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A-stage - An early stage of polymerization of thermosetting resins in which the material is still soluble in certain liquids and fusible. (See also B-stage, C-stage.)

Ablative - Describes a 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.

Addition - Polymerization reaction in which no byproducts are formed.

Additives - Ingredients mixed into resin to improve properties.

Adhesive - Substance applied to mating surfaces to bond them together by surface attachment.

Amorphous - Polymers with no crystalline component.

Angle-ply laminate - Any balanced laminate consisting of plies at angles of plus and minus theta, where theta is an acute angle with the principal laminate axis.

Anisotropic - Not isotropic. Exhibiting different properties when tested along axes in different directions within the material.

Aramid - Aromatic polyamide fibers. (Often referred to as Kevlar, DuPont’s trademark.)

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

Aspect ratio - Ratio of length to diameter of a fiber.

Autoclave - Closed vessel for applying fluid pressure, with or without heat, to an enclosed object.

Autoclave molding - Molding technique in which an entire assembly (lay up and tooling) is placed into an autoclave and subjected to heat and elevated pressure for consolidation and/or curing while removing entrapped air and volatiles.

Automated tape laying - Fabrication process in which prepreg material, typically unidirectional tape, is laid across the surface of a mold in multiple layers and directions by an automated tape-application machine to form a structure.

Axial winding - Filament winding wherein the filaments are parallel or at a small angle to the axis of rotation.

B-stage - Intermediate stage in the polymerization reaction of some thermosets in which the material softens with heat and is plastic and fusible but does not entirely dissolve or fuse. The resin of an uncured prepreg or premix is usually in this state. (See also A-stage, C-stage.)



Bag molding - Molding technique in which the composite structure is placed in a rigid mold and covered with a flexible impermeable layer of film and the edges sealed, followed by consolidation and/or curing with pressure applied by vacuum, autoclave, press or inflation of the bag.

Balanced laminate - Any laminate that contains one ply of minus theta orientation with respect to the principal axis of the laminate for every identical ply with a plus theta orientation.

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

Batch - Material made by the same process at the same time having identical characteristics throughout. Same as lot.

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

Biaxial fabric - Fabric with two non-interwoven layers - a unidirectional warp (0°) layer and a unidirectional weft (90°) layer - which are bonded together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also triaxial fabric, quadraxial fabric.)

Biaxial winding - Filament winding wherein helical bands are laid in sequence, side by side, with no fiber crossover.

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

Bismaleimide (BMI) - Type of thermoset polyimide that cures by an additional reaction, thus avoiding formation of volatiles. Exhibits temperature capabilities between those of epoxy and polyimide.

Bleeder cloth - 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 of the composite structure, particularly during filament winding.

BMI - See bismaleimide.

Bond ply - Ply or fabric patch that comes in contact with the honeycomb core during repair.

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 - Fiber produced by chemical vapor deposition of boron onto a core material, usually a tungsten-filament. Because of the deposition process, a boron fiber is of a fairly large diameter, typically about 0.4 mils, and is thus often referred to as a wire.

Braiding - Textile process that intertwines into a pattern three or more strands, yarns or tapes, typically into a tubular shape.

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

Breather - Loosely woven material that does not come in contact with the resin but serves as a continuous vacuum path over a part in production.

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

Buckling - Failure mode usually characterized by unstable lateral deflection rather than breaking under compressive action.

Bundle - General term for a collection of essentially parallel filaments.

C-stage - Final step in the cure of a thermoset resin, resulting in irreversible hardening and insolubility. (See also A-stage and B-stage.)



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

Carbon fiber - Reinforcing fiber produced by the pyrolysis of an organic precursor fiber, such as PAN, rayon or pitch, in an inert environment at temperatures above 1,800°F. The term carbon is often used interchangeably with the term graphite, but the fibers differ. Carbon fibers are typically carbonized at about 2,400°F and contain 93 percent to 95 percent carbon. Carbon fibers can be converted to graphite fibers by graphitization at 3,450°F to 4,500°F, after which they contain more than 99 percent elemental carbon. Carbon fibers are known for their light weight, high strength and high stiffness.

Carbon/carbon - Composite of carbon fiber in a carbon matrix.

Catalyst - Substance that promotes or controls curing of a compound without being consumed in the reaction. (See also hardener.)

