Graphite Fiber Composite Materials Terminology

Graphite, Glass and Aramid Fiber Composites - Terms and Definitions

Aug 23, 2009

Graphite fiber composites are used in the manufacture of many products. Some definitions are needed to introduce the jargon related to graphite composite fibers.

Composite fiber materials differ in a number of ways from homogeneous materials such as steel, aluminum or most plastics without reinforcement. One obvious difference is the vocabulary associated with graphite fiber composite materials. The following terms and definitions are intended to be a brief, non-technical introduction to graphite fiber composites terminology.

Composite Material

A composite material is a product of two or more dissimilar materials which will have structural properties that the original materials did not have. Fiber and resin are an example of components of a composite material and are sometimes referred to as "string and glue" by those who manufacture composites.

Typical fiber types that would be encountered in manufacturing would be glass fiber (fiberglass), aramid fiber, carbon fiber and graphite fiber. Aramid fiber is commonly referred to by the brand names of Kevlar(tm) and Nomex(tm). Aramid fibers are best known for their impact resistance. Aramid fibers have a greater tensile strength than fiberglass, but less tensile strength than graphite fibers.

Graphite Fibers

A precursor is a material which is processed into a fiber which is capable of being carbonized and graphitized. These are typically fibers of PAN. Rayon or pitch may also be used as a precursor for fibers used in composites.

PAN is an acronym for polyacrylonitrile. PAN is a polymer which can be spun into fibers and used as a precursor for some carbon fibers.

Carbonization is the process by which a stabilized precursor such as PAN fiber is heated to a typical temperature of around 1200 degrees C. With the proper time and temperature the fiber becomes approximately 94% carbon.

Graphitization is very similar to carbonization where a carbonized fiber is taken to a typical temperature in excess of 2000 degrees C. With the proper time and temperature the fiber becomes approximately 99% carbon.

Carbon fiber is the resulting fiber from a precursor fiber which has completed the carbonization process.

Graphite fiber is the resulting fiber from a carbonized fiber which has completed the graphitization process. Generally speaking, graphite fibers will be stronger than carbon fibers.

Note: Carbon and graphite are terms which are sometimes used interchangeably. They are technically different in both the processing and the resulting properties of the fibers. Graphite fibers are significantly stronger fibers, as measured by tensile strength.

Typical Terminology for Most Types of Composite Fibers

A filament is the smallest "thread" with the smallest cross section that is made up of miniscule fibers used in composite structures. A filament is extremely long and will typically provide a continuous strand to the finished extremity of a part.

A tow or strand, is made up of thousands of filaments. Typical of graphite fibers, some manufacturers may refer to a tow with a cross section made up of less than 24,000 filaments as a "small tow". As well, any tow with a cross section made up of more than 48,000 filaments would be referred to as a "large tow". The size of a tow is referred to as the number of filaments in a cross section, such as 6K, 12K, 48K and etc. In a dry fiber situation a roving, strand or tow is typically spooled like sewing thread, but on a much larger scale.

A roving is usually considered to be a number of tows or strands put together in the same direction with little or no twisting. Occasionally a roving is used synonymously with tow or strand.

A little more precise than the name "chopped" would imply, chopped fibers are strands or rovings of glass fibers which are cut to a specific length to be used in the spray up of fiber reinforced plastic.

Cloth, in the manufacture of composites, refers to tows woven at 90 degree angles.

Unidirectional fiber is a series of tows, strands or rovings laid up side by side and all oriented in the same direction. A typical lay up would be on a 4 ft. wide roll of poly. Once the fiber is placed where desired, the poly would be removed.

Broadgoods is a term generally referring to prepreg cloth or unidirectional materials which are uncured and laid up to greater than 12 inches width. Prepreg is a contracted form of the term pre-impregnated. It refers to a tow, strand, roving or cloth that is impregnated with resin and maintained in a tacky, uncured state until ready for final forming and cure.

Graphite Fiber Composites Terminology

The copyright of the article Graphite Fiber Composite Materials Terminology in Manufacture Engineering is owned by Jake Crum. Permission to republish Graphite Fiber Composite Materials Terminology in print or online must be granted by the author in writing.