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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
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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
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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
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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