While it's always disturbing to hear about the unexpected collapse of a bridge or building, technology is being developed to lessen the chances of such incidents occurring. Increasingly, this is taking the form of sensors that are implanted within structures, that measure and transmit data on the stresses that the surrounding material is experiencing. If the mechanical strain causes one of those sensors to break, however, it won't be much good anymore - depending on its location, it also may be impossible to replace. Fortunately, researchers at North Carolina State University have created a self-healing structural stress sensor.

The business end of the sensor consists of two horizontally-aligned glass optical fibers, set in a reservoir of ultraviolet (UV)-curable liquid resin, with a small gap between their two ends. This gap is bridged by a thin polymer filament. Focused beams of UV and infrared (IR) light are fed into one of the fibers, with the IR beam proceeding to flow through the filament, and then on through the other fiber, creating a closed circuit.

As the whole assembly stretches and compresses with the material around it, the IR beam measures the resulting changes in its own length, and relays this data. The numbers are then interpreted to indicate amounts of mechanical strain.

Should the sensor be pushed too far, however, the polymer filament will break. Because the surrounding resin is UV-curable, however, the UV beam will cause the resin that flows into the gap to harden into more polymer, thus rebuilding the filament. The circuit will then once again be closed, and the IR beam can continue its work.

"Events that can break a sensor, but don't break the structure being monitored, are important," said Dr. Kara Peters, an associate professor of mechanical and aerospace engineering at NC State. "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 research was recently published in the journal Smart Materials And Structures.