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OCR for page 26
Advanced
Composite
Monads
Right: A strong, ultralight leg
prosthesis of graphite/epoxy
composite helps an athlete
compete in world-class
bicycle races.
26
The unusual design of the
Starship business plane-
slender wings, vertical wing
tips, down-hanging rudder,
and wingless on the nose
masks the real revolution
behind this aircraft. Its body
and wings are made entirely
of advanced composite
materials far lighter and
stronger than the aluminum
most aircraft are made of
today. As a result, the plane
needs less maintenance than
conventional aircraft and flies
as far on a gallon of fuel as
planes much smaller.
Advanced composites like those in the
Starship were unheard of 25 years ago. Yet
they have already spread throughout the
transportation industry and into everyday
objects where higher performance provides
an advantage. They add lightness and
strength to racing boats and cars, golf clubs,
and electric guitars.
Composites are simply a matrix of one
material reinforced by fibers or particles of
another. Humans have been making com-
posites since the ancients discovered that
putting straw into mud bricks makes the
I E N G I N E E R I N G A N D T H E A D VA N C E M E h1 ~ ~ F H U M A N W E ~ EA ~ E
bricks stronger. Today steel rods are used to
reinforce concrete highways, bridges, and
buildings. Resin reinforced by glass fibers is
probably the most widely used composite
today.
Advanced composites, however, are in a
class alone. Most were originally developed
to provide lighter, stronger, more tempera-
ture-resistant materials for military aircraft
and spacecraft. Skyrocketing fuel prices in
the 1970s made the new materials attractive
to civil aviation, where their lower weight
I has helped cut the cost of operating airliners. I
The broadening market and more efficient
processing techniques have reduced the cost
of composites to a level where they can now
OCR for page 27
be used in many other products, especially
sporting goods.
The matrix of an advanced composite is
often an organic material, such as epoxy
resin, but can also be metal or ceramic. High-
strength fibers, such as graphite or Kevlar,
are frequently used as reinforcements. But
many combinations of matrix and reinforce-
ment are created for particular uses. Knowl-
edge about their interactions how fibers or
particles function within a matrix is the key
to designing these high-performance
materials. And it has ushered in an era of
designing new materials to meet new needs.
Advanced composites came directly out
of Air Force efforts in the early 1960s to find
A D VA N C ~ D C O M P O S ~ T E M AT E R 9 A L 5
Graphite/epoxy and other
advanced composite materials
are used to construct the
entire body and wings of the
Starship business plane and
account for more than
70 percent of its overall
structural weight.
materials with higher strength-to-weight and
stiffness-to-weight ratios than aluminum.
The most promising was boron, which is
stronger than steel but weighs less than
aluminum. The challenge, however, was to
put it into a usable foals i. The best, easiest,
most cost-effective method turned out to be
making it into a fiber that could be embed-
ded in a matrix of epoxy. Epoxy holds the
fibers in place, distributes the load among
them, and protects them from the environ-
ment. To make an aircraft part, strips of
epoxy tape with fibers running through it are
laid in remold in directions that will reinforce
27
OCR for page 28
Layers of woven Kewlar
fibers strengthen composite
downhill snow skis while
reducing weight and damping
vibration. Advanced
composite materials are
used in many sporting goods,
whose high performance
provides a competitive
advantage.
sections where strength is critical. High heat
and pressure transform the epoxy into a
solid, lightweight part. Such composites
were introduced to operational aircraft in tail
sections of the F-14, which was delivered to
the Navy in 1972.
! Boron fiber is expensive. So parallel
work in England and Japan focused on
graphite as a less expensive high-strength
fiber. Graphite is a form of carbon, and its
fibers are made by transforming organic
fibers such as rayon, acrylic, or pitch.
The first high-strength graphite fibers
produced in the late 1960s were made by a
time-consuming, labor-intensive process and
cost around $400 per pound. It began with an
acrylic fiber that was wound on steel racks to
stretch the fiber and align its long molecules
into stronger, parallel orientation. The fiber
was heated at low temperature to stabilize
the orientation, then cut into strands and
slowly baked in a furnace with inert
gas-usually nitrogen to burn away
impurities. The remaining graphite fibers
were treated with chemicals to help them
bond to a resin matrix.
In 1971 a U.S. company started making
graphite fibers by a continuous process that
took less than eight hours. These fibers cost
only $125 per pound, and their price
dropped steadily over the next decade. The
first production graphite-composite parts
went into F-15 aircraft delivered to the Air
Force in 1974. And today's Starship business
plane is made primarily of graphite/epoxy.
The first commercial graphite-composite
products, though, were sporting goods. In
1972 a California company began making
golf club shafts of graphite/epoxy, which is
stronger and lighter than the steel in conven-
tional shafts. The new shafts appeared in
Japanese golf clubs the following year.
Graphite composite increased stiffness and
reduced shaft weight by about 40 percent,
allowing golfers to swing the club faster and
drive the ball farther than they could with
heavier clubs. Composite tennis rackets
appeared next, followed by fishing rods,
race-car chassis, and other sports equipment.
Auto manufacturers are investigating
graphite composite to replace steel in
passenger cars, but its price is still generally
too high to make it economical for mass-
produced vehicles. However, since 1988 a
graphite composite has been used to rein-
force aluminum drive shafts on some light
trucks.
Another high-strength fiber, Kevlar, is
widely used in composites where high
tensile strength resistance to being pulled
apart is important but the stiffness of
graphite fibers is unnecessary. Kevlar is a
trade name for an aromatic polyamide, or P-
aramid, fiber that is derived from petroleum.