Caul plate - Plate or sheet the same size and shape as the composite lay-up with which it will be used. The caul plate is placed in immediate contact with the lay-up during curing to transmit normal pressure and provide a smooth surface on the finished part.

Ceramic-matrix composites (CMC) - Materials consisting of a ceramic or carbon fiber surrounded by a ceramic matrix, primarily silicon carbide.

Chemical vapor deposition (CVD) - Process in which the reinforcement material is deposited from the vapor phase onto a continuous core such as boron or tungsten.

Circumferential winding - Process of winding fiber perpendicular to the axis during filament winding.

Cloth - See fabric.

CMC - Ceramic-matrix composite.

Cocured - Cured and simultaneously bonded to another prepared surface.

Coefficient of thermal expansion (CTE) - A material’s fractional change in length 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.

Coin tap - Tapping a laminate with a coin in different spots to detect a change in sound, indicating the presence of a defect that may require repair.

Commingled yarn - Hybrid yarn made with two types of materials intermingled in a single yarn (for example, thermoplastic filaments intermingled with carbon filaments to form a single yarn).

Composite - Three-dimensional combination of at least two materials differing in form or composition, with a distinct interface separating the components. Composite materials are usually manmade and created to obtain properties that cannot be achieved by any of the components acting alone.

Compression molding - 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 strength - Resistance to a crushing or buckling force, the maximum compressive load a specimen sustains divided by its original cross-sectional area.

Condensation - Polymerization reaction in which simple by-products (for example, water) are released.

Consolidation - Processing step that compresses fiber and matrix to 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 - Individual, small-diameter reinforcement that is flexible and indefinite in length.

Continuous roving - Large bundle of parallel filaments coated with sizing, gathered together into single or multiple strands, and wound into a cylindrical package. May be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding compounds (may also be used chopped).

Coordinate axes - See laminate coordinate axes.

Core - In sandwich construction, the central component to which inner and outer skins are attached; also refers to a section of a complex mold that forms undercut parts.

Core crush - Compression damage of the core.

Core depression - Gouge or indentation in the 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 of two core segments by bonding them together.

Cowoven fabric - Reinforcement fabric woven with two different types of fibers in individual yarns (for example, thermoplastic fibers woven side by side with carbon fibers).

Crazing - Region of ultrafine cracks that may develop on or under a resin surface.

Creep - Time-dependent dimensional change in a material under physical load.

Crimp - Degree of waviness of a fiber, which determines its capacity to cohere.

Critical length - Minimum length of a fiber necessary for matrix shear loading to develop ultimate fiber strength.

Cross-laminated - Laminated with some of the layers oriented at one or more angles to the other layers with respect to the principal laminate axis.

Crossply laminate - Laminate having plies oriented only at 0° and 90°. May or may not be symmetric.

Crosslinking - Polymerization reactions that branch out from the main molecular chain to form a networked pattern of chemical links.

Crystalline - Having a molecular structure in which the atoms are arranged in an orderly, three-dimensional pattern.

CTE - See coefficient of thermal expansion.

Cure - To change the physical properties of a material irreversibly by chemical reaction via heat and/or catalysts, with or without pressure.

Cure temperature - Temperature at which a material attains final cure.

Curing agent - Catalytic or reactive agent that brings about polymerization when added to a resin.

CVD - See chemical vapor deposition.



Damage tolerance - Measure of the ability of structures to retain load-carrying capability after exposure to sudden loads (for example, ballistic impact).

Damping - Diminishing the intensity of vibrations.

Debond - Deliberate separation of a bonded joint or interface, usually for repair or rework purposes. (See also disbond.)

Delamination - Separation of plies in a laminate due to adhesive failure. This may be local or may cover a large area. Also includes the separation of layers of fabric from the core structure.

Demold - To remove a part from a tool, or a tool from an intermediate model.

Denier - Numbering system for continuous yarn and continuous filaments in which the yarn number is equal to the weight in grams per 9,000 meters of yarn; the finer the yarn, the lower the denier.

Design allowable - Limiting value for a material property that can be used to design a structural or mechanical system to a specified level of success with a specific level of statistical confidence.

Dielectric - Nonconductor of electricity; the ability of a material to resist the flow of an electric current.

