A-Basis : The mechanical property value above which 99 percent of the population of values is expected to fall with a confidence of 95 percent.
Ablative Plastic: A material that absorbs heat (with low material loss and char rate) through a decomposition process (pyrolysis). Absorption takes place at or near the surface exposed to the heat.
Abrasion: Wearing away by friction. Glass is highly resistant to abrasion from other materials, but can be damaged through contact with itself. Lubrication during processing and fabrication helps prevent abrasion.
ABS: Acrylonitrile Butadiene Styrene (Thermoplastic Resin).
Absorption: A process in which one material takes in or absorbs another.
Accelerated Test: Procedure in which conditions are magnified to reduce the time required to obtain a result, or to reproduce the deteriorating effects of normal service conditions in a very short time period.
Accelerator (Promoter): A highly active oxidizing agent used to speed up the chemical reaction (curing) between a catalyst and resin. Examples include diethylaniline, cobalt naphthanate and cobalt octoate.
Acrylic: Thermoplastic polymer made by the polymerization of esters of acrylic acid and its derivatives.
Actual End Count: The number of bundles or splits that are actually counted in one doff of roving. (This is less than the theoretical end count due to splitting efficiencies of less that 100%)
Addition Polymerization: Chemical reaction in which simple molecules (monomers) are added to each other to form long-chain molecules (polymers) without byproducts.
Additive: Any substance added to another substance, usually to improve properties, such as plasticizers, initiators, light stabilizers, and flame-retardants.
Adherend: 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 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. In the handbook, the term is used specifically to designate structural adhesives, those which produce attachments capable of transmitting significant structural loads.
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.
Admixture: Addition and homogeneous dispersion of discrete components, before cure.
Aggregate: Hard, coarse material usually of mineral origins used in composite tools. Also used in flooring or as a surface medium.
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.
Air Entrapment: Occlusion of air in a resin or resin glass system, giving rise to blisters, bubbles or voids in the system.
Air Splice: The coupling between two roving doffs which is made by a jet of air entwining/snarling the two strands together. The air splice is used instead of a knot.
Air Vent: Small outlet to prevent entrapment of gases in a molding or tooling fixture.
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.
Alkyd Plastics: Resin composed principally of polymeric esters, in which the recurring ester groups are an integral part of the main polymer chain or are part of the cross links present between chains.
Alligatoring: Visible cosmetic defect in exposed gel coat resembling wrinkled or alligator-like skin.
Alloy: In plastics, a blend of polymers or copolymers blended with other polymers or elastomers under select conditions.
Ambient: Surrounding environmental conditions, such as pressure, temperature, or relative humidity.
Amine Resins: A synthetic resin derived from the reaction of urea, thiourea, melamine or allied compounds with aldehydes, particularly formaldehyde.
An Isotropic: Exhibiting different properties when tested along axes in differentdirections.
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.
Angle-Ply Laminate: A laminate with fibers of adjacent plies oriented at alternating angles.
Anisotropy: The tendency of a material to exhibit different along the directions parallel to the length or width into the lamination planes; or parallel to the thickness into the planes perpendicular to the lamination.
Antioxidant: Substance that, when added in small quantities to resin, prevents oxidation and degradation while maintaining the resin’s properties.
Anti-Static Agents: Agents which, when added to the molding material or applied on the surface of the molded object, make it less conducting (thus hindering the fixation of dust).
Aramid: Aromatic polyamide fibers characterized by excellent high-temperature, flame-resistance, and electrical properties.
Arc Resistance: Ability to withstand exposure to an electric voltage. Also, the total time in seconds that an intermittent arc may play across a plastic surface without rendering it conductive.
Areal Weight: The weight of fiber per unit area (width x length) of tape or fabric.
Ash Content: The solid residue remaining after a reinforcing substance has been incinerated (or strongly heated).
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.
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.
ASTM: American Society of Test Methods.
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.
Autoclave Molding: Process in which an assembly is placed in a heated autoclave, usually at 50 to 200 psi, after lay-up, winding or wrapping. Additional pressure permits higher density and helps remove volatiles from the resin. Lay-up is usually vacuum bagged with a bleeder and release cloth.
Axial Winding: A type of filament winding in which the filaments are parallel to the axis.
Bag Molding: A process in which the consolidation of the material in the mold is effected by the application of fluid or gas pressure through a flexible membrane.
Bag Side: The side of the part that is cured against the vacuum bag.
Bagging: Applying an impermeable layer of film over an uncured part and sealing the edges so that a vacuum can be drawn.
Balanced Construction: Equal parts of warp and fill in fiber fabric. Construction in which reactions to tension and compression loads result in extension or compression deformations only and in which flexural loads produce pure bending of equal magnitude in axial and lateral directions.
Band Width: In filament winding, the width of the reinforcement as it is applied to the mandrel.
Barcol-Shore Rockwell Hardness: A material’s ability to be indented. The Rockwell method measures the amount of penetration caused when a steel point is forced into the material. The suffix (alphabetic letter) in the Rockwell reading describes the shape of the point and units describe how much load was applied during the test. The letter and number cannot be separated. Higher numbers with the same letter indicate harder materials with greater resistance to penetration by another substance.
