Far above, a much larger and faster craft crosses the sky. This one carries nearly 600 people across the Atlantic Ocean and around the world. It is a huge aircraft, and in some ways it reminds one of a double-decked bus. Yet it is more efficient than a traditional aircraft because of the special materials used in its construction. Other aircraft streak by the huge air carrier. One is undetectable by radar, and its shape is like nothing that has flown before, looking for all the world like a piece broken from the corner of a square table. The other one flies so fast that it must be maneuvered by actually changing the shape of its wings and body instead of by the use of standard flaps and rudders. Still farther above, a new satellite containing a very delicate instrument package is lifted gracefully from the cargo bay of a space shuttle by a large, remotely-controlled manipulator arm and set into orbit.

On the ground below the little one-person seaplane, a car speeds around a racetrack. The car weighs only a few hundred pounds and contains almost no metal parts at all. Spectators can be seen crossing a suspended footbridge as they make their way from the parking lot to the stands of the racetrack.

The bridge is nearly a hundred feet long, forty feet in height, and was assembled in a matter of hours from prefabricated parts. It contains no metal parts or fasteners. On the other side of the parking lot, a four-lane highway passes through a region of shallow gullies that discourage traditional road building methods. Instead, a stretch of several miles of high-traffic roadway constructed of plastic and glass fibers carry the steady flow of vehicles smoothly across the rough terrain.

The pilot of the little seaplane banks his craft about in order to fly over a secluded bay of the lake he has just risen from. Several pleasure boats are moored at docks built neither of wood nor concrete. The smooth surface of the water is split by sleek racing craft, powered only by the rhythmic strokes of the eight oarsmen they each carry. The combined weight of each crew, plus that of their coxswain and the oars, is just under 2000 pounds, yet their sleek racing shell weighs little more than one of the oarsmen and barely makes an impression in the smooth waters of the bay.

Flights of fancy? Not at all. The "Seawind", the Airbus A-380-550, the B-2 stealth bomber, and the F-22 all fly in our skies right now. The "CanadArm" has been providing remote manipulator service on shuttle flights for almost as long as there have been shuttle flights. Formula 1 racing has already tested cars that don't even have a metal drive shaft or metal parts in the engines. Bridges and roads like those described are in daily use in many different places in the world. And anyone who has ever watched rowing competitions in the Summer Olympic Games and elsewhere knows that the maximum weight of racing shells is strictly limited, and that the "eight with cox" is a rowing 'main event'. All these things are made possible through the use of advanced composite materials.

What are composite materials? A composite material is any material in which two or more separate materials have been combined to make a single construct having more desirable properties. What many people don't realize is that composites are probably the most common structural materials in the world, and have always been an essential part of their lives. Concrete, paper, corrugated cardboard, plywood, fiberglass, bamboo, cornstalks, trees, bricks... all are composite materials. Far from being a new invention, composite materials are the main structural elements of nature. Take a close look at the grain and structure of a piece of wood, and you will see how its strength comes from a structure of fibers bound together side by side.

For the purposes of this article, composite materials are those in which fibers, or some type of linear structures, are bound tightly in a solid matrix, such as plastic or concrete. The matrix material, while having its own strength and structural characteristics, serves primarily to hold the fibers or reinforcing structures in place.

Man's first use of such composite materials was probably the adobe brick. Mud or clay can be shaped and dried into a hard block, but that kind of block has little load bearing strength and can be easily crushed by the weight of other blocks on top of it. At some point in time, Man found that mixing dried grass or straw into the mud produced a brick with superior properties; a brick that could bear much greater loads without being crushed than a brick of plain dried mud could bear. Thus was born the ability to construct large, secure buildings that were the foundation of cities and of society. Adobe bricks are still in common use in many areas of the world today, essentially unmodified after thousands of years.

Plywood is another example of Man's attempts to capture and employ Nature's inherent wisdom. In plywood, thin sheets, or 'plies' of wood are laminated together. In each ply, the wood fibers (the grain of the wood) runs in one particular direction, and each ply is aligned in a different direction than the adjacent plies. This gives the resulting stack of wood plies an optimum strength in all directions, and plywood is a very versatile and useful structural material.

A third example of a composite material is reinforced concrete, as is used in the construction of bridges and buildings. Steel rods ("rebar", short for "reinforcing bars") are encased in a matrix of concrete, producing reinforced concrete, which has much better strength and load-bearing properties than concrete that has not been reinforced. Curiously, the load-bearing capacity of reinforced concrete lies in the steel rods that it contains, and not in the concrete itself. We think of the steel rods as reinforcing the concrete, when in fact the reverse is true. While the concrete itself is strong, its actual purpose is to hold the steel rods in place. The steel rods bear the load, and the concrete actually reinforces the steel rods!

