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

The degradation, decomposition and erosion of material caused by high temperature, pressure, time, percent oxidizing species and velocity of gas flow. A controlled loss of material to protect the underlying structure.

Ablative plastic
A material that absorbs heat (with a low material loss and char rate) through a decomposition process (pyrolysis) that takes place at or near the surface exposed to the heat.

The penetration into the mass of one substance by another. The capillary or cellular attraction of adherend surfaces to draw off the liquid adhesive film into the substrate.

Accelerated test
A test procedure in which conditions are increased in magnitude to reduce the time required to obtain a result. To reproduce in a short time the deteriorating effect obtained under normal service conditions.

A material that, when mixed with a catalyst or resin, will speed up the chemical reaction between the catalyst and the resin (either polymerizing of resins or vulcanization of rubbers). Also called promoter.

Acceptance test
A test, or series of tests, conducted by the procuring agency upon receipt of an individual lot of materials to determine whether the lot conforms to the purchase order or contract or to determine the degree of uniformity of the material supplied by the vendor, or both.

In an FRP context, acetone is primarily useful as a cleaning solvent for removal of uncured resin from applicator equipment and clothing. This is a very flammable liquid.

Acoustic emission
A measure of integrity of a material, as determined by sound emission when a material is stressed. Ideally, emissions can be correlated with defects and/or incipient failure.

An additive used to promote and reduce the curing time of resins. See also accelerator.

Any substance added to another substance, usually to improve properties, such as plasticizers, initiators, light stabilizers and flame retardants.

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

Adhesion Strength
The force required to cause a separation of two bonded surfaces.

Advanced composites
Strong, tough materials created by combining one or more stiff, high strength reinforcing fiber with compatible resin system. Advanced composites can be substituted for metals in many structural applications with physical properties comparable or better than aluminum.

Aerial Weight
Weight of cloth generally in gsm (gram per square metre)

aging The effect on materials of exposure to an environment for an interval of time. The process of exposing materials to an environment for a 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 Vent
Small outlet, to prevent entrapment of gases.

Air-bubble void
ir entrapment within and between the plies of reinforcement or within a bondline or encapsulated area; localized, noninterconnected, spherical in shape.

Air-inhibited resin
A resin by which surface cures will be inhibited or stopped in the presence of air.

Property values used for design with a 95 percent confidence interval: the “A” allowable is the minimum value for 99 percent of the population; and the “B” allowable, 90 percent.

Alternating stress
A stress varying between two maximum values which are equal but with opposite signs, according to a law determined in terms of the time.

Alternating stress amplitude
A test parameter of a dynamic fatigue test: one-half the algebraic difference between the maximum and minimum stress in one cycle.

Ambient conditions
Prevailing environmental conditions such as the surrounding temperature, pressure and relative humidity.

Amine Resins
A synthetic resin derived from the reaction of urea, thiourea, melamine or allied compounds with aldehydes, particularly formaldehyde.

Dependence of elastic strain on both stress and time. This can result in a lag of strain behind stress. In materials subjected to cyclic stress, the anelastic effect causes internal damping.

Not isotropic. Exhibiting different properties when tested along axes in different directions.

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).

A substance that, when added in small quantities to the resin during mixing, prevents its oxidative degradation and contributes to the maintenance of its properties.

A type of highly oriented organic material derived from polyamide (nylon) but incorporating aromatic ring structure. Used primarily as a high-strength high-modulus fiber. Kevlar® and Nomex® are examples of aramids.

Areal weight
The weight of fiber per unit area (width x length) of tape or fabric.

Artificial weathering
The exposure of plastics to cyclic, laboratory conditions, consisting of high and low temperatures, high and low relative humidities, and ultraviolet radiant energy, with or without direct water spray and moving air (wind), in an attempt to produce changes in their properties similar to those observed in long-term continuous exposure outdoors. The laboratory exposure conditions are usually intensified beyond those encountered in actual outdoor exposure, in an attempt to achieve an accelerated effect

Ash Content
The solid residue remaining after a reinforcing substance has been incinerated (or strongly heated).

Aspect ratio
The ratio of length to diameter of a fiber or the ratio of length to width in a structural panel.

The ability of two contiguous surfaces of the same material, when pressed together, to form a strong bond which prevents their separation at the place of contact.

A closed vessel for conducting and completing a chemical reaction or other operation, such as cooling, under pressure and heat.

Autoclave Bonding
A process in which an assembly consisting of cured composite parts, or a combination of cured composite and metal parts, is bonded using the pressure bag technique. The fall assembly is covered with a pressure bag and loaded in an autoclave capable of providing heat and pressure to cure the adhesive.

Automatic Mold
A mold, e.g., for compression, transfer or injection molding, that is equipped to perform all operations of the entire molding cycle, including ejection of the molded parts, in a completely automatic manner

Axial Winding
A type of filament winding in which the filaments are parallel to the axis.

Axial Yarn
A yarn used parallel to the braid axis and included within a braided layer.

An intermediate stage in the reaction of a thermosetting resin in which the material melts when heated and dissolves in certain solvents. Materials are usually precured to this stage to facilitate handling and processing prior to final cure.

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.

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.

Balanced Laminate
A composite in which all laminae at angles other than 0-deg and 90-deg occur only in æ pairs (not necessarily adjacent) and are symmetrical around the centerline.

Barcol hardness
A hardness value obtained by measuring the resistance to penetration of a sharp steel point under a spring load. The instrument, called a Barcol impressor, gives a direct reading on a scale of 0 to 100. The hardness value is often used as a measure of the degree of cure of a plastic.

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

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

Felted fabrics or structures built by the interlocking action of compressing fibers, without spinning, weaving, or knitting.

The mat of chopped glass fibers deposited over a layer of resin mix on carrier film following a chopping operation.

Bedding compound
White lead or one of a number of commercially available resin compounds used to form a flexible, waterproof base to set fittings.

Bi-Directional Laminate
A reinforced laminate in which the fibers are oriented in 2 directions.

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

Biaxial load
A loading condition in which a laminate is stressed in two different directions in its plane.

Biaxial Winding
A type of filament winding in which the helical band is laid in sequence, side by side, with no crossover of the fibers.

Bidirectional laminate
A reinforced plastic laminate with the fibers oriented in two directions in its plane. A cross laminate.

The resin or cementing constituent (of a plastic compound) that holds the other components together. The agent applied to fiber mat or preforms to bond the fibers before laminating or molding.

A large collection of continuous glass bundles which is tangled up and will not run through the guide eye into roving creel. In the field, a large tangled collection of roving which does not run through the tube or guide eyes to the chopper.

The difference between the two principal refractive indices (of a fiber) or the ratio between the retardation and thickness of a material at a given point.

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.

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 choppability. If it falls off, it usually does not wet-through, and this can cause blisters or porosity.

Bleeder cloth
A woven or nonwoven 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 and is not part of the final composite.

