Composite Terminology

Aramid – The term aramid denotes a class of aromatic polyamide fibers and includes Kevlar® and Nomex®, both products of the Dupont Company and Lumat®. This fabric is 5 times stronger than steel by weight and is yellow/gold in color, but can be dyed to make other colors.

Bleeder Cloth – A nonstructural layer of material used in the manufacture of composite parts to allow the escape of excess gas and resin during cure. The bleeder cloth is removed after the curing process is complete and is not part of the final composite.

Breather – A loosely woven material, such as glass fabric, which serves as a continuous vacuum path over a part but does not come in contact with the resin. The breather is removed after the curing process is complete and is not part of the final composite.

Bundle – A general term for a collection of essentially parallel filaments or fibers

Carbon Fibers – Fibers produced via pyrolytic degradation of synthetic organic fibers, rayon or polyacryionitrile (PAN), which have about 92-99% carbon content. The fibers have moduli greater than or equal to 70 GPa (10 million psi), tensile strengths up to 5.8 GPa (840,000 psi) and strain-to-fracture up to 2%.

Composite Material – A material composed of two or more different constituent materials. Composite materials may be classified as follows:

1. Fiberous composites – fibers in a matrix
2. Laminate composites – layers of the same or various materials
3. Particulate composites – particles in a matrix
4. Hybrid composites – Composed of more than one kind of fiber/matrix material system.

Composites are considered to be combinations of materials differing in composition or form. The materials retain their identities in the composite; that is, they do not dissolve or otherwise merge completely into each other although they act together. Normally, the composites can be physically identified between components.

Core – A sandwich filler material, generally cellular in nature resembling a honeycomb. The core material may be paper, nylon, phenolic, aluminum foil, or Nomex®.

Crimp – The amount of extra yarn required to allow for warp and weft (fill) to make a meter (yard) of fabric. Often expressed as a percent.

Cure – To change the properties of a thermosetting resin irreversibly by vulcanization or chemical reaction. Cure may be accomplished by the curing (cross-linking) agents, with or without a catalyst and with or without heat or pressure.

Denier – A numbering system for yarn and filament in which yarn number is equal to weight in grams of 9000 meters of yarn. A one denier fiber means that 9000 meters of the fiber weighs one gram.

Density – Weight per unit volume. Commonly expressed in grams per cubic centimeter, pounds per cubic inch, and pounds per cubic foot.

Drape – The ability of a fabric to conform to a contoured shape.

E-Glass – One of the oldest and most widely used types of fiberglass. It derives its name from its ability to resist electrical current and is made from sodium silicate.

Elongation – The amount of deformation of the fiber caused by breaking tensile force (stretching), expressed as a percentage of the original length.

Ends – Refers only to the fibers in the WARP direction of composite fabrics. Meaning, if there are 12 WARP ENDS in a fabric, then there are 12 fibers per-inch of width of the fabric, so it has a COUNT of “12-ENDS per-inch”

Fiber Strength – The resistance to stress forces expressed as (1) tenacity, in grams per denier, a specific measure of strength; or (2) tensile strength, in Kilopascals (pounds per square inch); used to measure the breaking strength of a fiber; an absolute measure of strength.

Filament – The smallest unit of a fibrous material. The basic units formed during drawing and spinning, which are gathered into strands of fiber for use in composites. Filaments usually are of extreme length and very small diameter, usually less than 25 mm (1 mil). Normally filaments are not used individually but bundled together to make strands.

Fill – The across or short direction of the fabric is called the FILL (also known as Weft). These fibers will only be as long as the width of the fabric, typically somewhere between 25″ to 60″ for fabrics, and 1″ to 16″ for tapes. It is customary to list the FILL specs after the Warp.

Gel Coat – A quick-setting color pigmented resin used in molding processes to provide an improved appearance and surface for the composite. It is applied to the mold after the mold release agent. The gel coat provides a finished surface that gives both weather and abrasion resistance.

Hand Layup – The process of assembling a component by placing and working successive plies on a mold by hand.

Homogenization of Fiber Alignment – Uniformly positioning the layers of Fiber in a layup.

Inch of Mercury – Inch of mercury (inHg and ″Hg) is a unit of measure for pressure. It is the pressure exerted by a column of mercury of 1 inch (25.4 mm) in height at the standard acceleration of gravity.

Interlaminar Shear Strength – Shear is basically a set of parallel forces which act in opposite directions.
Interlaminar shear strength is measured in the layered materials or composites, where the connection between two layers provides the necessary shear strength. Isotropic – Having properties that are uniform in all directions at any point within the material.

Mil – The unit used in measuring the diameter of glass fiber strands, wire, and so forth (1 mil = 0.001 in.).

