Fibers are usually circular or nearly circular and are significantly stronger in the long direction because they are normally made by either drawing or pulling during the manufacturing process. Drawing orients the molecules so that tension loads on the fibers pull more against the molecular chains themselves than against a mere entanglement of chains. Due to the strength and stiffness advantages of fibers, they are the predominant reinforcement for advanced composites. Fibers may be continuous or discontinuous, depending on the application and manufacturing process.
Fiber terminology
Before going over various types of fibers used as composite reinforcements, the major terminology used for fiber technology will be reviewed. Fibers are produced and sold in many forms
Fiber - the general term for a material that has a long axis that is many times greater than its diameter. The term aspect ratio, which refers to fiber length divided by diameter (l/d), is frequently used to describe short fiber lengths. Aspect ratios are normally greater than 100 for fibers.
Filament - the smallest unit of fibrous material. For spun fibers, this is the unit formed by a single hole in the spinning process. The term filament is synonymous with fiber.
End - the term used primarily for glass fibers that refer to a group of filaments in long parallel lengths.
Strand - another term associated with glass fibers that refer to a bundle or group of untwisted filaments. Continuous strand rovings provide good overall processing characteristics through fast wet-out (penetration of resin into the strand), even tension, and abrasion resistance during processing. They can be cut cleanly, and they disperse evenly throughout the resin matrix during molding.
Tow - similar to a strand of glass fiber, the tow is used for carbon and graphite fibers to describe the number of untwisted filaments produced at one time. Tow size is usually expressed in thousands, denoted by "k"; for example, a 12k tow contains 12,000 filaments.
Roving - the number of strands or tows collected into a parallel bundle without twisting. Rovings can be chopped into short fiber segments for sheet molding compound, bulk molding compound, or injection molding.
Yarn - a number of strands or tows collected into a parallel bundle with twisting. Twisting improves handleability and makes processes such as weaving easier, but the twist also reduces the strength properties.
Band - the thickness or width of several rovings, yarns, or tows as it is applied to a mandrel or tool; a common term used in filament winding.
Tape - the composite product form in which a large number of parallel filaments (such as tows) are held together with an organic matrix material (such as epoxy) commonly referred to as prepreg (pre-impregnated with resin). The length of the tape in the direction of the fibers is much greater than the width, and the width is much greater than the thickness. Typical tape product forms are several hundred feet long, 6 to 60 in. (15 cm to 1.5 m) wide, and 0.005 to 0.010 in. (125 to 255 mm) thick.
Woven Cloth - another composite product form made by weaving yarns or tows in various patterns to provide reinforcement in two directions, usually zero and 90 degrees. Typical two-dimensional woven cloth is several hundred feet long, 24 to 60 in. (60 cm to 1.5 m) wide, and 0.010 to 0.015 in. (255 to 380) mm thick. Woven cloth is normally supplied either without resin (dry) or as prepreg with resin.
GSM stands for Grams per Square Meter (g/m2) - the weight of the fabric if you take a sheet of material that is one meter by one-meter square and weigh it in grams.
It is a benchmark specification to meet production manufacturing requirements. It is also a standard (one of many) upon which different materials are compared.
Types of Fiber Reinforcement
There are many different types of fibers that can be used to reinforce polymer matrix composites. The most common are carbon fibers (AS4, IM7, etc.) and fiberglass (S-glass, E-glass, etc.). As with the matrix, the fiber chosen will be determined by the end application.
Carbon Fibers
Carbon fibers are conductive, have an excellent combination of high modulus and high tensile strength, have a very low (slightly negative) CTE, and offer good resistance to high temperatures.
Figure 1. Carbon Fiber Composites Examples
Carbon fibers are frequently categorized using tensile modulus. There are five categories of carbon fibers generally used in composites; low modulus, standard modulus, intermediate modulus, high modulus, and ultra-high modulus. The exact cut-off for these categories will vary depending on the reference consulted, but in general, low modulus fibers have a tensile modulus of less than 30Msi and ultra-high-modulus fibers have a tensile modulus greater than 75Msi. As a point of comparison, steel has a tensile modulus of 29Msi.
As the modulus increases, the fibers tend to get more brittle, more expensive, and harder to handle. Further, the tensile strength of the fibers generally increases as the modulus increases from low to intermediate but then tends to fall off in the high and ultra-high modulus fibers. I.e. the tensile strength of carbon fibers tends to be the greatest for the intermediate modulus fibers. For these reasons, standard and intermediate modulus fibers tend to give the best overall performance, unless the application is very stiffness oriented. This is illustrated even more clearly when fiber price and availability are also taken into consideration.
Fiberglass or Glass fiber
Fiberglass is, as its name implies, glass that has been spun into the form of fibers. Fiberglass is not as strong or stiff as carbon fibers, but it has characteristics that make it desirable in many applications. Fiberglass is non-conductive (i.e. an insulator) and it is generally invisible to most types of transmissions. This makes it a good choice when dealing with electrical or broadcast applications.
Figure 2. The application of fiberglass in infrastructure efficiency and sustainability (Source: CompositesWorld)
There are five major types of fiberglass.
A-glass (alkali glass) has good chemical resistance, but lower electrical properties.
C-glass (chemical glass) has very high chemical resistance.
E-glass (electrical glass) is an excellent insulator and resists attacks from the water.
S-Glass (structural glass) is optimized for mechanical properties.