Bare Glass: Glass in fiber form as it flows from the bushing before a binder or sizing is applied.
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.
Batch Oven: Large temperature-controlled oven used to heat-clean rolls of glass fiber fabric.
Batt: Felted fabrics or structures built by the interlocking action of compressing fibers, without spinning, weaving, or knitting.
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.
Beam: A spool on which parallel ends of single or plied yarns are wound for use in weaving or similar processing operations.
Bearing Load: A compressive load on an interface.
Bearing Strength: The maximum amount of stress that can be sustained. Also, the point on the stress-strain curve where the tangent is equal to the bearing stress divided by n% of the bearing hole diameter.
Bearing Stress: Applied load in pounds divided by the bearing area. Maximum bearing stress is the number of pounds that can be sustained, divided by the original bearing area.
Bearing Yield Strength: The bearing stress at which a material exhibits a specified limiting deviation from a linear stress-strain relationship.
Bed: The mat of chopped glass fibers deposited over a layer of resin mix on carrier film following a chopping operation.
Bias Fabric: A fabric in which warp and fill fibers are at an angle to the length.
Biaxial Load: Loading condition in which a laminate is stressed in two different directions in its plane. Also, a loading condition of a pressure vessel under internal pressure and with unrestrained ends.
Biaxial Winding: Filament winding in which the helical band is laid in sequence, side by side, without any fibers crossing over each other.
Bi-directional Laminate: A reinforced plastic laminate whose fibers are oriented in two directions in its plane.
Bi-directional: Reinforcing fibers arranged in two directions, usually at right angles.
Binder: A coating applied to the surface of a chopped glass mat or preform which is then cured. The binder holds the previously sized glass bundles or ends together in the roving operation into a stable shape or form.
Birdnest: A large, tangled up collection of continuous glass bundles unable to run through the guide eye into roving creel. In the field, it can also be a large tangled collection of roving which does not run through the tube or guide eyes to the chopper.
Bismaleimide (BMI): A polyamide that cures through an addition rather than a condensation reaction, thus avoiding problems with volatiles forming. It is produced by a vinyl-type polymerization of a pre-polymer terminated with two maleimide groups. BMI has an Intermediate temperature capability (between epoxy and polyamide).
Bisphenol A: A condensation product formed by reaction of two (bis) molecules of phenol with acetone (A). This polyhydric phenol is a standard resin intermediate along with epichlorohydrin in the production of epoxy resins.
Bladder: An elastomeric lining for the containment of pressurization medium in filament-wound structures, or for the manufacture of composite structures.
Blade Packing: Glass bundles or chopper fuzz which build up and pack between the blades of a chopper. This blade packing can cause poor chop ability. If it falls off, it usually does not wet-through, and this can cause blisters or porosity.
Blanket: Fiber or fabric plies that have been laid up in a complete assembly and placed on or in the mold all at one time (flexible bag process). Also, the form of bag in which the edges are sealed against the mold.
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.
Blister: Undesirable rounded elevation of the surface of a plastic, whose boundaries may be more or less sharply defined, resembling in shape a blister on the human skin. The blister may burst and become flattened.
BMC: Bulk Molding Compound. Thermosetting resin mixed with strand reinforcement, fillers, and so on, into a viscous compound for compression or injection molding. See also sheet molding compound.
Bobbin: The spool or shipping package on to which textile yarns are wound.
Bond: The adhesion of one surface to another, with or without the use of an adhesive as a bonding agent.
Bond Strength: 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.
Boron Fibre: A fiber usually of a tungsten-filament core with elemental boron vapor deposited on it to impart strength and stiffness.
Braid/Braider: A narrow tubular or flat fabric produced by intertwining a single set of yarns according to a definite pattern.
Braiding: Weaving of fibers into 3-dimensional shapes instead of flat tape or fabric.
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.
Breathing: Opening and closing a mold so that gas can escape early in the molding cycle. Also called “degassing”; sometimes called “bumping” in Phenolic molding.
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.
Broken End: In the roving operation, a broken or severed strand (bundle) which causes the forming cake to stop running.
B-STAGE: An intermediate stage in the reaction of a thermosetting resin; that is, partial cure.
Buckling (Composite): Failure mode generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.
Build-Up: Glass bundles or chopper fuzz which collect on the chopper, cot, static bars or machine frame.
Bulk Factor: The ratio of the volume of a raw molding compound, reinforcement or powdered plastic to the volume of the finished solid piece produced therefrom. The ratio of the density of the solid plastic object to the apparent or bulk density of the loose molding powder or fabric.
Bundle: A discrete collection of many parallel glass filaments. A collection of Individual filaments, a sub-strand.
Burst Strength: (1) Hydraulic pressure required to burst a vessel of given thickness. Commonly used in testing filament-wound composite structures. (2) Pressure required to break a fabric by expanding a flexible diaphragm or pushing a smooth spherical surface against a securely held circular area of fabric. The Mullen expanding diaphragm and Scott ball burst machine are examples of equipment used for this purpose.
