Appendix A—Glossary

A|B|C|D|E|F|G|H|I|J|K|L|M|N|O|P|Q|R|S|T|U|V|W|X|Y|Z

Ablative—Material that absorbs heat through a decomposition process called pyrolysis at or near the exposed surface.

Accelerator—Chemical additive that hastens cure or chemical reaction.

Acoustic Emission—A measure of integrity of a material, as determined by sound emission when a material is stressed. Ideally, emissions can be correlated with defects and/or incipient failure.

Additive—Ingredients mixed into resin to improve properties. Examples include plasticizers, initiators, light stabilizers and flame retardants.

Adhesive—Substance applied to mating surfaces to bond them together by surface attachment. An adhesive can be in liquid, film or paste form.

Anisotropic—Fiber directionality where different properties are exhibited when tested along axes in different directions.

Antimony Trioxide—Fire retardant additive for use with resins.

Aramid—High-strength, high-stiffness aromatic polyamide fibers, such as DuPont's Kevlar.

Areal Weight—Weight of a fiber reinforcement per unit area (width times length) of tape or fabric.

Aspect Ratio—The ratio of length to diameter of a fiber.

Axial Winding—Filament winding wherein the filaments are parallel to the axis.

Bag Molding—An airtight film used to apply atmospheric force to a laminate.

Balanced Laminate—A laminate in which all laminae except those at 0/90 degrees are placed in plus/minus pairs (not necessarily adjacent) symmetrically around the lay-up centerline.

Basket Weave—Woven reinforcement where two or more warp threads go over and under two or more filling threads in a repeat pattern. This weave is less stable than the plain weave but produces a flatter, stronger, more pliable fabric.

Batch (or Lot)—Material made with the same process at the same time having identical characteristics throughout.

Bias Fabric—A fabric in which warp and fill fibers are at an angle to the length.

Biaxial Winding—Filament winding wherein helical bands are laid in sequence, side by side, with no gaps or overlap between the fiber.

Bidirectional Laminate—A laminate with fibers oriented in more than one direction on the same plane.

Bismaleimide (BMI)—A type of polyimide that cures by an additional reaction to avoid formation of volatiles. BMIs exhibit temperature capabilities between those of epoxy and polyimide.

Bleeder Cloth—A layer of woven or nonwoven material, not a part of the composite, that allows excess gas and resin to escape during cure.

Bleedout—Excess liquid resin appearing at the surface, primarily occurring during filament winding.

Blister—A rounded elevation of the pultruded surface with boundaries that may be more or less sharply defined.

Blooming, Fiber—A pultrusion surface condition exhibiting a fiber prominence or fiber show that usually has a white or bleached color and a sparkling appearance.

Bond Strength—As measured by load/bond area, the stress required to separate a layer of material from another material to which it is bonded. The amount of adhesion between bonded surfaces.

Boron Fiber—A fiber usually of a tungsten-filament core with elemental boron vapor deposited on it to impart strength and stiffness.

Braid—Woven tubular shape used instead of a flat fabric for composite reinforcement.

Breakout—Separation or breakage of fibers when the edges of a composite part are drilled or cut.

Broadgoods—Fibers woven or stitched into fabrics that may or may not be impregnated with resin; usually furnished in rolls.

B-Stage—Intermediate stage in the polymerization reaction of thermosets. After B-stage, material softens with heat and is plastic and fusible. Also called resistal. The resin of an uncured prepreg or premix is usually B-stage. See A-stage, C-stage.

Buckling—A failure mode usually characterized by fiber deflection rather than breaking because of compressive action.

Cable—A ropelike, multistrand assembly of composite rods or steel wire or fiber.

CAD/CAM—Computer-aided design/computer-aided manufacturing.

Carbon Fiber—Reinforcing fiber known for its light weight, high strength and high stiffness. Fibers are produced by high temperature treatment of an organic precursor fiber based on PAN (polyacrylonitrile), rayon or pitch in an inert atmosphere at temperatures above 1,800 degrees Fahrenheit. Fibers can be graphitized by removing still more of the non-carbon atoms by heat treating above 3,000 degrees Fahrenheit.

Carbon/Carbon—A composite of carbon fiber in a carbon matrix.

