Compiled by Photonics Spectra staff
RALEIGH, N.C. – A new fiber optic strain sensor can heal itself, which will help engineers
collect data in the wake of earthquakes, explosions or other unexpected events and
make informed decisions about structural safety.
Strain sensors measure the force exerted on materials that make
up civil infrastructure, airplanes and buildings. These sensors can provide data
on, for example, how an airplane wing performs during flight and give advance notice
to maintenance authorities when the wing may be near failure. This strain data provides
a chance to address an issue before it becomes a problem.
Historically, these sensors have broken easily under stress and
thus no longer were able to provide information; moreover, as in the airplane example,
their inaccessibility means they are difficult or impossible to replace.
To address this problem, engineers at North Carolina State University
have developed a sensor that automatically repairs itself. The research, which was
funded by the National Science Foundation, appeared in the May 13 issue of Smart
Materials and Structures (doi: 10.1088/0964-1726/20/6/065005).
The top image shows a polymer filament connecting the glass fibers in
a 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 –
repairing itself. Courtesy of Kara Peters, North Carolina State
The new sensor can stretch and compress along with the material
it monitors. Infrared light passing through it detects the resulting changes in
length, offering details about how much strain the material is undergoing. The sensor
contains two glass optical fibers that run through a reservoir filled with ultraviolet-curable
resin. The ends of the glass fibers are aligned with each other but separated by
a small gap.
To demonstrate how the sensor works, the investigators focused
beams of IR and UV light through one fiber. When the resin is hit by the tightly
focused UV beam, it hardens, creating a thin polymer filament that connects the
glass fibers and forms a closed circuit for the IR light. The rest of the resin
in the reservoir remains in liquid form, surrounding the filament.
The liquid resin is important if the polymer filament breaks under
stress. In this instance, the remaining resin would rush into the gap, where it
would come into contact with the UV beam and harden, repairing the sensor automatically.
The scientists hope that their design will help collect data about
the structure being monitored.