HIGH-PERFORMANCE COMPOSITE PRODUCTS SINCE 1945
 
 

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

A

ABS: Acrylonitrile Butadiene Styrene (Thermoplastic Copolymer).

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.

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.

Acoustic Transmission: The ability of a material to conduct or pass sound from one source to another. The transmission loss of a structure is a measure of its sound insulation. It is defined as the ratio, expressed in decibels, of the acoustic power incident on the structure and the acoustic power transmitted by the structure to the receiving side.

Acrylic: Thermoplastic polymer made by the polymerization of esters of acrylic acid and its derivatives.

Additive: A material used to modify the properties of polymer resins. Examples include plasticizers, initiators, light stabilizers and flame-retardants.

Adhesion: The state in which two surfaces are held together by an interlocking action or force.

Adhesive: A film, liquid or paste capable of holding the surfaces of two materials together.

Aggregate: Hard, coarse material usually of mineral origins used in composite tools. Also used in flooring or as a surface medium.

Aging: The process of exposing materials to an environment for an interval of time.

Alloy: In plastics, a blend of polymers under select conditions that results in a plastic material having unique properties as compared to the individual components. In metals, a blend of two or more elements under select conditions so as to enhance the material properties of the resultant metal, as compared to the individual elements.

Ambient: The surrounding environmental conditions, such as pressure, temperature, or relative humidity of a specific location.

Antioxidant: Substance that, when added in small quantities to resin, prevents oxidation and degradation while maintaining the resin's properties.

Antistatic Agents: Agents added to a molding material or applied to the surface of a molded object to make it more conductive and prevent the fixation of dust or buildup of electrical charge.

Aramid: A class of polymers characterized by repeating units of aromatic amide. They are generally produced as highly oriented, high-strength, high-modulus fibers that can then be converted to a variety of use forms including fabrics, mats, papers, etc. Examples include Kevlar and Nomex (DuPont Registered Trade names) and Twaron (Teijin Company).

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.

A-Stage: Early stage in the polymerization reaction of certain thermosetting resins (especially Phenolic) in which the material, after application to the reinforcement, is still soluble in certain liquids and is fusible. Also called resole. see B-Stage and C-Stage

ASTM: American Society of Test Methods.

Autoclave: Closed vessel for conducting and completing a chemical reaction or other operation under pressure and heat.

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.



Bagging: Applying an impermeable layer of film over an uncured part and sealing the edges so that a vacuum can be drawn.

Bag Molding: Process in which fluid or gas is applied through a flexible membrane to consolidate material in a mold.

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.

Balanced Laminate: Composite laminate whose 0 and 90 angles occur only in + or - pairs (not necessarily adjacent) and are symmetrical around the centerline.

Bare Glass: Glass in fiber form as it flows from the bushing before a binder or sizing is applied.

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.

Bias Fabric: Warp and fill fibers placed at an angle to the length of the fabric.

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: Reinforcing fibers arranged in two directions, usually at right angles.

Bi-directional Laminate (Cross Laminate): A reinforced plastic laminate whose fibers are oriented in two directions in its plane.

Binder: Coating which is applied to the surface of a chopped glass mat or preform and then cured to hold bundles or ends together in a stable form during the roving operation.

Bismaleimide (BMI): A polyimide that cures through an addition rather than a condensation reaction, thus avoiding problems with volatiles forming during the cure process. BMI's have the advantage of processing more like epoxy resins while having higher thermal capability. BMI has an Intermediate temperature capability (between epoxy and condensation polyamides).

Bleeder Cloth: Woven or non-woven layer of material used in composite parts manufacturing that allows excess gas and resin to escape during cure. The bleeder cloth is removed after the curing process and is not part of the final composite.

Blister: Flaw either between layers of laminate or between the gel coat film and laminate.

Bobbin: The spool or shipping package on which textile yarns are wound.

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.

Braid/Braider: A narrow tubular or flat fabric produced by intertwining a single set of yarns according to a definite pattern.

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.

B-Stage: Intermediate stage in the reaction of certain thermosetting resins in which the material softens when heated, is plastic and is fusible but may not entirely dissolve or fuse. Also called "resistol" or "resitol." Resin in an uncured prepreg or premix is usually in this stage.

Buckling (Composite): Failure mode generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.



Capacitance: A measure of the amount of electrical charge stored (or separated) for a given electrical potential (voltage).

Carbon: Element that provides the backbone for all organic polymers. Graphite is a more ordered form of carbon. Diamond is the densest crystalline form of carbon.

Carbon-Carbon: Composite material consisting of carbon or graphite fibers in a carbon or graphite matrix.

Carbon Fiber: Fiber produced by the pyrolysis of organic precursor fibers, such as rayon, polyacrylonitrile (PAN), and pitch, in an inert environment.

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: A substance that acts as an agent to aid or speed the cure of a resin, by decomposing in the presence of a promoter to release an active radical or other chemical moiety that lowers the activation energy of the reacting resin.

Catastrophic Failures: Totally unpredictable failures of a mechanical, thermal, or electrical nature.

Caul Plates: Smooth metal plates free of surface defects with the same size and shape as a composite lay-up that contacts the lay-up during curing. Caul plates transmit normal pressure and temperature to the finished laminate while providing it with a smooth surface.

Cell: The thin-walled, columnar cavity of honeycomb core that can have a variety of sizes and shapes, depending on the specific process of manufacturing.