Catalyst—A substance that promotes or controls curing of a compound without being consumed in the reaction.

Centipoise (cps)—Unit of measure used to designate a fluid's viscosity. At 70 degrees Fahrenheit, water is 1 cps; peanut butter is 250,000 cps.

Centrifugal Casting—A processing technique for fabricating cylindrical structures, in which the composite material is positioned inside a hollow mandrel designed to be heated and rotated as resin is cured.

Ceramic-Matrix Composites (CMC)—Materials consisting of a ceramic or carbon fiber surrounded by a ceramic matrix, usually silicon carbide.

Chopped Strand—Continuous roving that is chopped into short lengths and then used in mats, spray-up or molding compounds.

Circumferential Winding—The process of winding filaments perpendicular to the axis during filament winding.

Cocured—Cured and simultaneously bonded to another prepared surface.

Coefficient of Expansion (COE)—Measure of the change in length or volume of an object.

Coefficient of Thermal Expansion (CTE)—A material's fractional change in length corresponding to for a given unit change of temperature.

Cohesion—Tendency of a single substance to adhere to itself. Also, the force holding a single substance together.

Composite—A material that combines fiber and a binding matrix to maximize specific performance properties. Neither element merges completely with the other. Advanced composites use only continuous, oriented fibers in polymer, metal and ceramic matrices.

Compression Molding—A technique for molding thermoset plastics in which a part is shaped by placing the fiber and resin into an open mold cavity, closing the mold, and applying heat and pressure until the material has cured or achieved its final form.

Compressive Modulus—A mechanical property description which measures the compression of a sample at a specified load. Described in the ASTM standard, D-695.

Compressive Strength—Resistance to a crushing or buckling force. The maximum compressive load a specimen sustains divided by its original cross-sectional area.

Compressive Strength—The stress that a given material can withstand when compressed. Described in ASTM D-695.

Condensation Polymerization—A polymerization reaction in which simple byproducts (for example, water) are formed.

Consolidation—A processing step that compresses fiber and matrix to remove excess resin, reduce voids and achieve a particular density.

Contaminant—Impurity or foreign substance that affects one or more properties of composite material, particularly adhesion.

Continuous Filament—An individual, small-diameter reinforcement that is flexible and indefinite in length.

Continuous Roving—Parallel filaments coated with sizing, gathered together into single or multiple strands, and wound into a cylindrical package. It may be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding compounds, or it may be used chopped.

Core—In sandwich construction, the central component to which inner and outer skins are attached. Foam, honeycomb, paper and wood are all commonly used as core material.

Core Orientation—Used on a honeycomb core to line up the ribbon direction, thickness of the cell depth, cell size and transverse direction.

Core Splicing—Joining two core segments by bonding them together.

Corrosion Resistance—The ability of a material to withstand contact with ambient natural factors or those of a particular artificially created atmosphere, without degradation or change in properties. For metals, corrosion can cause pitting or rusting; for composites, corrosion can cause crazing.

Cowoven Fabric—A reinforcement fabric woven with two different types of fibers in individual yarns. For example, thermoplastic fibers woven side by side with carbon fibers.

Crack—A visual separation that occurs internally or penetrates down from the pultruded surface to the equivalent of one full ply or more of reinforcement.

Crazing—Select region of fine cracks that may develop on or under a resin surface.

Creel—A device for holding the required number of roving spools.

Creep—Dimensional change over time the material of a finished part that is under physical load, beyond instantaneous elastic deformation.

Crimp—A fiber's waviness, which determines its capacity to cohere.

Critical Length—The minimum length of a fiber necessary for matrix shear loading to develop fiber ultimate strength by a matrix.

Cross Laminated—Material laminated so that some of the layers are oriented at various angles to the other layers with respect to the laminate grain. A cross-ply laminate usually has plies oriented only at 0/90 degrees (see Fiber Architecture).

Cross-Linking—The chemical bonding of molecules during polymerization that occurs during curing as the resin transitions from a liquid to a solid.

Crystallinity—The quality of having a molecular structure in which the atoms are arranged in an orderly, three-dimensional pattern.

