Researchers developed sensors that automatically
self-repair
Researchers from North Carolina State University 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 us 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 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.
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.
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.
Source: NCSU
