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A-BASIS (OR A-VALUE) - The mechanical property value above which 99 percent of the population of values is expected to fall with a confidence of 95 percent.

A-STAGE - An early stage in the reaction of thermosetting resins in which the material is still soluble in certain liquids and may be liquid or capable of becoming liquid upon heating.

ABSORPTION - A process in which one material takes in or absorbs another.

ADDITIVE - An ingredient blended into a resin to enhance or impart additional physical properties. Common filler additives include fire retardants, pigments, and ultraviolet inhibitors, among many others.

ADHESION - The state in which two surfaces are held together at an interface by forces or interlocking action or both.

ADHESIVE - A substance capable of holding two materials together by surface attachment. In the handbook, the term is used specifically to designate structural adhesives, those which produce attachments capable of transmitting significant structural loads.

AGING - The effect on materials of exposure to an environment for a period of time; the process of exposing materials to an environment for an interval of time.

AMBIENT - The surrounding environmental conditions such as pressure or temperature.

ANELASTICITY - A characteristic exhibited by certain materials in which strain is a function of both stress and time, such that while no permanent deformations are involved, a finite time is required to establish equilibrium between stress and strain in both the loading and unloading directions.


ANISOTROPIC - Not isotropic; having mechanical and/or physical properties which vary with direction relative to natural reference axes inherent in the material.

ARAMID - A type of highly oriented organic material derived from polyamide (nylon) but incorporating aromatic ring structure.

AREAL WEIGHT OF FIBER -- The weight of fiber per unit area of prepreg. This is often expressed as grams per square meter.

ASPECT RATIO -- In an essentially two-dimensional rectangular structure (e.g., a panel), the ratio of the long dimension to the short dimension. However, in compression loading, it is sometimes considered to be the ratio of the load direction dimension to the transverse dimension. Also, in fiber micro-mechanics, it is referred to as the ratio of length to diameter.

AUTOCLAVE - A closed vessel which applies heat and pressure to objects inside, such as a bagged laminate. The pressurizing medium is a gas, usually nitrogen or carbon dioxide.


B-BASIS (OR B-VALUE) - The mechanical property value above which at least 90 percent of the population of values is expected to fall with a confidence of 95 percent.

B-STAGE - An intermediate stage in the reaction of a thermosetting resin; that is, partial cure.

BAGGING - The process of applying an impermeable layer of film over a part and sealing the edges so that a vacuum can be drawn. The bag permits a pressure differential to exist between the pressurizing medium (usually the working fluid of the autoclave or hydroclave) and the part, thereby applying pressure to the part.

BALANCED LAMINATE - A composite laminate in which all laminae at angles other than 0° and 90° occur only in ± pairs (not necessarily adjacent).

BATCH (OR LOT) - In general, a quantity of material formed during the same process and having identical characteristics throughout. A batch of prepreg is defined as a quantity which is produced from a single batch of matrix material and fiber. The prepreg batch is produced at one time in the same equipment under identical conditions.

BEARING AREA - The product of the pin diameter and the specimen thickness.

BEARING LOAD - A compressive load on an interface.

BEARING YIELD STRENGTH - The bearing stress at which a material exhibits a specified limiting deviation from a linear stress-strain relationship.

BLEEDER CLOTH - Material, such as fiberglass, 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 and is not part of the final composite.

BOND - The adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.

BRAIDING - Weaving of fibers into 3-dimensional shapes instead of flat tape or fabric.

BREATHER CLOTH - A layer or layers of open weave cloth used to enable the vacuum to reach the area over the laminate being cured, such that volatiles and air can be uniformly removed. The uniform application of vacuum is required to evenly apply pressure over the surface of the laminate.

BRIDGING - Separation of fiber layers in an inside radius of an angle. Special techniques must be used so that the fibers will move into radii and corners; otherwise, they "bridge" the gap, resulting in dimensional control problems and voids. Care must also be taken to prevent bridging of separators, bleeders, perforated films, venting layers and bagging.

BROADGOODS - A term loosely applied to prepreg material greater than about 12 inches in width, usually furnished by suppliers in continuous rolls. The term is currently used to designate both collimated uniaxial tape and woven fabric prepregs.

BUCKLING (COMPOSITE) - A mode of structural response characterized by an out-of-plane material deflection due to compressive or shear load on the structural element involved. Buckling may take the form not only of conventional general instability and local instability but also a micro-instability of individual fibers.


C-SCAN - The record of the through transmission ultrasonic inspection, a nondestructive inspection (NDI) technique for finding voids, delaminations, defects in fiber distribution, etc.

CARBON FIBERS - Fibers produced by the pyrolysis of organic precursor fibers such as rayon, polyacrylonitrile (PAN), or pitch in an inert atmosphere. The term is often used interchangeably with "graphite"; however, carbon fibers and graphite fibers differ in the temperature at which the fibers are made and heat-treated, and the amount of carbon produced. Carbon fibers typically are carbonized at about 2400°F (1300-C°) and assay at 93 to 95% carbon, while graphite fibers are graphitized at 3450°F to 5450°F (1900 to 3000°C) and assay at more than 99% elemental carbon.

CATALYST - A chemical which promotes a chemical reaction without becoming a part of the molecular structure of the product. In resin systems, catalysts and accelerators lower the temperature at which significant amounts of reaction occur, affecting reaction rate and changing the characteristics of the cure cycle.