C-Stage—Final step in the cure of a thermoset resin, resulting in irreversible hardening and insolubility.

Cure—To irreversibly change the molecular structure and physical properties of a thermosetting resin by chemical reaction via heat and catalysts alone or in combination, with or without pressure.

Cure Temperature—The temperature at which a material attains final cure.

Curing Agent—A catalytic or reactive agent that brings about polymerization when added to a resin. Also called hardener.

Damage Tolerance—A measure of the ability of structures to retain load-carrying capability after exposure to sudden loads (e.g., ballistic impact).

Damping—Diminishing the intensity of vibrations.

Debond—An unplanned nonadhered or unbonded region in a structure.

Deformation—Projections or indentations on rebar that are designed to increase mechanical bonding.

Delaminate—The separation of ply layers because of adhesive failure. This also includes the separation of layers of fabric from the core structure. A delamination may be associated with bridging, drilling and trimming.

Delamination—In-plane separation of a laminate ply or plies due to adhesive failure. For pultruded composites, the separation of two or more layers or plies of reinforcing material within a pultrusion.

Denier—A numbering system for yarn and filament in which yarn number is equal to weight in grams of 9,000 meters of yarn.

Design Allowable—A limiting value for a material property that can be used to design a structural or mechanical system to a specified level of success with 95-percent statistical confidence.

Dielectric—Nonconductor of electricity; the ability of a material to resist the flow of an electrical current.

Dielectric Constant—The property of a material that determines the relative speed that an electrical signal will travel through that material. A low dielectric constant will result in a high signal propagation speed. A high dielectric constant will result in a much slower signal propagation speed. Signal speed is roughly inversely proportional to the square root of the dielectric constant.

Dielectric Strength—Voltage required to penetrate insulating material. Material with high dielectric strength offers excellent electrical insulating properties.

Doubler—Extra layers of reinforcement for added stiffness or strength where fasteners or other abrupt load transfers occur.

Drape—The ability of fabric (or prepreg) to conform to the shape of a contoured surface.

Dry Fiber—A condition in which fibers are not fully encapsulated by resin during pultrusion.

Dry Winding—A filament winding operation in which resin is not used.

Ductility—The ability of a material to be plastically deformed by elongation, without fracture.

E-Glass—Stands for electrical glass and refers to borosilicate glass fibers most often used in conventional polymer matrix composites.

Elastic Limit—The greatest stress a material is capable of sustaining without permanent deformation remaining after complete release of the stress.

Elastic Modulus—See Modulus of Elasticity.

Elasticity—The property of materials allowing them to recover their original size and shape after deformation.

Elastomer—A material that substantially recovers its original shape and size at room temperature after removal of a deforming force.

Elongation—The fractional increase in length of a material stressed in tension. When expressed as a percentage of the original length, it is called percent elongation.

End—A strand of roving consisting of a given number of filaments gathered together. The group of filaments is considered an end or strand before twisting.

End Count—An exact number of strands contained in a roving.

Epoxy Resin—A polymer resin characterized by epoxide molecule groups.

Exothermic—Term used for a chemical reaction that releases heat.

Extenders—Low-cost materials used to dilute or extend high-cost resins without extensive lessening of properties.

Fabric, Nonwoven—A material formed from fibers or yarns without interlacing (for example, stitched non-woven broadgoods).

Fabric, Woven—A material constructed of interlaced yarns, fibers or filaments.

Fabrication—The process of making a composite part or tool.

Fatigue—The failure of a material's mechanical properties as a result of repeated stress over time.

Fatigue Life—The number of cycles of deformation require to fail a test specimen under a given set of oscillating stresses and strains.

Fatigue Limit—The stress level below which a material can be stressed cyclically for an infinite number of times without failure.

Fatigue Strength—Maximum cyclical stress withstood for a given number of cycles before a material fails.

FEA—Finite element analysis.

Fiber—A general term used to refer to filamentary materials. Often, fiber is used synonymously with filament.

Fiber Architecture—The design of a fibrous part in which the fibers are arranged in a particular orientation to achieve the desired result. This may include braided, stitched or woven fabrics, mats, rovings or carbon tows.

Fiber Bridging—Reinforcing fiber material that is found bridging across an inside radius of a pultruded product.

