanotube composite sensor is cured to become a “mechanically robust,” electromechanically sensitive two-dimensional sensor that can be adhered to virtually any substrate and any shape, according to the duo.

Assuming that damage to the sensor directly affects its conductivity, they say, the sensor can detect damage and show its location within the material or structure.

‘A True 2-D Algorithm’

Although Thostenson and Schumacher reportedly began to work with this “smart skin” years ago, they have since refined their approach with the application of a technique called electrical impedance tomography (EIT).

EIT uses surface electrode measurements to create an image of the conductivity of a material or structure. (The technology has been in use since the 1980s as a noninvasive medical imaging technique, but until now has been overlooked by the structural health monitoring community, the school noted.)

“While the feasibility of employing carbon-nanotube-based composites as sensors has been validated, the typical approach is to use a series of one-dimensional measurements collected from a two-dimensional sensing area,” said Thostenson.

“The problem is that this confines the possible damage locations to the grid points of the measurements,” he explained. “EIT, on the other hand, is a true 2-D algorithm.”

Seattle seawall, viaduct
© iStock.com / David_Johnson

Although damage from earthquakes and weather incidents is sometimes obvious, the UD research suggests that a nanotube composite sensor placed on in-use structures can detect unseen damage and show its location within the material.

Schumacher, a structural engineering researcher who envisions using the technique on in-service structures, noted that the sensing technique’s primary benefits are that it can be scaled up and that it is relatively inexpensive, as it doesn’t require a large quantity of carbon nanotubes.

Testing and Future Applications

The UD team’s paper documents their initial evaluations of the sensing method. They first worked with well-defined damage for analysis before introducing a more realistic damage scenario. This helped to show the capability of the approach to detect impact damage on a composite laminate, they said.

The resulting EIT maps were then compared to visual inspection and thermograms taken with an infrared camera.

“Although we did encounter some issues with the size of cracks being overestimated and their shapes not being well represented, overall our EIT methodology was able to detect the initiation of damage well before it was visible with infrared thermography,” Schumacher said.

“We are in the process of making improvements to the EIT algorithm to increase its accuracy. After that, we plan to demonstrate it in the laboratory, with an aim toward scaling it up for future monitoring of real structures.”

Research Support

The UD team’s research received support from the National Science Foundation’s Civil, Mechanical and Manufacturing Innovation (CMMI) Division. CMMI focuses on advancing the future of manufacturing, the design of innovative materials and building technologies, infrastructure resilience and sustainability, and tools and systems for decision-making, robotics and controls.

Hongbo Dai and Gerard J. Gallo coauthored the published paper with Thostenson and Schumacher.

   

Tagged categories: Asia Pacific; Bridges; carbon nanotubes; Colleges and Universities; EMEA (Europe, Middle East and Africa); Infrastructure; Latin America; North America; Quality Control; Research and development; Roads/Highways

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