Disbond - Unplanned non-adhered or unbonded area within a bonded interface. Can be caused by adhesive or cohesive failure, may occur at any time during the life of the structure and may arise from a wide variety of causes. The term is also sometimes used to describe a delamination.

Doubler - Extra layers of reinforcement for added stiffness or strength in laminate areas that incur abrupt load transfers.

Drape - The ability of prepreg to conform to the shape of a contoured surface.

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



E-glass - Denotes "electrical glass," so called because of its high electrical resistivity. Refers to borosilicate glass fibers most often used in conventional polymer matrix composites.

Elasticity - The property of materials to recover immediately their original size and shape when load is removed after deformation.

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

End - General term for a continuous, ordered assembly of essentially parallel, collimated filaments, with or without twist.

Epoxy - Thermoset polymer containing one or more epoxide groups, curable by reaction with amines or other compounds.

Exotherm - Heat released during a chemical reaction. Uncontrolled exotherm can lead to extreme heat build up and possibly violent explosion.



Fabric - Planar textile. Also known as cloth.

Fabric, nonwoven - Planar textile constructed by bonding or interlocking, but not interlacing, by mechanical, chemical, thermal or solvent means.

Fabric, woven - Planar textile constructed by interlacing in a weaving process.

Fabrication - Process of making a composite part or tool.

Fatigue - Failure or deterioration of a material’s mechanical properties as a result of repeated cyclic loading or deformation over time.

Fatigue strength - Maximum cyclical stress withstood for a given number of cycles before a material fails. The residual strength after being subjected to fatigue loading.

FEA - Finite-element analysis.

Fiber - One or more filaments in an ordered assemblage.

Fiber architecture - Design of a fibrous preform or part in which the fibers are arranged (braided, stitched, woven, etc.) in a particular way to achieve the desired result.

Fiber content - Amount of fiber present in a composite expressed either as a percent by weight or percent by volume. Also sometimes stated as a fiber volume fraction.

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 greater strength in that direction.

Fiber placement - 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. Differs from filament winding in several ways: There is no limit on fiber angles; compaction takes place online via 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-reinforced plastics (FRP) - General term used for a polymer-matrix composite that is reinforced with cloth, mat, strands or any other fiber form. Often used to designate mid-range, glass-fiber reinforced composites.

Fiber volume fraction - See fiber content.

Filament - Polycrystalline or amorphous individual fiber unit with a length-to-diameter ratio greater than one. The minimum diameter of a filament is not limited, but the maximum diameter may not exceed 0.010 inches. Filaments greater than about 0.002 inches in diameter are often referred to as wires.

Filament count - Number of filaments in the cross-section of a fiber bundle.

Filament winding - Process of fabricating composites in which continuous reinforcing fibers, either preimpregnated with resin or drawn through a resin bath, are wound under controlled tension around a rotating form to make a structure. (See also winding, mandrel.)

Fill - Fiber bundles in a woven fabric that run transverse to the warp yarns; also known as weft or woof.

Filler - Solid constituent, usually inert, added to the matrix to modify the composite properties - such as increase viscosity, improve appearance or lower density - or to lower cost.

Filler ply - Additional patch to fill in a depression in repair or to build up an edge.

Film adhesive - 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 textiles to improve the bond between the fiber and matrix; applied after sizing is removed.

Finite element analysis - Process of selecting the optimum combination of materials in a composite based on computational modeling and analysis.

Flexural modulus - Ratio, within the elastic limit, of the applied stress on a test sample in flexure to the corresponding strain in the outermost fibers of the sample.

Flexural strength - Strength of a material in bending, usually expressed in force per unit area, as the stress of a bent test sample at the instant of failure.

Fracture - Rupture of the surface of a laminate due to external or internal forces; may or may not result in complete separation.

Fracture toughness - Measure of the damage tolerance of a material containing initial flaws or cracks.

FRP - Fiber-reinforced plastic.



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 transition - Reversible change in an amorphous polymer between a viscous condition and a hard, relatively brittle condition.

Glass-transition temperature (Tg) - Approximate temperature at which increased molecular mobility results in significant changes in properties of a cured resin. The measured value of Tg can vary, depending on the test method.

Graphitization - Process of pyrolysis at very high temperatures (up to 5,400°F) that converts carbon to its crystalline allotropic form.

