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Composite Terms

Terms in Composites

ADDITIVE - Any substance added to another substance, usually to improve properties, such as plasticizers, initiators, light stabilizers, and flame retardants. See also filler.

ADHERED - A body that is held to another body, usually by an adhesive. A detail or part prepared for bonding.

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

ADHESION PROMOTER - A coating applied to a substrate before it is coated with an adhesive, to improve the adhesion of the plastic. Also called primer.

ADHESIVE - A substance capable of holding two materials together by surface attachment. Adhesive can be in film, liquid, or paste form.

ADHESIVE FAILURE - Rupture of an adhesive bond such that the separation appears to be at the adhesive-adherend interface.

ADHESIVE FILM - A synthetic resin adhesive, with or without a film carrier fabric, usually of the thermosetting type, in the form of a thin film of resin, used under heat and pressure as an interleaf in the production of bonded structures.

ADHESIVE JOINT - The location at which two adherends or substrates are held together with a layer of adhesive. The general area of contact for a bonded structure.

ADHESIVE STRENGTH - Strength of the bond between an adhesive and an adhered.

AIR-BUBBLE VOID - Air entrapment within and between the plies of reinforcement or within a bond line or encapsulated area; localized, non interconnected, spherical in shape.

ASSEMBLY TIME - The time interval between the spreading of the adhesive on the adherent and the application of pressure and/or heat to the assembly.

A-STAGE - An early stage in the reaction of a thermosetting resin in which the material is still soluble and fusible.


BALANCED LAMINATE - A composite laminate in which all laminate at angles other than 0o and 90o occur only in + pairs (not necessarily adjacent) and are symmetrical around the centerline.

BATCH - In general, a quantity of material formed during the same process or in one continuous process and having identical characteristics throughout. Also called a lot.

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

BLEED OUT - The excess liquid resin appearing at the surface primarily occurring during filament winding.

BOND STRENGTH - The amount of adhesion between bonded surfaces. The stress required to separate a layer of material from the base to which it is bonded, as measured by load/bond area. See also peel strength.

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


CARBON FIBERS - Fibers produced from pyrolytic degradation of synthetic organic fibers.

CATALYST - A substance that markedly speeds up the cure of a compound when added in minor quantity as compared to the amounts of primary reactants.

CAVITY - The space inside a mold in which a resin or molding compound is poured or injected. The female portion of a mold. That portion of the mold that encloses the molded article (often referred to as the die). Depending on the number of such depressions, molds are designated as single cavity or multiple cavity.

CELL - In honeycomb core, a cell is a single honeycomb unit, usually in a hexagonal shape.

COHESION - The propensity of a single substance to adhere to itself. The internal attraction of molecular particles toward each other. The ability to resist partition of itself. The force holding a single substance together.

COHESIVE FAILURE - Failure of an adhesive joint occurring primarily in an adhesive layer.

COIN TAP TEST - Using a coin to tap a laminate in different spots, listening for a change in sound, which would indicate the presence of a defect. A surprisingly accurate test in the hands of experienced personnel.

COMPOSITE - A material created from a fiber (or reinforcement) and an appropriate matrix material in order to maximize specific performance properties. The constituents do not dissolve or merge completely but retain their identities as they act in concert.

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

COMPRESSIVE STRENGTH - A material's ability to resist a force that tends to crush or buckle; maximum compressive load a specimen sustains divided by the specimen's original cross sectional area.

CONTACT MOLDING - A molding technique in which reinforcement and resin are placed in a mold, with cure taking place at room temperature with a catalyst/promoter system or in a heated oven. No additional pressure is used.

CORE - The central component of a sandwich construction to which the sandwich faces or skins are attached; also, part of a complex mold that forms undercut parts.

CREEP - The dimensional change in a material under physical load over time beyond instantaneous elastic deformation.

CROSS LAMINATED - Material laminated so that some of the layers are oriented at various angles to the other with respect to the laminate grain. A cross-ply laminate usually has plies oriented only at 0/90 degrees.

CURE - To change the properties of a thermosetting resin irreversibly by chemical reaction, i.e., condensation, ring closure, or addition. Cure may be accomplished by addition of curing (cross-linking) agents, with or without catalyst, and with or without heat.

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

CURE STRESS - A residual intenal stress produced during the curing cycle of composite structures. Normally, these stresses originate when different components of a wet lay-up have different thermal coefficients of expansion.

CURING AGENT - A catalytic or reactive agent that brings about polymerization when it is added to a resin.