CAUL PLATES - Smooth plates, free of surface defects, used during the curing process to transmit normal pressure and/or to provide a controlled surface on the finished laminate.

CERAMIC TOOLING - Use of a castable ceramic to make a tool shape. Ceramic tooling is seldom used unless a very large number of complex parts are to be made; otherwise, tooling such as graphite tooling is more cost effective.

CLOTH - A woven product made from continuous yarns or tows of fiber. "Cloth" and "fabric" are usually used interchangeably.

COCURING - The act of curing a composite laminate and simultaneously bonding it to some other prepared surface during the same cure cycle.

COEFFICIENT OF LINEAR THERMAL EXPANSION - The change in length per unit length resulting from a one-degree rise in temperature.

COMPOSITE - A matrix material reinforced with continuous filaments. The constituents retain their identities in the composite; they do not dissolve or merge completely into each other although they act in concert.

COMPOSITE SHEET - A sheet material comprised of a fiber reinforced composite core sandwiched on either side by sheet metal. Typical sheet metals used in composite panels include aluminum, steel, and stainless steel.

COMPOUND - An intimate mixture of polymer or polymers with all the materials necessary for the finished product.

COMPRESSION MOLDING - Putting a reinforced resin into a mold cavity, closing the mold, and applying pressure and heat in order to force the material to completely fill the mold cavity and to cure the material.

CONSOLIDATION - In metal matrix or thermoplastic composites, the diffusion bonding operation in which an oriented stack of plies is transformed under heat and pressure into a finished composite laminate.

CONSTITUENT - In general, an element of a larger grouping. In advanced composites, the principal constituents are the fibers and the matrix.

CONTINUOUS FILAMENT - A yarn or strand in which the individual filaments are substantially the same length as the strand.

CORROSION BARRIER - A laminate applied to a substrate to enhance its corrosion resistance properties. In FRP products, corrosion barriers typically consist of a resin-rich layer that improves resistance to corrosion, and often chemicals and abrasion, as well.

COUPLING AGENT - Any chemical substance designed to react with both the reinforcement and matrix phases of a composite material to form or promote a stronger bond at the interface. Coupling agents are applied to the reinforcement phase from an aqueous or organic solution or from a gas phase, or added to the matrix as an integral blend.

CRAZING - The development of a multitude of very fine cracks in the matrix material. CREEP -- The time dependent part of strain resulting from an applied stress.

CROSSLINKING - Chemical reaction between molecules resulting in the formation of a three-dimensional network of molecules. Crosslinking requires that at least one of the molecules involved in the reaction have three or more reactive groups; otherwise, the reaction only results in forming a longer molecule (chain extension).

CROSSPLY - Any filamentary laminate which is not uniaxial. In some references, the term crossply is used to designate only those laminates in which the laminae are at right angles to one another while the term angleply is used for all other layup combinations. In this manual, the terms crossply and angleply are used synonymously.

CRYSTALLINITY - Polymers, such as nylon, form localized areas of crystallinity (highly ordered sections) formed by alignment of sections of a polymer chain (by folding, etc.) or of adjacent molecules. The localized areas of crystallinity change the physical behavior of the polymer.

CURE - To change the properties of a thermosetting resin irreversibly by chemical reaction. Cure may be accomplished by addition of curing agents, with or without catalyst, and with or without heat and pressure.

CURE CYCLE - The time/temperature/pressure cycle used to cure a thermosetting resin system or prepreg.

CURE STRESS - A residual internal stress produced during the curing cycle of composite structures. Normally, these stresses originate when different components of a lay-up have different thermal coefficients of expansion. DAM - Boundary support used to prevent excessive edge bleeding of a laminate and to prevent crowning of the bag.


DEBOND - A deliberate separation of a bonded joint or interface, usually for repair or rework purposes. (see Disbond, Unbond).

DEBULKING - Using pressure and/or vacuum to remove the air gaps and resin pockets in the prepreg during laminate fabrication. This technique is used to prevent thickness tolerances of each ply from building up.

DEFORMATION - The change in shape of a specimen caused by the application of a load or force.

DEGRADATION - A deleterious change in chemical structure, physical properties or appearance.

DELAMINATION - The separation of the layers of material in a laminate. This may be local or may cover a large area of the laminate. It may occur at any time in the cure or subsequent life of the laminate and may arise from a wide variety of causes.

DENIER - A textile term for the weight, in grams, of 9000 meters of fiber tow.

DENSITY - The mass per unit volume.

DESORPTION - A process in which an absorbed or adsorbed material is released from another material. Desorption is the reverse of absorption, adsorption, or both.

DIELECTRIC CONSTANT - The ratio of the capacity of a condenser having a dielectric constant between the plates to that of the same condenser when the dielectric is replaced by vacuum; a measure of the electrical charge stored per unit volume at unit potential.

DIELECTRIC STRENGTH - The average potential per unit thickness at which failure of the dielectric material occurs.

DIELECTROMETRY - Use of electrical techniques to measure the changes in loss factor (dissipation) and in capacitance during cure of the resin in a laminate.

DISBOND - A lack of proper adhesion in a bonded joint. This may be local or may cover a majority of the bond area. It may occur at any time in the cure or subsequent life of the bond area and may arise from a wide variety of causes.

