Engineers Develop Carbon Nanotube “Smart Skin” to Sense Structural Damage

The carbon nanotube “smart skin” is coated onto fabric (left) and is robust yet flexible (right). (Image courtesy of Journal of Nondestructive Evaluation.)

Engineers at the University of Delaware are developing a new method for monitoring the structural health of infrastructure based on the use of a “smart skin” made of a carbon nanotube composite. The composite is mechanically robust and can adhere to practically any shape, meaning it can be used to monitor the health of roads, bridges and other structures.

Electrical Impedance Tomography

The smart skin uses a technique called electrical impedance tomography (EIT) to indicate which areas of the structure may be compromised. EIT makes use of surface electrode measurements to map the two-dimensional conductivity of the skin’s surface. The researchers tested a square of the carbon nanotube composite, with 32 evenly spaced electrodes along the boundary, to measure the surface conductivity.

The idea is simple: you create a 2D image of the skin’s electrical conductivity at two different times, such as before and after an earthquake. By comparing the images, you can map the differences in conductivity between them. This map gives you an indication of the structural damage resulting from the earthquake.

(a) A photo of the experimental carbon nanotube sensor, with a small scratch meant to simulate a crack. (b) EIT conductivity change map from before and after applying the scratch. (c) Conventional temperature map from infrared thermography, for comparison. (Image courtesy of Journal of Nondestructive Evaluation.)

The engineers performed tests on a few different types of simulated structural damage, including multiple square holes in the skin, a scratch in the skin to simulate a crack and impact damage inflicted with a drop weight tester.

"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," said structural engineer Thomas Schumacher. "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."

Preventing Tragedy

Most engineers are familiar with our profession’s horror stories—I think it’s practically curriculum that engineering students are shown this video of the 1940 collapse of the Tacoma Narrows Bridge, colloquially known as Galloping Gertie, as a dire warning about the consequences of poor engineering.

But bridges don’t collapse because the engineers who build them are careless or lazy. There’s a host of factors beyond our control that must be constantly guarded against, such as aging, deterioration and natural disasters such as earthquakes and hurricanes. Often, the effects of these and other factors are invisible, so it’s necessary to devise methods for monitoring the health of our structures so they don’t end up being used as a case study for future engineers.

Although there’s still work to be done on the carbon nanotube smart skin, it could eventually help make tragedies like bridge collapses a thing of the past.

You can check out the team’s paper here, or for more infrastructure news, read about developing smarter concrete with biomaterials.