Fiber Content—Amount of fiber in a composite expressed as a ratio to the matrix. Strength generally increases as the fiber content ratio increases.

Fiber Orientation—Direction of fiber alignment in a nonwoven or mat laminate wherein most of the fibers are placed in the same direction to afford higher strength in that direction.

Fiber Placement—A continuous process for fabricating composite shapes with complex contours and/or cutouts by means of a device that lays preimpregnated fibers (in tow form) onto a nonuniform mandrel or tool. It differs from filament winding in several ways: there is no limit on fiber angles; compaction takes place online by heat, pressure, or both; and fibers can be added and dropped as necessary. The process produces more complex shapes and permits a faster putdown rate than filament winding.

Fiber Prominence—A visible and measurable pattern of the reinforcing material on the surface of pultruded product.

Fiber Show—Strands or bundles of fibers that are not covered by resin and that are at or above the surface of a reinforced plastic pultrusion.

Fiber-Reinforced Plastics (FRP)—A general term for a composite material or part that consists of a resin matrix containing reinforcing fibers, such as glass or carbon, having greater strength or stiffness than the resin. FRP is most often used to denote glass fiber-reinforced plastics; the term "advanced composite" usually denotes high-performance aramid or carbon fiber-reinforced plastics.

Filament Winding—An automated process for fabricating composites in which continuous roving or tows, either preimpregnated with resin or drawn through a resin bath, are wound around a rotating mandrel.

Filaments—Individual fibers of indefinite length used in tows, yarns or roving.

Fill Threads—Also known as the weft. These are the crosswise fibers woven at 90 degrees to the warp fibers.

Filler—Material added to the mixed resin to increase viscosity, improve appearance, and/or lower the density and cost.

Film Adhesive—An adhesive in the form of a thin, dry resin film with or without a carrier; commonly used for adhesion between laminate layers.

Finish—Material applied to fibers (after sizing is removed) to improve bonding between resin and fiber.

Fish Eye—The effect of surface contamination which causes a circular separation of a paint or gel coat.

Flexural Modulus—An engineering measurement that determines how much a sample will bend when a given load is applied (ASTM D-790).

Flexural Strength—The strength of a material in bending expressed as the stress of a bent test sample at the instant of failure. Usually expressed in force per unit area.

Folded Reinforcement—An unintentional or unspecified misalignment of mat or fabric reinforcing material in relation to the contour of a pultruded section.

Fracture—Cracks, crazing, or delamination, or a combination thereof, resulting from physical damage.

Gel Coat—Pigmented or clear coating resins applied to a mold or part to produce a smooth, more impervious finish on the part.

Gel Time—Period of time from initial mixing of liquid reactants to the point when gelation occurs as defined by a specific test method.

Glass Fiber—Reinforcing fiber made by drawing molten glass through bushings. The predominant reinforcement for polymer matrix composites, it is known for its good strength, processability and low cost.

Glassiness—A glassy, marbleized, streaked appearance at the pultruded surface.

Glass-Transition Temperature (Tg)—Approximate temperature above which increased molecular mobility causes a material to become rubbery rather than brittle. The measured value of Tg can vary, depending on the test method.

Graphitization—The process of pyrolization at very high temperatures (up to 5,400 degrees Fahrenheit) that converts carbon to its crystalline allotropic form.

Grooving—Long narrow grooves or depressions in a surface of a pultrusion running parallel to its length.

Hand Lay-up—A fabrication method in which reinforcement layers—preimpregnated or coated afterward—are placed in a mold or on a structure by hand, then cured to the formed shape.

Hardener—Substance that reacts with resin to promote or control curing action.

Heat—Term used colloquially to indicate any temperature above ambient (room) temperature, to which a part or material is or will be subjected.

Heat-Distortion Temperature (HDT)—Temperature at which a test bar deflects a certain amount under specified temperature and stated load.

Helical—Ply laid onto a mandrel at an angle, often a 45-degree angle.

Honeycomb—Lightweight cellular structure made from either metallic sheet materials or non-metallic materials (for example, resin-impregnated paper or woven fabric) and formed into hexagonal nested cells, similar in appearance to the cross section of a beehive.

