New sensor to measure structural stresses can heal itself when broken

Engineers use sensors to measure the strain, or forces, exerted on materials used to build everything from airplanes to civil infrastructure. For example, these sensors can tell us how an airplane wing is performing in flight, and give maintenance authorities advance notice when the wing may be near failure. In other words, it gives you a chance to address an issue before it becomes a problem.

Historically, one flaw in such sensors is that they can break under stress. That means the sensor can no longer provide information to users, but it doesn't necessarily mean that the material they were monitoring has been irreparably harmed. And, as in the airplane example, the 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," says Dr. Kara Peters, an associate professor of mechanical and aerospace engineering at NC State and co-author of a paper describing the research.

The sensor can stretch and compress along with the material it monitors. An infrared (IR) light wave runs through the sensor and detects these changes in length, which tells us how much strain the material is undergoing.

The sensor contains two glass optical fibers that run through a reservoir filled with ultraviolet(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 says. "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."

The paper, "A self-repairing polymer waveguide sensor," is published in the June issue of Smart Materials And Structures and was co-authored by Peters and NC State Ph.D. student Young Song. The research was funded by the National Science Foundation.

NC State's Department of Mechanical and Aerospace Engineering is part of the university's College of Engineering.