In this segment, we will continue the investigation of ACMs by taking a closer look at specific fiber materials used in ACMs.

Recall that an ACM consists of two parts: reinforcing fibers, and a matrix material in which the fibers are bound. The specific strength characteristics and behaviour properties of an ACM are determined by the relative orientations of layers of fibers within the matrix, and to a lesser extent by the rigidity and toughness of the matrix material.

Fiber Materials In ACMs:

As the name suggests, 'fibers' are just filaments or threads of certain materials. They are supplied in various forms ranging from sheets of parallel fibers, to plain-woven and satin- or harness-woven fabrics. The individual fibers of any fiber material are produced in different 'weights' or thickness, as are the various fabrics made from them. Many different grades of fibers of different materials can be used within the same fabric or the same structure. In essence, a fabric may be 'custom tailored' to fit specific uses and applications.

The fiber materials in most common use today are glass fibers, aramid fiber (the fiber formerly known as Kevlar...), carbon fiber, and boron fiber. The use of basalt fibers as a composite structural material, taking the place of asbestos, is also becoming common.

It might appear that the range of available fibers is quite limited, and in some ways that is true. Fiber reinforcement materials must generally meet some very demanding requirements in order to see widespread application. Fibers, and hence the materials from which the fibers are made, must have a significant strength along their length. They must be highly resistant to compressive or tensile loads, or both. A fiber that breaks readily under light loads is not at all useful. Consider aramid fiber as an example. This material has very poor resistance to compressive loads, but it has an immense resistance to stretching (tensile load), allowing it to absorb and dissipate impact energy effectively, which is why aramid fibers are at the heart of every bullet-proof vest and blast mitigation shield in use today.

In addition to having some particular physical strength, fibers must be chemically and thermally stable. A fiber material that becomes brittle or soft with relatively small fluctuations in temperature will have greatly limited conditions under which it may be used. Imagine an airplane sitting on a runway in a hot, sunny locale such as near a desert. Heat reflected from the runway can expose the body of that airplane to temperatures of 50 degrees C or more. The airplane takes off and ascends to an altitude of 35,000 feet. In a matter of but a few minutes, materials in the structure of that airplane will be exposed to temperatures of -30 degrees C, with the wind-chill factor associated with 250 mph winds. The temperature range is essentially the same as that between boiling water and ice! Fibers used in ACM structures exposed to these conditions must be able to withstand such fast temperature variations with little or no change in their properties or dimensions. That is, they must be thermally stable.

Similarly, the fibers must not interact in a chemical sense with the matrix material surrounding them, nor with any other materials with which they are in contact. In a composite structure, the fibers and the surrounding matrix must remain entirely separate entities. This is essential to the strength properties of a composite structure. If a fiber material reacts with a matrix material, a number of things can happen: the two materials may become one; the fiber material may become degraded and weakened; the matrix material may become degraded and weakened adjacent to the fibers; or a new material may be formed between the fiber and the matrix, to the detriment of both. All of these possibilities end with the failure of the composite structure to perform as it should. Should that structure happen to be flying across the ocean at 35,000 feet, the results would be catastrophic indeed!

Fiber materials also have to be chemically inert to external materials that may come into contact with the composite structure. This is sometimes not the case, however, and when such undesirable contact occurs the composite structure is severely damaged.

Non-composite structures can also be severely damaged as a result. Some years ago, an incident occurred in which the aluminum fuselage panels of a fleet of passenger aircraft were assembled in contact with the carbon fiber in adjacent composite structures inside the aircraft. This resulted in the formation of an oxidation-reduction pair junction similar to the junction that powers a battery. The resulting oxidation reaction that occurred in the aluminum caused such corrosion damage to the aircraft that major overhaul and reconstruction efforts were necessary. The entire fleet had to be taken out of service and stripped down so that the damage could be repaired and the construction error responsible for the damage could be corrected. The cost of having a fleet of passenger-carrying aircraft grounded was tremendous.

Since composite structures are usually in constant contact with external materials (air and water to name only two), being chemically inert is a very important consideration, and one that also acts to restrict the range of materials that can be used. A material that breaks down readily from reaction with water or through oxidation from contact with air can not be useful as either a fiber or a matrix material.