Bleeder Ply
The bleeder ply creates a path for the air and volatiles to escape from the repair. Excess resin is collected in the bleeder. Bleeder material could be made of a layer of fiberglass, nonwoven polyester, or it could be a perforated Teflon® coated material. The structural repair manual (SRM) indicates what type and how many plies of bleeder are required. As a general rule, the thicker the laminate, the more bleeder plies are required.

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

An elevation on the surface of an adherend containing air or water vapor, somewhat resembling in shape a blister on the human skin. Its boundaries may be indefinitely outlined, and it may have burst and become flattened.

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.

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

The adhesion at the interface between two surfaces. To attach materials together by means of adhesives.

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

Bonding angles
An additional FRP laminate, or an extension of the laminate used to make up the joined member, which extends onto the existing laminate to attach additional items such as framing, bulkheads and shelves to the shell or to each other.

Boron Fiber
A fiber usually of a tungsten-filament core with elemental boron vapor deposited on it to impart strength and stiffness.

Boundary conditions
Load and environmental conditions that exist at the boundaries. Conditions must be specified to perform stress analysis.

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

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.

Breather Ply
A thick fabric that regulates the pressure within the vacuum bagging system. The breather ply is often made of the same material as the bleeder ply, and a single piece of fabric may be used as both the bleeder and the breather.

Condition in which fibers do not move into or conform to radii and corners during molding, resulting in voids and dimensional control problems.

Broad Goods
Fiber woven to form fabric up to 1270 mm (50 in.) wide. It may or may not be impregnated with resin and is usually furnished in rolls of 25 to 140 kg. (50 to 300 lb).

Broken End
In the roving operation, a broken or severed strand (bundle) which causes the forming cake to stop running.

A mode of failure generally characterized by an unstable lateral material deflection due to compressive action on the structural element involved.

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

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.

Bulk Molding Composite (BMC)
Thermosetting resin mixed with short strand reinforcement, filler, and other materials to form a viscous compound for compression or injection molding.

Bulk molding compound (BMC)
Thermo-set resin mixed with strand reinforcement, fillers, etc. into a viscous compound for compression or injection molding.

A discrete collection of many parallel glass filaments. A collection of Individual filaments, a sub-strand.

Burst Strength
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.

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.

Butt joint
A type of edge joint in which the edge faces of the two adherends are at right angles to the other faces of the adherends.

A non-porous synthetic rubber that offers better protection from certain chemicals, such as some ketones, than other rubber or rubber-like materials. Safety gloves made of butyl provide good protection against resins and hardeners.

A special type of glass used as a fiber reinforcement, made and applied specifically for high chemical resistance. The composition is 64.6% SiO2, 4.1% Al2O3·Fe2O3, 13.4% CaO, 3.3% MgO, 9.6% Na2O·K2O, 4.7% B2O3 and 0.9% BaO. The symbol C was originally chosen for chemical resistance.

The back-and-forth scanning of a specimen with ultrasonics. A nondestructive testing technique for finding voids, delaminations, defects in fiber distribution, and so forth.

The final stage of the curing of a thermosetting resin in which the material has become infusible and insoluble in common solvents.

Cabled Yarn
Yarn that is plied more than once; yarn made by plying two or more previous plied yarns.

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.

The 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 fiber
Fiber produced by the pyrolysis of organic precursor fibers, such as rayon, polyacrylonitrile (PAN), and pitch, in an inert environment. The term is often used interchangeably with the term graphite; however carbon fibers and graphite fibers differ. The basic differences lie in the temperature at which the fibers are made and heat treated, and in the amount of elemental carbon produced. Carbon fibers typically are carbonized in the region of 2400°F and assay at 93 to 95% carbon, while graphite fibers are graphitized between 3450° and 4500°F and assay to more than 99% elemental carbon.

A composite of carbon fiber in a carbon matrix.

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.

Carpet plot
A design chart showing the uniaxial stiffness or strength as a function of arbitrary ratios of 0, 90, and æ 45 degree plies.

A substance that changes the rate of a chemical reaction without itself undergoing permanent change in composition or becoming a part of the molecular structure of the product. A substance that markedly speeds up the cure of a compound when added in minor quantity.

Smooth metal, plastic, or rubber plates free of surface defects, used in contact with the lay-up during the curing process to transmit normal pressure and provide a smooth surface on the finished part. A caul plate must be the appropriate size and shape for the composite lay-up with which is will be used.

A plate or pad placed over a vacuum bagging assembly. Cauls apply pressure to a specific area and prevent bridging.

Caul Plate
A caul plate made from aluminum is often used to support the part during the cure cycle. A mold release agent, or parting film, is applied to the caul plate so that the part does not attach to the caul plate. A thin caul plate is also used on top of the repair when a heat bonder is used. The caul plate provides a more uniform heated area and it leaves a smoother finish of the composite laminate.

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

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

Cell size
The diameter of an inscribed circle within the cell of a honeycomb core.

Ceramic Fibers
Continuous fibers of metal oxides or refractory oxides which are resistant to high temperatures (2000-3000°F). This class of fibers includes alumina, beryllia, magnesia, thoria, zirconia, silicon carbide, quartz and high silica reinforcements. Although glass is also a ceramic material, glass fibers are not generally included. Ceramic fibers are produced by chemical vapor deposition, melt drawing, spinning and extrusion. Their main advantage is high strength and modulus.

Carbon fiber-reinforced plastic

Chain plates
The metallic plates, embedded in or attached to the hull or bulkhead, used to evenly distribute loads from shrouds and stays to the hull of sailing vessels.

Charpy impact test
A test for shock loading in which a centrally notched sample bar is held at both ends and broken by striking the back face in the same plane as the notch.

Chemical Size
A surface finish applied to the fiber that contains some chemical constituents other than water.

A very soft and filling plain woven Silk texture consisting of the Finest Singles which are hard twisted and woven in the gum condition. The cloth is afterward degummed.

The ease of chopping/cutting the glass fibers to a uniform length.

Chopped strand
Continuous strand yarn or roving cut up into uniform lengths, usually from 1/32 inch long. Lengths up to 1/8 inch are called milled fibers.

Chopper Gun
A special piece of equipment used in the manufacture of reinforced plastic parts, which chops glass and sprays resin and catalyst simultaneously onto a molded surface.

closed cell foam
Cellular plastic in which individual cells are completely sealed off from adjacent cells.

In the co-bonding process, one of the detail parts is precured with the mating part being cured simultaneously with the adhesive. Film adhesive is often used to improve peel strength

Cured and simultaneously bonded to another prepared surface.

Co-curing is a process wherein two parts are simultaneously cured. The interface between the two parts may or may not have an adhesive layer. Co-curing often results in poor panel surface quality, which is prevented by using a secondary surfacing material co-cured in the standard cure cycle or a subsequent fill-and-fair operation. Co-cured skins may also have poorer mechanical properties, requiring the use of reduced design values. A typical co-cure application is the simultaneous cure of a stiffener and a skin. Adhesive film is frequently placed into the interface between the stiffener and the skin to increase fatigue and peel resistance. Principal advantages derived from the co-cure process are excellent fit between bonded components and guaranteed surface cleanliness.