Hoop—Ply laid onto a mandrel at a 90-degree angle.

Hoop Stress—Circumferential stress in a cylindrically shaped part as a result of internal or external pressure.

Hybrid Composite—A composite made with two or more types of reinforcing fibers.

Hygroscopy—A material's readiness to absorb or retain moisture.

Impact Strength—A material's ability to withstand shock loading as measured by fracturing a specimen.

Impregnate—To saturate the voids and interstices of a reinforcement with a resin.

Impregnated Fabric—See Prepreg.

Inclusion—Any foreign matter or particles that are either encapsulated or embedded in the pultrusion.

Inhibitor—Chemical additive that slows or delays cure cycle.

Injection Molding—Method of forming a plastic to the desired shape by forcibly injecting the polymer into a mold.

Interface—Surface between two materials: in glass fibers, for instance, the area at which the glass and sizing meet; in a laminate, the area at which the reinforcement and laminating resin meet.

Interlaminar—Existing or occurring between two or more adjacent laminae.

Interlaminar Shear—Shearing force that produces displacement between two laminae along the plane of their interface.

Internal Shrinkage Cracks—Longitudinal cracks in the pultrusion that are found within sections of roving reinforcement.

Intumescent—A fire-retardant technology which causes an otherwise flammable material to foam, forming an insulating barrier when exposed to heat.

Isophthalic—A polyester resin based on isophthalic acid, generally higher in properties than a general purpose or orthophthalic polyester resin.

Isotropic—Fiber directionality with uniform properties in all directions, independent of the direction of applied load.

Kevlar—Strong, lightweight aramid fiber trademarked by DuPont, used as a reinforcing fiber.

Laminate—The structure resulting from bonding multiple plies of reinforcing fiber or fabric.

Laminate Ply—One fabric/resin or fiber/resin layer that is bonded to adjacent layers in the curing process.

Lay-up—Placement of layers of reinforcement in a mold.

Liquid-Crystal Polymers (LCP)—High-performance melt-processible thermoplastic with improved tensile strength and high-temperature capability.

Mandrel—Elongated mold around which resin-impregnated fiber, tape or filaments are wound to form structural shapes or tubes.

Mat—A fibrous reinforcing material composed of chopped filaments (for chopped-strand mat) or swirled filaments (for continuous-strand mat) with a binder applied to maintain form; available in blankets of various widths, weights, thicknesses and lengths.

Matrix—Binder material in which reinforcing fiber of a composite is embedded; the binder is usually a polymer, but may also be metal or a ceramic.

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

Milled Fiber—Continuous glass or carbon strands hammer milled into very short fibers.

Modulus—The physical measurement of stiffness in a material, which equals the ratio of applied load (stress) to the resultant deformation of a material, such as elasticity or shear. A high modulus indicates a stiff material.

Modulus of Elasticity—An engineering term used to describe a material's ability to bend without losing its ability to return to its original physical properties.

Moisture Absorption—Assimilation of water vapor from air by a material. Refers to vapor withdrawn from the air only, as distinguished from water absorption, which is weight gain due to absorption of water by immersion.

Mold—The cavity into or on which resin/fiber material is placed, and from which a finished part takes form.

Monomer—A single molecule that can react with like or unlike molecules to form a polymer.

Multifilament—A yarn consisting of many continuous filaments.

Nomex—Trademark of DuPont for nylon paper-treated material that is made into honeycomb core.

Nondestructive Inspection (NDI)—Determining material or part characteristics without permanently altering the test subject. Nondestructive testing (NDT) and nondestructive evaluation (NDE) are broadly considered synonymous with NDI.

Nonwoven Roving—A reinforcement composed of continuous fiber strands loosely gathered together.

One-Part Resin—A resin system in which the neat resin and catalyst are mixed together by the material supplier as part of the resin production operation, thereby eliminating the mix step during composite fabrication.

Outgassing—Release of solvents and moisture from composite parts under a vacuum.

Out-Time—Period of time in which a prepreg remains handleable with properties intact outside a specified storage environment (a freezer, in the case of thermoset prepregs).

PAN—See Polyacrylonitrile.