DISTORTION - In fabric, the displacement of fill fiber from the right angle it should be at, relative to the warp fiber. In a laminate, the displacement of the fibers (especially in radii), relative to their idealized location, because of motion during lay-up and cure.

DOUBLE TEE BEAM - A load-bearing structural component that resembles two T-beams, side by side. A double tee beam consists of two vertical legs and a horizontal flange along the deck. The flange and legs are integrally molded for to create an exceptionally reliable structure capable of supporting heavy loads.

DRAPE - The ability of a prepreg to conform to a contoured surface. If the resin becomes hard because of loss of solvent or staging, the prepreg becomes stiff and loses its drape characteristics.

DRY FIBER AREA - Area of fiber not totally encapsulated by resin.

DUCTILITY - The ability of a material to deform plastically before fracturing.


EDGE BLEED - Removal of volatiles and excess resin through the edge of the laminate, as in matched die molding of a laminate. In autoclaved parts, edge bleeding is not recommended since excess resin will only be removed from the area near an edge, resulting in uneven resin distribution.

ELASTICITY - The property of a material which allows it to recover its original size and shape immediately after removal of the force causing deformation.

ELASTOMERIC TOOLING - A tooling system utilizing the thermal expansion of rubber materials to form composite hardware during cure.

ELONGATION - The increase in gage length or extension of a specimen during a tension test, usually expressed as a percentage of the original gage length.

END - A single fiber, strand, roving or yarn incorporated into a product. An end may be an individual wrap yarn or cord in a woven fabric. In referring to aramid and glass fibers, an end is usually an untwisted bundle of continuous filaments.

EPOXY RESIN - Resins which may be of widely different structures but are characterized by the presence of the epoxy group. (The epoxy or epoxide group is usually present as a glycidyl ether, glycidyl amine, or as part of an aliphatic ring system. The aromatic  epoxy resins are normally used in composites.)

EXTENSOMETER - A device for measuring linear strain.


FABRIC - A material constructed of interlaced yarns, fibers or filaments. Used interchangeably with "cloth".

FABRICATION - (1) Secondary processes performed on FRP sheets/panels. These additional operations may include cutting to size, drilling holes, assembly, and more. (2) The process of mixing plastic resin with thin glass fibers and additives to create fiber reinforced polymer.

FIBER - A single homogeneous strand of material, essentially one-dimensional in the macrobehavioral sense, used as a principal constituent in composites because of its high axial strength and modulus.

FIBER CONTENT - The amount of fiber present in a composite. This is usually expressed as a percentage volume fraction or weight fraction of a cured composite.

FIBER DIRECTION - The orientation or alignment of the longitudinal axis of the fiber with respect to a stated reference axis.

FIBERGLASS - The generic name for glass fibers and for composites using glass fibers for reinforcement.

FIBER REINFORCED PLASTIC - See “Fiber Reinforced Polymer.”

FIBER REINFORCED POLYMER - An engineered material consisting of reinforcement fibers, polymer resin, and additives to achieve targeted performance properties. This combination creates an extremely strong and durable material that can be used for applications ranging from equipment parts to large infrastructure components.

FIBER SYSTEM - The type and arrangement of fibrous material which comprises the fiber constituent of an advanced composite. Examples of fiber systems are collimated filaments or filament yarns, woven fabric, randomly oriented short-fiber ribbons, random fiber mats, whiskers, etc.

FIBER TOW - A loose, untwisted bundle of continuous fibers. In composite technology, "tow" is often used interchangeably with "yarn", the twisted version.

FIBER VOLUME - The volume percent of fiber in a composite.

FIBERGLASS - Created by spinning glass as it melts to turn it into glass fibers. Though sometimes used on its own as a standalone material, fiberglass is a critical component of Fiber Reinforced Polymers.

FILAMENT - Fibers characterized by extreme length, such that there are normally no filament ends within a part except at geometric discontinuities. Filaments are used in filamentary composites and are also used in filament winding processes which require long continuous strands.

FILAMENT COMPOSITES - A major form of advanced composites in which the fiber constituent consists of continuous filaments. Specifically, a filamentary composite is a laminate comprised of a number of laminae, each of which consists of a nonwoven, parallel, uniaxial, planar array of filaments ( or filament yarn) embedded in the selected matrix material. Individual laminae are directionally oriented and combined into specific multi-axial laminates for application to specific envelopes of strength and stiffness requirements.

FILAMENT WINDING - An automated process in which continuous filament (or tape) is treated with resin and wound on a removable mandrel in a prescribed pattern.

FILAMENT WOUND - Pertaining to an object created by the filament winding method of fabrication.

FILL - Yarn oriented at right angles to the warp in a woven fabric.

FINISH (OR SIZE SYSTEM) - A material, with which filaments are treated, which contains a coupling agent to improve the bond between the filament surface and the resin matrix in a composite material. In addition, finishes often contain ingredients which provide lubricity to the filament surface, preventing abrasive damage during handling, and a binder which promotes strand integrity and facilitates packing of the filaments.

FLAME-SPRAYED TAPE - A form of metal matrix preply in which the fiber system is held in place on a foil sheet of matrix alloy by a metallic flamespray deposit. Each flame-sprayed preply is usually combined in the layup stack with a metal cover foil and/or additional metal powder to ensure complete encapsulation of the fibers. During consolidation, all the metallic constituents are coalesced into a homogeneous matrix.