A resin produced by copolymerization, the process where unlike molecules are arranged in alternate sequence in a chain.

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.

The act of curing a composite laminate and simultaneously bonding it to some other prepared surface. See also secondary bonding.

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

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

The application of a temporary vacuum bag and vacuum to remove trapped air and compact the lay-up.

Measurement of softness as opposed to stiffness of a material. It is a reciprocal of the Young’s modulus, or an inverse of the stiffness matrix.

A homogeneous material created by the synthetic assembly of two or more materials (a 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.

Composite material
A combination of two or more materials (reinforcing elements, fillers and composite matrix binder), differing in form or composition on a macroscale. The constituents retain their identities; that is, they do not dissolve or merge completely into one another although they act in concert. Normally, the components can be physically identified and exhibit an interface between one another.

Compression Mold
A mold which is open when the material is introduced and which shapes the material by heat and by the pressure of closing.

Compression molding
A mold that is open when the material is introduced and that shapes the material by the presence of closing and heat.

Compressive strength
The ability of a material to resist a force that tends to crush or buckle. The maximum compressive load sustained by a specimen divided by the original cross-sectional area of the specimen.

Compressive stress
The normal stress caused by forces directed toward the plane on which they act.

Subjecting a material to a prescribed environmental and/or stress history before testing.

Ability of the mat to conform to difficult shapes without causing wrinkles or leaving excessively resin-rich or glass-rich radii, which may craze.

Constituent materials
Individual materials that make up the composite material; e.g., graphite and epoxy are the constituent materials of a graphite/epoxy composite material.

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 heating in an oven, without additional pressure.

Continuous Filament
An individual, small-diameter reinforcement that is flexible and indefinite in length.

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. It also can be chopped (see Chopped Strand).

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.

A long chain molecule formed by the reaction of two or more dissimilar monomers.

The central member of a sandwich construction to which the faces of the sandwich are attached. A channel in a mold for circulation of heat-transfer media. Male part of a mold which shapes the inside of the mold.

Correspondence Principle
Known as the elastic-viscoelastic correspondence principle, it states that the expressions for the effective complex moduli of a viscoelastic hetrogeneous material can be obtained from the corresponding expressions for the effective elastic moduli of an associated hetrogeneous elastic material simply by replacing phase elastic moduli by phase complex moduli.

Corrosion resistance
The ability of a material to withstand contact with ambient natural factors or those of a particular artificially created atmosphere, without degradation or change in properties. For metals, this could be pitting or rusting; for organic materials, it could be crazing.

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 agent designed to react with both the reinforcement and matrix phases of a composite material to form or promote a stronger bond at the interface

Region of ultrafine cracks, which may extend in a network on or under the surface of a resin or plastic material. May appear as a white band.

A device for holding the required number of roving balls or supply packages in desired position for unwinding onto the next processing step.

The change in dimension of a material under load over a period of time, not including the initial instantaneous elastic deformation. (Creep at room temperature is called cold flow.) The time dependent part of strain resulting from an applied stress.

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.

Cross Linked PVC Foams
Polyvinyl foam cores are manufactured by combining a polyvinyl copolymer with stabilizers, plasticizers, cross-linking compounds and blowing agents. The mixture is heated under pressure to initiate the cross-linking reaction and initiate the cross-linking reaction and then submerged in hot water tanks to expand to the desired density. Cell diameters range from .0100 to .100 inches (as compared to .0013 inches for balsa)[16, 22]. The resulting material is thermoplastic, enabling the material to conform to compound curves of a hull. PVC foams have almost exclusively replaced urethane foams as a structural core material, except in configurations where the foam is “blown” in place. A number of manufacturers market cross-linked PVC products to the marine industry in sheet form with densities ranging from 2 to 12 pounds per ft3.As with the balsa products, solid sheets or scrim backed block construction configurations are available.

Applied to polymer molecules, the setting-up of chemical links between the molecular chains. When extensive, as in most thermosetting resins, cross-linking makes one infusible supermolecule of all the chains.

The setting up of chemical links between molecule chains. This occurs in all thermosetting resins. Styrene monomer is a crosslinking agent in polyester resins.

To irreversibly change the properties of a thermosetting resin by chemical reaction, i.e. condensation, ring closure or addition. Curing may be accomplished by addition of curing (crosslinking) agents, with or without heat.

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

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

Curing Temperature
Temperature at which a cast, molded, or extruded product, a resin-impregnated reinforcement, an adhesive, etc., is subjected to curing.

Curing Time
The period of time during which a part is subjected to heat or pressure, or both, to cure the resin; interval of time between the instant of cessation of relative movement between the moving parts of a mold and the instant that pressure is released. (Further cure may take place after removal of the assembly from the conditions of heat or pressure.)

The complete, repeating sequence of operations in a process or part of a process. In molding, the cycle time is the period (or elapsed time) between a certain point in one cycle and the same point in the next.

Damage tolerance
A design measure of crack growth rate. Cracks in damage tolerant designed structures are not permitted to grow to critical size during expected service life.

Area of separation within or between plies in a laminate, or within a bonded joint, caused by contamination, improper adhesion during processing or damaging interlaminar stresses.

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.

Separation of the layers of material in a laminate, either local or covering a wide area. Can occur in the cure or subsequent life.

A yarn and filament numbering system in which the yarn number is numerically equal to the weight in grams of 9000 meters. Used for continuous filaments where the lower the denier, the finer the yarn.

Dimensional stability
Ability of a plastic part to retain the precise shape to which it was molded, cast or otherwise fabricated.

Small sunken dots in the gel coat surface, generally caused by a foreign particle in the laminate.

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 sizings on textile yarns that allow them to be resin compatible.

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.

a sandwich construction which has faces made of materials which have different properties. For example, one face of aluminum and one of graphite–epoxy, or one titanium and one aluminum.

Double-Sided Bagging
A vacuum bagging technique during which the vacuum bag surrounds the entire part rather than simply covering the exposed side of the part.

Draft angle
The angle of a taper on a mandrel or mold that facilitates removal of the finished part.

The ability of a fabric or a prepreg to conform to a contoured surface.

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

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

Dry Loft
Height of the bed of chopped fibers.

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

The amount of plastic strain that a material can withstand before fracture. Also, the ability of a material to deform plastically before fracturing.

A family of glasses with a calcium aluminoborosilicate composition and a maximum alkali content of 2.0%. A general-purpose fiber that is most often used in reinforced plastics, and is suitable for electrical laminates because of its high resistivity. Also called electric glass.

loads applied in the plane of the sandwich panel (in the L–W (x–y) plane), are considered edgewise loads.