Graphite fibers - Carbon fiber that has been graphitized by heating and stretching at temperatures above 3,000°F.



Hand layup - Fabrication method in which reinforcement layers, preimpregnated or coated afterwards, are placed in a mold by hand, prior to cure to the formed shape.

Hard tool - Tool made of metal or any "hard" material that is generally impervious to damage during normal use.

Hardener - Substance used to promote or control curing action by participating in and being consumed by the cure reaction. (See also catalyst.)

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 deflection occurs under specified temperature and stated load.

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

High-performance composites - Composites offering properties better than conventional structural metals, typically on a strength-to-weight or stiffness-to-weight basis. Such composites use continuous, oriented fibers in polymer, metal or ceramic matrices to achieve their superior properties.

Honeycomb - Resin-impregnated material, most commonly manufactured in hexagonal cells, that serves as a core in sandwich structure. May also be a metal or a polymer in rigid, open-cell structure.

Hoop - Ply laid onto a mandrel at a 90° angle.

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

Hot-bond repair - Repair made on a hot-patch bonding machine to cure and monitor curing. Typically includes heat and vacuum source.

Hybrid composite - Composite containing at least two distinct types of matrix or reinforcement. The matrix or reinforcement types can be distinct because of their physical properties, mechanical properties, material form and/or chemical composition.



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 - Physical and mechanical discontinuity occurring within a material or part.

Integral heating - System in which heating elements are built into a tool, forming part of the tool and usually eliminating the need for an oven or autoclave as a heat source.

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 in a laminate.

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

Intralaminar - Existing or occurring within a single lamina in a laminate.

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





Kevlar - Trademark of DuPont for high-performance para-aramid fibers used as reinforcements.

Knit - Textile process that interlocks, in a specific pattern, loops of yarn by means of needles or wires.



Lamina - Subunit of a laminate consisting of one or more adjacent plies of the same material with identical orientation.

Lamina orientation - See ply orientation.

Laminate - Any fiber- or fabric-reinforced composite consisting of laminae with one or more orientations with respect to some reference direction.

Laminate coordinate axes - Set of coordinate axes, usually right-handed Cartesian, used as a reference in describing the directional properties and geometrical structure of the laminate. Usually the x-axis and the y-axis lie in the plane of the laminate and the x-axis is the reference axis from which ply angle is measured. The x-axis is often in the principal load direction of the laminate and/or in the direction of the laminate principal axis. (See also principal axis, off-axis laminate, x-axis.)

Layup - Process of placing layers of reinforcing material placed in position in the mold. The reinforcing materials placed in the mold.

Layup code - Designation system for abbreviating the stacking sequence of laminated composites.

Liquid-crystal polymers (LCP) - High-performance melt-processible thermoplastics that develop high orientation in the melt and after molding, resulting in very high tensile strength and high-temperature capability.

Lot - See batch.



Mandrel - A form, fixture or male mold used as the base for production of a part in processes such as lay-up or filament winding.

Mat - An unwoven textile fabric made of fibrous reinforcing material such as chopped filaments (to produce chopped-strand mat) or swirled filaments (to produce continuous-strand mat) with a binder applied to maintain form. Available in blankets of various widths, weights, thicknesses and lengths. May be oriented.

Matrix - Material in which reinforcing fiber of a composite is imbedded: polymer, metal or ceramic.

Matrix content - Amount of matrix present in a composite expressed either as a percent by weight or percent by volume. For polymer-matrix composites this is the resin content. (See also fiber content.)

Metal-matrix composites (MMC) - Continuous carbon, silicon carbide, or ceramic fibers embedded in a metallic matrix material.

Midplane - Plane that is equidistant from both surfaces of the laminate.

Microcracking - Microscopic cracks formed in composites when thermal stresses locally exceed the strength of the matrix.

MMC - Metal-matrix composite.

Modulus - Measure of the ratio of applied load (stress) to the resultant deformation of a material. May be represented by a number or in descriptive terms as low, intermediate, high or ultrahigh. (See also stiffness, Young’s modulus.)

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


Monomer - A single molecule that reacts with like or unlike molecules to form a polymer.

Monofilament - Single continuous filament strong enough to function as a fiber in textile or other operations.

Multifilament - Yarn or tow consisting of many continuous filaments.



NDE, NDI, NDT - Non-destructive evaluation, non-destructive inspection, non-destructive testing.