FLASH - Excess material which forms at the parting line of a mold or die, or which is extruded from a closed mold.

FRACTURE DUCTILITY - The true plastic strain at fracture.

FRP PANELS - Panels used to build lightweight structures, such as walls and containers. Specialized combinations of fiber and polymers can create ballistic panels.


GAGE LENGTH - The original length of that portion of the specimen over which strain or change of length is determined.

GEL - The initial jelly-like solid phase that develops during formation of a resin from a liquid. Also, a semi-solid system consisting of a network of solid aggregates in which liquid is held.

GELCOAT - A resin applied to the mold to provide an improved surface for the composite.

GEL POINT - The stage at which a liquid begins to exhibit pseudo-elastic properties. (This can be seen from the inflection point on a viscosity-time plot.)

GEL TIME - The period of time from a pre-determined starting point to the onset of gelation (gel point) as defined by a specific test method.

GLASS - An inorganic product of fusion which has cooled to a rigid condition without crystallizing. In the handbook, all reference to glass will be to the fibrous form as used in filaments, woven fabric, yarns, mats, chopped fibers, etc.

GLASS CLOTH - Conventionally-woven glass fiber material (see Scrim).

GLASS FIBERS - A fiber spun from an inorganic product of fusion which has cooled to a rigid condition without crystallizing.

GLASS TRANSITION - The reversible change in an amorphous polymer or in amorphous regions of a partially crystalline polymer from (or to) a viscous or rubber condition to (or from) a hard and relatively brittle one.

GLASS TRANSITION TEMPERATURE (Tg) - One method of describing the temperature at which increased molecular mobility results in significant changes in the properties of a cured resin system. The glass transition temperature (Tg) can be defined as the inflection point on a plot of modulus vs. temperature. Another definition is the intersection point of the coefficient of thermal expansion slopes below and above Tg (i.e., in the "glassy" region and in the " rubbery/leathery" region). Tg is defined as the inflection point; properties can decrease significantly before Tg is reached.

GRAPHITE - The crystalline, allotropic form of carbon. In bulk form, used for advanced composite tooling and for such items as the lead in pencils. See GRAPHITE FIBERS.

GRAPHITE FIBERS - Technically, a highly oriented form of graphite. In common usage, however, it also includes highly oriented carbon fibers which have only a small amount of graphite content.

GRAPHITE FIBERS - See Carbon Fibers.

GRAPHITIZATION - Conversion of carbon to its crystalline allotropic form by use of very high temperatures (2500 - 4500° F). Diamond is also a crystallizing allotropic form of carbon, but requires extremely high pressures (over one million psi) in addition to very high temperatures in order to be formed.


HAND LAY-UP - A process in which components are applied either to the mold or on a working surface and the successive plies are built up and worked by hand.

HARDENER - The component which reacts with a resin to form the crosslinked (thermoset) plastic.

HARDNESS - Resistance to deformation; usually measured by indention. Types of standard tests include Brinell, Rockwell, Knoop, and Vickers.

HARNESS SATIN - Describes a set of weaving patterns which produce a fabric having a satin appearance. "8HS" describes a harness satin weave where the warp fiber tows go over seven fill tows and then under one fill tow, for a repeating total of 8. By itself, "8HS" is not a complete description, because there are many possible patterns of where the crossover points of adjacent tows are located.

HEAT DISTORTION TEMPERATURE (HDT) - A measure of the softening point of a material. For unreinforced materials, HDT correlates reasonably well with the glass transition temperature. The test consists of applying a load to a specimen in flexure and slowly increasing the temperature until the bar deflects 0.010 inch. HDT is normally reported for stress levels of 66 PSI and/or 264 PSI. Because the stress levels are so low, HDT is not a particularly useful number for continuously reinforced materials that will be used at high stress levels.

HETEROGENEOUS - Descriptive term for a material consisting of dissimilar constituents separately identifiable; a medium consisting of regions of unlike properties separated by internal boundaries. (Note that all nonhomogeneous materials are not necessarily heterogeneous).

HOMOGENEOUS - Descriptive term for a material of uniform composition throughout; a medium which has no internal physical boundaries; material whose properties are constant at every point, i.e.. constant with respect to spatial coordinates (but not necessarily with respect to directional coordinates).

HUMIDITY, RELATIVE - The ratio of the pressure of water vapor present to the pressure of saturated water vapor at the same temperature.

HYBRID - A composite laminate comprised of laminae of two or more composite material systems. Or, a combination of two or more different fibers such as carbon and glass or carbon and aramid into a structure (tapes, fabrics and other forms may be combined).

HYGROSCOPIC - Capable of absorbing and retaining atmospheric moisture.

HYSTERESIS - The energy absorbed in a complete cycle of loading and unloading.


INCLUSION - A physical and mechanical discontinuity occurring within a material or part, usually consisting of solid, encapsulated foreign material. Inclusions are often capable of transmitting some structural stresses and energy fields, but in a noticeably different manner from the parent material.

INTEGRAL COMPOSITE STRUCTURE - Composite structure in which several structural elements, which would conventionally be assembled by bonding or with mechanical fasteners after separate fabrication, are instead laid up and cured as a single, complex, continuous structure: e.g. spars, ribs, and the stiffened cover of a wing box fabricated as a single integral part. The term is sometimes applied more loosely to any composite structure not assembled by mechanical fasteners.

