Composites Overview - Fiber

Fibers
Fibers encapsulated within the resin matrix account for the strength advantage that reinforced composites have over unreinforced plastics. Strong and stiff fibers carry loads while the resin transfers the load between the fibers. A wide variety of properties can be achieved through proper selection of fiber type, fiber orientation and fiber reinforcement form (e.g., rovings, mats, fabrics, preforms, etc.)

The different fiber types include:

•  Glass
•  Carbon (also referred to as graphite)
•  Aramid
•  Quartz
•  Boron
•  Basalt
•  Natural fibers (hemp, jute, kenaf)

Glass
Glass fiber is the most commonly used fiber because it offers the best value (good performance at a low cost). Glass accounts for over 90% of the fiber usage. Most of the fibers are E-glass which gets its callout from its early application in electrical products such as circuit boards. Its attributes include:

•  High Specific Strength
•  Moderate Specific Modulus
•  Good Damage Resistance
•  Isotropic
•  Low Cost

Carbon
Carbon fiber provides high performance, but costs more. It is used where there is a strong need for high stiffness or strength at lowest weight. The term graphite is frequently used as a generic name for carbon, but graphite is a high crystalline form of carbon. Most of the fibers are carbon and its attributes include:

•  Low Density
•  High Strength and Modulus
•  Very Low Coefficient of Thermal Expansion
•  Good Fatigue Resistance
•  Good Damping
•  Good Heat Conductor

Other
Aramid fibers have a high strain to failure and are most used in damage resistant applications such as ballistic protection. Its attributes include:

•  Excellent Tensile Properties
•  Very Poor Compression
•  Extemely Tough
•  Good Chemical Resistance
•  Good Thermal Resistance
•  Hygroscopic
•  Easily Woven

Boron fiber attributes are:

•  high strength and modulus
•  thermally stable
•  forming restrictions/difficult to handle
•  high cost

Quartz fiber attributes are:

•  very high temperature capability
•  high tensile strength
•  lower density than glass
•  high cost

Reinforcement Forms
Fibers come in a variety of forms to choose from. The form selection depends on cost and manufacturing process.

At the most basic level the fiber filaments come bundled in groups of different quantities. For glass, the bundles are called rovings or yarns. For carbon, the bundles are called tows.

In some manufacturing processes the rovings or tows are the direct input. For the RTM and Vacuum Infusion processes, the rovings (or tows) are converted into two or three dimensional reinforcement forms. These include mats, fabrics and braids. These forms provide the orientation for the fiber and better handling. The desired amount of reinforcements at the correct orientation are placed in the molding tool for resin infusion.

Mats
Mats are nonwoven fabrics that provide equal strength in all directions. Chopped mats contain randomly distributed fibers that have been cut to lengths around two inches long and are held together with a chemical binder. Continuous-strand mat is formed by swirling continuous strands of fiber onto a moving belt. Lightweight mats are often used as surface veils to provide resin-rich finish for improved cosmetics.

Woven Fabrics
Woven fabrics are fabricated using textile processes in a wide variety of weights, weaves and widths. Most fabrics are bi-directional to deliver equivalent properties in the 0- and 90-degree directions.

Knitted Fabrics
Knitted fabrics are fabricated by placing rovings in the desired alignment and stitching them together. The rovings can be placed in any direction including all unidirectional. The rovings lay on top of each other and are not crimped like woven fabrics. This results in a slightly better properties and a more pliable fabric. A wide variety of orientations and weights can be achieved to meet product requirements and simplify the layup process.

Braids
Braid materials provide good material properties in a form that can be easily conformed to the part shape. Braids come in a flat form or as a tubular sock that can be pulled completely around the part shape. Braids are generally more expensive than woven materials due to a more complex manufacturing process.

Preforms
Preforms are fiber assemblies that are consolidated in an operation before molding.  A binder is used to hold the fibers in the desired shape.  Preforms are primarily used for complex shapes and they greatly simplify the tooling loading process.  Preforms may include combinations of the previous reinforcement forms or may be specifically created for the application such as directed fiber preforms of chopped fibers.