Chalking: Surface phenomenon indicating degradation of a cosmetic surface. Chalking is a powdery film that appears lighter than the original color.

Cloth: Fabric reinforcement made by weaving strands of glass, carbon, aramid or polymer fiber yarns.

Coefficient of Thermal Expansion (CTE): How much a material's shape will change per each degree of temperature change.

Cold Flow: Distortion that occurs in a material under continuous load within its working temperature range and without a phase or chemical change.

Compaction: Applying a temporary vacuum bag and vacuum to remove trapped air from the lay-up; compaction can also be achieved by pressing with external pressure such as a press, a squeegee paddle, an iron, etc.

Compatibility: The ability of two or more substances to be combined in order to form a composition of useful plastic properties; for example, the suitability of a sizing or finish for use with certain general resin types.

Composite Fabrication: A framework used to hold spools of fiberglass roving or other fiber (tow), from which the fibers (tows) are dispensed and "threaded up" through guide eyes and combs into a prepregging process.

Composite Material: Chemical or mechanical joining of dissimilar materials such as glass fiber and polyester resin, whose cumulative properties are superior to the individual materials.

Compression Molding: An open molding process in which material is introduced and shaped by the pressure of closing the mold and by heat.

Compressive Modulus: Ratio of compressive stress to compressive strain below the proportional limit. Theoretically equal to Young's modulus determined from tensile experiments, but buckling effects often result in lower actual measurement. It is the measure of the resistance to a load applied normal to a specimen surface. Units are usually millions of pounds per square inch (msi) in the English system or Giga Pascals (GPa) in the metric system. 1 GPa is equivalent to approximately 0.145 msi

Compressive Strength: The amount of static load, applied normal to the specimen, that a material can sustain take before it fails. Units are usually thousands of pounds per square inch (ksi) in the English system or Mega Pascals (MPa) in the metric system. 1 MPa is equivalent to approximately 0.145 ksi

Conductivity: Reciprocal of volume resistivity. The electrical or thermal conductance of a unit cube of any material (conductivity per unit volume).

Conformability: A material's ability to conform to difficult shapes without wrinkling or leaving excessively resin-rich or resin-starved areas. (See Drape)

Continuous Filaments: Filaments that extend substantially throughout the length of the yarn.

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 laminating can also be accomplished by using preimpregnated reinforcements.

Continuous Rovings: Rovings supplied in a package that allows for continuous processing.

Continuous Strand: Fiberglass mat of very long individual fibers that have a regular crossed pattern and are loosely held together with a binder.

Copolymer: A resin produced by a polymerization process in which segments of dissimilar prepolymers are combined into one polymer chain, either arranged in alternate sequence or randomly.

Core: (1) A material used between two facings (skins) that serves to rigidly separate the facings by a fixed or variable thickness. Examples are end-grain balsa wood, urethane foam, PVC foam and various honeycomb materials. The core becomes the central member in a sandwich construction, to which the faces of the sandwich are attached or bonded.

(2) A device on which fabric or prepreg is wound.

(3) The opening or cavity of a mold.

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 organic materials, it could be crazing.

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.

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: Glass Fiber Manufacturing - A framework used to hold forming cakes so they can be wound or roved into roving doffs. Creels generally hold 10 to 33 forming cakes that are replaced randomly when they run out or as doffs are roved.

Creep: The slow movement of a plastic material with time.

Creep, Rate of: Rate of the slope of the creep-time curve at a given time. Deflection with time under a given static load.

C-Stage: Final stage in the reaction of certain thermosetting resins in which the material is practically insoluble and infusible.

Cure: Cross-linking or total polymerization of a resin's molecules that alters a material's properties, changing it from a liquid to a solid.

Cure Cycle: The time/temperature/pressure cycle used to cure a thermosetting resin system or prepreg.

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.

Curing Agent: A catalytic or reactive agent that, when added to a resin, causes polymerization. Also called hardener.

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.



Daylight: The distance, in the open position, between the moving and fixed tables or the platens of a hydraulic press. In a multi-platen press, daylight is the distance between adjacent platens. Daylight provides space so a molded part can be removed from the mold.

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 part.

Deflection Temperature Under Load: Temperature at which a simple beam has deflected a given amount under load (formerly called heat distortion temperature).

Deformation Under Load: 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'.)

Delamination: Separation of composite layers, either local or covering a wide area. Can occur in the cure process or by damage in service.

Denier: A direct numbering system for expressing linear density, equal to mass in grams per 9000m of yarn, filament, fiber, or other textile strand. See Tex.

Density: A material's mass per unit volume. Measurements may be in pounds per cubic foot (pcf) in the English system or grams per cubic centimeter (g/cc) or kilograms per cubic meter (kg/m3) in the metric system. Higher numbers indicate denser materials that appear to be heavier. 1 g/cc is equivalent to approximately 62.4 pcf; 1 pcf is equivalent to approximately 16 kg/m3

Die: A material forming tool used in extrusion, pultrusion and vacuum forming processes.

Dielectric: A nonconductor of electricity. A material's ability to resist the flow of electrical current.

Dielectric Constant: An assembly's ratio of capacitance when its two electrodes are separated solely by a plastic insulating material to its capacitance when the electrodes are separated by air.

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" in a table of dielectric strength properties means that the material conducts electricity and therefore has no dielectric strength.

