Definitions /Dictionary/Glossary - nptel

Definitions /Dictionary/Glossary
This glossary of terms is not totally comprehensive but it does represent nearly all commonly
used terms. Where exceptions are made, they are noted in the text and tables. For ease of
identification the definitions have been organized alphabetically.
A-Basis (or A-Value): A statistically-based material property; a 95% lower confidence
bound on the first percentile of a specified population of measurements. Also a 95% lower
tolerance bound for the upper 99% of a specified population.
A-Stage: An early stage in the reaction of thermosetting resins in which the material is still
soluble in certain liquids and may be liquid or capable of becoming liquid upon heating
(Sometimes referred to as resol). See also B-stage, C-stage.
Ablation: The degradation, decomposition and erosion of a material caused by high temperature,
pressure, time and velocity of gas flow.
Ablative: A material that absorbs heat through a decomposition process called pyrolysis at or
near the exposed surface.
Abrasive Water Jet: It is similar to a conventional water jet except that a fine grit is mixed into
a high pressure water stream. This is useful for cutting cured organic matrix and metal matrix
composite materials. The main cutting action is an accelerated erosion process.
Absorption: A process in which one material (the absorbent) takes in or absorbs another (the
absorbate).
Accelerator: A material which, when mixed with a catalyzed resin, will speed up the chemical
reaction between the catalyst and the resin; either in polymerizing of resins or vulcanization of
rubbers. It is also known as "promoter".
Accuracy: The degree of conformity of a measured or calculated value to some recognized
standard or specified value. Accuracy involves the systematic error of an operation.
Acetone: In an FRP context, acetone is primarily useful as a cleaning solvent for removal of uncured
resin from applicator equipment and clothing. It is very flammable liquid.
ACI: The Air Catalyst Integrator valve is mounted on Binks spray guns. It functions as a point
where catalyst and air are introduced, internally mixed and atomized in preparation for external
mixing with the resin or gel coat in a spray pattern.
Acoustic Emission: It is a measure of integrity of a material, as determined by sound emission
when a material is stressed. Ideally emission can be correlated with defects and/or incipient
failure.

Acrylic Plastic: A thermoplastic polymer made by the polymerization of esters of acrylic acid
and its derivatives.
Activation: The chemical process of making a surface more receptive to bonding.
Activator: See accelerator.
Addition: A polymerization reaction in which no by-products are formed.
Adherend: A body that is held to another body, usually by an adhesive.
Addition Polymerization: Polymerization by a repeated addition process in which monomers
are linked together to form a polymer without splitting off of water or other simple molecules.
Additives: Ingredients mixed into resin to improve properties.
Adhesion: The state in which two surfaces are held together at an interface by forces or
interlocking action or both.
Adhesion Failure: A rupture of an adhesive bond that appears to be a separation at the
adhesive/adherend interface.
Adhesion-to-core-Failure: It is a failure caused by adhesive not adhering to the core. Adhesionto-facing
Failure- Failure caused by adhesive not adhering to facing.
Adhesion Promoter: A coating applied to a substrate, before it is coated with an adhesive to
improve the adhesion of the plastic
Adhesive: A substance capable of holding two materials together by surface attachment. In the
handbook, the term is used specifically to designate structural adhesives, those which produce
attachments capable of transmitting significant structural loads.
Adhesive Assembly: It is a group of materials or parts, including adhesive, that are placed
together for bonding or that have been bonded together.
Adhesive Bonding: A material joining process in which an adhesive, placed between facing
surfaces, solidifies to bond the surface together.
Adhesive Failure: Rupture of an adhesive bond such that the separation appears to be at the
adhesive-adherend interface.
Adhesive Film: It is a synthetic resin adhesive, with or without a film carrier fabric, usually of
the thermosetting type, in the form of a thin film of resin, used under heat and pressure as an
interleaf in the production of bonded Structures.

