American engineers have developed a multi-layered 'sensing skin” able to detect cracks or corrosive substances in structures.
The sensing skin consists of three layers, which can be painted onto the surface of a structure or pre-assembled and attached to the surface like wallpaper.
The first layer is electrically conductive and is used solely to detect cracks. The second layer serves as a buffer between the first and third layers. The third layer detects cracks, but is also engineered to detect specific chemicals of interest.
This third layer incorporates metal nanoparticles whose conductivity changes in the presence of specific ions. By changing the composition of the metal nanoparticles, this layer can be engineered to respond to any particular chemical.
“We’ve created a skin that can be applied to the surface of almost any structure and be used to monitor the structure’s integrity remotely and in real time, identifying potential problems long before they become catastrophic,” said Professor Mohammad Pour-Ghaz, from the Department of Civil, Construction and Environmental Engineering at North Carolina State University.
Electrodes are applied around the perimeter of a structure and the sensing skin is then applied to the structure, over the electrodes.
A computer program then runs a small current between two of the electrodes at a time, cycling through a number of possible electrode combinations. When the current runs between two electrodes, a computer monitors and records the electrical potential at all of the electrodes on the structure for both the first and third layers of the sensing skin.
This data is then used to calculate the sensing skin’s spatially distributed electrical conductivity on both layers. The researchers have developed a suite of algorithms that can use changes in conductivity measured by the first and third layers of the skin to detect and locate both damage and the presence of target chemicals.
In a proof-of-concept study, the team applied their sensing skin to reinforced concrete. They exposed the concrete to corrosive elements and subjected the concrete to strain in order to simulate the failure of real-world structures. For this study, the third layer of the sensing skin was engineered to detect chlorides, which can cause corrosion in reinforced concrete.
“The skin performed really well,” said Pour-Ghaz. “
We were able to detect cracks as small as a few hundred micrometers; and we could very accurately detect any instances in which chlorides came into contact with the skin."
He said they want people to be able to detect problems very early on.
“And while this proof-of-concept looked at concrete, the technology – if properly applied – could be used on structural materials from metals to polymers,” he said.
[The skin is applied to a polymeric substrate with a physical crack and a reservoir for exposing the sensing skin to aggressive ions. Photo: Julie Williams Dixon]