Dimensional Stability: A part's ability to retain the precise shape to which it was molded, cast, or otherwise fabricated.

Dispersion: Degree to which any filler material separates into discrete particles or units. For example, a roving separates into discrete bundles after being chopped. Good dispersion is characterized by a bed of bundles 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.

In a resin formulation, hardener and flame retardant are often in small particulate form that need to be well dispersed in order for the formulation to cure properly and have consistent properties.

Distortion: Change in shape from that which is intended. Symptomatic of laminating difficulties, curing problems, tooling problems or resin shrinkage.

Draft: The taper or slope of a mold's vertical surface allowing molded parts to be removed.

Drape: The ability of pre-impregnated broad goods to conform to an irregular shape; textile conformity. (See Conformability)

Dry Spot: An area of resin starvation or poor bonding between the reinforcement and resin matrix in a FRP composite material.

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.



Elasticity: A material's ability to recover its original size and shape after the force deforming it has been removed.

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.

Elastomer: A material that substantially recovers its original shape and size at room temperature after a deforming force is removed. Rubber is an example.

Elongation: As mentioned under tensile modulus, when a test specimen is pulled it gets longer. The elongation tells how much longer it gets before it breaks.

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.

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 resin containing one or more epoxide groups cured by its reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, or mercaptans. An important matrix resin in structural composites and adhesives. Epoxies generally have higher physical properties than polyester resins or phenolic resins, but may be more costly to process.

Even Tension: Applying the same amount of tension to each end of roving in a ball, or to each tow in a creel.

Exotherm: The liberation or evolution of heat during the curing of a plastic product. An undesired and potentially hazardous rapid polymerization of a resin material that generates excessive heat and leads to degradation of the material, including toxic smoke, fire, and possibly explosion.

Exothermic Heat: Heat given off during polymerization by chemical ingredients as they react to form chemical bonds causing the resin to cure.

Extruder: Machine that pushes molten plastic through a die to form fibers, films or other desired shapes.



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).

Fatigue Limit: The maximum level under which a material can be stressed cyclically for an infinite number of times before it fails.

Fatigue Strength: The maximum cyclical stress a material can withstand for a given number of cycles before failure occurs. The residual strength after being subjected to fatigue.

Fiber: The major reinforcement material component in a composite matrix. Often, fiber is used synonymously with filament. Fiber may be any material that is drawn or spun into a long, continuous, small diameter filament. Examples are glass, carbon, Kevlar, polypropylene, nylon, cotton, hemp.

Fiber Content: The amount of fiber present in a composite. This is usually expressed as a percentage volume fraction or weight fraction of the composite.

Fiber Glass: Primarily means glass in fiber form. However, it has become somewhat of a generalized term that can refer to the entire composite material made of polyester resin and fiber glass reinforcement, or the process of making such a composite.

Fiber Glass Reinforcement: Major material used to reinforce plastic. Available in mat, roving, fabric and other forms, it is incorporated into both thermosets and thermoplastics.

Fiber Reinforced Plastic (FRP): A general term for a composite that consists of a thermoset or thermoplastic matrix resin reinforced with cloth, mat, strands, or any other fiber form.

Filler: A substance that is added to plastic resins to extend volume, improve specific properties, or lower cost. Generally a filler does not react or combine chemically with the matrix resin.

Fillet: A concave easing of an interior corner of a part design. In adhesive bonds it is the rounded filling of the internal angle between the two bonded surfaces.

Fire Retardants: Chemicals that reduce a resin's tendency to burn.

Flammability: A measure of a materials ability to support a flame at ambient conditions. Generally this is measured by a test in which a flame is applied to one end of a strip of material for a specified length of time and then removed. The length of time the material continued to burn is measured, the amount of material burned is measured by length from the point of flame application, and any other burning characteristics are noted, such as dripping, glow after the flame extinguishes, etc.

Flash: Residue from a molding or curing process that flows or is extruded from the mold or press during the process. Flash must be removed before the part is considered finished.

Flash Point: Lowest temperature at which a substance gives off enough vapors to form a flammable mixture.

Flexible Molds: Rubber or elastomeric plastic molds used for casting plastics. They can be stretched to remove cured pieces with undercuts.

Flexural Modulus: A measure of a material's stiffness when subjected to a bending load. Units are normally millions of pounds per square inch (msi) in the English system, or Giga Pascals (GPa) in the metric system. The higher the modulus, the stiffer a material is. 1 GPa is equivalent to approximately 0.145 msi

Flexural Strength: Also known as bending strength. A measure of how much static load can be applied to a beam before it yields or breaks. Units are normally thousands of pounds per square inch (ksi) in the English system or Mega Pascals (MPa) in the metric system. Higher numbers indicate stronger materials that can withstand a heavier load. 1 MPa is equivalent to approximately 0.145 ksi

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. In prepreg physical properties, it is a measure of how much resin is lost when the material is pressed at a given pressure and temperature until it cures.

Flow Line: A mark on a molded piece made by the meeting of two flow fronts during molding. Also called striae or striations, 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.

Force: The load applied to a test specimen in a specific direction. Force is measured in pounds (lb) in the English system or in Newtons (N) in the metric system. 1 lb is equivalent to approximately 4.45 N

Fracture: When a composite material ruptures or fails by either delamination or fiber breakage.

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.