Bushing: Plate with holes through which molten glass is pulled to produce glass fibers.
Bushing Tip: Small tapered protrusions on the bottom of bushings each containing an orifice through which molten glass flows, from which continuous filaments are drawn.
Cabled Yarn: Yarn that is plied more than once or made by plying two or more previously plied yarns.
Cake: A term applied to the glass package that is produced in the forming department. Also used as forming cake package, forming cake and cake package. All terms are synonymous.
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.
Carbon-Carbon: Composite material consisting of carbon or graphite fibers in a carbon or graphite matrix.
Carding: The process of untangling and partially straightening fibers by passing them between two closely spaced surfaces which are moving at different speeds, and at least one of which is covered with sharp points, thus converting a tangled mass of fibers to a filmy web.
Casting: Process of pouring resin, fillers and/or fibers into a mold vs. building up layers through lamination. Casting results in physical properties that are different than those resulting from lamination.
Catalyst (Hardener): A substance that markedly speeds up the cure of a compound by decomposing in the presence of a promoter to release active oxygen radical. Catalyst content can vary from 0.2% to 2.0% with higher catalyst levels giving faster gel times. Examples are methyl ethyl ketone peroxide, benzoyl peroxide.
Catastrophic Failures: Totally unpredictable failures of a mechanical, thermal, or electrical nature.
Catenary: A measure of the difference in length of strands in a specified length of roving as a result of unequal tension; the tendency of some strands in a taut horizontal roving to sag lower than the others.
Caul Plates: Smooth metal, plastic, or rubber plates free of surface defects. It is used in contact with the lay-up during curing process to transmit normal pressure and to provide a smooth surface on finished part. A caul plate must be of appropriate size and shape for the composite lay-up with which it is to be used.
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 cavities.
Cell: In honeycomb core, a cell is a single honeycomb unit, usually in a hexagonal shape.
Ceramic Tooling: Use of a cast-able 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.
CFRP: Carbon fiber-reinforced plastic.
C-Glass: Glass with a soda-lime-borosilicate composition that maintains chemical stability in corrosive environments.
Chalking: Surface phenomenon indicating degradation of a cosmetic surface. Chalking is a powdery film that appears lighter than the original color.
Chemical Size: A surface finish applied to the fiber that contains some chemical constituents other than water.
Chop-ability: The ease of chopping/cutting the glass fibers to a uniform length.
Chopped Strand: Continuous roving that is chopped into short lengths for use in mats, spray-up or molding compounds.
Chopper Gun: A special equipment used in the manufacture of reinforced plastic parts, which chops glass and sprays resin and catalyst simultaneously onto a molded surface.
Clamping Pressure: In injection molding and transfer molding, the pressure that is applied to the mold to keep it closed in opposition to the fluid pressure of the compressed molding material.
Cloth: Fiberglass reinforcement made by weaving strands of glass fiber yarns.
Clump: A group of chopped bundles of glass fibers that has collected on the SMC machine and then fallen into the bed of glass. The clump produces areas of high glass content which may not wet-through.
Co-Cured: Cured and simultaneously bonded to another prepared surface.
Coefficient of Linear Thermal Expansion: The change in length per unit length resulting from a one-degree rise in temperature.
Cold Flow: Distortion that occurs in a material under continuous load within its working temperature range and without a phase or chemical change.
Collet: A spool on which the gathered strands from the bushing are wound for further processing.
Compaction: Applying a temporary vacuum bag and vacuum to remove trapped air and compact the lay-up; also removing air in SMC machines prior to roll-up.
Compatibility: The ability of two or more substances to be combined in order to form a homogeneous composition of useful plastic properties; for example, the suitability of a sizing or finish for use with certain general resin types.
Composite: A homogeneous material created by the synthetic assembly of two or more materials (selected filler or reinforcing elements and compatible matrix binder) to obtain specific characteristics and properties. Composites are subdivided into classes on the basis of the form of the structural constituents; Laminar – Composed of layer or laminar constituents; Particular -The dispersed phase consists of small particles; Fibrous -The dispersed phase consists of fibers; Flake -The dispersed phase consists of flat flakes; Skeletal -Composed of a continuous skeletal matrix filled by a second material.
Compound: An intimate mixture of polymer or polymers with all the materials necessary for the finished product.
Compression Molding: A technique for molding thermoset plastics in which a part is shaped by placing the fiber and resin into an open mold cavity, closing the mold, and applying heat and pressure until the material has cured or achieved its final form.
Compressive Modulus: Ratio of compressive stress to compressive strain below the proportional limit. Theoretically equal to Young’s modulus determined from tensile experiments.
Compressive Strength: The amount of nonmoving load that a bar can take before it is crushed. Units are normally thousands of pounds per square inch. (103 psi) – Mega Pascal (mPa). Higher numbers indicate stronger materials that can withstand a heavier load before they break.
Condensation Polymerization: A chemical reaction in which two or more molecules combine, with the separation of water or some other simple substance.