Ejection / Demolding
The process of removing a molding from the molding impression; by mechanical means, by hand, or by the 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 deformation
The part of the total strain in a stressed body that disappears upon removal of the stress.

elastic limit
The greatest stress a material is capable of sustaining without permanent strain remaining after the complete release of the stress. A material is said to have passed its elastic limit when the load is sufficient to initiate plastic, or nonrecoverable, deformation.

That property of materials by virtue of which they tend to recover their original size and shape after removal of a force causing deformation.

A material that substantially recovers its original shape and size at room temperature after removal of a deforming force.

Deformation caused by stretching. The fractional increase in length of a material stressed in tension. (When expressed as percent-age of the original gage length, it is called percentage. elongation.)

The enclosure of an item in plastic. Sometimes used specifically in reference to the enclosure of capacitors or circuit board modules.

A strand of roving consisting of a given number of filaments gathered together. The group of filaments is considered an "end" or strand before twisting, a "yarn" after twist has been applied. An individual warp yarn, thread, fiber, or roving.

Envelope Bagging
Envelope bagging is a process in which the part to be repaired is completely enclosed in a vacuum bag or the bag is wrapped around the end of the component to obtain an adequate seal. It is frequently used for removable aircraft parts, such as flight controls, access panels, etc., and when a part’s geometry and/or the repair location makes it very difficult to properly vacuum bag and seal the area in a vacuum.

Epoxy plastic
A polymerizable thermoset polymer containing one or more epoxide groups and curable by reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans. An important matrix resin in composites and structural adhesive.

Epoxy Resins
Epoxy resins are a broad family of materials that contain a reactive functional group in their molecular structure. Epoxy resins show the best performance characteristics of all the resins used in the marine industry. Aerospace applications use epoxy almost exclusively, except when high temperature performance is critical.

The liberation or evolution of heat during the curing of a plastic product.

Exotherm heat
The heat given off as the result of the action of a catalyst on a resin.

Exothermic Reaction
A chemical reaction that releases heat. Mixing resin and hardener may cause an exothermic reaction within the container.

Fabric, Nonwoven
A material formed from fibers or yarns without interlacing (e.g., stitched bonded, nonwoven broadgoods).

Failure criterion
Empirical description of the failure of composite materials subjected to complex state of stresses or strains. The most commonly used are the maximum stress, the maximum strain, and the quadratic criteria.

Failure envelope
Ultimate limit in combined stress or strain state defined by a failure criterion.

A member or structure, the primary function of which is to streamline the flow of a fluid by producing a smooth outline and to reduce drag, as in aircraft frames and boat hulls.

The failure or decay of mechanical properties after repeated applications of stress. Fatigue tests give information on the ability of a material to resist the development of cracks, which eventually bring about failure as a result of a large number of cycles.

Fatigue limit
The stress limit below which a material can be stressed cyclically for an infinite number of times without failure.

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

Faying surface
The surfaces of materials in contact with each other and joined or about to be joined together.

Feathered Edge
A fabric edge that tapers down in weight instead of abruptly ending.

felt A fibrous material made up of interlocking fibers by mechanical or chemical action, pressure or heat. Felts may be made of cotton, glass or other fibers.

A general term used to refer to filamentary materials. Often, fiber is used synonymously with filament. It is a general term for a filament with a finite length that is at least 100 times its diameter, which is typically 0.004 to 0.005 inches. In most cases it is prepared by drawing from a molten bath, spinning, or deposition on a substrate. A whisker, on the other hand, is a short single-crystal fiber or filament made from a variety of materials, with diameters ranging from 40 to 1400 micro inches and aspect ratios between 100 and 15000. Fibers can be continuous or specific short lengths (discontinuous), normally less than 1/8 inch.

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 count
The number of fibers per unit width of ply present in a specified section of a composite.

Fiber direction
The orientation or alignment of the longitudinal axis of the fiber with respect to a stated reference axis.

Fiber 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.

Fiber-reinforced plastic (FRP)
A general term for a composite that is reinforced with cloth, mat, strands or any other fiber form.

An individual filament made by drawing molten glass. A continuous filament is a glass fiber of great or indefinite length. A staple fiber is a glass fiber of relatively short length, generally less than 17 inches, the length related to the forming or spinning process used.

Fiberglass chopper
Chopper guns, long cutters and roving cutters cut glass into strands and fibers to be used as reinforcement in plastics.

Fiberglass reinforcement
Major material used to reinforce plastic. Available as mat, roving, fabric, and so forth, it is incorporated into both thermosets and thermoplastics.

Fick’s equation
Diffusion equation for moisture migration. This is analogous to the Fourier’s equation of heat conduction.

The smallest unit of 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 1 mil. Normally, filaments are not used individually. Some textile filaments can function as a yarn when they are of sufficient strength and flexibility.

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 or mandrel in a prescribed way to meet certain stress conditions. Generally, the shape is a surface of revolution and may or may not include end closures. When the required number of layers is applied, the wound form is cured and the mandrel is removed.

A phenomenon in which a coated strand breaks up into loose individual filaments.

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

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.

A rounded filling or adhesive that fills the corner or angle where two adherends are joined.

Filling yarn
The transverse threads or fibers in a woven fabric. Those fibers running perpendicular to the warp. Also called weft.

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.

A mixture of materials for treating glass or other fibers. It contains a coupling agent to improve the bond of resin to the fiber, and usually includes a lubricant to prevent abrasion, as well as a binder to promote strand integrity. With graphite or other filaments, it may perform any or all of the above functions.

First ply or ply group that fails in a multidirectional laminate. The load corresponding to this failure can be the design limit load.

Fish eye
A circular separation in a gel coat film generally caused by contamination such as silicone, oil, dust or water.

Flame retardants
Certain chemicals that are used to reduce or eliminate the tendency of a resin to burn.

Measure of the extent to which a material will support combustion.

loads applied normal to the plane of the sandwich panel (in the T(z) direction)

Flexural modulus
The ratio, within the elastic limit, of the applied stress on a test specimen in flexure to the corresponding strain in the outermost fibers of the specimen.

Flexural Rigidity
a measure of the sandwich panel’s resistance to bending, corresponds to the bending rigidity (EI) of monolithic beam theory.

Flexural strength
The maximum stress that can be borne by the surface fibers in a beam in bending. The flexural strength is the unit resistance to the maximum load before failure bybending, usually expressed in force per unit area.

The movement of resin under pressure, allowing it to fill all parts of the mold. The gradual but continuous distortion of a material under continued load, usually at high temperatures; also called creep.

Loose filaments of fiber that have broken from their parent strand during processing and are freely floating in the air.

Refers to the deposition of foams when the foaming machine must be brought to the work that is “in place,” as opposed to bringing the work to the foaming machine. Also, foam mixed in a container and poured in a mold, where it rises to fill the cavity.

Fracture toughness
A measure of the damage tolerance of a material containing initial flaws or cracks. Used in aircraft structural design and analysis.

