Process could lead to alloys that change aircraft-wing shape in flight

By Neal Singer

"Smart" wires have been embedded in a graphite-epoxy matrix
similar to that used in fighter aircraft wings, providing the
simplest means yet for changing a wing's shape to conform to
different flight conditions, says the leader of a UI research
team.

The wires, created from a nickel-titanium alloy known as nitinol,
are called smart because they "remember" two shapes and can be
made to change into the shapes at different temperatures, said
principal investigator Scott White, a UI aeronautical and
astronautical engineer.

Because the metal changes shape forcefully, the wires can, in
effect, do the work of a motor. So far, researchers have used the
wire's unique characteristics to activate levers and other
machines.

"This may be the first use for two-memory nitinol that does not
require additional machinery," White said. "What we want is to
have the embedded wires by themselves change the shape of the
wing to match whatever the plane is doing, in order to achieve
more efficient flight. A wing that uses the same shape for steady
flight, rolls, takeoffs and landings one day may be considered
primitive."

The discovery that nitinol could remember two shapes was made in
the late 1970s.

Until now, embedding smart materials in a plastic matrix was
problematic - because bubbles formed around the metal rods as the
plastic set. Bubbles diminish the ability of the wires to change
the shape of the plastic.  Bubbles also encourage the growth of
cracks in the matrix. White, however, found he could use a unique
curing and surface pretreatment process to prevent bubbles from
forming around the embedded wires.

Training the wires after embedding them produced an unexpected
bonus: It shortened the time it takes for the nitinol wires to
"learn" what they're supposed to do, White said. "Ordinarily it
takes 50 training cycles for the wires to remember their shapes."
Encased in plastic, the wires were able to remember a shape after
only a few training sessions.

Nitinol is trained by by being stretched when cold, White said.
The deformed metal is clamped so that it cannot move. It is
heated, and then allowed to return to the original temperature of
the training session, which may be approximately room
temperature. The cycle is repeated a number of times. The
repetition changes the crystalline structure of the wire and
makes the lengthened nitinol a "remembered" state to which the
composite will forcefully return at cold temperatures. At still
higher temperatures, the metal will return to the shape it was in
when first formed.

The work builds upon research that began in 1959 with William J.
Buehler's finding at the Naval Ordnance Laboratory in White Oak,
Md., that nitinol had single-shape memory.



UIUC -- Inside Illinois -- 1995/02-02-95