Conditioning: Subjecting a material to a prescribed environmental and/or stress history before testing.
Conductivity: Reciprocal of volume resistivity. The electrical or thermal conductance of a unit cube of any material (conductivity per unit volume).
Conformability: Ability of the mat to conform to difficult shapes without causing wrinkles or leaving excessively resin-rich or glass-rich radii, which may craze.
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.
Contact Molding: A process for molding reinforced plastics in which reinforcement and resin are placed on a mold, cure is either at room temperature using a catalyst-promoter system or by heat in an oven and no additional pressure is used.
Continuous Filament: A yarn or strand in which the individual filaments are substantially the same length as the strand.
Continuous Heat Resistance: Maximum temperature to which material should be subjected in a continuous application. Below this temperature, the material is acceptable. At temperatures above the maximum, the material may decompose, melt, or otherwise fail in an application. Units – degrees Fahrenheit (°F) – degrees Centigrade (°C). Higher numbers mean the material can be used continuously at higher temperatures.
Continuous Laminating: Process for forming panels and sheeting in which fabric or mat is passed through a resin bath, brought together between covering sheets, and passed through a heating zone for cure. Squeeze rolls control thickness and resin content as the various plies are brought together.
Continuous Roving: Parallel filaments coated with sizing, gathered together in single or multiple strands and wound into a cylindrical package. It can be used to provide continuous reinforcement in woven roving, filament winding, pultrusion, prepregs or high-strength molding components.
Conventional Roving: Roving that is assembled from several forming packages using a creel and a roving winder. Typical characteristics are multiple ends, 3-inch diameter centers, a tube core and some catenary.
Co-Polymer: Resins produced by copolymerization, the process where unlike molecules are arranged in alternate sequence in a chain.
Core: A low-density material used between two FRP skins. Examples are end-grain balsa wood, urethane foam, PVC foam and various honeycomb materials. The central member, usually foam or honeycomb, has a sandwich construction to which the faces of the sandwich are attached or bonded.
Coronizing: Continuous heat cleaning and weave setting.
Corrosion Resistance: A material’s ability to withstand ambient natural factors or those of a particular artificially created atmosphere, without degrading or changing in properties. For metals, this could be pitting or rusting; for or organic materials, it could be crazing.
Count: For fabric, number of warp and filling yarns per inch in woven cloth. For yarn, size based on relation of length and weight.
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; a bonding link.
Co-Woven Fabric: A reinforcement fabric woven with two different types of fibers in individual yarns; for example, thermoplastic fibers woven side by side with carbon fibers.
Crazing: Cracking of gel coat or resin due to stress. Region of ultra-fine cracks, which may extend in a network on or under the surface of a resin or plastic material. May appear as a white band. Often found in a filament-wound pressure vessel or bottle.
Creel: A device for holding the required number of roving balls or supply packages in desired position for unwinding onto the next processing step.
Creep: The slow movement of a plastic material with time.
Crimp: A fiber’s waviness, which determines the capacity of the fiber to cohere.
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.
Crosslinking: The setting up of chemical links between molecule chains. This occurs in all thermosetting resins. Styrene monomer is a crosslinking agent in polyester resins.
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 angle ply is used for all other layup combinations. In this manual, the terms crossply and angle-ply 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.
C-Scan: The record of the through transmission ultrasonic inspection, a nondestructive inspection (NDI) technique for finding voids, delamination, defects in fiber distribution, etc.
C-Stage: The final stage of the curing of a thermosetting resin in which the material has become infusible and insoluble in common solvents.
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 wet lay-up have different thermal coefficients of expansion.
Cure Temperature: Temperature at which a cast, molded, or extruded product, resin-impregnated reinforcement, adhesive or other material is subjected to curing.
Cure Time: The time required for liquid resin to reach a cured or fully polymerized state after catalyst has been added.
Curing Agent: A catalytic or reactive agent that, when added to a resin, causes polymerization. Also called hardener.
Cut Ends on Doff: Severed ends generally caused by abrasion during shipping or by careless use of a knife when the package is removed from the pallet.
Cycle: The complete, repeating sequence of operations in a process or part of a process. In molding, cycle time is the period (or elapsed time) between a certain point in one cycle and the same point in the next.
Dam: Boundary support used to prevent excessive edge bleeding of a laminate and to prevent crowning of the bag.
Damping: The decay with time of the amplitude of free vibrations of a specimen.
DAP: Diallyl Phthalate (Thermoset Resin).
Daylight: The distance, in the open position, between the moving and fixed tables or the platens of a hydraulic press. In the case of a multiday light press, daylight is the distance between adjacent platens.
De-bond: A deliberate separation of a bonded joint or interface, usually for repair or rework purposes.
De-bulking: Compacting of a thick laminate under moderate heat and pressure and/or vacuum to remove most of the air, to ensure seating on the tool, and to prevent wrinkles.
Deep-Draw Mold: A mold whose core is long in relation to its wall thickness.
Deflashing: A finishing technique used to remove excess, unwanted material (flashing) on a plastic molding.