Friction stir processing
A method of changing the properties of a metal through intense, localized plastic deformation.

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

Detached and broken glass fiber that has collected on processing equipment.

Where molten thermoplastic or liquid thermoset resin enters the cavity in a tool.

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

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

Gel Point
The stage at which a liquid begins to exhibit pseudoelastic properties. (This stage may be conveniently observed from the inflection point on a viscosity-time plot). Also, ’gel time.’

Gel Time
Time required to change a flowable liquid resin into a non-flowing gel.

Gelation time
That interval of time, in connection with the use of synthetic thermosetting resins, extending from the introduction of a catalyst into a liquid adhesive system until the start of gel formation. Also, the time under application of load for a resin to reach a solid state.

Glass fiber-reinforced plastic, polymer or polyester.

Glass finish
A material applied to the surface of a glass reinforcement to improve the bond between the glass and the plastic resin matrix.

Glass Resin Ratio
The amount of glass by weight compared to the amount of resin by weight in a finished laminate or molding.

Glass transition
The reversible change in an amorphous polymer or in an amorphous regions of a partially crystalline polymer from, or to, a viscous or rubbery condition to, or from, a hard to a relatively brittle one.

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.

To crystalline allotropic form of carbon.

Green Strength
The ability of a material, while not completely cured, set or sintered, to undergo removal from the mold and handling without distortion.

Glass-reinforced plastic, polymer or polyester.

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 of resin-impregnated reinforcement in position on a mold by hand.

A substance or mixture added to a plastic composition to promote or control the curing action by taking part in it.

Harness satin
Weaving pattern producing a satin appearance. “Eight-harness” means the warp tow crosses over seven fill tows and under the eighth (repeatedly).

Heat Blanket
A heat blanket is a flexible heater. It is made of two layers of silicon rubber with a metal resistance heater between the two layers of silicon. Heat blankets are a common method of applying heat for repairs on the aircraft. Heat blankets may be controlled manually; however, they are usually used in conjunction with a heat bonder. Heat is transferred from the blanket via conduction. Consequently, the heat blanket must conform to and be in 100 percent contact with the part, which is usually accomplished using vacuum bag pressure.

Heat build-up
The temperature rise in part resulting from the dissipation of applied strain energy as heat.

Heat resistance
The property or ability of plastics and elastomers to resist the deteriorating effects of elevating temperatures

Descriptive term for a material of uniform composition throughout. A medium that has no internal physical boundaries. A material whose properties are constant at every point, that is, constant with respect to spatial coordinates (but not necessarily with respect to directional coordinates).

Manufactured product of resin impregnated sheet material (paper, glass fabric and so on) or metal foil, formed into hexagonal shaped cells. Used as a core material in sandwich constructions.

Hoop stress
The circumferential stress in a material of cylindrical form subjected to internal or external pressure.

Hopkinson Bar
Split Hopkinson bar; also called a Kolsky bar, is an inexpensive device for performing high strain-rate experiments. Measures stress pulse propagation in a metal bar.

Hull liner
A separate interior hull unit with bunks, berths, bulkheads, and other items of outfit preassembled then inserted into the hull shell. A liner can contribute varying degrees of stiffness to the hull through careful arrangement of the berths and bulkheads

A composite laminate consisting of laminae of two or more composite material systems. 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; usually only the fibers differ.

Hygrothermal effect
Change in properties due to moisture absorption and temperature change.

The energy absorbed in a complete cycle of loading and unloading. This energy is converted from mechanical to frictional energy (heat).

Ignition loss
The difference in weight before and after burning. As with glass, the burning off of the binder or size.

Impact strength
The ability of a material to withstand shock loading. The work done on fracturing a test specimen in a specified manner under shock loading.

Impact test
Measure of the energy necessary to fracture a standard notched bar by an impulse load.

In reinforced plastics, to saturate the reinforcement with a resin.

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 degree from the parent material.

A material added to a resin to slow down curing. It also retards polymerization, thereby increasing shelf life of a monomer.

An ingredient used to facilitate chemical reactions. The operator must add an initiator to the gel coat before applying it to the mold.

Injection molding
Method of forming a plastic to the desired shape by forcing the heat softened plastic into a relatively cool cavity under pressure.

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 laminae.

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

Descriptive term pertaining to an object (for example, voids), event (for example, fracture), or potential field (for example, temperature gradient) existing entirely within a single lamina without reference to any adjacent laminae.

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.

The width of a cut made by a saw blade, torch, water jet, laser beam and so forth.

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

Knitted fabrics
Fabrics produced by interlooping chains of yarn.

A single ply or layer in a laminate made up of a series of layers (organic composite). A flat or curved surface containing unidirectional fibers or woven fibers embedded in a matrix.

Plural of lamina

To unite laminae with a bonding material, usually with pressure and heat (normally used with reference to flat sheets, but also rods and tubes). A product made by such bonding.

Lap joint
A joint made by placing one adherend partly over another and bonding the overlapped portions.

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

Lay-up Molding
A manual molding process during which reinforcement in the form of a fabric or a mat is positioned into the mold and saturated with a resin. Also called hand lay-up.

Load-deflection curve
A curve in which the increasing tension, compression, or flexural loads are plotted on the ordinate axis and the deflections caused by those loads are plotted on the abscissa axis.

Loss on ignition
Weight loss, usually expressed as percent of total, after burning off an organic sizing from glass fibers, or an organic resin from a glass fiber laminate.

Low-pressure laminates
In general, laminates molded and cured in the range of pressures from 400 psi down to and including pressure obtained by the mere contact of the plies.

Structural behavior of composite laminates using the laminated plate theory. The fiber and matrix within each ply are smeared and no longer identifiable.

A device for distributing vacuum pressure more evenly through the vacuum system. Manifolds are typically used for vacuum bagging large parts.

Mastic Tape
A sticky strip of material used to seal the vacuum bag to the mold. Mastic tape has protective backing that should be left on the tape until the operator is ready to seal the bag to the mold.

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. Available in blankets of various widths, weights and lengths. Also, a sheet formed by filament winding a single-hoop ply of fiber on a mandrel, cutting across its width and laying out a flat sheet.

The essentially homogeneous resin or polymer material in which the fiber system of a composite is embedded. Both thermoplastic and thermoset resins may be used, as well as metals, ceramics and glass.

Matt Weave or Basket weave
Ttwo or more yarns are used in both the warp and filling direction. These groups of yarns are woven as one, producing a basket effect.

Mechanical adhesion
Adhesion between surfaces in which the adhesive holds the parts together by interlocking action.

Mechanical properties
The properties of a material, such as compressive or tensile strength, and modulus, that are associated with elastic and inelastic reaction when force is applied. The individual relationship between stress and strain.

Mek peroxide (MEKP)
Abbreviation for Methyl Ethyl Ketone Peroxide; a strong oxidizing agent (free radical source) commonly used as the catalyst for polyesters in the FRP industry.