It is lighter than fiberglass but five times
stronger than steel on a pound-for-pound
basis. It appeared commercially as a replace-
ment for steel cord in radial tires in 1972. A
high-strength form of Kevlar is used in resin-
matrix composites for aircraft bodies, sailboat !
hulls, snow skis, and artificial limbs.
P-aramid fibers were discovered in 1965,
but the breakthrough came five years later in
learning how best to convert the substance
into much stronger fibers. P-aramid would
not melt like other plastics nor would it
dissolve easily in any normal solvent used in
making synthetic fibers. The substance, it
turned out, needs an especially strong
solvent 100 percent sulfuric acid to
dissolve it. Normal sulfuric acid solvent I
contains about 3.5 percent water, but this is
enough to keep P-aramid from dissolving
init.
28 E N G I N E E R I N G A N D T H E A D VA N C E M E N T O F H U M A N W E L FA R E
OCR for page 29
In addition, P-aramid molecules would
not align themselves in strong, parallel
orientation during a normal fiber-producing
process. Usually a solution containing a fiber
substance is forced through a plate, called a
spinneret, with hundreds of tiny holes. The
emerging fibers are pulled directly into a
bath that leaches out the solvent, then
stretched to align the molecules before being
wound around a spindle. This did not work
with P-aramid. The problem was solved,
however, by stretching the fibers-and
aligning their molecules in an air gap just
as they emerge from the spinneret. Then they
are pulled into a cold-water bath that leaches
out sulfuric acid, gels the fibers, and fixes
molecular orientation. Finally the fibers are
dried and wound onto a spindle.
High-strength fibers and particles are
also embedded in metal matrices for use at
high temperatures that would melt organic
matrices such as epoxy. Metal-matrix
composites, however, have a rather slim
range of current applications in diesel and jet
engines, spacecraft structures, and high-
performance sports equipment.
The first commercial use of a metal-
matrix composite was in high-performance
diesel engines by a Japanese automaker in
1982. The composite, made of aluminum
with various reinforcements, forms a
reinforcing ring around the crown of the
pistons. It resists wear as well as steel but is
much lighter. Many automakers are investi-
gating metal-matrix composites for use in
pistons and other moving engine parts.
Lightweight composite parts would use less
energy and would reduce the total weight of
the engine.
The first actual use of a metal-matrix
composite, though, was probably the
boron/aluminum structural tubing used in
the space shuttle, which first orbited the
earth in 1981. Aluminum-oxide/aluminum
is now being used in handlebars of
lightweight racing bicycles. And although
silicon-carbide/aluminum is relatively
expensive, it still costs less than half as much
as the beryllium it is replacing in an instru-
ment housing for the inertial navigation
system of a Navy ballistic missile.
Composites with ceramic matrices are
theoretically superior to metal-matrix
composites for high-temperature applica-
lions. Ceramics materials that are neither
metallic nor organic also have great
strength and light weight, but brittleness
often limits their use. Reinforcements,
however, can toughen them. Since the mid-
1980s, aluminum oxide reinforced with
silicon-carbide whiskers has been used in
cutting tools. Similar composites are being
developed as armor to protect helicopters,
armored personnel carriers, tanks, soldiers,
1
and police. These composites are up to five
times tougher than unreinforced ceramics,
lighter than steel, and less expensive than
other advanced materials now used for
armor.
Ceramic composites are also being
studied for use in car and jet engines, where
their light weight and heat resistance would
substantially boost fuel economy. They may
find wider use in low-temperature applica-
tions where their ability to withstand hostile
environments gives them an advantage over
other materials. For example, ceramic-matrix
composites might be used in valves and
reactor vessels handling corrosive chemicals
or in structures, such as satellites, that must
endure the harshness of space.
Carbon is similar to a ceramic, and
composites made of graphite fibers embed-
ded in a matrix of graphite are extremely
heat resistant. This carbon/carbon composite
is used in the nose and leading edges of the
space shuttle to protect against the searing
heat of atmospheric reentry. And, because of
its light weight and durability, it is being
used increasingly for wheel brake linings in
military and commercial aircraft.
Special composite materials are being
developed for the experimental National
Aero-Space Plane, which is designed to take
off and land on runways, cruise at hyperson-
ic speeds of Mach 6 (six times the speed of
sound) or greater in the upper atmosphere,
and reach Mach 25 while climbing into orbit.
I The plane's skin, frame, and engines will
need to be extremely light, strong, and heat
resistant. The skin, in particular, will have to
withstand repeated exposure to extreme heat
and cold and must be far thinner than the
composite tiles that protect the space shuttle.
The plane is scheduled to fly in the late
l990s. But before it does, a new generation of
advanced composites will have to be born.
A D VA N C E D C O M P O 51 T E M AT E R I A ~ S
it.
.
A strong, light ceramic-
composite tool cuts a thin
chip from a fast turning
bar of tough nickel alloy.
The durability of this
ceramic composite at high
temperatures allows it to
withstand the heat produced
in high-speed cutting.
~ _
a_ ~
~'~'~
ma
A_
,_
The National Aero-Space
Plane flies far above the
earth in an artist's
drawing. New advanced
composite materials are
being developed for its skin,
engines, and other
components that will allow
I the space plane to climb into
orbit or cruise in the
atmosphere at more than six
times the speed of sound.
;
29
Representative terms from entire chapter:
graphite fibers