FRP: Acronym for fiber glass-reinforced or fiber-reinforced plastic, polymer or polyester.



Galvanic Corrosion:  An electrochemical process in which one metal corrodes preferentially to another when both metals are in electrical contact and immersed in an electrolyte. The same galvanic reaction is exploited in primary batteries to generate a voltage.

Gelation: The point during resin cure when viscosity has increased so much that resin barely moves when probed with a sharp instrument  See also Gel Point.

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: When a liquid begins to exhibit pseudo-elastic properties. This stage may be conveniently observed from the inflection point on a viscosity time plot.

GFRP: Glass fiber-reinforced plastic, polymer or polyester.

Glass Content: Percentage of glass in the compound, either by weight or by volume.

Graphite Fiber: Fiber made from a precursor by oxidation, carbonization and graphitization process (which provides a graphitic structure).

Green: Resin, which has not completely cured and is still rather soft and rubbery.

Green Strength: That ability of the material, while not completely cured, to undergo removal from the mold and handling without tearing or permanent distortion.

GRP: A derivation commonly used in Europe referring to glass-reinforced plastic, polymer or polyester.



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. Method of molding room temperature curing thermosetting polymers, mainly epoxies and polyesters, in association with glass, mineral, or fiber reinforcements. Catalyzed resin mixtures are sprayed, brushed, or spatulated 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.

Hardener: A substance or mixture added to a polymer composition to promote or control curing by taking part in it.

Hardness: Resistance to surface indentation usually measured by the depth of penetration (or arbitrary units related to the depth of penetration) of a blunt point under a given load using a particular instrument according to a prescribed procedure.

Heat Distortion Temperature: The temperature at which a material will bend under a given load. It was developed for thermoplastic materials, which soften considerably when heated. It has relatively little value as a design figure for thermosetting reinforced plastics. During this test, a load is applied in bending to cause 264 psi stress in the material. The temperature of the material is then raised until the material bends one-tenth of an inch at the center. Units are measured in degrees Fahrenheit (F) minus degrees Centigrade (C). Higher numbers mean that the material can be heated to a higher temperature before it deflects one-tenth of an inch under the arbitrary load of 264 psi.

Heat Resistance: The ability of plastics and elastomers to resist deterioration due to elevated temperatures.

Homogeneous: Describes a material with a uniform composition.

Homopolymer: A compound produced by polymerization of a single monomer without any additional hardener or cross linker.

Honeycomb: Manufactured product of resin-impregnated sheet material (paper, glass, fabric) or metal foil, formed into hexagonal-shaped cells. Used as a core material in sandwich construction.

Hybrid: A composite laminate consisting of two or more composite material systems. Two or more different fibers, such as carbon and glass or carbon and aramid, combined into a structure.

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 Test: Measure of the energy necessary to fracture a standard sample by an impulse load. There are many different methods for determining impact resistance or impact strength and each method will lead to a different result for a given material. It is important to the details of the test when comparing results.

Impregnate: In reinforced plastics, to saturate a reinforcement, especially fiberglass, with a resin.

Inhibitor: A substance that retards polymerization, thus extending shelf life of a monomer. Also used to influence gel time and exotherm.

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.

Instron: A leading manufacturer of instruments for determining the mechanical properties of materials. The manufacturer name is often used to describe virtually any mechanical test machine.

Interface: A surface that lies between two different materials.

Interlaminar: Descriptive term pertaining to an object (for example, voids), event (for example, fracture), or potential field (for example, shear stress) referenced as existing or occurring between two or more adjacent laminate.

Interlaminar Shear: Shearing force that produces a relative displacement between two laminae in a laminate along the plane of the interface.

Intumescent: Fire-retardant technology causing an otherwise flammable material to foam, forming an insulating barrier when exposed to heat.

Isotropic: Having uniform properties in all directions.

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.



Jackstrawing: Visual effect of glass fiber 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 in the class of aromatic polyamides having a para-type orientation (refers to the relative positions of substitutions on a benzene ring). Kevlar is DuPont Company's brand name for a particular light but very strong aramid fibre. It can be spun into ropes or sheets of fabric that can either be used as-is, or used in the construction of composite components. It is now used in a wide range of applications - from bicycles to bulletproof jackets - due to its high strength-to-weight ratio, five times greater than that of steel.



Laminate: Primarily means a composite material system made with layers of fiber reinforcement in a resin. Sometimes used as a general reference for composites, regardless of how made. Examples of usage: laminate consumption by market, compression-molded laminate.

Lay-up: Act of building up successive layers of polymer and reinforcement. Layers of catalyzed resin and fiberglass or other reinforcements are applied to a mold in order to make a part. Also refers to the reinforcing material placed in position in the mold, the process of placing reinforcing material in position in the mold, or the resin-impregnated reinforcement.

Liquid-Crystal Polymer: A newer thermoplastic polymer that is melt process capable and develops high orientation in molding, resulting in tensile strength and high-temperature capability.

Loom: A mechanical device that interlaces fibers at right angles with varying degrees of weave construction (weight, thickness and design). More modern looms are air jet but rapier and more traditional shuttle equipment is still in use.

Low-Pressure Laminates: Laminated, molded, and cured using pressures from 400 psi down to and including the pressure obtained by mere contact of the plies.



Mandrel: The core tool around which resin-impregnated paper, fabric or fiber is wound to form pipes, tubes or structural shell shapes.