INTEGRALLY HEATED - Referring to tooling which is self-heating through use of electrical heaters, such as cal rods. Most hydroclave tooling is integrally heated; some autoclave tooling is integrally heated to compensate for thick sections, to provide higher heatup rates, or to permit processing at a higher temperature than the capability of the autoclave.

INTERFACE - The boundary between the individual, physically distinguishable constituents of a composite.

INTERLAMINAR - Descriptive term pertaining to the location of some object (e.g., voids), event (e.g., fracture), or potential field (e.g., shear stress) referenced as existing or occurring between two or more adjacent laminae.

INTERLAMINAR SHEAR - Shearing force tending to produce a relative displacement between two laminae in a laminate along the plane of their interface.

INTRALAMINAR - Descriptive term pertaining to the location of some object (e.g., voids), event (e.g., fracture), or potential field (e.g., shear stress) referenced as existing or occurring within a single lamina.

ISOTROPIC - Having uniform properties in all directions. The measured properties of an isotropic material are independent of the axis of testing.



KEVLAR - An organic polymer composed of aromatic polyamides having a parallel type orientation. (Parallel chain extending bonds from each aromatic nucleus).


LAMINA -- A single ply or layer in a laminate.

LAMINAE - Plural of lamina.

LAMINATE - A product made by bonding together two or more layers of laminae of material or materials.

LAMINATE ORIENTATION - The configuration of a crossplied composite laminate with regard to the angles of crossplying, the number of laminae at each angle, and the exact sequence of the lamina lay-up.

LAY-UP - A process of fabrication involving the assembly of successive layers of resin impregnated material.


MACRO -- In relation to composites, denotes the gross properties of a composite as a structural element but does not consider the individual properties or identity of the constituents.

MACROSTRAIN - The mean strain over any finite gage length of measurement which is large in comparison to the material's interatomic distance.

MANDREL - A form fixture or male mold used for the base in the production of a part by layup or filament winding.

MAT - A fibrous material consisting of randomly oriented chopped or swirled filaments loosely held together with a binder.

MATCHED DIE - A mold, in two or more pieces, which is capable of producing parts with two or more dimensionally controlled surfaces.

MATERIAL SYSTEM - A specific composite material made from specifically identified constituents in specific geometric proportions and arrangements and possessed of numerically defined properties.

MATRIX - The essentially homogeneous material in which the fiber system of a composite is embedded.

MECHANICAL PROPERTIES - The properties of a material that are associated with elastic and inelastic reaction when force is applied, or the properties involving the relationship between stress and strain.

MELTING RANGE - Thermoplastics whose makeup includes a distribution of molecular weights will not have a well defined melting point, but have a melting range.

MICRO - In relation to composites, denotes the properties of the constituents, i.e., matrix and reinforcement and interface only, as well as their effects on the composite properties.

MICROCRACKING - Microcracks are formed in composites when residual thermal stresses locally exceed the strength of the matrix. Since most microcracks do not penetrate the reinforcing fibers, microcracks in a crossplied tape laminate or in a laminate made from cloth prepreg are usually limited to the thickness of a single ply.

MICROSTRAIN - The strain over a gage length comparable to the material's interatomic distance.

MODULUS, INITIAL - The slope of the initial straight portion of a stress-strain or load-elongation curve.

MODULUS, SECANT - The ratio of change in stress to change in strain between two points on a stress-strain curve, particularly the points of zero stress and stress at a particular strain.

MODULUS, TANGENT - The ratio of change in stress to change in strain derived from the tangent to any point on a stress-strain curve.

MODULUS, YOUNG'S - The ratio of change in stress to change in strain below the elastic limit of a material. (Applicable to tension and compression).

MODULUS OF RIGIDITY (ALSO SHEAR MODULUS OR TORSIONAL MODULUS) - The ratio of stress to strain below the proportional limit for shear or torsional stress.

MOISTURE CONTENT - The amount of moisture in a material determined under prescribed conditions and expressed as a percentage of the mass of the moist specimen; i.e., the mass of the dry substance plus the moisture present.

MOISTURE EQUILIBRIUM - The condition reached by a sample when it no longer takes up moisture from, or gives up moisture to, the surrounding environment.

MOLD RELEASE AGENT - A lubricant applied to mold surfaces to facilitate release of the molded article.

MOLDED FIBERGLASS GRATING - Created by layering the resin and fiberglass in a mold and curing it. Molded grating is highly corrosion-resistant, making it popular for many marine applications. It is often used as an alternative to iron or steel grates in commercial kitchens, marine vessels, pool decks, outdoor platforms, and more.

MOLDING - The forming of a composite into a prescribed shape by the application of pressure during the cure cycle of the matrix.

MONOLAYER - The basic laminate unit from which crossplied or other laminates are constructed.


NDE - Nondestructive Evaluation. Broadly considered synonymous with NDI.

NDI - Nondestructive Inspection. A process or procedure for determining the quality or characteristics of a material, part, or assembly without permanently altering the subject or its properties.

NDT - Nondestructive Testing. Broadly considered synonymous with NDI.

NECKING - A localized reduction in cross-sectional area usually due to plastic deformation which may occur in a material under tensile stress.

NOMINAL SPECIMEN THICKNESS - The nominal ply thickness multiplied by the number of plies.