Deflection Temperature:Temperature at which a simple beam has deflected a given amount under load (formerly called heat distortion temperature).
Deformation: Dimensional change of a material under load for a specific time following the instantaneous elastic deformation caused by the initial application of the load. (Also, ‘cold flow’ or ‘creep’.)
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 numbering system for yarn and filament in which yarn number is equal to weight in grams of 9000 meters of yarn.
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: A nonconductor of electricity. A material’s ability to resist the flow of electrical current.
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 Heating: Heating materials by dielectric loss in a high-frequency electrostatic field.
Dielectric Strength: An electrical property indicating how well a material acts as an electrical insulator. It describes how much of an electrical voltage can be built up on one side of the material before it is communicated to the other side. Units are measured in volts per mil of thickness (volts/mil). Higher numbers indicate materials with better insulation properties. C means that the material conducts electricity and therefore has no dielectric strength.
Dielectrometry: Use of electrical techniques to measure the changes in loss factor (dissipation) and in capacitance during cure of the resin in a laminate.
Dimensional Stability: A plastic part’s ability to retain the precise shape to which it was molded, cast, or otherwise fabricated.
Direct Wound Roving: A roving made directly at the bushing that does not go through a roving process. Typical characteristics are single-end roving, coreless, 6-inch diameter centers and no catenary.
Direct-Sized Yarn: Specially formulated sizing on textile yarns that allow them to be resin compatible.
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.
Dispersion: The degree to which the roving separates into discrete bundles after being chopped. Good dispersion is characterized by a bed of bundles that are uniform in width. Poor dispersion is characterized by a wide distribution in the widths of various bundles in the bed. Poor dispersion can cause poor wet-through and wet-out.
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.
Doctor Blade or Bar: A straight piece of material used to spread resin, as applying a thin film of resin for use in hot melt prepreg or as an adhesive film. Also called paste metering blade.
Doff Collapse: The failures of the roving doff to maintain its shape and stability during run-out or storage. Doff collapse generally occurs when there is only a 1/2″ to 1/4″ ring of roving left from the original doff.
Draft: The tape or slope of the vertical surfaces of a mold designed to facilitate removal of molded parts.
Drape: A property indicating the stiffness of a product. The stiffness of a mat, for example, determines the ease of conformity to molded surfaces during initial processing.
Dry Fiber Area: Area of fiber not totally encapsulated by resin.
Dry Laminate: A laminate containing insufficient resin for complete bonding of the reinforcement.
Dry Lay-Up: Construction of a laminate by the layering of pre-impregnated reinforcement (partly cured resin) in a female mold or on a male mold, usually followed by bag molding or autoclave molding.
Dry Loft: Height of the bed of chopped fibers.
Dry Spot: Anarea of incomplete surface film on laminated plastics; in laminated glass, an area over which the interlayer and the glass have not become bonded.
Dry Winding: A filament winding operation in which resin is not used.
Ductility: The ability of a material to deform plastically before fracturing.
Dwell: A pause in the application of pressure or temperature to a mold, made just before it is completely closed, allowing gas to escape from the molding material.
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.
E-Glass: A borosilicate glass; the type most used for glass fibers for reinforced plastics; suitable for electrical laminates because of its high resistivity.
Ejection / Demolding: Removing a molded part from the mold by hand, mechanical means or use of compressed air.
Ejection Plate: A metal plate used to operate ejector pins; designed to apply a uniform pressure to them in the process of ejection.
Elastic Limit: The greatest stress a material can sustain without permanent strain after the stress has been completely released. A material is said to have passed its elastic limit when the load is sufficient to initiate plastic, or non-recoverable, deformation.
Elasticity: The property of a material which allows it to recover its original size and shape immediately after removal of the force causing deformation.
Elastomer: A material that substantially recovers its original shape and size at room temperature after a deforming force is removed.
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.
Encapsulating: Completely surrounding an object with resin or a fiber resin composite. Sometimes used specifically in reference to the enclosure of capacitors or circuit board modules.
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.
End Count: The number of strands contained in a roving.
Environment: The aggregate of all conditions (such as contamination, temperature, humidity, radiation, magnetic and electric fields, shock, and vibration) that externally influence the performance of an item.
Epoxy: A polymerizable thermoset polymer containing one or more epoxide groups cured by its reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans. An important matrix resin in composites and structural adhesive. Epoxies generally have higher physical properties than polyester resins. They are also more costly and difficult to process, and less able to withstand sunlight.
Even Tension: Applying the same amount of tension to each end of roving in a ball.
Exotherm: The liberation or evolution of heat during the curing of a plastic product.
Exothermic Heat: Heat given off during polymerization by chemical ingredients as they react and the resin cures.
Extenders: To add fillers or low-cost materials in an economy producing endeavor. To add inert materials to improve void-filling characteristics and reduce crazing.
Extensometer: A device for measuring linear strain.
Extruder: Machine that pushes molten plastic through small holes to form fibers.
Fabric: A material constructed of interlaced yarns, fibers or filaments. Used interchangeably with “cloth”.May be woven or non-woven.