Mat: A fibrous material for reinforced plastic consisting of randomly oriented chopped filaments, short fibers (with or without a carrier fabric) or swirled filaments loosely held together with a binder.

Mat Binder: Resin applied to glass fiber and cured during the manufacture of mat that holds fibers in place and maintains the mat's shape.

Matched Metal Molding: A reinforced plastics manufacturing process in which matching male and female metal molds are used (also called compression molding) to form the part with time, pressure and heat.

Mat Strength: The mat's ability to resist being pulled apart under tension during impregnation and molding.

Mechanical Properties: A material's properties, such as compressive and tensile strength and modulus, that are associated with elastic and inelastic reactions when force is applied. The individual relationship between stress and strain.

Metallic Fiber: Manufactured fiber composed of metal, plastic-coated metal, metal-coated plastic or core completely covered by metal.

Micro balloons: Microscopic bubbles of glass, ceramic or Phenolic, used as a filler or to create syntactic foam or putty mixtures.

Micro cracking: Crack formed in composites when thermal stresses locally exceed the strength of the matrix.

Micron: A millionth of a meter. One micron = .001 millimeter = .00003937 inch.

Mil: Unit used to measure the diameter of glass fiber strands (1 mil = 0.001 in.).

Milled Fiber: Continuous glass strands hammer milled into very short glass fibers. Useful as inexpensive filler or anti-crazing reinforcing filler for adhesives.

Modulus, Initial: The slope of the initial straight portion of a stress strain or load-elongation curve.

Modulus of Elasticity: How much a material can bend without losing its ability to return to its original physical properties. Also known as Young's Modulus, is a measure of material resistance to axial deformation. Its value is obtained by measuring the slope of the axial stress-strain curve in the elastic region. It is named after the English scientist Thomas Young. It is usually denoted by E, and has units of giga pascal, GPa, or millions of pounds per square inch, msi. 1 GPa is equal to 0.145 msi.

Moisture Content: The amount of water in a material determined under prescribed conditions, expressed as a percentage of the mass of the moist specimen; that is, the mass of the dry substance plus the moisture present.

Mold: The cavity or matrix into or on which the plastic composition is placed and from which it takes form. The tool used to fabricate the desired part shape.

Molding: The forming of a polymer or composite into a solid mass of prescribed shape and size. Pressing between flat platens of a press is a type of molding.

Mold-Release Agent: Lubricant, liquid or powder (often silicone oils and waxes) that prevents molded articles from sticking in the cavity.

Monomer: A simple molecule capable of reacting with like or unlike molecules to form a polymer; the smallest repeating structure of a polymer (mers); for addition polymers, this represents the original unpolymerized compound.



NOMEX: is the registered brand name of a flame retardant meta-aramid material marketed and first discovered by DuPont in the 1970s. It can be considered an aromatic "nylon". It is sold in both fiber and sheet forms and is used as a fabric wherever resistance from heat and flame is required. Nomex sheet is actually a calendared paper and made in a similar fashion. The paper is used in electrical laminates such as circuit boards and transformer cores as well as fireproof honeycomb structures where it is coated with a phenolic resin. Honeycomb structures are used extensively in aircraft construction to save weight. Both the firefighting and vehicle racing industries use NOMEX to create clothing and equipment that can stand up to intense heat. It is the meta variant of the para-aramid Kevlar. Both aramids are heat and flame resistant but Kevlar, having a para-orientation can be molecularly aligned and gives high strength. Meta aramid polymer cannot align during filament formation and has less strength.

Non-Woven Fabric: A textile structure produced by bonding or interlocking fibers, or both, accomplished by mechanical, chemical, thermal and/or solvent means.

Notch Sensitivity: The extent to which a material's sensitivity to fracture is increased by a surface in homogeneity such as a notch, a sudden change in section, a crack or a scratch. Low notch sensitivity is usually associated with ductile materials and high notch sensitivity with brittle materials.



Orange Peel: Gel coated or painted finish that is not smooth and is patterned similar to an orange's skin.

Organic: Matter originating in plant or animal life, or composed of chemicals of hydrocarbon origin, either natural or synthetic.

Orientation: Position with relation to the predominant direction of the fiber, in case of a laminate; the core ribbon direction, in the case of a sandwich structure; the warp direction of a fabric.

Overlay Sheet: Non-woven fibrous mat (of glass, synthetic fiber or other material) used as the top layer in a cloth or mat lay-up to provide a smoother finish, minimize the appearance of a fibrous pattern, or permit machining or grinding to a precise dimension. Also called surfacing mat.



PBT: Polybuthlene Therephthalate (Thermoplastic Polyester Resin).

PET: Polyethylene Terephthalate (Thermoplastic Polyester Resin).

Phenolic Resin: Thermosetting resin produced by the condensation of an aromatic alcohol with an aldehyde, particularly of phenol with formaldehyde. Used in high-temperature applications with various fillers and reinforcements. Phenolic resins find wide usage in applications where excellent flammability resistance, low smoke generation and low toxicity are needed. Parts made with phenolic resin matrices generally have acceptable heat release properties also.

Pigment: Colorant added to gel coat or resin.

Pinholes: Small holes on the exposed gel coated surface that are about the diameter of common pins and may be easily counted.

Pit: Small regular or irregular crater in the surface of a plastic, usually of equal width and depth.