NORMALIZED STRESS - Stress calculated by multiplying the raw stress value by the ratio of measured fiber volume to the nominal fiber volume. This ratio is often approximated by the ratio of the measured specimen thickness to the nominal specimen thickness. Stresses for fiber-dominated failure modes are often normalized.


ORTHOTROPIC - Having three mutually perpendicular planes of elastic symmetry.

OVEN DRY - The condition of a material that has been heated under prescribed conditions of temperature and humidity until there is no further significant change in its mass.


PAN - Polyacrylonitrile, used in fiber form as a precursor for making carbon/graphite fibers.

PEEL PLY - A layer of open weave material, usually fiberglass or heat-set nylon, applied directly to the surface of a prepreg lay-up. The peel ply is removed from the cured laminate immediately before bonding operations, leaving a clean, resin rich surface which needs no further preparation for bonding, other than application of a primer where one is required.

PEEK - Short for polyetheretherketone. A semi-crystalline thermoplastic polymer used as a composite matrix material.

PERFORATED FILM - The layer of film used to permit removal of air and volatiles from a composite lay-up during cure while minimizing resin loss.

PHENOLIC - Any of several types of synthetic thermosetting resins obtained by the condensation of phenol or substituted phenols with aldehydes such as formaldehyde.

PICK COUNT - The number of filling yarns per inch of woven fabric.

PITCH FIBERS - Fibers derived from a special petroleum pitch.

PITCH - High molecular weight material left as a resin after processing of petroleum (crude oil). After further purification, can be processed into fiber form; useful as a precursor for production of carbon/graphite fibers.

PLAIN WEAVE - A weaving pattern where the warp and fill fibers alternate; i.e., the repeat pattern is warp/fill/warp. Both faces of a plain weave are identical. Properties are significantly reduced relative to a weaving pattern with fewer crossovers.

PLASTIC - A material that contains one or more organic polymers of large molecular weight, is solid in its finished state, and, at some state in its manufacture or processing into finished articles, can be shaped by flow.

PLY - A single layer of prepreg. Used synonymously with LAMINA.

POISSON'S RATIO - The absolute value of the ratio of transverse strain to corresponding axial strain resulting from uniformly distributed axial stress.

POLYMER - An organic material composed of long molecular chains consisting of repeating chemical units. Also see RESIN.

POROSITY - A condition of trapped pockets of air, gas, or voids within a cured laminate, usually expressed as a percentage of the total non-solid volume to the total volume (solid + non-solid) of a unit quantity of material. See VOID.

POSTCURE - Additional elevated temperature cure, usually without pressure, to improve final properties or complete the cure or both.

POT LIFE - The period of time during which a reacting thermosetting composition remains suitable for its intended processing after mixing with a reaction initiating agent.

PRECURSOR - In carbon/graphite fiber technology, the organic fiber which is the starting point for making carbon or graphite fibers. In resin technology, sometimes used to describe the polymers present at an intermediate stage in the formulation of a cured resin.

PREMOLDING - The layup and partial cure at an intermediate cure temperature of a laminated or chopped fiber detail part to stabilize its configuration for handling and assembly with other parts for final cure.

PREPREG - Ready to mold or cure material in sheet form which may be fiber, cloth, or mat impregnated with resin and stored for use. The resin is partially cured to a B-stage and supplied to the fabricator for lay-up and cure.

PRESS CLAVE - A simulated autoclave made by using the platens of a press to seal the ends of an open chamber, providing both the force required to prevent loss of the pressurizing medium and the heat to cure the laminate inside.

PRESSURE - The force or load per unit area

PRESSURE INTENSIFIER - A layer of flexible material (usually a high temperature rubber) used to assure that sufficient pressure is applied to a location, such as a radius, in a lay-up being cured.

PROPORTIONAL LIMIT - The maximum stress that a material is capable of sustaining without any deviation from the proportionality of stress to strain (also known as Hooke's law).

PULTRUDED FIBERGLASS GRATING - Created using profiles that are created through a pultrusion process. Pultruded profiles are then assembled into grates. The pultrusion process produces less waste than molding.

PULTRUSION - A process to continuously process structural shapes or flat sheet by drawing prepreg materials through forming dies to produce the desired constant cross sectional shape while simultaneously curing the resin.


QUASI-ISOTROPIC LAMINATE - A laminate approximating isotropy with equal amounts of plies oriented in several directions.


REDUCTION OF AREA - The difference between the original cross sectional area of a tension test specimen and the area of its smallest cross section, usually expressed as a percentage of the original area.

REINFORCED PLASTIC - A plastic with relatively high stiffness or very high strength fibers embedded in the composition. This improves some mechanical properties over that of the base resin.

REINFORCEMENT - Specialized fibers incorporated into FRP materials to impart additional characteristics. Typical reinforcement fibers include aramid, carbon, glass, polyester, and some natural fibers.

RELEASE AGENT - See Mold Release Agent.

RELEASE FILM - An impermeable layer of film which does not bond to the resin being cured. See SEPARATOR.

RESIN - A form of plastics/polymers commonly used in manufacturing. In FRP, resins provide the polymer component of Fiber Reinforced Polymer. The resin choice largely determines the properties of the FRP product. Most resins used in FRP are some type of thermoset.

RESIN CONTENT - The amount of matrix present in a composite either by percent weight or percent volume.