Calculation of the effective ply properties as functions of the fiber and matrix properties. Some numerical approaches also provide the stress and strain within each constituent and those at the interface.

The unit used in measuring the diameter of glass fiber strands, wire, etc. (1 mil = 0.001 inch).

Mil Gage
A device used to measure the thickness of gel coat.

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

Modulus of elasticity
The ratio of stress or load applied to the strain or deformation produced in a material that is elasticity deformed. If a tensile strength of 2 ksi results in an elongation of 1%, the modulus of elasticity is 2.0 ksi divided by 0.01 or 200 ksi. Also called Young’s modulus.

Moisture absorption
The pickup of water vapor from air by a material. It relates only to vapor withdrawn from the air by a material and must be distinguished from water absorption, which is the gain in weight due to the take-up of water by immersion.

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, that is, the mass of the dry substance plus the moisture present.

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

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

A single molecule that can react with like or unlike molecules to form a polymer. The smallest repeating structure of a polymer (mer). For additional polymers, this represents the original unpolymerized compound.

Netting analysis
Treating composites like fibers without matrix. It is not a mechanical analysis, and is not applicable to composites.

Non-air-inhibited resin
A resin in which the surface cure will not be inhibited or stopped by the presence of air. A surfacing agent has been added to exclude air from the surface of the resin.

Non-volatile material
Portion remaining as solid under specific conditions short of decomposition.

Nondestructive evaluation (NDE)
Broadly considered synonymous with nondestructive inspection (NDI). More specifically, the analysis of NDI findings to determine whether the material will be acceptable for its function.

Nondestructive inspection (NDI)
A process or procedure, such as ultrasonic or radiographic inspection, for determining the quality of characteristics of a material, part or assembly, without permanently altering the subject or its properties. Used to find internal anomalies in a structure without degrading its properties.

Nonwoven fabric
A planar textile structure produced by loosely compressing together fibers, yarns, rovings, etc. with or without a scrim cloth carrier. Accomplished by mechanical, chemical, thermal, or solvent means and combinations thereof.

Normal stress
The stress component that is perpendicular to the plane on which the forces act.

Notch sensitivity
The extent to which the sensitivity of a material to fracture is increased by the presence of a surface nonhomogeneity, 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 is usually associated with brittle materials.

Nut Factor
The K, or nut factor, can be thought of as anything that increases or decreases the friction within the threads of the nut; or, anything and everything that affects the relationship between torque and preload.

Open Hole Tension

Open Mold
An open cavity that has no rigid second piece to enclose the part and apply pressure. Vacuum bagging is typically done on parts that have been created on open molds.

Orange peel
Backside of the gel coated surface that takes on the rough wavy texture of an orange peel.

Having three mutually perpendicular planes of elastic symmetry.

The designation of a section of FRP shell plating, of either single-skin or sandwich construction, bonded by longitudinal and transverse stiffeners or other supporting structures.

Paste Wax
A release material applied to composite molds. Paste wax is soft and never fully dries so it is easy to remove.

polyethyl ether ketone

Peel ply
A layer of resin-free material used to protect a laminate for later secondary bonding.

Peel strength
Adhesive bond strength, as in pounds per inch of width, obtained by a stress applied in a peeling mode.

Perforated Release Film
Perforated parting film is used to allow air and volatiles out of the repair, and it prevents the bleeder ply from sticking to the part or repair. It is available with different size holes and hole spacing depending on the amount of bleeding required.

Permanent set
The deformation remaining after a specimen has been stressed a prescribed amount in tension, compression or shear for a definite time period. For creep tests, the residual unrecoverable deformation after the load causing the creep has been removed for a substantial and definite period of time. Also, the increase in length, by which an elastic material fails to return to original length after being stressed for a standard period of time.

A continuous process for manufacturing composites that have a constant cross-sectional shape. The process consists of pulling a fiber-reinforcing material through a resin impregnation bath and through a shaping die, where the resin is subsequently cured.

Plain Weave
Most simple and most common type of construction Inexpensive to produce, durable, Flat, tight surface is conducive to printing and other finishes. The simplest of all patterns is the plain weave. Each weft yarn goes alternately over and under one warp yarn. Each warp yarn goes alternately over and under each weft yarn.

Prepreg is pre-impregnated material i.e. impregnated with mixed epoxy resin.

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

Ordering of laminates by strength, stiffness or others.

Reaction injection molding (RIM)
A process for molding polyurethane, epoxy, and other liquid chemical systems. Mixing of two to four components in the proper chemical ratio is accomplished by a high-pressure impingement type mixing head, from which the 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, fabrics, and so forth, have been added before the forming operation to provide some strength properties greatly superior to those of the base resin.

A solid or pseudosolid organic material, usually of high molecular weight, that exhibits a tendency to flow when subjected to stress. It usually has a softening or melting range, and fractures conchoidally. Most resins are polymers. In reinforced plastics, the material used to bind together the reinforcement material; the matrix. See also polymer.

Resin content
The amount of resin in a laminate expressed as either a percentage of total weight or total volume.

Resin transfer molding (RTM)
A process whereby catalyzed resin is transferred or injected into an enclosed mold in which the fiberglass reinforcement has been placed.

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.

Rib weave
the filling yarns are larger in diameter than the warp yarns. A rib weave produces fabrics in which fewer yarns per square centimeter are visible on the surface.

Room Temperature Curing
Room temperature curing is the most advantageous in terms of energy savings and portability. Room temperature cure wet layup repairs do not restore either the strength or the durability of the original 250 °F or 350 °F cure components and are often used for wet layup fiberglass repairs for noncritical components. Room temperature cure repairs can be accelerated by the application of heat. Maximum properties are achieved at 150 °F. A vacuum bag can be used to consolidate the plies and to provide a path for air and volatiles to escape.

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

A magnesium aluminosilicate composition that is especially designed to provide very high tensile strength glass filaments. S-glass and S-2 glass fibers have the same glass composition but different finishes (coatings). S-glass is made to more demanding specifications, and S-2 is considered the commercial grade.

S-N diagram
A plot of stress (S) against the number of cycles to failure (N) in fatigue testing. A log scale is normally used for N. For S, a linear scale is often used, but sometimes a log scale is used here, too. Also, a representation of the number of alternating stress cycles a material can sustain without failure at various maximum stresses.

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

The size or weight dimensions of the members which make up the structure of the vessel.

Secondary bonding
The joining together, by the process of adhesive bonding, of two or more already cured composite parts, during which the only chemical or thermal reaction occurring is the curing of the adhesive itself.

Secondary structure
Secondary structure is considered that which is not involved in primary bending of the hull girder, such as frames, girders, webs and bulkheads that are attached by secondary bonds.

Self-extinguishing resin
A resin formulation that will burn in the presence of a flame but will extinguish itself within a specified time after the flame is removed.