Fabrication: The process of glass fiber production during which forming cakes are put into creels and “roved” or fabricated onto doffs.
Fall: Shape or pattern of chopped fibers as they drop from the chopper to the bed.
Fan or Curtain: Chopped bundles that fall or are thrown off the chopper and cot.
Fatigue: Failure or decay of mechanical properties after repeated applications of stress. Fatigue tests indicate a material’s ability to resist cracking, which eventually causes failure due to a large number of cycles.
Fatigue Life: How many cycles of deformation it takes before a test specimen will fail under a given set of oscillating conditions (stresses and strains).
Feathered Edge: A fabric edge that tapers down in weight instead of abruptly ending.
Fibre: 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.
Fibre Architecture: The design of a fibrous part in which the fibers are arranged in a particular way to achieve the desired result. This can include braided, stitched or woven fabrics, or mats, roving or carbon tows.
Fibre Content: The amount of fiber present in a composite. This is usually expressed as a percentage volume fraction or weight fraction of the composite.
Fibre Diameter: A term used to denote the diameter of continuous glass filaments. Their diameter can vary depending on the purpose for which they are to be used. Can be expressed in letter designation, microns or inches.
Fibre Direction: The orientation or alignment of the longitudinal axis of the fiber with respect to a stated reference axis.
Fibre Glass Reinforcement: Major material used to reinforce plastic. Available in mat, roving, fabric and other forms, it is incorporated into both thermosets and thermoplastics.
Fibreglass: Primarily means glass in fiber form. However, “fiberglass” is also used to describe composite processing and applications. Examples of usage: fiber glass molding plant, fiber glass car.
Fibre Orientation: The fiber alignment in a nonwoven or a mat laminate in which most of the fibers are in the same direction, thereby affording higher strength in that direction.
Fibre Pattern: Visible fibers on the surface of laminates or moldings; the thread size and weave of glass cloth.
Fibre Prominence: The appearance of reinforcement fibers in the surface of a molded part. Can also be termed pattern print-through, strike-through or fiber pattern.
Fibre Reinforced Plastics (FRP): A general term for composite materials or parts that consist of a resin matrix that contains reinforcing fibers such as glass or fiber and have greater strength or stiffness than the resin. FRP is most often used to denote glass fiber-reinforced plastics.
Fibre 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.
Fibre Tow: A loose, untwisted bundle of continuous fibers. In composite technology, “tow” is often used interchangeably with “yarn”, the twisted version.
Fibre Volume: The volume percent of fiber in a composite.
Filament: Fibres 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: A process for fabricating a composite structure in which continuous reinforcements (filament, wire, yarn, tape, or other), either previously impregnated with a matrix material or impregnated during the winding, are placed over a rotating and removable form.
Filament Yarn: A yarn composed of continuous filaments assembled with or without twist.
Filament: The smallest unit of a fibrous material. The basic units formed during drawing and spinning, which are gathered into strands of fiber for use in composites. Filaments usually are of extreme length and very small diameter, usually less than 25 micron. Normally filaments are not used individually. Some textile filaments can function as yarn when they are of sufficient strength and flexibility.
Filamentization: A phenomenon in which a coated strand breaks up into loose individual filaments.
Fill: That part of a woven fabric in which the strands are perpendicular to the main direction of the fabric (warp), the strands running from selvage to selvage . Also called weft.
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.
Fillet: A rounded filing of the internal angle between two surfaces of a plastic molding.
Film Adhesive: An adhesive in the form of a thin, dry, resin film with or without a carrier, commonly used for adhesion between layers of laminates.
Fines: Bundles that have been split apart into smaller bundles composed of only a few or single filaments. Fuzz is usually made of fines.
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.
Fish Eye: Effect of surface contamination causing a circular separation of paint or gel coat.
Flame Retardants: Certain chemicals that are used to reduce or eliminate a resin’s tendency to burn.
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.
Flammability: How fast a plastic material will burn when subjected to a particular ASTM test. In this test, a flame is applied to one end of a strip of material. When the material starts burning the flame is removed and the time to consume a given amount of material is measured. Units are measured in inches per minute (in/min.). Higher numbers indicate that the material will burn faster under this particular test’s conditions. S.E. means self extinguishing; S.E. classified materials stop burning when the flame is taken away.
Flash: Excess material which forms at the parting line of a mold or die, or which is extruded from a closed mold.
Flash Point: Lowest temperature at which a substance gives off enough vapors to form a flammable mixture.
Flexural Modulus: A number referring to a material’s stiffness. It is used to calculate how far a bar will bend when a bending load is applied. Units are normally millions of pounds per square inch. (106 psi) – Giga Pascal (gPa). In two materials of equal thickness, the one with a higher number is more resistant to deflection.
Flexural Strength: Also known as bending strength. Describes how much nonmoving load can be applied to a bar before it yields or breaks. Units are normally thousands of pounds per square inch. (103 psi) – Mega Pascal (mPa). Higher numbers indicate stronger materials that can withstand a heavier load.