RESIN RICHNESS - An area of excess resin, usually occurring at radii, steps, and the chamfered edge of core.

RESIN STARVED - An area deficient in resin usually characterized by excess voids and/or loose fibers.

RESIN SYSTEM - A mixture of resin, with ingredients such as catalyst, initiator, diluents, etc. required for the intended processing and final product. S-BASIS (OR S-VALUE) -- The mechanical property value which is usually the specified minimum value of the appropriate government specification or SAE Aerospace Material Specification for this material.

ROVING - A number of strands, tows, or ends collected into a parallel bundle with little or no twist.

RUBBER - Crosslinked polymers whose glass transition temperature is below room temperature and which exhibit highly elastic deformation and have high elongation.


SANDWICH CONSTRUCTION - A structural panel consisting in its simplest form of two relatively thin, parallel sheets of structural material (face sheets) bonded to and separated by a relatively thick, lightweight core.

SATURATION -- An equilibrium condition in which the net rate of absorption under prescribed conditions falls essentially to zero.

SCRIM (ALSO CALLED GLASS CLOTH, CARRIER) - A low cost fabric woven into an open mesh construction, used in the processing of tape or other B-stage material to facilitate handling.

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

SELVAGE - The woven edge portion of a fabric parallel to the warp.

SEMI-CRYSTALLINE - In plastics, refers to materials which exhibit localized crystallinity. See CRYSTALLINITY.

SEPARATOR - A permeable layer which also acts as a release film. Porous Teflon-coated fiberglass is an example. Often placed between lay-up and bleeder to facilitate bleeder system removal from laminate after cure.

SET - The strain remaining after complete release of the force producing .The deformation.

SHEAR FRACTURE (FOR CRYSTALLINE TYPE MATERIALS) - A mode of fracture resulting from translation along slip planes which are preferentially oriented in the direction of the shearing stress.

SHELF LIFE - The length of time a material, substance, product, or reagent can be stored under specified environmental conditions and continue to meet all applicable specification requirements and/or remain suitable for its intended function.

SIZE SYSTEM - See Finish.

SLENDERNESS RATIO - The unsupported effective length of a uniform column divided by the least radius of gyration of the cross-sectional area.

SLIVER - A continuous strand of loosely assembled fiber that is approximately uniform in cross-sectional area and has no twist.

SPECIFIC GRAVITY - The ratio of the weight of any volume of a substance to the weight of an equal volume of another substance taken as standard at a constant or stated temperature. Solids and liquids are usually compared with water at 4°C (39°F).

SPECIMEN - A piece or portion of a sample or other material taken to be tested. Specimens normally are prepared to conform with the applicable test method.

STAGING - Heating a premixed resin system, such as in a prepreg, until the chemical reaction (curing) starts, but stopping the reaction before the gel point is reached. Staging is often used to reduce resin flow in subsequent press molding operation.

STOPS - Metal pieces inserted between die halves; used to control the thickness of a press molded part. Not a recommended practice, since the resin will end up with less pressure on it and voids can result.

STRAIN - the per unit change, due to force, in the size or shape of a body referred to its original size or shape. Strain is a non-dimensional quantity, but it is frequently expressed in inches per inch, meters per meter, or percent.

STRAND - Normally an untwisted bundle or assembly of continuous filaments used as a unit, including slivers, tow, ends, yarn. etc. Sometimes a single fiber or filament is called a strand.

STRENGTH - the maximum stress which a material is capable of sustaining.

STRESS - The intensity at a point in a body of the forces or components of forces that act on a given plane through the point. Stress is expressed in force per unit area (pounds-force per square inch, megapascals, etc.).

STRESS RELAXATION - The time dependent decrease in stress in a solid under given constraint conditions.

STRESS-STRAIN CURVE (DIAGRAM) - A graphical representation showing the relationship between the change in dimension of the specimen in the direction of the externally applied stress and the magnitude of the applied stress. Values of stress usually are plotted as ordinates (vertically) and strain values as abscissa (horizontally).

SURFACING MAT - A thin mat of fine fibers used primarily to produce a smooth surface on an organic matrix composite.

SYMMETRICAL LAMINATE - A composite laminate in which the sequence of plies below the laminate midplane is a mirror image of the stacking sequence above the midplane.


TACK - Stickiness of the prepreg.

TACKING - To locally join together layers of thermoplastics by localized melting of the resin. (Also known as Tack Welding).

TAPE - Prepreg fabricated in widths up to 12 inches wide for carbon and 3 inches for boron. Cross stitched carbon tapes up to 60 inches wide are available commercially in some cases.

THERMAL CONDUCTIVITY - Ability of a material to conduct heat. The physical constant for quantity of heat that passes through unit cube of a substance in unit time when the difference in temperature of two faces is one degree.

THERMOFORMING - Forming a thermoplastic material after heating it to the point where it is soft enough to be formed without cracking or breaking reinforcing fibers.

THERMOPLASTIC - A plastic that repeatedly can be softened by heating and hardened by cooling through a temperature range characteristic of the plastic; in the softened stage, it can be shaped by flow into articles by molding or extrusion.

THERMOSET - A plastic that is substantially infusible and insoluble after having been cured by heat or other means.

TOLERANCE LIMIT - A lower (upper) confidence limit on a specified percentile of a distribution. For example, the B-basis value is a 95% lower confidence limit on the tenth percentile of a distribution.