Adhesive Joint: It is the location at which two adherends or substrates are held together with a
layer of adhesive.
ADK: Notation used for the k-sample Anderson-Darling statistic, which is used to test the
hypothesis that k batches have the same distribution.
Adsorption: The adhesion of the molecules of gases, dissolved substances or liquids in more or
less concentrated form, to the surfaces of solids or liquids with which they are in contact.
Advanced Ceramics: These are the Ceramic materials that exhibit superior mechanical
properties such as corrosion/oxidation resistance, or electrical, optical and/or magnetic
properties. This term includes many monolithic ceramics as well as particulates, whiskers and
fibre reinforced glass, glass-ceramics and ceramic-matrix composites.
Advanced Composites: These are the Composite materials that are reinforced with continuous
fibres having a modulus higher than that of fibre glass fibres. The term includes metal-matrix and
ceramic-matrix composites as well as carbon-carbon composites.
Aerodynamic Tailoring: It is the design of an aerodynamic surface whose strength and stiffness
are matched to the aerodynamic loads that will be imposed upon it.
Aging: The effect, on materials, of exposure to an environment for a period of time; the process
of exposing materials to an environment for an interval of time.
Air-Bubble-Void: Air entrapment within and between the plies of reinforcement; noninterconnected,
spherical in shape.
Air Dry: To cure at room temperature with the addition of catalyst but without the assistance of
heat and pressure.
Air-Inhibited Resin: A resin by which surface cures will be inhibited or stopped in the presence
of air.
Air Splice: The coupling between two roving doffs which is made by a jet of air
entwining/snarling two strands together. The air splice is used instead of a knot.
Air Vent: It is a small outlet, to prevent entrapment of gases.
Aliquot: A small, representative portion of a larger sample.
Alkyd Plastics: Plastics based on resins composed principally of polymeric esters, in which the
recurring ester groups are an integral part of the main polymer chain, and in which ester groups
occur in most cross links that may be present between chains.
Alligatoring: Wrinkling of the gel coat film resembling alligator hide. It sucks.

Allotropy: The existence of a substance in one or more forms.
Alloy: It will made by mixing traditional polymers, which have already been formed. The
mixing results in a single-phase material because the polymers have some interaction that
combines them together. The properties of the alloy depend upon the physical interactions of the
polymers.
Alternating Stress: According to law determined in terms of the time, it is a stress varying
between two maximum values that is equal but with opposite signs.
Ambient: The surrounding environmental conditions such as pressure or temperature.
Amine Resins: A synthetic resin derived from the reaction of urea, thiourea, melamine or allied
compounds with aldehydes, particularly formaldehyde.
Amorphous: Describes polymers that have no order to their molecules, thus no crystalline
component.
Anelasticity: A characteristic exhibited by certain materials in which strain is a function of both
stress and time, such that, while no permanent deformations are involved, a finite time is
required to establish equilibrium between stress and strain in both the loading and unloading
directions.
Angle ply: Same as Cross ply.
Anisotropic: Not isotropic; having mechanical and/or physical properties which vary with
direction relative to natural reference axes inherent in the material. It is also defined as the
tendency of a material to exhibit different properties in response to stresses applied along axes in
different directions.
Anisotropic Laminate: One in which the strength properties are different in different directions.
Anisotropy of Laminates: The difference of the properties along the directions parallel to the
length or width into the lamination planes; or parallel to the thickness into the planes
perpendicular to the lamination.
Antimony Trioxide: An additive used to give special flammability characteristics to polyester.
Antioxidant: It is 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.
Anti-Static Agents: Agents which, when added to the molding material or applied on the
surface of the molded object, make it less conducting (thus hindering the fixation of dust).

Aramid: A manufactured fiber in which the fiber-forming substance consisting of a long-chain
synthetic aromatic polyamide in which at least 85% of the amide (-CONH-) linkages are attached
directly to two aromatic rings.
Aramid: (Kevlar(R)) Aromatic polyamide fibers characterized by excellent flame-resistance,
high-temperature, and electrical properties. Aramid fibers are used to achieve high-strength,
high-modulus reinforcement in plastic composites.
Arcing: This spray method should normally be avoided as it consists of directing spray passes
by gun rotation at the wrist (arcing), as opposed to the conventional stroke from the shoulder,
keeping the fan pattern perpendicular to the mold.
Arial Weight: The weight of fiber per unit area (width times the length) of tape or fabric.
Arial Weight of Fiber: The weight of fiber per unit area of prepreg. This is often expressed as
grams per square meter.
Aromatic: Unsaturated hydrocarbon with one or more benzene ring structures in the molecule.
Artificial Weathering: Exposure to laboratory conditions which may be cyclic, involving
changes in temperature, relative humidity, radiant energy and any other elements found in the
atmosphere in various geographical areas.
Ash Content: The solid residue remaining after a reinforcing substance has been incinerated (or
strongly heated).
ASTM: American Society of Test Methods.
Aspect Ratio: In an essentially two-dimensional rectangular structure (e.g., a panel), the ratio of
the long dimension to the short dimension. However, in compressive loading, it is sometimes
considered to be the ratio of the load direction dimension to the transverse dimension. Also, in
fiber micro-mechanics, it is referred to as the ratio of length to diameter.
Assembly line: The time interval between the spreading of the adhesive on the adherend and the
application of pressure and/or heat to the assembly.
Autoclave: A closed vessel for producing an environment of fluid pressure, with or without heat,
to an enclosed object which is undergoing a chemical reaction or other operation.
Autoclave Molding: A process similar to the pressure bag technique. The lay-up is covered by a
pressure bag, and the entire assembly is placed in an autoclave capable of providing heat and
pressure for curing the part. The pressure bag is normally vented to the outside.
Auto- Ignition Temperature: The lowest temperature required to initiate or cause self-sustained
combustion in the absence of a spark or a flame.

