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 (Graphite) 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.

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 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 is also taken into consideration.

Image #1: Mounting plate made from carbon fiber reinforced composite.  The carbon fiber was chosen for its very low CTE and high strength.

Fiberglass:

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.

There are five major types of fiberglass.  A-glass (alkali glass) which has good chemical resistance, but lower electrical properties.  C-glass (chemical glass) which has very high chemical resistance.  E-glass (electrical glass) which is an excellent insulator and resists attacks from water.  S-Glass (structural glass) which is optimized for mechanical properties.  D-glass (dielectric glass) which has the best electrical properties but lacks in mechanical properties when compared to E and S glass.

E-glass and S-glass are, by far, the most common types found in composites.  These types have good combinations of chemical resistance, mechanical properties and insulating properties.  Of the two, E-glass offers the more attractive economics, and S-glass offers better mechanical performance.

Image #2: Fiberglass reinforced composite insulating sleeve.  In this case the fiber was chosen for its insulating properties and strength to transfer loads between metal components.

Other Fibers:

While carbon fibers and fiberglass are the most common reinforcements in thermoplastic composites, there are other options.  Aramid fibers (such as Kevlar ® and Twaron ®) and boron fibers have been used in composites and offer some beneficial properties (excellent toughness and compressive strength, respectively).  However they have characteristics that have limited their use (susceptibility to light/difficulty machining and brittleness, respectively).  Still others include ceramic fibers like SiC or aluminum oxide.  These may be attractive for their compression, insulating, or high-temperature properties.

Automated Dynamics' staff can assist with the selection of the best fiber for your application based on program needs, availability, economics, and other considerations.