The structural properties of composite materials are derived primarily from the fiber reinforcement. In a composite, the fiber contributes high tensile strength, enhancing properites in the final part, such as stregnth and stiffness while minimizing weight.
Glass fibers
The vast majority of all fibers used in the composites industry are glass. Glass fibers are the oldest and, by far, the most common reinforcement used in nonaerospace applications to replace heavier metal parts. Glass weighs more than carbon, but also is more impact-resistant. Depending upon the glass type, filament diameter, sizing chemistry and fiber form, a wide range of properties and performance levels can be achieved.
Fiber properties are determined by the fiber manufacturing process and the ingredients and coatings used in the process. During glass fiber production, raw materials are transformed into delicate and highly abrasive filaments, ranging in diameter from 3.5 to 24 micrometers. Silica sand is the primary raw ingredient, typically accounting for more than 50 percent of glass fiber weight. Metal oxides and other ingredients can be added to the silica, and processing methods can be varied to customize the fibers for particular applications.
Glass filaments are supplied in bundles called strands. A strand is a collection of continuous glass filaments. Roving generally refers to a bundle of untwisted strands, packaged like thread on a large spool. Single-end roving consists of strands containing continuous, multiple glass filaments that run the length of the strand. Multiple-end roving contains lengthy but not entirely continuous strands, which are added or dropped in a staggered arrangement during the spooling process. Yarns are collections of strands that are twisted together.
Electrical or E-glass, so named because its chemical composition makes it an excellent electrical insulator, is particularly well suited to applications in which radio-signal transparency is desired, such as aircraft radomes, antennae and computer circuit boards. However, it is also the most economical glass fiber for composites, offering sufficient strength in most applications at a relatively low cost. It has become the standard form of fiberglass, accounting for more than 90 percent of all glass-fiber reinforcements. At least 50 percent of E-glass fibers are silica oxide; the remainder are oxides of aluminum, boron, calcium and other compounds, including limestone, fluorspar, boric acid and clay.
When greater strength is desired, high-strength glass, first developed for military applications in the 1960s, is an option. Variously known as S-glass in the U.S., R-glass in Europe and T-glass in Japan, its strand tensile strength is 700 ksi, with a tensile modulus of 14 Msi. S-glass has appreciably higher silica oxide, aluminum oxide and magnesium oxide content than E-glass and is 40 to 70 percent stronger than E-glass. E-glass and S-glass lose up to half of their tensile strength as temperatures increase from ambient to 538°C/1000°F, although both fiber types still exhibit generally good strength in this elevated temperature range.
While glass fibers have relatively high chemical resistance, they can be eroded by leaching action when exposed to water. For example, an E-glass filament 10 microns in diameter typically loses 0.7 percent of its weight when placed in hot water for 24 hours. The erosion rate, however, slows significantly as the leached glass forms a protective barrier on the outside of the filament; only 0.9 percent total weight loss occurs after seven days of exposure. To slow erosion, moisture-resistant coatings, such as silane compounds, are applied during fiber manufacturing.
Corrosion-resistant glass, known as C-glass or E-CR glass, loses much less of its weight when exposed to an acid solution than does E-glass. However, E-glass and S-glass are much more resistant to sodium carbonate solution (a base) than is C-glass. A boron-free glass fiber, with performance and price comparable to E-glass, demonstrates greater corrosion resistance in acidic environments (like E-CR glass), higher elastic modulus and better performance in high temperatures than does E-glass.
High-performance fibers
High-strength fibers used in advanced composites include not only carbon, glass and aramid, but high-modulus polyethylene (PE), boron, quartz, basalt, ceramic, newer fibers such as poly p-phenylene-2,6-benzobisoxazole (PBO), and hybrid combinations, as well. The basic fiber forms for high-performance composite applications are bundles of continuous fibers called tows. A carbon fiber tow consists of thousands of continuous, untwisted filaments, with the filament count designated by a number followed by “K,” indicating multiplication by 1,000 (e.g., 12K indicates a filament count of 12,000). Tows may be used directly, in processes such as filament winding or pultrusion, or may be converted into unidirectional tape, fabric and other reinforcement forms.
Carbon fiber — by far the most widely used fiber in high-performance applications — is produced from a variety of precursors, including polyacrylonitrile (PAN), rayon and pitch. The precursor fibers are heated and stretched to create the high-strength fibers. The first high-performance carbon fibers on the market were made from rayon precursor. PAN- and pitch-based fiber have replaced rayon-based fiber in most applications, but the latter’s “dogbone” cross-section often makes it the fiber of choice for carbon/carbon (C/C) composites. PAN-based carbon fibers are the most versatile and widely used. They offer an amazing range of properties, including excellent strength — to 1,000 ksi — and high stiffness. Pitch fibers, made from petroleum or coal tar pitches, have high to extremely high stiffness and low to negative axial CTE. Their CTE properties are especially useful in spacecraft applications that require thermal management, such as electronic instrumentation housings.
Although they are stronger than glass or aramid fibers, carbon fibers are not only less impact-resistant but also can experience galvanic corrosion in contact with metal. Fabricators overcome the latter problem by using a barrier material or veil ply — often fiberglass/epoxy — during laminate layup.
Aramid fibers, composed of aromatic polyamide, provide exceptional impact resistance and tensile strength. Standard high-performance aramid fiber has a modulus of about 20 Msi and tensile strength of approximately 500 ksi. Renowned for performance in bulletproof vests and other armor and ballistic applications, aramid fiber has been in increasing demand, due, in part, to growth of the personnel protection and military armor markets spurred by conflicts around the world. Aramid’s properties also make the fiber an excellent choice for helicopter rotor blades, solid rocket motors, compressed natural gas (CNG) tanks and other parts that must withstand high stress and vibration.