Automated Tape Laying: It is a fabrication process in which prepreg tape is laid side by side or
overlapped in various configurations to form a structure.
Automatic Mold: A mold (for injection or compression molding) that cycles repeatedly through
the injection phase without human assistance.
Automatic Press: A press for compression molding or an injection machine that operates
continuously is being controlled mechanically, electrically or hydraulically.
Axial Strain: It is the linear strain in a plane parallel to the longitudinal axis of the specimen.
Axial Winding: In filament-wound reinforced plastics, a winding with the filaments parallel to,
or at a small angle to, the axis.
Axis of Braiding: The direction in which the braided form progresses.
Axial Winding: A type of filament winding in which the filaments are parallel to the axis.
B-Basis (or B-Value): A statistically-based material property; a 95% lower confidence
bound on the tenth percentile of a specified population of measurements. Also a 95% lower
tolerance bound for the upper 90% of a specified population.
B-Stage: An intermediate stage in the reaction of a thermosetting resin in which the material
softens when heated and swells when in contact with certain liquids but does not entirely fuse or
dissolve. Materials are usually precured to this stage to facilitate handling and processing prior to
final cure (Sometimes referred to as resitol). See A-stage, C-stage.
Back Pressure: Resistance of a material, because of its viscosity, to continued flow when mold
is closing.
Bagging: It is an operation consisting of thermocouple placement, bleeder cloth and blanket
arrangement over a laminate lay-up, installation of vacuum bag and vacuum lines and sealing of
the vacuum bag.
Bag Molding: It is a method of molding or laminating which involves the application of fluid
pressure to a flexible material which transmits the pressure to the material being molded or
bonded. Fluid pressure usually is applied by means of air, steam, water or vacuum.
Balanced Construction: Equal parts of warp and fill in fibre fabric.
Balanced Design: In filament-wound reinforced plastics, a winding pattern so designed that, the
stresses in all filaments are equal.
Balanced In-plane Contour: In a filament wound part, a head contour in which the filaments
are oriented within a plane and the radii of curvature are adjusted to balance the stresses along
the filaments with the pressure loading.

Balanced Laminate: A composite laminate in which all identical laminae at angles other than 0
degrees and 90 degrees occur only in ± pairs (not necessarily adjacent).
Band Density: In filament winding, the quantity of fibre glass reinforcement per inch of band
width, expressed as strands per inch.
Band Thickness: In filament winding, the thickness of the reinforcement as it is applied to the
mandrel.
Band Width: In filament winding, the width of the reinforcement as it is applied to the mandrel.
Barcol Hardness: Hardness value obtained by measuring the resistance to penetration of a sharp
steel point under a spring load. The instrument, called the Barcol Impressor, gives a direct
reading on a 0-100 scale. The hardness value is often used as a measure of the degree of cure of a
plastic.
Bare Glass: Glass (yarns, rovings, fabrics) from which the sizing or finish has been removed;
also, such glass before the application of sizing or finish.
Barrier Coat: An exterior coating applied to the composite structure to provide protection.
Barrier Film: The layer of film used during cure to permit removal of air and volatiles from a
composite lay-up while minimizing resin loss.
Basket Weave: It is Woven reinforcement where two or more warp threads go over and under
two or more filling threads in a repeat pattern; less stable than the plain weave but produces a
flatter, stronger, more pliable fabric.
Batch (or Lot): For fibers and resins, a quantity of material formed during the same process and
having identical characteristics throughout. For prepregs, laminae, and laminates, material made
from one batch of fiber and one batch of resin.
Beam Equation:
The Euler beam equation arises from a combination of four distinct subsets of beam theory: the
kinematics, constitutive, force resultant, and equilibrium definition equations.
The outcome of each of these segments is summarized here:
Kinematics:
Constitutive:

Resultants:
Equilibrium:
To relate the beam's out-of-plane displacement w to its pressure loading ‘p’, we combine the
results of the four beam sub-categories in the order shown,
Kinematics →Constitutive →Resultants →Equilibrium=Beam
Equation
We'll demonstrate this hierarchy by working backwards. We first combine the 2 equilibrium
equations to eliminate ‘V’,
Next replace the moment resultant ‘M’ with its definition in terms of the direct stresses,
Use the constitutive relation to eliminate s in favor of the strain ‘e’, and then use kinematics to
replace ‘e’ in favor of the normal displacement ‘w’,

