Engineers at
Missiles are sometimes damaged when struck by rocks and
debris kicked up by helicopter rotors or when mishandled during shipping or
maintenance.
Unlike missiles made of metallic alloys, which often show
external signs of damage such as cracks or dents, damage in the new "filament
wound" composite materials may not reveal telltale signs, said Douglas Adams, an
associate professor of mechanical engineering.
The new monitoring system uses a mathematical model to
pinpoint the location and severity of impacts based on vibration data collected
by a sensor called a triaxial accelerometer.
"We have shown that 98 percent of the time we can detect,
locate and quantify the force of impacts,"
Findings were detailed in a research paper presented March
21 during an International Society for Optical Engineering conference in
The research focuses on the missile casing, a 7-inch-wide,
30-inch-long central segment located between the rocket motor and
warhead.
"The casing is essentially a cylinder that holds the solid
rocket fuel, and it has to withstand the high pressures created as the fuel
burns," said
The researchers are using a 15-foot-tall "impact tower" that
rams a steel rod into the casing with enough force to punch holes in military
armor. The testing simulates impacts from debris, tools striking the casing or
mishaps, such as personnel accidentally dropping a
missile.
"These sorts of impacts can cause damage that cannot be
detected by visual inspections,"
The casings are made out of carbon fibers, Kevlar or other
materials wound in layers. Missiles made of the composite materials perform
better than their metallic counterparts because the casings are up to 40 percent
lighter than those made from aluminum alloys. Their light weight also makes them
less expensive to ship and easier to handle.
In addition to detecting damage caused by an accident, the
technique also could determine how durable the material is after long-term
storage and exposure to the environment.
"The military stores weapons systems for a long time,
sometimes years, and then they deploy them, so you want to know how they are
affected by factors such as humidity, ultraviolet radiation and other
conditions,"
Another goal is to gather data to help engineers design more
impact-resistant casings in the future.
Research shows that some designs are more prone to damage
because the force of impact concentrates in a small area.
"You want a material system that distributes that impact
load along many fibers so that no one fiber is carrying too much load, which
could cause it to break,"
The more damage, the less pressure the casing can withstand
before bursting.
A major challenge is designing an effective monitoring
system that uses only one sensor.
"You can't be putting a lot of sensors everywhere because
there are weight restrictions,"
The triaxial accelerometer measures vibrations from three
directions. When engineers strike the casing at various points with a hammer,
vibrations travel through the structure and are recorded by the sensor. Data
from those measurements are then used to fine tune a mathematical model needed
for the monitoring system.
Recent findings indicate the system effectively tracks the
location and severity of impacts using only one sensor.
Future testing might simulate "ballistic impacts" from
bullets or shrapnel from improvised explosive devices,
The same technique could be applied to commercial aircraft
and spacecraft, as well as bridges, railways and other elements of
The research is funded by the U.S. Army Research Office.