Plastic: Material of which an essential ingredient is an organic polymer of large molecular weight and also contains hardeners, fillers and reinforcements; is solid in its finished state; and has been shaped by flow during some stage of its manufacture or processing. A plastic may be either thermoplastic or thermoset.

Plastic Deformation: Change in dimensions of an object under load that is not recovered when the load is removed; opposed to elastic deformation.

Plasticizers: Material added to increase a plastic's workability and flexibility. Normally used in thermoplastics. Also a lower molecular weight material added to epoxy to reduce stiffness and brittleness, thereby resulting in a lower glass transition temperature for the polymer.

Plastic Tooling: Tools (mostly for metal forming trades) constructed of plastics, generally laminates or casting materials.

Platens: Mounting plates of a press to which the entire mold assembly is bolted.

Polyamide: A polymer in which the structural units are linked by amide or thioamide groupings. Many polyamides are fiber-forming.

Polyester (Unsaturated): Product of an acid-glycol reaction commonly blended with a monomer to create a polymer resin. In its thermosetting form it is the most common resin used in the FRP industry.

Polymer: Chain molecule composed of many identical groups, commonly found in plastics.

Polymerization: Chemical bonding of polymer molecules during the curing reaction.

Porosity: Entrapped gas bubbles or voids in a gel coat film.

Pot Life: The length of time that catalyzed resin retains a viscosity low enough to be used in processing.

PPO: Polyphenylene Oxide (Thermoplastic Resin).

PPS: Polyphenylene Sulfide (Thermoplastic Resin).

Preform: Preshaped fibrous reinforcement formed when chopped fibers are distributed by air, water flotation or vacuum over the surface of a perforated screen to the approximate contour and thickness desired in the finished part. Also, a preshaped fibrous reinforcement of mat or cloth formed to the desired shape on a mandrel or mock-up prior to being placed in a mold press. Also, a compact "pill" formed by compressing premixed material to facilitate handling and control the uniformity of charges for mold loading.

Premix: A compound prepared prior to and apart from the molding operation containing all components required for molding: resin, reinforcement, fillers, catalysts, release agents and other compounds.

Prepreg: Either ready-to-mold material in sheet form or ready-to-wind material in roving form, which may be cloth, mat, unidirectional fiber or paper impregnated with resin and stored for use. The resin is partially cured to a B-stage and supplied to the fabricator, who lays up the finished shape and completes the cure with heat and pressure. The two distinct types of prepreg available are (1) commercial prepregs, where the roving is coated with a hot melt or solvent system to produce a part meeting specific customer requirements, and (2) wet prepreg, where the basic resin is installed without solvents or preservatives but has limited room-temperature shelf life.

Press: A manufacturing process in which a composite material is cured by the application of heat and pressure, usually in a flat form. Hydraulic pressure is applied by rams to stiff platens, between which the composite assemblies are placed, and appropriate cure cycles are then performed.

Print Through: Distortion in a part's surface through which the pattern of the core or fiberglass reinforcement is visible. Also known as print out, telegraphing or read through.

Pultrusion: Continuous process for manufacturing composites with a constant cross-sectional shape. The process consists of pulling a fiber-reinforcing material through a resin impregnation bath and shaping die, where the resin is subsequently cured.



Reaction Injection Molding (RIM): Process for molding polyurethane, epoxy and other liquid chemical systems. Combining two to four components in the proper chemical ratio is accomplished by a high-pressure impingement-type mixing head, from which mixed material is delivered into the mold at low pressure, where it reacts (cures).

Reinforced Plastics: Molded, formed, filament-wound, tape-wrapped or shaped plastic parts consisting of resins to which reinforcing fibers, mats and fabrics have been added before the forming operation to provide strength properties greatly superior to those of the base resin.

Reinforced Reaction Injection Molding (RRIM): A reaction injection molding with reinforcement added. See Reaction Injection Molding

Resin: Solid or pseudo-solid organic material, usually of high molecular weight, that tends to flow when subjected to stress. Most resins are polymers. In reinforced plastics, the material used to bind together the reinforcement material.

Resin Content: The amount of resin in a laminate, expressed either as a percent of total weight or total volume.

Resin-Rich Area: Localized area filled with resin and lacking reinforcing material.

Resin-Starved Area: Localized area of insufficient resin, usually identified by low gloss, dry spots or fiber showing on the surface.

Resin Tearing: Separation of pigments in a gel coat affecting cosmetic appearance.

Resin Transfer Molding (RTM): A process in which catalyzed resin is transferred or injected into an enclosed mold where fiberglass reinforcement has been placed.

Rib: Reinforcing member of a fabricated or molded part.

Room Temperature Curing Adhesives: Adhesives that set to handling strength within an hour at 68 to 86 degrees F, and later reach full strength without heating.

Roving Doff or "Doff": The final product sold to the customer. Made by roving or pulling together a group of forming cakes. (the number of which depends upon the product being made)

RP: Reinforced plastic, polymer or polyester.

RTP: Sometimes used to distinguish reinforced thermoplastic from reinforced thermosetting plastic.

Rule-of-Mixtures: A composite's properties are the combination of properties in its two constituent materials. The composite property equals the amount of fiber property multiplied by the volume percentage of fiber, plus the amount of matrix property multiplied by the volume percentage of matrix.



SAN: Styrene Acrylonitrile (Thermoplastic Resin).