DELAMINATION - The separation of a laminated plastic material along the plane of its layers.

DRY LAMINATE - A laminate containing insufficient resin for complete bonding of the reinforcement. See also resin starved.

DRY LAY-UP - Construction of a laminate by the layering of preimpregnated reinforcement (partly cured resin) in a female mold or on a male mold, usually followed by bag molding or autoclave molding.


E-GLASS - "Electrical glass"; the borosilicate glass most often used for the glass fibers in conventional reinforced plastics.


FIBER-REINFORCED PLASTIC (F.R.P.) - A general term for a composite that is reinforced with cloth, mat, strands, or any other fiber form.

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

FILLER - A relatively inert substance added to a material to alter its physical, mechanical, thermal, electrical, and other properties or to lower cost or density. Sometimes the term is used specifically to mean particulate additives.

FRACTURE - A rupture of the surface of a laminate because of external or internal forces, with or without complete separation.


GEL - The initial jellylike solid phase that develops during the formation of a resin from a liquid. A semisolid system consisting of a network of solid aggregates in which liquid is held.

GEL COAT - A quick setting resin applied to the surface of a mold and gelled before lay-up. The gel coat becomes an integral part of the finished laminate, and is usually used to improve surface appearance and bonding.

GEL TIME - The time required for a liquid material to form a gel under specified conditions of temperature as measured by a specific test.

GLASS CLOTH - Conventionally woven glass fiber material; certain lightweight glass fabrics are also called scrims.

GRAPHITE FIBERS - A group of carbon fibers which have a carbon content of about 99% and also have high modulus values. This term is used interchangeably with "carbon fibers" throughout the industry.


HAND LAYUP - A fabrication method in which reinforcement layers, preimpregnated or coated afterwards, are placed in a mold by hand, then cured to the formed shape.

HARDENER - A substance used to promote or control curing action by taking part in it; as opposed to catalyst.

HONEYCOMB - Resin-impregnated material manufactured in, usually, hexagonal cells that serves as a core material in sandwich constructions. Honeycomb may also be metallic or polymer materials in a rigid, open-cell structure.


IMPACT STRENGTH - A material's ability to withstand shock loading as measured by the work done in fracturing a specimen.

IMPREGNATE - To saturate the voids and interstices of a reinforcement with a resin.


KEVLAR® - Registered trademark of E.I. Dupont de Nemours, Inc. for a strong organic fibers similar to fiberglass but having a higher strength-to-weight ratio. When woven into cloth and impregnated with a thermosetting epoxy resin, it produces a material having high impact resistance and low radio frequency attenuation. Generic term: aramid.


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

LAY-UP - The reinforcing material placed in position in the mold. The process of placing the reinforcing material in position in the mold. The resin-impregnated reinforcement. A description of the component materials, geometry, and so forth, of a laminate.


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

MILLED FIBER - Continuous glass strands hammer milled into very short glass fibers. Useful as inexpensive filler or anticrazing reinforcing fillers for adhesives.

MOLD - The cavity or matrix into or on which the plastic composition is placed and from which it takes form. To shape plastic parts or finished articles by heat and pressure. The assembly of all the parts that function collectively in the molding process.

MOLD-RELEASE AGENT - A lubricant, liquid, or powder (often silicone oils and waxes), used to prevent sticking of molded articles in the cavity.

MOLD SURFACE - The side of a laminate that faced the mold (tool) during cure in an autoclave or hydroclave.


ORANGE PEEL - An uneven surface somewhat resembling that of an orange peel; said of injection moldings that have inintentionally ragged surfaces.


PEEL PLY - Layer of material applied to a prepreg layup surface that is removed from the cured laminate prior to bonding operations and leaves a clean resin-rich surface ready for bonding.

PLY - In general, fabrics or felts consisting of one or more layers (laminates, and so forth). The layers that make up a stack. Yarn resulting from twisting operations (three-ply yarn, and so forth). A single layer of prepreg. A single pass in filament winding (two plies forming one layer).

POT LIFE - The length of time a catalyzed thermosetting resin system retains a viscosity low enough for it to be suitable for processing.

PREPREG, PREIMPREGNATED - A combination of mat, fabric, nonwoven material, or roving with resin, usually advanced to the B-stage, ready for curing.

PRESSURE BAG MOLDING - A process for molding reinforced plastics in which a tailored, flexible bag is placed over the contact lay-up on the mold, sealed, and clamped in place. Fluid pressure, usually provided by compressed air or water, is placed against the bag, and the part is cured.

