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: North Carolina State Univ.
Researchers from North Carolina State Univ. have designed a sensor that can
measure strain in structural materials and is capable of healing itself—an important
advance for collecting data to help make informed decisions about structural
safety in the wake of earthquakes, explosions, or other unexpected events.
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 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 users a chance to address an issue before it becomes a
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 users 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 Smart
Materials And Structures and was co-authored by Peters and NC State PhD
student Young Song.