Carbon Fiber Terminology

Axial
Reference made to a characteristic in the direction of the fiber orientation.

B-Stage
The partially crosslinked form of a thermosetting resin. This curing stage is used in Prepreg materials to facilitate storage and transport before final processing.

Bobbin (Spool)
The individual package in which a carbon fiber roving is wound onto a core.

Carbonization
The thermal process whereby stabilized (oxidized) PAN or pitch fibers are heated to a temperature of 1200 – 1500°C, in order to develop the ultimate strength and modulus of the fibers.

CFRP (Carbon Fiber Reinforced Plastics)
A composite material consisting of carbon fiber as the primary reinforcement within a polymer matrix. The mechanical and physical properties of CFRP will depend upon both the type of matrix and carbon fiber used.

Denier
A direct numbering system for measuring fiber linear density (yield), equal to the mass in grams per 9000 meters of fiber roving.

Elongation
The increase in length of a material during a tensile test, usually expressed as a percentage of the original length.

Graphitization
The process of heat treating carbonized precursor fiber in an inert atmosphere, such as nitrogen, at temperatures of 2,000~3,000°C. This process produces carbon fibers with fine graphite crystalline structures and a range of high modulus and strength properties.

IM Fiber
A PAN-based carbon fiber with a nominal tensile modulus of 42 Msi (Intermediate Modulus).

Laminate
The multiple layers or plies of a fabric or prepreg constituting a composite structure.

MIL-HDBK-17
A three volume handbook, focusing on continuously reinforced (advanced) polymer composites, which provides methods for the development of standardized, statistically-based material property data for polymer matrix composite materials. Also provided are data summaries for a number of relevant composite material systems for which available data meets specific MIL-HDBK-17 requirements for publication. Summaries of supporting engineering and manufacturing technologies and common practices to composite materials are also provided. The handbook is primarily intended for use by the aerospace and defense (USDoD) industries, and is available through ASTM.

Modulus
The change in stress divided by the change in strain of a loaded material specimen within its elastic (non-yielded) range. A measure of a material’s stiffness.

Oxidation (Stabilization)
The thermal process whereby the PAN precursor is converted into a cross-linked, thermoset material that is capable of maintaining its shape during the carbonization process.

PAN
The acronym for polyacrylonitrile, the raw material (precursor) for the most common type of carbon fibers. PAN-based carbon fibers are especially suited for use in structural applications due to their high tensile and compressive strengths relative to pitch-based carbon fibers.

Precursor
Raw material for carbon fiber. In case of PAN-based carbon fibers, polyacrylonitrile fiber is the precursor. Pitch-based carbon fibers use petroleum or coal tar-based precursors.

Preform
An assembly of dry fabric and/or fibers which has been prepared for a matrix addition process. A preform is usually stitched or otherwise stabilized in some way to maintain its shape before final processing.

Prepreg
Intermediate sheet material of reinforcing fibers impregnated with a polymer matrix resin system. Matrices are typically thermosetting resins in a partially cured (B-staged) form, though thermoplastic resins are also employed. Carbon fibers may be incorporated into a Prepreg directly as rovings (i.e, unidirectional tape), or as a fabric, either woven or non-woven. In addition to the constituents, prepregs are classified by their aerial weight, typically in grams per square meter (gsm) or ounces per square yard (oz/yd2).

SACMA
An organization of advanced material suppliers no longer in existence. SACMA developed multiple test methods which remain in use today as testing guidelines (Suppliers of Advanced Composite Materials Association).

Sizing
A polymeric coating added to carbon fibers in order to improve handling and processing. Sizing can also improve compatibility between reinforcing fibers and matrix resin systems.

Specific Modulus
Modulus divided by weight per unit volume.

Specific Strength
Strength divided by weight per unit volume.

Surface Treatment
The electrochemical treatment which partially oxidizes the fiber surface, creating chemically active sites that promote adhesion to a resin matrix. Different surface treatment processes may be employed to impart differing fiber surface characteristics, as desired.

TEX
A measure of the weight of fiber per unit length, expressed as grams per 1000 meters of roving or yarn. The lower the TEX, the finer the roving. More generally referred to as yield.

Tow (Roving, Strand)
A large grouping of carbon fiber filaments packaged together onto a single spool, or bobbin. Also referred to as a roving or strand. The term “small tow” refers to carbon fiber rovings that contain 24,000 (24K) or fewer filaments. “Large tow” refers to carbon fiber rovings that contain on the order of 48,000 to 320,000 (48K – 320K) filaments or more.

Transverse
Reference made to a characteristic in a direction 90° to the fiber orientation.

Unidirectional (UD) Sheet or Tape (Unitape)
A roll of carbon fiber prepreg with its fiber arranged in parallel and impregnated by matrix. Prepreg using this is called UD (unidirectional) prepreg or unitape, which offers high fiber content, low aerial weight, and maximum mechanical features in the direction of fiber.

Warp
Fibers in a fabric running lengthwise. Weft (Fill)Fibers in a fabric running across the fabric width.

Yield
A measure of a rovings linear density, expressed as the weight of fiber per unit length. When expressed as grams per 1000 meters, the yield is equal to the “TEX.”