PRIMER - A coating applied to a surface, before the application of an adhesive, lacquer, enamel, and so forth, to improve the adhesion performance or load-carrying ability of the bond.


REINFORCEMENT - A material added to the matrix to provide the required properties; ranges from short fibers through complex textile complex textile forms.

RELEASE AGENTS - Materials that are used to prevent cured matrix material from bonding to tooling.

RELEASE FILM - An impermeable layer of film that does not bond to the resin being cured. See also separator.

RESIN - A material, generally a polymer, that has an indefinite and often high molecular weight and a softening or melting range and exhibits a tendency to flow when it is subjected to stress. Resins are used as the matrices to bind together the reinforcement material in composites.

RESIN CONTENT - The amount of resin in a laminate expressed as either a percentage of total weight or total volume.

RESIN RICH - Localized area filled with resin but lacking reinforcement fiber.

RESIN STARVED - Localized area lacking sufficient resin for wetout of the fibers.


SANDWICH CONSTRUCTION - A composite composed of lightweight core material (usually honeycomb or foamed plastic) to which two relatively thin, dense, high strength, functional, or decorative skins (also called faces) are adhered.

SCRIM - A low-cost reinforcing fabric made from continuous filament yarn in an open-mesh construction. Used in the processing of tape or other B-stage material to facilitate handling. Also used as a carrier of adhesive, to be used in secondary bonding.

S-GLASS - Structural glass; a magnesia/alumina/silicate glass reinforcement designed to provide very high tensile strength.

SHEAR STRENGTH - The maximum shear stress that a material is capable of sustaining. Shear strength is calculated from the maximum load during a shear or torsion test and is based on the original cross-sectional area of the specimen.

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.

SHRINKAGE - The relative change in dimension from the length measured on the mold when it is cold to the length of the molded object 24 hrs after it has been taken out of the mold.

SKIN - A layer of relatively dense material used in a sandwich construction of the surface of the core.

SPRAY-UP - Technique in which a spray gun is used as an applicator tool. In reinforced plastics, for example, fibrous glass and resin can be simultaneously deposited in a mold. In essence, roving is fed through a chopper and ejected into a resin stream that is directed at the mold by either of two spray systems. In foamed plastics, fast-reacting urethane foams or epoxy foams are fed in liquid streams to the gun and sprayed on the surface. On contact, the liquid starts to foam.

STORAGE LIFE, POT LIFE - The period of time during which a liquid resin, packaged adhesive, or prepreg can be stored under specified temperature conditions and remain suitable for use. Also called shelf life.

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

STRUCTURAL ADHESIVE - Adhesive used for transferring required loads between adherends exposed to service environments typical for the structure involved.

STRUCTURAL BOND - A bond that joins basic load-bearing parts of an assembly. The load may be either static or dynamic.

SURFACE PREPARATION - Physical and/or chemical preparation of an adherend to make it suitable for adhesive bonding.


TACK - The stickiness of a prepreg.

TEMPLATE - A pattern used as a guide for cutting and laying plies.

TENSILE STRENGTH - The maximum load or force per unit cross-sectional area, within the gage length, of the specimen. The pulling stress required to break a given specimen.

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


VACUUM BAG MOLDING - A process in which a sheet of flexible transparent material plus bleeder cloth and release film are placed over the lay-up on the mold and sealed at the edges. A vacuum is applied between the sheet and the lay-up. The entrapped air is mechanically workded out of the lay-up and removed by the vacuum, and the part is cured with temperature, pressure, and time. Also called bag molding.

VENT - A small hole or shallow channel in a mold that allows air or gas to exit as the molding material enters.

VISCOSITY - The tendency of a material to resist flow.

VOIDS - Air or gas that has been trapped and cured into a laminate. Porosity is an aggregation of microvoids. Voids are essentially incapable of transmitting structural stresses or nonradiative energy fields.


WEATHERING - Exposure of plastics to the outdoor environment.

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

WET-OUT - The condition of an impregnated roving or yarn in which substantially all voids between the sized strands and filaments are filled with resin.

WORKING TIME - The period of time during which a liquid resin or adhesive, after mixing with catalyst, solvent, or other compounding ingredients, remains usable.

WRINKLE - A surface imperfection in laminated plastics that has the appearance of a crease or fold in one or more outer sheets of the paper, fabric, or other base, which has been pressed in. Also occurs in vacuum bag molding when the bag is improperly placed, causing a crease.






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