Self-healing sensor measures strain in structural materials

The sensor could be used to help make decisions about structural safety in the wake of earthquakes, explosions or other events.

Engineers use sensors to measure the strain exerted on materials, but a flaw in such sensors is that they can break under stress. However, a sensor that can no longer provide information to users doesn’t necessarily mean that the material they were monitoring has been irreparably harmed.

‘To address this problem, we’ve developed a sensor that automatically repairs itself, in the event that it is broken,’ said 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 reportedly 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 how much strain the material is undergoing.

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

If the polymer filament breaks under stress, more liquid resin moves 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,’ said Peters. ‘These events could be bird strikes to an aeroplane 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, entitled ‘A self-repairing polymer waveguide sensor’, is published in the June issue of Smart Materials and Structures.