Near-net shape - Part fabrication resulting in final dimensions that require minimal machining, cutting or other finishing.

Net shape - Part fabrication resulting in final dimensions that do not require machining or cutting.

Nomex - Trademark of DuPont for moderate-performance meta-aramid material that is often used in paper-form to make honeycomb core.

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

Nonwoven roving - Reinforcement composed of continuous rovings loosely gathered together.

Off-axis laminate - Laminate whose principal axis is oriented at an angle theta other than 0° or 90° with respect to a reference direction, usually related to the principal load or stress direction.



One-off - Fabrication process in which a single part is fabricated.

One-part resin system - Resin system (often used in resin transfer molding) in which the neat resin and catalyst are mixed together by the materials supplier as part of the resin production operation.

Original equipment manufacturer (OEM) - Companies that design and build products bearing their name, such as Boeing 777 aircraft or Prince tennis racquets.

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

Outgassing - Release of solvents and moisture from composite parts under the hard vacuum of space.


PAN - Same as polyacrylonitrile.

Part consolidation - Process of composites fabrication in which multiple discrete parts are designed and fabricated together into a single part, thus reducing the number of fabricated parts and the need to join those parts together.

Peel ply - Layer of material applied to a prepreg layup 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 a condensation reaction of an aromatic alcohol with an aldehyde (usually phenol with formaldehyde).

Pin holes - Small holes that penetrate the surface of a cured part.

Pitch - Residual petroleum product used as a precursor in the manufacture of certain carbon fibers.

Planar winding - Filament winding in which the filament path lays on a plane that intersects the winding surface.

Plied yarn - Two or more yarns collected together with or without twist.

Ply - Constituent single layer used in fabricating or occurring within a composite structure. Also, the number of single yarns twisted together to form a plied yarn.

Ply orientation - Acute angle (theta) - including 90° - between a reference direction and the ply principal axis. The ply orientation is positive if measured counterclockwise from the reference direction and negative if measured clockwise.

Ply schedule - Layup of individual plies or layers to form a laminate. Plies may be arranged in alternating fiber orientation to produce multidirectional strength in a part.

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.

Polyimide - Highly heat-resistant polymer resin.

Polymer - Large organic molecule formed by combining many smaller molecules (monomers) in a regular pattern.

Polymerization - Chemical reaction that links monomers to form polymers.

Porosity - Presence of visible voids within a solid material into which either air or liquids may pass.

Postcure - Additional exposure to elevated temperature, often occurring without tooling or pressure, that improves mechanical properties.

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

Precure - Full or partial setting of a resin or adhesive before the clamping operation is complete or before pressure is applied.

Precursor - Material from which carbon fiber is made by pyrolysis. Common precursors are polyacrylonitrile (PAN), rayon and pitch.

Preform - Pre-shaped fibrous reinforcement, normally without matrix, but often containing a binder to facilitate manufacture; formed by distribution of fibers to the approximate contour and thickness of the finished part, typically on a mandrel or mock-up.

Prepreg - Admixture of fibrous reinforcement and polymeric matrix used to fabricate composite materials in a form that can be stored for later use. It may be sheet, tape, tow or fabric. For thermosetting matrices the resin is usually partially cured or otherwise brought to a controlled viscosity, called B-stage. Additives such as catalysts, inhibitors and flame retardants can be added to obtain specific end-use properties and improve processing, storage and handling characteristics.

Primary structure - An aerospace critical load-bearing structure; if damaged the air- or spacecraft cannot fly.

Prime contractors - Referred to as "primes"; companies that are awarded government contracts and usually work with subcontractors (or "subs") who provide individual and specific components or systems relevant to the contract. Primes often team on contracts, sharing portions of the contract funding.

Principal axis - Laminate coordinate axis that coincides with the direction of maximum inplane Young’s modulus. Within a ply, for a balanced weave fabric either warp or fill direction may be chosen. (See also laminate coordinate axes and x-axis.)

Prototype - Process of creating a test part not intended for commercial release that establishes design, material and fabrication parameters for a new product. May entail multiple iterations to arrive at final/commercial part design.

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

Pultrusion - Continuous process for manufacturing composites in rods, tubes and structural shapes having constant cross sections. After the reinforcement is passed through the resin-impregnation bath, it is drawn through a shaping die to form the desired cross section; curing takes place before the laminate can depart from that cross section.

