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  • Oh, Snap: Stressed Sensor Self-Heals
Jun 2011
RALEIGH, N.C., June 16, 2011 — One flaw of strain sensors in that they can break under stress. But a new fiber optic strain sensor can heal itself — so in the wake of earthquakes, explosions or other catastrophic events it continues collecting data, helping engineers make informed decisions about structural safety.

Engineers use strain sensors to measure the force exerted on materials used to build everything from airplanes to civil infrastructure. These sensors can provide data on how, for example, an airplane wing is performing in flight and give maintenance authorities advance notice when the wing may be near failure. In other words, strain data provides a chance to address an issue before it becomes a problem.

The top image shows the polymer filament connecting the glass fibers in the sensor. The middle image shows where the filament has snapped off. The bottom image shows where the resin has rushed into the gap, been exposed to UV light and reconnected the filament — effectively repairing itself. (Image: Kara Peters, North Carolina State University)

Historically, one flaw in such sensors is that they can break under stress. That means the sensor can no longer provide information to users, and, as in the airplane example, such sensors may be inaccessible — making them difficult or impossible to replace.

“To address this problem, we’ve developed a sensor that automatically repairs itself, in the event that it is broken,” said Kara Peters, an associate professor of mechanical and aerospace engineering at North Carolina State University and co-author of a paper describing the research, which was published in the June issue of Smart Materials and Structures.

The sensor can stretch and compress along with the material it monitors. IR light passing through the sensor detects the resulting changes in length.

The sensor contains two glass optical fibers that run through a reservoir filled with UV-curable resin. The ends of the glass fibers are aligned with each other but separated by a small gap. Focused beams of IR and UV light run through one of the fibers. When the tightly focused UV beam hits the resin, the resin hardens, creating a thin polymer filament that connects the glass fibers — creating a closed circuit for the IR light. The rest of the resin in the reservoir remains in liquid form, surrounding the filament.

The remaining liquid resin is important. If the polymer filament breaks under stress, more liquid resin rushes into the gap, comes into contact with the UV beam and hardens — repairing the sensor automatically.

“Events that can break a sensor, but don’t break the structure being monitored, are important,” Peters said. “These events could be bird strikes to an airplane wing or earthquake damage to a building. Collecting data on what has happened to these structures can help us make informed decisions about what is safe and what is not. But if those sensors are broken, that data isn’t available. Hopefully, this new sensor design will help us collect this sort of data in the future.”

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