Peel Ply—Layer of material applied to a prepreg lay-up surface that is removed from the cured laminate prior to bonding operations leaving a clean, resin-rich surface ready for bonding.

Peel Strength—Strength of an adhesive bond obtained by stress that is applied "in a peeling mode."

Phenolic Resin—Thermosetting resin produced by condensation of an aromatic alcohol with an aldehyde, particularly phenol with formaldehyde.

Planar Winding—Filament winding in which the filament path lies on a plane that intersects with the winding surface.

Plastic—A high molecular weight thermoplastic or thermosetting polymer that can be molded, cast, extruded or laminated into objects. A major advantage of plastics is that they can deform significantly without rupturing.

Ply—One of the layers that makes up a laminate. Also, the number of single yarns twisted together to form a plied yarn.

Ply Schedule—Lay-up of individual plies or layers to build an FRP laminate. Plies may be arranged (scheduled) in alternating fiber orientation to produce in a multidirectional strength laminate (see Fiber Architecture).

Polar Winding—Filament winding in which the filament path passes tangent to the polar opening at one end of the chamber and tangent to the opposite side of the polar opening at the other end of the chamber.

Polyacrylonitrile (PAN)—Base material in the manufacture of some carbon fibers.

Polymer—Large molecule formed by combining many smaller molecules or monomers in a regular pattern.

Polymerization—Chemical reaction that links monomers together to form polymers.

Porosity—The presence of numerous pits or pin holes on or beneath the pultruded surface.

Postcure—An additional elevated temperature exposure often performed without tooling or pressure to improve mechanical properties.

Post-Tension—To compress cast concrete beams or other structural members to impart the characteristics of prestressed concrete.

Pot Life—Length of time in which a catalyzed thermosetting resin retains sufficiently low viscosity for processing.

Precursor—For carbon fibers, the rayon, PAN or pitch fibers from which carbon fibers are made.

Prepreg—Resin-impregnated cloth, mat or filaments in flat form that can be stored for later use in molds or hand lay-up. The resin is often partially cured to a tack-free state called B-staging. Additives such as catalysts, inhibitors, flame retardants and others can be added to obtain specific end-use properties and improve processing, storage and handling characteristics.

Prestress—To apply a force to a structure to condition it to withstand its working load more effectively or with less deflection.

Pretension—Precasting concrete beams with tension elements embedded in them.

Promoter—See Accelerator.

Puckers—Local areas on prepreg where material has blistered and pulled away from the separator film or release paper.

Pultrusion—An automated, continuous process for manufacturing composite rods, tubes and structural shapes having a constant cross section. Roving and/or tows are saturated with resin and continuously pulled through a heated die, where the part is formed and cured. The cured part is then cut to length.

Pyrolysis—Decomposition or chemical transformation of a compound caused by heat.

Quasi-isotropic—Approximating isotropy by orienting plies in several directions.

Ramping—Gradual programmed increase/decrease in temperature or pressure to control cure or cooling of composite parts.

Reinforcement—Key element added to matrix to provide the required properties (primarily strength and stiffness); ranges from short fibers and continuous fibers through complex textile forms.

Release Agent—Used to prevent cured matrix material from bonding to molds or forms. It is usually sprayed or painted on mold.

Release Film—An impermeable film layer that does not bond to the composite during cure.

Resin—Polymer with indefinite and often high molecular weight and a softening or melting range that exhibits a tendency to flow when subjected to stress. As composite matrices, resins bind together reinforcement fibers.

Resin Starved—Localized areas lacking sufficient resin for fiber wetout.

Resin Transfer Molding (RTM)-A molding process in which catalyzed resin is pumped into a two-sided, matched mold where a fibrous reinforcement has been placed. The mold and/or resin may or may not be heated.

Resin-Rich Area-Localized area filled with excess resin as compared to consistent resin/fiber ratio. A resin-rich area is beneficial when the composite is exposed to a corrosive environment, as long as sufficient reinforcement is present to carry structural loads.

Ribbon Direction-On a honeycomb core, the way the honeycomb can be separated. The direction of one continuous ribbon.

Rod-A thin, round bar made of composites or metal.

Roving-A collection of bundles of continuous filaments either as untwisted strands or as twisted yarn.

