Students develop large area structural strain sensor Friday, 01 July 2016

Two American engineering students have developed a sensor sheet to detect strains over large areas of structures.

Matthew Gerber is studying civil and environmental engineering at Princeton while Campbell Weaver is majoring in electrical engineering.

Weaver's supervisor, Professor James Sturm, said engineers have been pursuing a large-area sensor to monitor strain on surfaces of buildings, bridges and airplane wings for years.

"Conventional sensors for this purpose measure only a single point per sensor, or at most a line of locations with a fiber optic sensor," said Sturm.

"Both of these can miss cracks beginning to form in one location, so we sought a sheet of sensors to cover the surface. We did some fundamental work on an approach several years ago, but had had little progress translating the electrical engineering end of it into a practical way forward."

The sensing sheets take advantage of the fact that the electrical resistance of metal wires changes as they stretch. So very thin wires arranged on polymer sheets will detect strains to an underlying surface as stretching wires trigger changes in resistance. Weaver believed that many of the problems could be solved by simplifying the electronics.

"Previous generations of sensing sheets showed that the physics behind the idea worked, but they could not consistently read strain because of electrical and mechanical problems," said Weaver.

Another problem was how to affix the sensing sheets to structures. The adhesive is critical because the wrong type can either muffle emerging problems or fail to hold the sheets in place. To deal with that, the electrical engineers turned to their colleagues in civil engineering and this is where Gerber came into the picture.

"We needed to find an adhesive that was strong so the sheet would not peel off, but it also had to be flexible" Gerber said. "If the glue is too rigid, it will transfer to much strain to the sheet and break the sensor."

After many rounds of testing, he chose a special type of epoxy produced by 3M.

Between them, Weaver and Gerber developed a system that they successfully deployed on the University's Streicker Bridge.

[A flexible sensing sheet designed to warn engineers of structural weakness. Photo: Princeton]

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