FIBER PRODUCTION

The production of fibers is fairly straightforward. The material from which the fibers are to be made is softened or liquefied. It can then be stretched or drawn, or extruded through dies to produce filaments. Every type of process used imparts a certain unique character to the functional properties of the fibers. For example, extrusion tends to produce fibers with a built-in longitudinal compression, while forming a fiber by pulling the material tends to produce fibers with a built-in longitudinal extension stress. As a result, all processes see use in the production of fibers for various applications.

The fibers are then processed into forms that facilitate their use in well-defined ways. Glass fiber is the most common fiber used and so it is set into the most diverse variety of forms, as readily usable fabrics. These fabrics include sheets of randomly-oriented fibers, sheets in which the fibers all run in a single direction ("unidirectional"), sheets in which fibers are woven plainly together like cloth, and sheets in which fibers are woven in various harness or satin-weave patterns. In the plain weaves and twills, the fibers are woven in a simple over-and-under pattern, such as over-one-under-one or over-two-under-two. In harness or satin weaves, the fibers are woven in more complex patterns, going over a certain number of fibers and under a certain different number of fibers. A four-harness weave, for example, uses an over-one-under-three pattern. Each type of weave has its own special characteristics that the properly trained ACM technician learns to use to advantage.

Like glass, specialty fibers - aramid, carbon, boron, and basalt - are also made into fabrics because of, or to make use of, their unique properties. Unlike glass fiber, however, these are never used in random fiber orientations, but only in unidirectional or woven fabrics. Accordingly, the specialty fibers are produced in unidirectional, plain weave and harness weave fabrics, with the bright yellow aramid plain weave and the distinctive black 2-by-2 twill pattern of carbon fiber being the most readily recognizable of the different forms.

Boron fiber is an exception. Boron fibers have an inherent stiffness like that of a fine sewing needle, and are produced in a manner that allows only one filament to be produced at a time. It is itself a composite structure, in which pure boron is deposited on a base filament of tungsten. Boron fibers are thus generally produced only as unidirectional tapes, although a limited number of specialty weaves are also available.

The common fiber materials are chemically stable relative to each other, and do not enter into chemical reactions with each other. This permits the production of an entire range of fabrics employing various combinations of these fibers. It is entirely possible to obtain combination fabrics of glass and aramid, glass and carbon, or glass, aramid, and carbon. It also permits the technician to fabricate an ACM structure using any combination of these materials.

Fiber Materials:

Glass:

Glass fibers are produced from, as you have no doubt guessed, glass, or silica. There is an immense number of different formulations for glass, with S glass and E glass being the most common formulations for glass fiber. Glass-making technology has been around for thousands of years, and the properties and behaviours of glass are well understood. Glass rods or tubes are readily softened with heat and drawn into long thin flexible fibers. Alternatively, molten glass can be readily extruded through fine dies. Extrusion also allows for the production of fibers of infinite length in a continuous process, whereas the drawing out of fibers from glass rods or tubes can only produce fibers of limited length.

Aramid:

Aramid was formerly known by the trade name of "Kevlar". It is a polymeric material based on terephthallic acid and para-phenylenediamine.

Several other types of aramid polymers are available in which different isomeric forms of terephthallic acid and phenylenediamine, or their derivatives, are used, and the properties of each formulation are very different from one to another.

The long polymer chains of aramid take on a rigid structure due to the physical restrictions of the amide links and the phenyl rings in the molecules. A highly coiled structure results in which different molecules readily intertwine. This provides the bulk material with a very high tensile strength (resistance to stretching). An effective visualization would be to intertwine a number of coil springs and then try to separate them by pulling on the ends of the resulting mess. (Anyone who has ever tried to untangle a 'Slinky' spring toy knows exactly how this works!). This is what allows composite structures of aramid fiber to absorb and dissipate a great deal of energy from an impact or collision. This ability to absorb energy works well for bullet-proof vests and ballistic shielding. It also works well against the impact of birds and other objects that may strike aircraft. Many important structural components of aircraft are thus made from ACMs based on aramid fiber.

Carbon:

Carbon fibers are produced by the pyrolysis of polyacrylonitrile (PAN) or of pitch, the tarry residue left over from petroleum distillation. This process drives hydrogen and other atoms out of the molecules and rearranges the bonding of the remaining carbon atoms into the graphite structure.