S Glass: A family of magnesium-alumina-silicate glasses with a certified chemical composition that conforms to an applicable material specification and produces high mechanical strength.

Sandwich Constructions: Panels composed of a lightweight core material, such as honeycomb or foamed plastic, to which two relatively thin, dense, high-strength or high-stiffness faces or skins are adhered.

Scrim: Light woven or non-woven fabric with relatively large openings between the yarns, used to reinforce paper and other products.

Set-Up: To harden, as in curing.

Shear: Engineering term referring to force normally applied to the surface of a given material. The movement between plies of a laminate is referred to as interlaminate shear.

Sheet Molding Compound (SMC): Composite of fibers, usually polyester resin, pigments, fillers and other additives, that have been compounded and processed into sheet form to facilitate handling in the molding operation.

Shelf Life: Allowable storage time before a product must be used.

Shore Hardness: A material's resistance to indentation from a spring-loaded indenter. Higher numbers indicate materials with greater resistance.

Size: Treatment applied to glass fiber that allows resin and glass to adhere to one another. Also allows glass fiber to be conveniently handled.

Soft Glass: A roving product whose sizing is moderately soluble in acetone or styrene, resulting in bundles that tend to open readily or filamentize in the matrix resin. The size is generally between 50% and 80% soluble in acetone.

Solid: The amount of sizing on glass expressed as a percentage of the total weight. The amount of resin formulation dissolved or dispersed in a solvent or carrier expressed as a percentage of the total weight.

Solvent Resistance: The non-swelling of a material; also, a material's ability to resist being dissolved by a particular solvent.

Specific Gravity: A material's weight in relation to the weight of an equal volume of water. For example, a material with a Specific Gravity of 2.0 weighs twice as much as an equal volume of water. Because specific gravity is a ratio of values for two materials, there are no units. Higher numbers indicate denser materials.

Specific Heat (Thermal Capacity): The amount of heat needed to raise the temperature of a unit mass of material one degree. Units are measured in BTUs per pound per degree (BTA/LB/F) - or Joules per kilogram per degree Kelvin (J/Kg/K). Higher numbers indicate more input heat energy is needed to raise the temperature of a material.

Specimen: An individual piece or portion of a sample used for a specific test; also, of specific shape and dimensions.

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.

Static: Buildup of an electrical charge causing the chopper roving to "cling" or stick to the chopper, line and/or people. The static level is quantified by measuring the electrical field strength in kilovolts per unit distance.

Stiffness: The relationship of load to deformation; a term often used when the relationship of stress to strain does not conform to the definition of Young's modulus.

Strength, Flexural: Maximum stress that can be borne by surface fibers in a beam in bending. Flexural strength is the unit resistance to maximum load prior to failure by bending, usually expressed in pounds per square inch, psi or in megapascals, MPa. 1 MPa is equal to approximately 145 psi.

Stress-Strain Curve: Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissa to obtain a stress-strain diagram.

Structural Reaction Injection Molding (S-RIM): Evolution of two other plastic molding processes, RIM and RTM. S-RIM uses the fast polymerization reactions of RIM-type polymers, its intensive resin mixing procedures and its rapid resin injection rates. S-RIM also employs preforms like RTM to obtain composite mechanical properties.

Styrene Monomer: A water-thin liquid monomer used to thin polyester resins and act as the cross-linking agent.

Surfacing Mat: Very thin mat, usually 7 to 20 mil (0.15 to 0.50 mm) thick, used primarily to produce a smooth, resin-rich surface on a reinforced plastic laminate, or for precise machining or grinding. See Veil

Surfactant: Chemicals used to modify or change the wetting characteristics of a layer of resin or polymer. Typically used in adhesives and coatings to improve the ability of the material to form a uniform layer on the substrate.



Tack: Surface stickiness.

Tack Free: Surface which is not sticky, either before or after cure.

Tangent Modulus: Tangent modulus is defined as the slope of a line tangent to the stress-strain curve at a point of interest. Tangent modulus can have different values depending on the point at which it is determined. For example, tangent modulus is equal to the Young's Modulus when the point of tangency falls within the linear range of the stress-strain curve.

Tenacity: Term used in yarn manufacture and textile engineering to denote the strength of a yarn or filament of a given size. Numerically, it is expressed as grams of breaking force per denier unit of yarn or filament size; grams per denier, gpd. The yarn is usually pulled at the rate of 12 inches per minute. Tenacity equals breaking strength (grams) divided by denier.

Tensile Elongation: Engineering term referring to the amount of stretch a sample experiences during tensile stress.

Tensile Load: Load applied away from and to opposite ends of a given sample.

Tensile Modulus: Is the measure of resistance to elongation when a specimen is put under a tension load. Units are normally millions of pounds per square inch. (msi) or Giga Pascals (GPa). Higher numbers indicate materials that do not elongate as much as others under equal tensile loading conditions.

Tensile Strength (Stress): The amount of nonmoving load a specimen can withstand before it breaks due to elongation. Units are normally thousands of pounds per square inch, ksi, or Mega Pascals (MPa). Higher numbers indicate materials that can withstand a stronger pull before breaking.

Tension Device: A mechanical or magnetic device that controls tension of fibers or fabrics in winding or prepregging processes.

TEX: A unit for expressing linear density, equal to the mass in grams of 1 kilometer of yarn, filament, fiber or other textile strand. See denier.