Flow Line: A mark on a molded piece made by the meeting of two flow fronts during molding. Also, know as ‘striae’, or ‘weld-mark,’ or ‘weld-line.’
Flow Marks: Wavy surface appearance of an object molded from thermoplastic resins, cased by improper flow of resin into the mold.
Flow: The movement of resin under pressure, allowing it to fill all parts of a mold. The gradual but continuous distortion of a material under continued load, usually at high temperatures; also called creep.
Fly: Loose filaments of fiber that have broken from their parent strand during processing and are freely floating in the air.
Foam: Lightweight, cellular plastic material containing glass-filled voids. Typical foams include urethane, PVC, and polyester.
Force: The male half of the mold that enters the cavity, exerting pressure on the resin and causing it to flow. Also called punch.
Forming Cakes or “Cakes”: Package of glass fibers produced during forming. This package is generally found on a tube placed on a forming carrier and sent through a drying/curing oven. “Cakes” are subsequently put into a roving creel and collected together into a roving doff.
Forming: Process of glass fiber production during which fibers are drawn, attenuated from molten glass and collected in forming cakes.
Fracture Ductility: The true plastic strain at fracture.
Fracture Stress: The true, normal stress on the minimum cross-sectional area at the beginning of fracture.
Fracture Toughness: The damage tolerance of a material containing initial flaws or cracks. Used in aircraft structural design and analysis.
Fracture: When a surface ruptures without the laminate completely separating, or where there is complete separation of a body because of external or internal forces.
FRP: Acronym for fiber glass-reinforced or fiber-reinforced plastic, polymer or polyester.
Fuzz: In Composite Fabrication, the broken filaments found around the glasscutter or chopper. Often referred as creel fuzz.
Fuzz Plug: Small, broken, compacted filaments of glass that collect inside the guide eye tubes feeding the chopper, preventing glass from running through.
Gage Length: The original length of that portion of the specimen over which strain or change of length is determined.
Gate: Point at which molten thermoplastic enters the injection molding tool cavity.
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.
Gel Coat: Surface coat of a specialized, quick-setting polyester resin, either colored or clear, providing cosmetic enhancement and weather ability to a fiberglass laminate. Gel coat is an integral part of the finished laminate.
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.
Gelation: The point during resin cure when viscosity has increased so much that resin barely moves when probed with a sharp instrument.
GFRP: Glass fiber-reinforced plastic, polymer or polyester.
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 Blends: When several different fiber types, i.e. different lengths and diameters, are blended in the fiber slurry.
Glass Cloth: Conventionally woven glass fiber material; certain lightweight glass fabrics are also called scrims.
Glass Content: Percentage of glass in the compound.
Glass Fiber: Reinforcing fiber made by drawing molten glass through bushings. The predominant reinforcement for polymer matrix composites, it is known for good strength, processability and low cost.
Glass Resin Ratio: The amount of glass by weight compared to the amount of resin by weight in a finished laminate or molding.
Glass Rich: An area of molded part that has an overabundance of glass reinforcement. The reinforcement may appear dry and unwet with the resin.
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.
Glint: A visual defect in a fiber glass reinforced cured organic (usually corrosion resistant resin) panel. The defect appears as many small visible unwet or foreign substances; a salt and pepper effect. The defect is not visible before cure but appears at exotherm of the panel.
Good Side: Side of molding in contact with a mold surface.
Graphite Fiber: Fiber made from a precursor by oxidation, carbonization and graphitization process (which provides a graphitic structure).
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.
Green Strength: The strength of a freshly molded part that hasn’t completely cured.
Guide Pin: A pin which guides mold halves into alignment on closing.
Gun Roving: A special type of roving reinforcement designed for use in a gun or chopper gun.
Hand Lay-up: The process of placing (and working) successive plies of reinforcing material or resin-impregnated reinforcement in position on a mold by hand. Room temperature curing thermosetting polymers, mainly epoxies and polyesters, in association with glass, mineral, or fiber reinforcements are used. Catalyzed resin mixtures are sprayed, brushed, or rolled on a mold. A precut reinforcing layer is laid on the wet resin. After the resin soaks into the reinforcement, subsequent layers are built up to the required thickness and are cured, removed from the mold and trimmed. Some variations of hand lay-up techniques are bag molding, drape molding, vacuum molding and spray-up molding. Typical parts are custom auto bodies and boat hulls.
Hardner: The substance (chemical) which reacts with a resin to form the cross-linked (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.
Heat Resistance: An ability of plastics and elastomers to resist deterioration due to elevated temperatures.
Heat Sink: A contrivance for the absorption or transfer of heat away from a critical element or part. Bulk graphite is often used as a heat sink.
Helical Winding: In filament wound items, a winding in which a filament band advances along a helical path, but not necessarily at a constant angle (except on cylinders).
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).
High-Pressure Laminates: Laminates molded and cured at pressures not lower than 6.9 MP (1.0 ksi), and more commonly between 8.3 to 13.9 Mpa (1.2 to 2.0 ksi).
Homogeneous: Describes a material with a uniform composition.