Carbon fiber ACMs have become the material of choice in aerospace and many other applications. The relative ease of manufacture and the versatility of structures made from carbon fiber ACMs is greatly enhanced by the unique nature of carbon itself. Carbon fiber has incredible strength, with a modulus of elasticity of about 70 million pounds per square inch. A carbon fiber rod just one half inch in diameter will support a weight of several tons, and a carbon fiber bar one inch wide, one foot long, and a mere three-sixteenths of an inch thick will completely defeat any strong man's attempts at breaking it. It has become an accepted, and highly successful technique in civil engineering to wrap new or degrading reinforced concrete components such as beams, support columns, and bridge spans, with a few layers of carbon fiber composite. This can enhance the strength of the member by as much as 50% and increase the lifetime of the structure by dozens of years. The innate strength of carbon fiber is truly amazing.

Carbon has another property that makes it extremely valuable for use in ACM structures: a negative coefficient of thermal expansion. Carbon fibers get shorter instead of longer when they are heated. This can induce incredible amounts of stress into a structure. But by engineering the arrangement of fibers so that the effects 'cancel out', ACM structures are produced that are extremely resistant to dimensional changes with temperature.

Boron:

A more recent development in the fiber technology of ACMs is boron fiber. It is somewhat lighter than carbon fiber, but considerably stiffer. Boron fiber is produced by chemical vapour deposition at high temperatures from the reaction between boron trichloride and hydrogen gas. A very fine 'seed' wire of tungsten is passed through a sealed reaction chamber containing the two reactant gases, at a temperature of 800 to 2000 degrees C, The reaction proceeds, producing elemental boron and hydrogen chloride, Layers of pure boron are thus deposited onto the tungsten 'seed' wire as it passes through the chamber at a rate that will yield fibers of a desired diameter. The resulting fibers are then subjected to a polishing operation, increasing their strength by 30% or more. Curiously, the process makes every boron fiber itself an ACM consisting of a single tungsten fiber within a boron matrix.

The conditions required for the generation of boron fiber restrict its production to single strands, making it a rather high-priced commodity. The rigidity of the fibers defeats weaving, so boron fiber is generally used only in unidirectional tapes and patches, although some simple woven materials are available on a small scale.

Boron fiber is so stiff and strong that it has been used to patch across the fuselage of jet fighter aircraft like a second skin, to literally hold the wings on and prevent the early fatigue and failure of the aircraft. It is also interesting to note that the primary structural member of the B-1 bomber is a single lengthwise beam of boron fiber construction.

Basalt:

Basalt fiber represents one of those little strokes of simple genius that appear once in a while. Basalt itself is familiar from the columnar formations in volcanic deposits. That same columnar structure is a clue to the molecular behaviour of basalt, a hint that it might be a viable fiber-forming material. Molten basalt can indeed be extruded into fibers, but what was basalt first if not just molten rock ejected from the vent (a volcano...) of a very large furnace (the Earth...)? Hold on a moment... where else do we see this happening? How about in the metals industry, where millions of tons of molten rock are ejected from somewhat smaller furnaces each year in the form of slag? Indeed, basalt fiber is now produced in quantity in two source grades: 'basalt', and 'modified basalt' or slag.

Basalt fibers can be processed into all the fabric forms currently available with glass fiber, and they can be substituted directly into any application for which glass fiber is suitable. Basalt fiber materials are proving to be a very useful alternative in applications calling for a more robust version of glass fiber, and in other applications that have traditionally been the domain of rock fibers such as asbestos. Since basalt is also a rock fiber it exhibits far better heat resistance than does glass fiber, withstanding conditions that would quickly destroy glass constructs. It also exhibits a significantly higher chemical stability than does glass fiber.

Being a recently developed material, research into potential applications of basalt fiber has really only just begun. The properties of basalt fiber will certainly guarantee that its major uses will be in the construction trades, but it will undoubtedly see far broader applications as well.

Advanced composite materials technology is a field that is growing both quickly and steadily. That new fiber materials and applications will be developed is the proverbial "no brainer", and it has been the intent of this article to indicate that considerable training is essential for those who would work in this field.

Informative links:

Renaissance Aeronautics Associates Inc.
www.raacomposites.com

Composite Materials
www.netcomposites.com

Composites Fabricators Association
www.cfa.org

Materials Web
www.matweb.com

Composites News
www.compositesnews.com

Aramid fiber
www.teijin=aramid.com/ENG/aramid_fiber.htm

Basalt Fiber
www.siteblazer.net/sudaglass

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