The irrecoverable or permanent deformation or creep after complete release of the force producing the deformation.

Set up
To harden, as in curing of a polymer resin.

An action or stress resulting from applied forces that causes or tends to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact. In interlaminar shear, the plane of contact is composed primarily of resin.

Shear modulus
The ratio of shearing stress to shearing strain within the proportional limit of the material.

Shear strain
The tangent of the angular change, caused by a force between two lines originally perpendicular to each other through a point in a body. Also called angular strain.

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

Shear stress
The component of stress tangent to the plane on which the forces act.

Sheet molding compound (SMC)
A composite of fibers, usually a polyester resin, and pigments, fillers, and other additives that have been compounded and processed into sheet form to facilitate handling in the molding operation.

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.

The watertight boundary of a vessel’s hull.

Short beam shear (SBS)
A flexural test of a specimen having a low test span-to-thickness ratio (for example, 4:1), such that failure is primarily in shear.

Shrink Tape
Another method of pressure application for oven cures is the use of shrink wrapping or shrink tape. This method is commonly used with parts that have been filament wound, because some of the same rules for application apply. The tape is wrapped around the completed layup, usually with only a layer of release material between the tape and the layup. Heat is applied to the tape, usually using a heat gun to make the tape shrink, a process that can apply a tremendous amount of pressure to the layup. After shrinking, the part is placed in the oven for cure. High quality parts can be made inexpensively using shrink tape.

Single Side Vacuum Bagging
This is the preferred method if the repair part is large enough for a vacuum bag on one side of the repair. The vacuum bag is taped in place with tacky tape and a vacuum port is placed through the bag to create the vacuum.

Single-Sided Bagging
A vacuum bagging technique during which a sheet of plastic is sealed to the outer rim of the mold.

Any treatment consisting of starch, gelatin, oil, wax, or other suitable ingredient applied to yarn or fibers at the time of formation to protect the surface and aid the process of handling and fabrication or to control the fiber characteristics. The treatment contains ingredients that provide surface lubricity and binding action, but unlike a finish, contains no coupling agent. Before final fabrication into a composite, the size is usually removed by heat cleaning, and a finish is applied.

Generally, a term used to describe all of the hull shell. For sandwich construction, there is an inner and outer skin which together are thinner than the single-skin laminate that they replace.

Skin coat
A special layer of resin applied just under the gel coat to prevent blistering. It is sometimes applied with a layer of mat or light cloth.

Solid Release Film
Solid release films are used so that the prepreg or wet layup plies do not stick to the working surface or caul plate. Solid release film is also used to prevent the resins from bleeding through and damaging the heat blanket or caul plate if they are used.

Specific gravity
The density (mass per unit volume) of any material divided by that of water at a standard temperature.

Allied Corporation developed a high strength/modulus extended chain polyethylene fiber called Spectra® that was introduced in 1985. Room temperature specific mechanical properties of Spectra®®are slightly better than Kevlar although performance at elevated temperatures falls off. Chemical and wear resistance data is superior to the aramids.

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

Spray-Up Molding
A manual molding process during which an operator uses a spray machine to simultaneously apply resin and chopped fiberglass strands to an open mold.

Spun roving
A heavy, low-cost glass fiber strand consisting of filaments that are continuous but doubled back on each other.

Starved area
An area in a plastic part which has an insufficient amount of resin to wet out the reinforcement completely. This condition may be due to improper wetting or impregnation or excessive molding pressure.

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

Elastic deformation due to stress. Measured as the change in length per unit of length in a given direction, and expressed in percentage or in./in.

The internal force per unit area that resists a change in size or shape of a body. Expressed in force per unit area.

Stress concentration
On a macromechanical level, the magnification of the level of an applied stress in the region of a notch, void, hole, or inclusion.

Stress corrosion
Preferential attack of areas under stress in a corrosive environment, where such an environment alone would not have caused corrosion.

Stress cracking
The failure of a material by cracking or crazing some time after it has been placed under load. Time-to-failure may range from minutes to years. Causes include molded in stresses, post fabrication shrinkage or warpage, and hostile environment.

Stress whitening
This condition is associated with plastic materials that are stretched near their yield point. The surface takes on a whitish appearance in regions of high stress.

Stress-strain curve
Simultaneous readings of load and deformation, converted to stress and strain, plotted as ordinates and abscissae, respectively, to obtain a stress-strain diagram.

Structural adhesive
Adhesive used for transferring required loads between adherends exposed to service environments typical for the structure involved.

A colorless and toxic hydrocarbon used in plastics. Many of the resins used in pultrusion emit styrene vapors that may be harmful to the operator if inhaled.

Surface Bagging
A vacuum bagging technique typically used for making repairs on composite parts. After holes have been filled or damaged areas replaced, the vacuum bag is placed over the repair and sealed around the perimeter of the repair area.

Surfacing mat
A very thin mat, usually 7 to 20 mils thick, of highly filamentized fiberglass, used primarily to produce a smooth surface on a reinforced plastic laminate, or for precise machining or grinding.

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

Stickiness of a prepreg; an important handling characteristic

Take-Up Twist
The change in length of a filament or yarn caused by twisting and expressed as a percent of the original untwisted length.

A composite ribbon consisting of continuous or discontinuous fibers that are aligned along the tape axis parallel to each other and bonded together by a continuous matrix phase.

Tape Test
A method for evaluating the adhesion of a cured coating on a substrate. Pressure-sensitive adhesive tape is applied to an area of the coating which is sometimes cross-hatched with scratched lines. Adhesion is considered to be adequate if no coating is pulled off by the tape when it is removed.

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

Tensile stress
The normal stress caused by forces directed away from the plane on which they act.

A thermocouple (TC) is a thermoelectric device used to accurately measure temperatures. It may be connected to a simple temperature reading device, or connected to a hot bonder, oven, or other type of controller that regulates the amount of heat. TCs consist of a wire with two leads of dissimilar metals that are joined at one end. Heating the joint produces an electric current, which is converted to a temperature reading with a TC monitor. Select the type of wire (J or K) and the type of connector that are compatible with the local temperature monitoring equipment (hot bonder, oven, autoclave, etc.). TC wire is available with different types of insulation; check the manufacturer’s product data sheets to ensure the insulation withstands the highest cure temperature. Teflon-insulated wire is generally good for 390 °F and lower cures; Kapton-insulated wire should be used for higher temperatures.

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

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

A plastic that, when cured by application of heat or chemical means, changes into a substantially infusible and insoluble material.

Thermoset Foams
Foamed plastics such as cellular cellulose acetate (CCA), polystyrene, and polyurethane are very light (about 2 lbs/ft) and resist water, fungi and decay. These materials have very low mechanical properties and polystyrene will be attacked by polyester resin. These foams will not conform to complex curves and their uses are generally limited to buoyancy rather than structural applications. Polyurethane is often foamed in-place when used as a buoyancy material.