Puncture - Break in composite skin in sandwich structure that may or may not go through to the core material or completely through the part.

Quadraxial fabric - Fabric with four non-interwoven layers - +45°, -45°, 0° and 90°, - which are bonded together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also biannual fabric, triaxial fabric.)



Quasi-isotropic laminate - A laminate approximating isotropy by orientation of plies in several or more directions.



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

Rate tools - Tools designed to be used repeatedly in a production setting to fabricate many parts rather than a single prototype or small number of demonstration parts.

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

Release agent - Used to prevent cured matrix material from bonding to tooling; usually sprayed or painted on mold.

Release film - Impermeable film layer that does not bond to the composite during cure.


Resin - Solid polymeric material, often of high molecular weight, which exhibits a tendency to flow when subjected to stress, usually has a softening or melting range, and usually fractures conchoidally. As composite matrices, resins bind together reinforcement fibers.

Resin content - See matrix content.

Resin-rich - Filled with excess resin and thus departing from a consistent resin/fiber ratio.

Resin-starved - Lacking sufficient resin for fiber wetout.

Resin transfer molding (RTM) - Molding process in which catalyzed resin is transferred into an enclosed mold into which a fibrous reinforcement has been placed. The mold and/or resin may or may not be heated. RTM combines relatively low tooling and equipment costs with the ability to consolidate large structural parts.

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

Roving - Large filament-count tow.

RTM - See resin-transfer molding.


S-glass - Denotes "structural glass" a magnesia/ alumina/silicate glass reinforcement designed to provide very high tensile strength. Used in high-performance composites.

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

Scrim - Low-cost, woven reinforcing fabric in an open mesh construction.

Sealant - Paste or liquid applied to a joint that hardens in place to form a seal.

Secondary structure - Aerospace structure that is not critical to flight safety.

Shear - 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. (See also storage life.)

Silicon carbide fiber - Reinforcing fiber with high strength and modulus; density is equal to that of aluminum. May be formed as wires by chemical vapor deposition onto a carbon-filament core, or as filaments. Used in both organic and metal-matrix composites.

Size - Material applied to textiles to facilitate subsequent operations such as weaving or braiding. Sizes may be used to bind together and stiffen warp yarns during weaving and/or to minimize abrasion and wear. Sizes are usually removed and replaced with finish before matrix application. Also called sizing.

Skin - Layer of relatively dense material used on the surface of the core of a sandwich structure.

Soft tool - Tool made of composites or a similar "soft" material that is vulnerable to damage during use, storage or transportation.

Solvent - Liquid used to dissolve and clean


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

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

Stacking sequence - Arrangement of ply orientations and material components in a laminate specified with respect to some reference direction.

Staple - Collection of short filaments of spinnable length.

Stiffness - Measure of the resistance of a material to deformation. The ratio of applied stress to resulting strain for a particular material.

Storage life - Amount of time a material can be stored and retain specific properties. (See also shelf life.)

Strain - Deformation resulting from applied stress. Measured as the change in length per unit of length in a given direction, and expressed in percentage or as inches per inch.

Strand - See tow.

Stress - Internal resistance to change in size or shape, expressed in units of force (load) per unit area.

Stress concentration - Magnification of applied stress in the region of a notch, void, hole or inclusion.

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

Structural adhesive - Adhesive used to transfer loads between adherents.

Structural bond - Bond joining load-bearing components of an assembly.

Structural repair manual (SRM) - Document prepared by an OEM that designates original structural materials (both composite and metal) used for a specific aircraft. It usually includes schematics for all parts and listings of fastener types and adhesives. It also suggests general repair methodology so that structural integrity can be maintained, including whether autoclave cure is required. Updated periodically by OEMs based on input from repair technicians.

Substrate - Material upon the surface of which an adhesive-containing substance is spread for any purpose, such as bonding or coating.

Symmetric laminate - Laminate in which the stacking sequence for the plies located on one side of the geometric midplane are the mirror image of the stacking sequence on the other side of the midplane.



Tack - Stickiness of an uncured prepreg.

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

Tensile strength - Maximum tensile stress sustained by a specimen before it fails in a tension test.

Tg - Glass-transition temperature.