TOLERANCE LIMIT FACTOR - The factor which is multiplied by the estimate of variability in computing the tolerance limit.

TOUGHNESS - A measure of a material's ability to absorb work, or the actual work per unit volume or unit mass of material that is required to rupture it. Toughness is proportional to the area under the load-elongation curve from the origin to the breaking point.

TOW - An untwisted bundle of continuous filaments. Commonly used in referring to man-made fibers, particularly carbon and graphite fibers, in the composites industry.

TRACER - A fiber, tow or yarn added to a prepreg for verifying fiber alignment and, in the case of woven materials, distinguishing warp fibers from fill fibers.

TRANSFORMATION - A transformation of data values is a change in the units of measurement accomplished by applying a mathematical function to all data values. For example, if the data is given by x, then y - x + 1, x2, l/x, log x, and cos x are transformations.

TRANSVERSELY ISOTROPIC - Descriptive term for a material exhibiting a special case of orthotropy in which properties are identical in two orthotropic dimensions. but not the third: having identical properties in both transverse directions but not the longitudinal direction.

TWIST - The number of turns about its axis per unit of length in a yarn or other textile strand. It may be expressed as turns per inch (tpi).

TWIST, DIRECTION OF - The direction of twist in yarns and other textile strands is indicated by the capital letters S and Z. Yarn has S twist if when held in a vertical position the visible spirals or helices around its central axis are in the direction of slope of the central portion of the letter S. Z twist is in the other direction.


UNBOND - An area within a bonded interface between two adherends in which the intended bonding action failed to take place. Also used to denote specific areas deliberately prevented from bonding in order to simulate a defective bond, such as in the generation of quality standards specimens. (See Disbond, Debond).

UNIDIRECTIONAL LAMINATE - A laminate with non-woven reinforcements and all layers laid up in the same direction.


VACUUM BAG - A process in which the lay-up is cured under pressure generated by drawing a vacuum in the space between the lay-up and a flexible sheet placed over it and sealed at the edges.

VACUUM INFUSION - Dry fiber materials are placed into a shaped mold and covered with a sealing plastic. Once the reinforcement materials are under complete vacuum conditions, precisely positioned tubes introduce liquid polymer resin into the mold. Though more time-consuming than other methods, vacuum infusion is a more customizable process that creates a maximum fiber-to-resin ratio. This method is used to produce large, high-strength FRP panels.

VENTING - In autoclave curing of a part or assembly, venting refers to turning off the vacuum source and venting the pressure difference between the pressure in the clave and atmospheric pressure. Venting is usually used to prevent the resin boiling that can occur when a resin is heated and simultaneously subjected to reduced pressure (vacuum).

VISCOSITY - the property of resistance to flow exhibited within the body of a material.

VOID - A physical and mechanical discontinuity occurring within a material or part which may be 2-D (e.g., disbonds, delaminations) or 3-D (e.g., vacuum-, air-, or gas-filled pockets). Porosity is an aggregation of micro-voids. Voids are essentially incapable of transmitting structural stresses or non-radiative energy fields . See INCLUSION.

VOLATILES - Refers to gaseous materials leaving a laminate that is being cured, and which were liquids or solids before the cure cycle started. Volatiles produced usually include residual solvents and absorbed or adsorbed water. Many materials also produce volatiles as by-products of the curing reactions.


WARP - The longitudinally oriented yarn in a woven fabric (see FILL); a group of yarns in long lengths and approximately parallel.

WET LAY-UP - A method of making a reinforced product by applying a liquid resin system while the reinforcement is put in place.

WET STRENGTH - The strength of an organic matrix composite after the composite has absorbed moisture.

WET WINDING - A method of filament winding in which the fiber reinforcement is coated with the resin system as a liquid just prior to wrapping on a mandrel.

WHISKER - A short single fiber or filament. Whisker diameters range from 1 to 25 microns with length-to-diameter ratios between 100 and 15,000.

WORK LIFE - The period during which a compound, after mixing with a catalyst, solvent, or other compounding ingredients, remains suitable for its intended use.

WOVEN FABRIC COMPOSITE - A major form of advanced composites in which the fiber constituent consists of woven fabric. A woven fabric composite normally is a laminate comprised of a number of laminae, each of which consists of one layer of fabric embedded in the selected matrix material. Individual fabric laminae are directionally oriented and combined into specific multi-axial laminates for application to specific envelopes of strength and stiffness requirements.


X-AXIS - In composite laminates, an axis in the plane of the laminate which is used as the 0° reference for designating the angle of a lamina.

X-Y PLANE - In composite laminates. the reference plane parallel to the plane of the laminate.


YARN - Generic term for strands of fibers or filaments in a form suitable for weaving or otherwise intertwining to form a fabric.

YARN, PLIED - Yarns made by collecting two or more single yarns together. Normally, the yarns are twisted together though sometimes they are collected without twist.

YIELD STRENGTH - The stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain.

Y-AXIS - In composite laminates, the axis in the plane of the laminate which is perpendicular to the x-axis. Z-AXIS -- In composite laminates, the reference axis normal to the plane of the laminate.


ZERO BLEED - A laminate fabrication procedure which does not allow loss of resin during cure. Also describes prepreg made with the amount of resin desired in the final part, such that no resin has to be removed during cure.

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