Sandwich Structure-Composite composed of light-weight core material (usually honeycomb or foam) to which two relatively thin, dense, high-strength, functional or decorative skins are adhered.

Scale-A condition wherein resin plates or particles are on the surface of a pultrusion.

Sealant-Applied to a joint in paste or liquid form that hardens in place to form a seal.

Secondary Bonding-The joining together, by the process of adhesive bonding, of two or more already cured composite parts.

S-Glass-Stands for structural glass, and refers to magnesia/alumina/silicate glass reinforcement designed to provide very high tensile strength. Commonly used in applications requiring an exceptionally high strength and low weight.

Shear-An action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relative to each other.

Shelf Life-Length of time in which a material can be stored and continue to meet specification requirements, remaining suitable for its intended use.

Silicon Carbide Fiber-Reinforcing fiber with high strength and modulus; its density is equal to that of aluminum. Used in organic metal-matrix composites.

Sizing-A solution of chemical additives used to coat filaments. The additives protect the filaments from water absorption and abrasion. They also lubricate the filaments and reduce static electricity.

Skin-A layer of relatively dense material used in a sandwich structure on the surface of the core.

Sluffing-A condition wherein scales peel off or become loose, either partially or entirely, from the pultrusion.

Specific Gravity-Density (mass per unit volume) of a material divided by that of water at a standard temperature.

Specification-The properties, characteristics or requirements a particular material or part must have to be acceptable to a potential user of the material or part.

Spray-Up-Technique in which continuous strand roving is fed into a chopper gun, which chops the roving into predetermined lengths. The gun sprays the chopped fiber, along with a measured amount of resin and catalyst, onto an open mold.

Stiffness-A material's ability to resist bending. Relationship of load to deformation for a particular material.

Stop Mark-A band, either dull or glossy, on the surface of a pultrusion, approximately ½ to 3 inches wide and extending around the periphery of a pultruded shape.

Storage Life-Amount of time a material can be stored and retain specific properties.

Strain-Elastic deformation resulting from stress.

Strand-One of a number of steel or composite wires twisted together to form a wire rope or cable.

Stress-Internal resistance to change in size or shape, expressed in force per unit area.

Stress Concentration-The magnification of applied stress in the region of a notch, void, hole or inclusion.

Stress Corrosion-Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion.

Stress Crack-External or internal cracks in a composite caused by tensile stresses; cracking may be present internally, externally or in combination.

Structural Adhesive-An adhesive used to transfer loads between two or more adherents.

Structural Bond-A bond joining load-bearing components of a structure.

Substrate-A material on which an adhesive-containing substance is spread for any purpose, such as bonding or coating.

Surfacing Veil-Used with other reinforcing mats and fabrics to enhance the quality of the surface finish. Designed to block out the fiber patterns of the underlying reinforcements and often adds ultraviolet protection to the structure.

Tack-Stickiness of an uncured prepreg.

Tape-Thin unidirectional prepreg in widths up to 12 inches.

Tendon-Broadly, any prestressing element, including one or more steel or composite seven-wire strands, composite rods or steel threaded bars.

Tensile Strength-Maximum stress sustained by a composite specimen before it fails in a tension test.

Thermal Conductivity-Ability to transfer heat.

Thermoplastic-A composite matrix capable of being repeatedly softened by an increase of temperature and hardened by a decrease in temperature.

Thermoset-Composite matrix cured by heat and pressure or with a catalyst into an infusible and insoluble material. Once cured, a thermoset cannot be returned to the uncured state.

Thixotropic-Materials that are gel-like at rest, but fluid when agitated. Having high static shear strength and low dynamic shear strength at the same time. Losing viscosity under stress.

Toughness-A measure of the ability of a material to absorb energy.

Tow-An untwisted bundle of continuous filaments, usually designated by a number followed by K, indicating multiplication by 1,000 (for example, 12K tow has 12,000 filaments.)

Twist-A condition of longitudinal progressive rotation found in pultruded parts.

Two-Part Resin-A resin, typically epoxy, that requires addition of a hardening agent before it will cure.

Unidirectional—Reinforcing fibers that are oriented in the same direction, such as unidirectional fabric, tape, or laminate.