Thermal Coefficient of Expansion: A material property that is the measure of how much a unit length of a material will change when it is heated or cooled. Units are expressed in inches per inch per degree Fahrenheit (in/in/F) or meters per meter per degree C, m/m/C. Higher numbers mean that the material will expand or lengthen more for each degree that its temperature increases. Smaller numbers indicate relative stability to changes in temperature.

Thermal Conductivity (K factor): is the intensive property of a material that indicates its ability to conduct heat. It is defined as the quantity of heat, Q, transmitted in time t through a thickness L, in a direction normal to a surface of area A, due to a temperature difference ?T, under steady state conditions and when the heat transfer is dependent only on the temperature gradient.
It is measured as BTUs (units of heat in the English system) per hour per unit area (square feet) for a thickness of one inch and a temperature difference of one degree Fahrenheit between both sides of the plate, BTU/hr/sq/F/inch. Or in the Metric system Watts per meter for every degree Kelvin, W/mK. Higher values mean that a material conducts heat more efficiently.

Thermoplastic: Capable of being repeatedly softened by an increase of temperature and hardened by a decrease in temperature. Applicable to those materials whose change upon heating is substantially physical rather than chemical, and that in the softened stage, they can be shaped by flow into articles by molding or extrusion.

Thermoplastic Polyesters: Class of thermoplastic polymers in which the repeating units are joined by ester groups. The two important types are (1) polyethylene terephthalate (PET), which is widely used as film, fiber and soda bottles; and (2) polybutylene terephthalate (PBT), primarily a molding compound.

Thermoset: A material that undergoes a chemical reaction caused by heat, catalyst or other condition, which results in the formation of a solid. Once it becomes a solid, it cannot be reformed.

Thermosetting Polyesters: Class of resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl-type active monomer such as styrene, methyl styrene, or diallylphthalate. Cure is affected through vinyl polymerization using peroside catalysts and promoters or heat to accelerate the reaction. The two important commercial types are (1) liquid resins that are cross-linked with styrene and used either as impregnants for glass or carbon fiber reinforcements in laminates, filament-wound structures and other built-up constructions, or as binders for chopped-fiber reinforcements in molding compounds, such as sheet molding compound (SMC), bulk molding compound (BMC) and thick molding compound (TMC); and (2) liquid or solid resins cross-linked with other esters in chopped-fiber and mineral-filled molding compounds (for example, alkyd and diallylphthalate).

Thickeners: Material added to resin to raise its viscosity index so that it will not flow as readily.

Translucent: Permits a percentage of light to pass but not optically clear like window glass.



Ultimate Tensile Strength: The ultimate or final stress sustained by a specimen in a tension test; the stress at moment of rupture.

Unidirectional: Strength lying mainly in one direction. A glass reinforcement in which the fiber is oriented in one direction.

Untreated: A descriptive term for glass fiber yarns having no applied chemicals or coatings other than minimal lubricant or binder for controlling intra-fiber abrasion.

UV Stabilizer: Chemical compound which improves resistance to degradation from ultraviolet radiation.



Vacuum Bag Molding: Process in which a sheet of flexible transparent material, bleeder cloth and release film are placed over the lay-up in the mold and sealed at the edges. A vacuum is applied between the sheet and the lay-up. Entrapped air is mechanically worked out of the lay-up and removed by the vacuum. The part is cured with temperature, pressure and time. Also called bag molding.

Vacuum Assisted Resin Transfer Molding: Also known as VARTM. A Low cost method of resin transfer molding in which a vacuum bag provides a flexible upper tool and also assists in the transfer of a low viscosity liquid resin into a perform of fiber, foam or other reinforcing substrates. Capable of producing 3-4 times as many moldings as that produced through open mold hand layups and with less scrap and less mess. Especially useful for large structures such as boat hulls.

Veil: Ultra thin mat similar to a surface mat.

Vinyl Esters: Thermosetting resins containing esters of acrylic and/or methacrylic acids, many of which have been made from epoxy resin. Cure is accomplished as with unsaturated polyesters by co-polymerization with other vinyl monomers, such as styrene.

Viscosity: Measure of a liquid's resistance to flow.

Void Content: The percentage of voids on a volume basis in a laminate.



Wet Strength (Hot Wet Strength): Mechanical property measured after water immersion or exposure to high humidity. Tests may be performed at room temperature or at sub-ambient or elevated temperatures. Exposure conditions and test temperatures vary depending on the end use of the material.

Wet Lay-Up: Reinforced plastic with liquid resin applied to a dry reinforcement at the time it is being put on the part tool. This is opposed to a prepreg layup (dry layup).

Wet-Out Rate: Time required for a plastic to fill the interstices of a reinforcement material and wet the surface of the reinforcement fibers.

Wrinkle: Surface imperfection pressed into laminated plastics similar to a crease or fold in paper, fabric or other base. Also occurs in vacuum bag molding when the bag is improperly placed, causing a crease.



Yield: Linear density of a roving or yarn, measured by the number of yards per pound.

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

Yield Strength: Stress at the yield point. Stress at which a material exhibits a specified limiting deviation from the proportionality of stress to strain. The lowest stress at which a material undergoes plastic deformation. Below this stress, material is elastic; above it, material is viscous. Often defined as the stress needed to produce a specified amount of plastic deformation (usually a 0.2% change in length).

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