Commercially available ultrahigh-strength, high-modulus polyethylene (PE) fibers are well known for their extremely light weight, excellent chemical and moisture resistance, outstanding impact resistance, antiballistic properties and low dielectric constant. However, PE fibers have relatively low resistance to elongation under sustained loading, and the upper limit of their use temperature range is about 98°C/210°F. PE fiber composites are used in racing boat hulls, ski poles, offshore mooring ropes and other applications that require impact and moisture resistance and light weight but do not need extreme temperature resistance. At least one aircraft manufacturer now uses high-modulus PE fibers for the bulletproof insert in cockpit doors.
Other fiber options
Quartz fibers, while more expensive than glass, have lower density, higher strength and higher stiffness than E-glass, and about twice the elongation-to-break, making them a good choice where durability is a priority. Quartz fibers also have a near-zero CTE; they can maintain their performance properties under continuous exposure to temperatures as high as 1050°C/1920°F and up to 1250°C/2280°F for short time periods. Quartz fibers possess significantly better electromagnetic properties than glass, a plus when fabricating parts such as aircraft radomes.
Ceramic fibers offer high to very high temperature resistance but low impact resistance and relatively poor room-temperature properties. Typically much more expensive than other fibers, ceramic, like quartz, is the fiber of choice when its advantages justify the extra cost. One application of ceramic fibers is for flame-resistant veil material in laminates for aircraft interiors, which must withstand 1093°C/2000°F for at least 15 minutes without flame penetration. Ceramic composites also are being considered for use in certain high-heat aircraft engine applications.
Poly p-phenylene-2,6-benzobisoxazole (PBO) is a relatively recently developed fiber, with modulus and tensile strength almost double that of aramid fiber and a decomposition temperature that is almost 100°C/212°F higher. Suitable for high-temperature applications, it is currently used in protective ballistic armor, sporting goods, insulation and tire reinforcements.
Also a newcomer, basalt fibers are inexpensive, golden brown-colored fibers, similar to glass, and currently produced primarily in Russia and Ukraine. Basalt exhibits better chemical and alkali resistance than glass, promising an additional choice for use in reinforcing concrete in infrastructure applications. Kamenny Vek (Dubna, Russia), TechnoBasalt (Kyiv, Ukraine) and Hengdian Group Shanghai Russia & Gold Basalt Fibre Co. (Shanghai, China) are three of the growing number of basalt fiber suppliers.
Boron fibers are five times as strong and twice as stiff as steel. They are made by a chemical vapor deposition process in which boron vapors are deposited onto a fine tungsten or carbon filament. Boron provides strength, stiffness and light weight, and possesses excellent compressive properties and buckling resistance. Uses for boron composites range from sporting goods, such as fishing rods, golf club shafts, skis and bicycle frames, to aerospace applications as varied as aircraft empennage skins, Space Shuttle truss members and prefabricated aircraft repair patches.
Fiber hybrids capitalize on the best properties of more than one fiber type and can reduce raw material costs. Hybrid composites that combine carbon/aramid or carbon/glass fibers have been used successfully in ribbed aircraft engine thrust reversers, telescope mirrors, driveshafts for ground transportation and infrastructure column-wrapping systems.
Natural fibers — abaca, coconut, flax, hemp, jute, kenaf and sisal are the most common — are derived from the bast or outer stem of certain plants. Natural fibers are enjoying increased use because of their “green” attributes (less energy to produce), light weight, recyclability, good insulation properties and carbon dioxide neutrality (when burned, natural fibers give off no more carbon dioxide than was consumed to grow the source plant). They also have the lowest density of any structural fiber but possess sufficient stiffness and strength for some applications.
The automotive industry, in particular, is using these fibers in traditionally unreinforced plastic parts and even employs them as an alternative to glass fibers. Natural fiber-reinforced thermosets and thermoplastics are most often found in door panels, package trays, seat backs and trunk liners in cars and trucks. European fabricators hold the lead in use of these materials, in part because regulations now require their automobile components to be recyclable. Natural fibers can be incorporated into molded or extruded parts and, more recently, have been used in the direct long fiber injection (D-LFT) process where kenaf, flax and natural fiber/glass hybrids are used to reinforce polypropylene. Studies are underway to determine the suitability of long natural fiber composites for structural applications.
Critical fiber sizing
To achieve desirable properties in composite components, adhesion between fiber and matrix must be optimized. Adhesion requires sufficient saturation with resin (wetout) at the fiber/matrix interface. To ensure good adhesion, attention must be given to fiber surface preparation, such as the use of a surface finish or coupling agent, often termed sizing. Sizing, applied to glass and carbon filaments immediately after their formation, actually serves three purposes: As it enhances the fiber/matrix bond, it also eases processing and protects the fibers from breakage during processing. Although it accounts for only 0.25 to 6.0 percent of total fiber weight, sizing is a dynamic force in fiber reinforcement performance. Sizing chemistry distinguishes each manufacturer’s product and can be optimized for manufacturing processes, such as pultrusion, filament winding and weaving. For example, developments in sizing formulations have variously resulted in more cleanly chopped glass with reduced fuzz, glass that wets out more efficiently, and glass fibers that contain no chromium compounds.
Historically, carbon fiber was sized only for compatibility with epoxy resin. Today, fiber manufacturers are responding to demands from fabricators and OEMs to produce carbon fiber forms that are compatible with a broader range of resins and processes, as carbon fiber use increases in applications outside the aerospace arena.