Vacuum Bag Molding—Molding technique wherein the part is cured inside a layer of film from which entrapped air is removed by vacuum.

Vacuum-Assisted Resin Transfer Molding (VARTM)—An infusion process where a vacuum draws resin into a one-sided mold; a cover, either rigid or flexible, is placed over the top to form a vacuum-tight seal.

Viscosity—Tendency of a material to resist flow. As temperature increases, the viscosity of most materials decreases.

Void—A pocket of entrapped gas that have been cured into a laminate. In a composite that has been cured properly, void content is usually less than one percent.

Volatile Organic Compounds (VOCs)—Carbon-containing chemical compounds (for example, solvents and styrene) that evaporate readily at ambient temperatures. Environmental, safety and health regulations often limit exposure to these compounds, so low VOC content is preferable.

Volatiles—Materials in a sizing or resin that can be vaporized at room or slightly elevated temperature.

Warp—Yarns running lengthwise and perpendicular to the narrow edge of woven fabric.

Warpage—Dimensional distortion in a composite part.

Water Absorption—Ratio of weight of water absorbed by a material to the weight of dry material.

Weave—Pattern by which a fabric is formed from interlacing yarns. In plain weave, warp and fill fibers alternate to make both fabric faces identical. In satin weave, pattern produces a satin appearance with the warp yarn over several fill yarns and under the next one (for example, eight-harness satin would have one warp yarn over seven fill yarns and under the eighth).

Weft—Yarns running perpendicular to the warp in a woven fabric. Also called woof.

Wet Lay-Up—Fabrication step involving application of a resin to dry reinforcement.

Wet Winding—Filament winding wherein fiber strands are impregnated with resin immediately before they contact the surface of the winding.

Wetout—Saturation with resin of all voids between strands and filaments.

Wetting Agent—A surface-active agent that promotes wetting by decreasing the cohesion within a liquid.

Wind Angle—The measure in degrees between the direction parallel to the filaments and an established reference line.

Winding Pattern—Regularly recurring pattern of the filament path in a filament winding after a certain number of mandrel revolutions.

Wire Mesh—Fine wire screen used to increase electrical conductivity. Typically used to dissipate the electrical charge from lightning.

Woven Roving—Heavy, coarse fabric produced by weaving continuous roving bundles.

X-Axis—The axis in the plane of the laminate used as 0-degree reference. The Y-axis is the axis in the plane of the laminate perpendicular to the X-axis. The Z-axis is the reference axis normal to the laminate plane.

Yarn—Continuously twisted fibers or strands that are suitable for weaving into fabrics.

Y-Axis—See X-axis.

Yield Point—The first stress in a material, less than the maximum attainable stress, at which the strain increases at a higher rate than the stress. The point at which permanent deformation of a stressed specimen begins to take place. Only materials that exhibit yielding have a yield point.

Yield Strength—The stress at the yield point. The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.

Young's Modulus—Ratio of normal stress to the corresponding strain for tensile or compressive stresses less than the proportional limit of the material.

Z-Axis—See X-axis.

Glossary References

Yellow Pages 1997—Composites Technology magazine's directory and buyer's guide for the FRP Industry, Ray Publishing, Inc.

SOURCEBOOK 1998—High-Performance Composites' guide to products and services for the Composites Industry, Ray Publishing, Inc.

Introduction to Composites (4th Ed.), Composites Institute of the Society of the Plastics Industry (Washing-ton, DC), 1998.

Guidelines and Recommended Practices for Fiberglass Reinforced Plastic Architectural Products, Composites Fabricators Association, Architectural Div. (Arlington, VA), 1995.

Standard Terminology Relating to Reinforced Plastic Pultruded Products, ASTM D 3918-96, American Society for Testing and Materials (W. Conshohocken, PA), 1996.

McGraw-Hill Dictionary of Scientific and Technical Terms (4th Ed.), McGraw-Hill (New York), 1993.

Handbook of Composites, George Lubin, editor, Van Nostrand Reinhold Co. (New York), 1982.

Whittington's Dictionary of Plastics (3rd Ed.), James F. Carley, editor, Technomics (Lancaster, PA), 1993.