Thermosetting polyesters
A class of resins produced by dissolving unsaturated, generally linear, alkyd resins in a vinyl-type active monomer such as styrene, methyl styrene, or diallyl phthalate. Cure is effected through vinyl polymerization using peroxide 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

Thixotropic (thixotropy)
Concerning materials that are gel-like at rest but fluid when agitated.Having high static shear strength and low dynamic shear strength at the same time. To lose viscosity under stress.

Tooling resin
Resins that have applications as tooling aids, coreboxes, prototypes, hammer forms, stretch forms, foundry patterns, and so forth. Epoxy and silicone are common examples.

Twisting stress

Torsional stress
The shear stress on a transverse cross section caused by a twisting action.

A property of a material for absorbing work. 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.

An untwisted bundle of continuous filaments. Commonly used in referring to manmade fibers, particularly carbon and graphite, but also glass and aramid. A tow designated as 140K has 140,000 filaments.

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

Transfer molding
Method of molding thermosetting materials in which the plastic is first softened by heat and pressure in a transfer chamber and then forced by high pressure through suitable sprues, runners, and gates into the closed mold for final shaping and curing.

Transition temperature
The temperature at which the properties of a material change. Depending on the material, the transition change may or may not be reversible

Hoechst-Celanese manufactures a product called Treveria®, which is a heat treated polyester fiber fabric designed as a “bulking” material and as a gel coat barrier to reduce “print-through.” Although polyester fibers have fairly high strengths, their stiffness is considerably below that of glass[10]. Other attractive features include low density, reasonable cost, good impact and fatigue resistance, and potential for vibration damping and blister resistance.

Triaxial Braid
A braid consisting of a biaxial braid with added axials in which the yarns are locked together without possible geometric arrangement. The axials lie within the fabric formed by the biaxial braiding yarns and generally have little crimp. Significant reinforcement is provided to the final structure since they lie in the zero degree direction.

Ultimate tensile strength
The ultimate or final (highest) stress sustained by a specimen in a tension test. Rupture and ultimate stress may or may not be the same.

Ultrasonic testing
A nondestructive test applied to materials for the purpose of locating internal flaws or structural discontinuities by the use of high-frequency reflection or attenuation (ultrasonic beam).

Uniaxial load
A condition whereby a material is stressed in only one direction along the axis or centerline of component parts.

Unidirectional fibers
Fiber reinforcement arranged primarily in one direction to achieve maximum strength in that direction.

Urethane plastics
Plastics based on resins made by condensation of organic isocyanates with compounds or resins that contain hydroxyl groups. The resin is furnished as two component liquid monomers or prepolymers that are mixed in the field immediately before application. A great variety of materials are available, depending upon the monomers used in the prepolymers, polyols, and the type of diisocyanate employed. Extremely abrasion and impact resistant. See also polyurethane.

Vacuum Bag
A thick, stretchy plastic bag that covers the composite laminate and the rest of the vacuum bagging materials. Vacuum bags must be able to withstand vacuum forces and curing temperatures without breaking.

Vacuum Bag Assembly
The collective term for a composite that has been covered with plies of vacuum bagging film and the vacuum bag.

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

Vacuum Compaction Table
A vacuum compaction table is a convenient tool for debulking composite layups with multiple plies. Essentially a reusable vacuum bag, a compaction table consists of a metal table surface with a hinged cover. The cover includes a solid frame, a flexible membrane, and a vacuum seal. Repair plies are laid up on the table surface and sealed beneath the cover with vacuum to remove entrapped air. Some compaction tables are heated but most are not.

An ultrathin mat similar to a surface mat, often composed of organic fibers as well as glass fibers.

Vinyl esters
A class of 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 copolymerization with other vinyl monomers, such as styrene.

The property of resistance to flow exhibited within the body of a material, expressed in terms of relationship between applied shearing stress and resulting rate of strain in shear. Viscosity is usually taken to mean Newtonian viscosity, in which case the ratio of shearing stress to the rate of shearing strain is constant. In non-Newtonian behavior, which is the usual case with plastics, the ratio varies with the shearing stress. Such ratios are often called the apparent viscosities at the corresponding shearing stresses.Viscosity is measured in terms of flow in Pa • s (P), with water as the base standard (value of 1.0). The higher the number, the less flow.

Void content
Volume percentage of voids, usually less than 1% in a properly cured composite. The experimental determination is indirect, that is, calculated from the measured density of a cured laminate and the “theoretical” density of the starting material.

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

Volatile content
The percent of volatiles that are driven off as a vapor from a plastic or an impregnated reinforcement.

Materials, such as water and alcohol, in a sizing or a resin formulation, that are capable of being driven off as a vapor at room temperature or at a slightly elevated temperature.

The yarn running lengthwise in a woven fabric. A group of yarns in long lengths and approximately parallel. A change in dimension of a cured laminate from its original molded shape

Water absorption
Ratio of the weight of water absorbed by a material to the weight of the dry material.

The exposure of plastics outdoors. Compare with artificial weathering.

The particular manner in which a fabric is formed by interlacing yarns. Usually assigned a style number.

The transverse threads or fibers in a woven fabric. Those running perpendicular to the warp. Also called fill, filling yarn, or woof.

Wet lay-up
A method of making a reinforced product by applying the resin system as a liquid when the reinforcement is put in place.

Wet strength
The strength of an organic matrix composite when the matrix resin is saturated with absorbed moisture, or is at a defined percentage of absorbed moisture less than saturation. (Saturation is an equilibrium condition in which the net rate of absorption under prescribed conditions falls essentially to zero.)

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

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

A short single crystal fiber or filament. Whisker diameters range from 1 to 25 microns, with aspect ratios between 100 and 15,000.

Working Life
The period of time during which a liquid resin or adhesive, after mixing with catalyst, solvent, or other compounding ingredients, remains usable

Woven roving
A heavy glass fiber fabric made by weaving roving or yarn bundles

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

The axis in the plane of the laminate used as 0-degree reference; the Y-axis is the axis in the plane of the laminate perpendicular to the X-axis; the Z-axis is the reference axis normal to the laminate plane in composite laminates.

In composite laminates, the axis in the plane of the laminate which is perpendicular to the x-axis.

Similar to Yield (see) but used to describe the linear density of "bare glass" or an unsized product. Yardage specifies the number of yards of glass required to weigh one pound. It is measured in hundreds. For example, K18 is a K fiber diameter that has 1800 yards in one pound of glass.

Yield point
The first stress in a material, less than the maximum attainable stress, at which the strain increases at a higher rate than the stress. 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
The stress at the yield point. The 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, the material is elastic; above it, the material is viscous. Often defined as the stress needed to produce a specified amount of plastic

Young’s modulus
The ratio of normal stress to corresponding strain for tensile or compressive stresses less than the proportional limit of the material. See also modulus of elasticity.

In composite laminates, the reference axis normal to the plane of the laminate.