Modulus – A measure of a material’s stiffness or its inherent resistance to flexing. Expressed in millions of pounds per square inch. The higher the modulus the stiffer the material.

Mold Release Agent – a lubricant applied to the mold surface to facilitate the release of the molded material.

Nominal – a designated or theoretical size that may vary from the actual : an approximate value.

PAN – Polyacrylonitrile, used as a source material for carbon fibers.

Peel Ply – A layer of resin-free material used to protect a laminate for later secondary bonding. No structural adhesion occurs and when pulled away leaves a surface ready for the next layer bonding or finishing.

Pic – a bundle of filaments running in the fill (Weft) direction.

Polyester resin – The term generally used for unsaturated polyesters, formed by the reaction of dibasic organic acids and polyhydric alcohol.

Porosity – a distribution of trapped pockets of air, gas, or vacuum within a solid material, usually expressed as a percentage of the total nonsolid volume to the total volume. Also referred to as voids.

Postcure – After the initial cure of a thermoplastic resin, additional elevated temperature exposures, usually above the cure temperature and without pressure can be made to improve final properties and/or complete the cure. In certain resins, complete cure and ultimate mechanical properties are attained only by exposure of the cured resin to a higher temperature than those of the first curing.

Pot Life (of a resin) – The length of time that a resin with its catalyst retains viscosity that is low enough to be used in processing. Length of time at room temperature before a catalyzed resin has polymerized (hardened) to an unworkable state.

PPV (Parts Per Volume) – When mixing resins, this is a mixing method where the liquid is measured by volume in a measuring container, not weighed using a scale.

PPW (Parts Per Weight) – When mixing resins, this is a mixing method where the liquid is measured using a scale by weight, not by volume.

Prepreg – A combination of mat, fabric, nonwoven material, or roving that is pre-impregnated with resin.
Quasi-isotropic – Isotropic means having the same properties in all directions.
Quasi-isotropic means having isotropic properties in-plane. A quasi-isotropic part has either randomly oriented fiber in all directions, or has fibers oriented such that equal strength is developed all around the plane of the part.
Generally, a quasi-isotropic laminate made from woven fabric has plies oriented at 0º, 90º +45º and –45º, with at least 12.5% of the plies in each of these four directions.
Quasi-isotropic properties can also be achieved with 0º, 60º and 120º oriented unidirectional plies.

RTM – Resin Transfer Molding.

Resin – Most resins are polymers. The four major types of resins are:

1. Polyesters
2. Vinylesters
3. Epoxies
4. Phenolics

Roving – A number of yarns, strands, tows, or ends collected into a parallel bundle with little or no twist.

S-Glass – Structural glass; a magnesia/alumina/silicate glass reinforcement designed to provide very high tensile strength. S-Glass has a higher tensile modulus (stiffness) and higher temperature resistance than E-Glass

Strand – Normally an untwisted bundle or assembly of continuous filaments used as a unit, including slivers, tows, ends, and yarn, for example. Sometimes a single fiber or filament is called a strand.

Tape – A narrow fabric loosely defined as a material that ranges in width from ¼ to 12 inches.

Tg – Glass transition temperature is the temperature at which higher molecular weight materials(polymers) transforms from glassy state to a viscous and rubbery state.

Tow – The bundle of individual carbon filaments used to weave carbon fabric. 50k tow means there are 48-50,000 carbon filaments in the tow. Smaller tow i.e. 12k, 6k, 3k and 1k are obtained by dividing the 50k tow into smaller bundles.

Vacuum Bag Molding – a fabrication process in which the layup is cured under pressure generated by drawing a vacuum in the space between the layup and a flexible sheet that is placed over it and sealed at the edges. Vacuum is then applied to pressurize laminate against mold during cure. This method of molding allows fabricating high strength parts without resorting to expensive tooling or an autoclave.

Veil – A non-woven glass fiber material used to enhance surface finish, weather-ability, appearance and smoothness to the touch (see surfacing mat).

Viscosity – Internal friction or resistance to flow of a liquid. For example honey has a higher viscosity than water.

Warp – Strands of material running in the lengthwise or long direction of the fabric. Fibers running in the WARP direction can be 100’s of yards long, depending on the length of the roll of fabric. When describing a fabric, it is traditional to stipulate the WARP specs first.

Weft – Strands of material running the width of the fabric.(0 Degree) Also referred to as Fill.

Weight – Estimated in ounces per square yard (oz./sq. yd.) or grams per square meter (gsm).

Work Life – The period during which a compound, after mixing with a catalyst, solvent, or other compounding ingredients, remains suitable for its intended use.

Yarn – Generic term for strands of fibers or filaments, usually twisted. A yarn is produced by twisting and plying either strands of fiber or continuous filaments in a form suitable for weaving.

Categories: General Composite Knowledge


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