As interesting as they are, these materials - bricks, plywood, reinforced concrete - are not advanced composite materials. Though the operating principles of the materials are the same, airplanes and other such structures are generally not built from artificial stone and plywood.

ADVANCED COMPOSITE MATERIALS, or ACMs, are a much more recent and an entirely man-made 'take' on Nature's style. They consist exclusively of man-made specialty fibers bound in a matrix of specialty plastics. The variety of such materials is nothing short of spectacular, and the development and application of new ACMs are among the fastest-growing sectors of modern technological endeavours.

Most people get their first introduction to the world of ACMs through 'fiberglass', a composite material in which fine glass fibers are bound into a thick sheet of polyester resin. Relatively light and strong, fiberglass is one of the most generally useful and therefore most common of ACMs. There are several types of glass fiber that can be used, and a variety of glass and other fibers see application in ACMs. In principle, any fiber can be used, on the condition that the fiber material is compatible with the matrix material. Similarly, any solid polymeric material can be used to form the matrix, on the condition that the matrix material is compatible with the fiber material. This relationship is essentially true, but in a practical sense only fibers that are easy to produce or that have certain properties see widespread use in ACMs. Similarly, only resins and plastics with certain properties of strength, durability, and formability see widespread use in ACMs. It goes without saying that the fiber materials and the matrix materials must not react chemically with each under under any circumstances.

Given these conditions, the materials list for fibers in general use is quite short. It includes all types of glass fiber, aramid (or 'Kevlar'), carbon, and boron. There is also growing development in the application of basalt fibers. Other materials for use as reinforcing fibers are constantly being investigated as well, but most applications are currently served by this 'short list' of fiber materials.

Far greater versatility is possible in the nature of the matrix material due to the vast variety of polymeric materials that is possible. Even metals can be used as a matrix material; the Airbus A-380 will make extensive use of an ACM called "GLARE", in which glass fibers are combined with layers of aluminum.

Generally, matrix materials are chosen either for the ability to be formed into a desired shape with heating (thermoplastic polymers) or for their loss of formability with heating (thermosetting plastics). Thermoplastics see general use in applications where the retention of shape and strength with temperature changes is not critical. Typical examples of thermoplastics include polyethylene, polypropylene, rubber, polystyrene, and many other plastics that will melt when heated. Thermosets, on the other hand, lose any mobile nature they may have when heated, and change from a liquid resin or 'pre-polymer' to a tough, rigid, highly cross-linked polymer. When heated sufficiently after having set, thermosetting plastics decompose rather than melt, and the material is thus destroyed. Various epoxy resin formulations are the most common examples of thermosetting matrix materials in use today.
The strengths and weaknesses of ACMs as structural materials all derive from the ordering of layers of fibers within the matrix. The fibers have no strength against forces that cut across their length, and part of the function of the matrix material is to protect the fibers from such forces. In all cases, the embedded reinforcements, be they fibers in an ACM or steel rebar in concrete, have strength properties only along their length. Thus, only under compressive loads (pushing the ends of the fibers toward each other), or under tensile loads (pulling the ends of the fibers away from each other) do composite materials perform well. It is perhaps a difficult concept to visualize, but flexing the length of a sheet of ACM compresses the fibers on one side of the sheet while at the same time puts the fibers on the other side of the sheet under tension. Having different layers of fibers in different orientations within the same sheet of ACM provides strength in all directions, and a great amount of engineering and calculation is involved in the production of any new load-bearing design to be constructed from ACMs. In the actual construction, layers of fibers will be laid down in a specific pattern, and the whole stack will then be consolidated with the matrix material to form the final, single product.

The repair and maintenance of such structures requires more or less specialized training, depending on the nature of the application. Aerospace applications of ACMs are of a particularly critical nature. The failure of an aircraft body panel repair patch while the aircraft is on the ground is not likely to have any serious consequences. However, if that failure occurs while the aircraft is in flight, the results can be catastrophic. Correspondingly, aircraft maintenance technicians require training from certified and authorized training centers in order to be qualified to carry out inspection, repair, and maintenance procedures on aerospace composite structures.

The fields of ACM technology and application continue to grow rapidly in scope and practice. New materials and novel methodologies are being developed almost daily. The world is made from composites, and the future is waiting!

Next time we will continue with "Advanced Composite Materials: A Closer Look At Fibers"

Radon, A Rare Element

Chemical Weapons
A Four Part Series

What is pH?

Composite Materials

How Can A Bullet-proof Vest Stop A Bullet?

For further information you may view the following useful links:

Renaissance Aeronautics Associates Inc.
www.raacomposites.com

SeaWind Aircraft Company
www.seawindsna.com

Composites Fabricators Association
www.cfa.com
www.netcomposites.com