Honeycomb: Lightweight, cellular structure made from either metallic sheet materials or non-metallic materials (e.g., resin-impregnated paper or woven fabric) and formed into hexagonal nested cells, similar in appearance to the cross section of a beehive, that serves as a core material in sandwich constructions. Honeycomb may also be metallic or polymer materials in a rigid, open-cell structure.
Hoop Stress: The circumferential stress in a material of cylindrical form subjected to internal or external pressure.
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).
Hydraulic Press: A press in which molding force is created from pressure exerted by a fluid.
Hygroscopic: Material that absorbs moisture from the air.
Hysteresis: The energy absorbed in a complete cycle of loading and unloading. Mechanical energy is converted into friction energy (heat).
Ignition Loss: With glass, the difference in weight before and after binder or size has been burned off.
Impact Strength: A moving load is one that is moving when it strikes a bar. The effect of such a load is denoted by the work “impact”. The impact strength of a material is a measure of how much energy is absorbed by the bar when it is broken by a moving weight.
Impact Test: Is the measure of energy necessary to fracture a standard sample by an impulse load.
Impregnate: In reinforced plastics, to saturate reinforcement, especially fiberglass, with a resin.
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.
Inhibitor: A substance that retards polymerization, thus extending shelf life of a monomer. Also used to influence gel time and exothermic reaction.
Initiator: Peroxides used as sources of free radicals. They are used in free-radical polymerization, for curing thermosetting resins, as cross-linking agents for elastomers and polyethylene, and for polymer modification.
Injection Molding: Method of forming plastic to the desired shape by forcing a heat-softened thermoplastic polymer into a relatively cool cavity under pressure or thermosetting polymer into a heated mold.
Inorganic Pigments: Natural or synthetic metallic oxides, sulfides, and other salts that impart heat and light stability, weathering resistance, color, or migration resistance to plastics.
Insert: An integral part of plastic molding consisting of metal or other material that may be molded into the part or pressed into position after the molding is completed.
In-Situ: In place. In the position which it will finally occupy, e.g. molding or forming foam.
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 or surface between two different, physically distinguishable media. On fibers, the contact area between fibers and sizing or finish. In a laminate, the contact area between the reinforcement and the laminating resin.
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.
Intumescent: Fire-retardant technology causing an otherwise flammable material to foam, forming an insulating barrier when exposed to heat.
Irreversible: Not capable of re-dissolving or re-melting. Chemical reactions that proceed in a single direction and are not capable of reversal (as applied to thermosetting resins).
Isophthalic: Polyester resin based on isophthalic acid, generally higher in properties than a general purpose or orthothatic polyester resin.
Isotropic: Having uniform properties in all directions. The measured properties of an isotropic material are independent of the axis of testing.
Izod Impact Test: A test for shock loading in which a notched specimen bar is held at one end and broken by striking, and the energy absorbed is measured. Izod is one of many different test methods for measuring impact. Units are measured in foot pounds per inch of width; sometimes given as foot pounds per inch of notch. Joules/Meter (J/M). Higher numbers mean that the material will absorb more energy before it is broken by a moving weight.
Jackstrawing: Visual effect of glass fibre turning white in a cured laminate. It may not affect the strength of a laminate, but could indicate air entrapment or water contamination.
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 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-cracking: 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.
Micro-strain: 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.
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: A 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.
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.
Polymer: An organic material composed of long molecular chains consisting of repeating chemical units.
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.
Post-cure: Additional elevated temperature curing, 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.
Pre-molding: 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).
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.
Release Film: An impermeable layer of film which does not bond to the resin being cured.
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.
Roving: A number of strands, tows, or ends collected into a parallel bundle with little or no twist.
Rubber: Cross-linked polymers whose glass transition temperature is below room temperature and which exhibit highly elastic deformation and have high elongation.
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.
Sandwich Construction: A composite part 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: A fabric woven into an open mesh construction, used in the processing of tape or other B-stage material to facilitate handling. Also called as Carrier.
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.
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.
Shear Fracture: 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.
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 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).
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.
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.
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 change in dimension of specimen in the direction of externally applied stress and magnitude of the applied stress. Values of stress usually are plotted as ordinates (vertically) and strain values as abscissa (horizontally).
Surface 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 laminate mid-plane is a mirror image of the stacking sequence above the mid-plane.
Tack: Stickiness of the laminate or prepreg.
Tacking: To locally join together layers of thermoplastics by localized melting of the resin.
Tape: Reinforcement or prepreg fabricated in small widths available commercially.
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.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.
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.
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.
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).
Un-bond: An area within a bonded interface between two adherents 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.
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.
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: 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, delamination) 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.
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/or absorbed 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. Sometime, know as Hand Lay-up.
Wet Strength: The strength of an organic matrix composite after the composite has absorbed moisture.
Wet Winding: Is a method of filament winding. 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 fibre constituent consists of woven fabric.A woven fabric composite normally is a laminate comprised of a number of lamina, each of which consists of one layer of fabric embedded in the selected matrix material. Individual fabric lamina 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 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.