Thermal conductivity - Ability to conduct heat.

Thermal stress cracking - Crazing and cracking of some thermoplastic resins from overexposure to elevated temperatures.

Thermocouple - Wire assembly used with a control device to sense temperature readings.

Thermoplastic - Class of plastics that can be repeatedly softened by heating and hardened by cooling through a temperature range characteristic of the plastic, and that in the softened state can be shaped by flow into articles by molding or extrusion.

Thermoset - Class of plastics that, when cured using heat, chemical or other means, changes into a substantially infusible and insoluble material. Once cured, a thermoset cannot be returned to the uncured state.

Thixotropic - Substance that is gel-like at rest, but fluid when agitated, and thus can be applied easily but clings to a vertical surface. Thixotropic substances have high static shear strength and low dynamic shear strength at the same time, and lose viscosity under stress.

Tool - The mold, either one- or two-sided and either open or closed, in or upon which composite material is placed in order to make a part.

Tooling resins - Plastic resins, chiefly epoxy and silicone, that are used as tooling aids.

Toughness - Measure of the ability of a material to absorb energy.

Tow - Continuous, ordered assembly of essentially parallel, collimated filaments, normally continuous filaments without twist. Same as strand.

Tow size - Designation indicating the number of filaments in a tow, usually a number followed by K, indicating multiplication by 1,000 (for example, 12K tow has 12,000 filaments).

Triaxial fabric - Fabric with three non-interwoven layers - +45°, - 45° and either 0° or 90° - which are bonded together, usually by through-the-thickness stitching, to form a single sheet of fabric. (See also biaxial fabric, quadraxial fabric.)

Twist - Measure of the number of turns per unit length that a fiber bundle makes around its axis. "Z"-twist denotes a right-handed twist, while "S"-twist denotes a left-handed twist. "U" is often used to represent no twist and "N" means never twisted.



Unidirectional (UD) - Referring to 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.

Viscosity - Tendency of a material to resist flow. Viscosity is measured in comparison with water. The higher the number, the less flow.

Void - Any pockets of enclosed gas or air within a composite.

Volatiles - Materials, such as water and alcohol, in a sizing or resin formulation that can be vaporized at room or slightly elevated temperatures.



Warp - Fiber bundles in a woven fabric that run parallel to the length of the loom, lengthwise along the long-dimension of the 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.

Water jet - High-pressure water stream used for cutting polymer composite parts.

Weave - Fabric pattern formed from interlacing yarns. In plain weave, warp and fill fibers alternate to make both fabric faces identical. A satin weave pattern is produced by a warp tow over several fill tows and under one fill tow (for example, eight-harness satin would have one warp tow over seven fill tows and under the eighth).


Wet layup - Application of a resin to a dry reinforcement in the mold.

Wet winding - Filament winding wherein fiber strands are impregnated with resin immediately before contact with the mandrel.

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

Wetting agent - Surface-active agent that promotes wetting by decreasing the cohesion within a liquid.

Wind angle - Measure in degrees between the direction parallel to the filaments and an established reference point.

Winding - Process in which continuous material is applied under controlled tension to a form in a predetermined geometric relationship to make a structure. A matrix material to bind the fibers together may be added before, during or after winding. Filament winding is the most common type.

Winding pattern - In filament winding, recurring pattern of the filament path after a certain number of mandrel revolutions.

Wire - Large diameter (greater than about 2 mils) high-performance fiber, such as boron or silicon carbide, usually made by chemical vapor deposition onto a filamentary substrate.

Wire mesh - Fine wire screen used to dissipate the electrical charge from lighting.

Woof - Same as fill.

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

Wrinkle - Imperfection in the surface of a laminate that looks like a crease in one of the outer layers. This occurs in vacuum-bag molding when the bag is improperly placed.



X-axis - Usually, the axis in the plane of the laminate used as 0° reference. Typically, the y-axis is the axis in the plane of the laminate perpendicular to the x-axis, and the z-axis is the reference axis normal to the laminate plane in the composite laminate. (See also laminate coordinate axes, off-axis laminate and principal axis.)



Y-axis - See x-axis.

Yarn - Continuous, ordered assembly of essentially parallel, collimated filaments, usually with a twist.

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.

Zero bleed - Laminate fabrication procedure that does not allow loss of resin during cure.

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