2
As a final step, recognizing that the integral over y is the definition of the beam's area moment
of inertia ‘I’,
Allows us to arrive at the Euler-Bernoulli beam equation,
Bearing Area: It is the product of the ‘pin diameter’ and the ‘specimen thickness’.
Bearing Load: It is the compressive load which acting on an interface.
Bearing Strength: It is the maximum bearing stress which not causes failure of the composite
(when applied through a cylindrical fastener surface).
Bearing Strain: It is the ratio of the deformation of the bearing hole (in the direction of the
applied force) to the pin diameter.
Bearing Stress: Applied load divided by bearing area (hole diameter times thickness).
Bearing Yield Strength: It is the bearing stress at which a material exhibits a specified limiting
deviation from the proportionality of bearing stress to bearing strain.
Bend Test: A test of ductility by bending or folding, usually with steadily applied forces. In
some instances the test may involve blows to a specimen having a cross section that is essentially
uniform over a length several times as great as the largest dimension of the cross section.
Bending Stiffness The sandwich panel stiffness deriving from the facing properties and distance
they are apart.
Benzoyl Peroxide (BPO): The catalyst used in conjunction with aniline accelerator or where
heat is used as an accelerator.
Bias Cut: Fabric cut at an angle (45o) to the warp & fill fibers.
Bias Fabric: A fabric in which warp and fill fibers are at an angle to the length.
Biaxial Load: It is a loading condition in which a laminate is stressed in two different directions
in its plane.

Biaxial Winding: In filament winding, a type of winding in which the helical band is laid in
sequence, side by side, with the crossover of the fibers eliminated.
Bi-Directional Laminate: A reinforced plastic laminate with the fibers oriented in two
directions in the plane of the laminate; a cross laminate.
Binder: A bonding resin used to hold strands together in a mat or preform during manufacture of
a molded object. The resin or cementing constituent of a plastic compound which holds the other
components together; the agent applied to glass mat or preform to bond the fibers prior to
laminating or molding.
Binomial Random Variable: The number of successes in independent trials where the
probability of success is the same for each trial.
Birefringence: 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.
Bisected Core: Corrugated honeycomb that 'has flat sheets in between the corrugated ribbons;
same as reinforced core.
Bismalcimide - (BMI): A type of thermoset polyimide that cures by an additional reaction,
avoiding formation of volatiles, and has temperature capabilities between those of epoxy and
polyimide.
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.
Blanket: Fibre or fabric plies that have been laid up in a complete assembly and placed on or in
the mold all at one time. Also, it is the type of bag in which the edges are sealed against the
mold.
Bleeder Cloth: A nonstructural layer of material used in the manufacture of composite parts to
allow the escape of excess gas and resin during cure. The bleeder cloth is removed after the
curing process and is not part of the final composite.
Bleeder Ply: Layer of porous material placed in a vacuum bag to absorb excess resin and allow
air and gas to escape.
Bleeding: -The removal of excess resin from a laminate during cure.
Bleed-out: The excess liquid resin appearing at the surface, primarily occurring during filament
winding.

Blend: Made by mixing traditional polymers which have already formed. The mixing results in a
multi-phase system. Physical interactions between the polymers are responsible for the properties
of the blend.
Blister: Undesirable rounded elevation of the surface of a plastic, whose boundaries may be
more or less sharply defined, resembling in shape a blister on the human skin. The blister may
burst and become flattened.
Block: It is the large expanded honeycomb piece before the slices are sawn.
BMC: Bulk Molding Compound (Thermoset).
Bobbin: A cylinder or slightly tapered barrel, with or without flanges, for holding tows, rovings,
or yarns.
Bond: The adhesion of one surface to another, with or without the use of an adhesive as a
bonding agent.
Bond Line: The layer of adhesive that attaches two adherends.
Bond Ply: The ply or fabric patch that comes in contact with the honeycomb core during repair.
Bond Strength: The amount of adhesion between bonded surfaces; a measure of the stress
required to separate a layer of material from the base to which it is bonded. The amount of
adhesion between bonded surfaces.
Boron Fiber: A fiber usually of a tungsten-filament core with elemental boron vapor deposited
on it to impart strength and stiffness.
Braid: A system of three or more yarns which are interwoven in such a way that no two yarns
are twisted around each other.
Braid Angle: The acute angle measured from the axis of braiding.
Braid, Biaxial: Braided fabric with two-yarn systems, one running in the +θ direction, the other
in the -θ direction as measured from the axis of braiding.
Braid Count: The number of braiding yarn crossings per inch measured along the axis of a
braided fabric.
Braid, Diamond: Braided fabric with an over one, under one weave pattern, (1 x 1).
Braid, Flat: A narrow bias woven tape wherein each yarn is continuous and is intertwined with
every other yarn in the system without being intertwined with itself.
Braid, Hercules: A braided fabric with an over three, under three weave pattern, (3 x 3).

Braid, Jacquard: A braided design made with the aid of a jacquard machine, which is a
shedding mechanism by means of which a large number of ends may be controlled
independently and complicated patterns produced.
Braid, Regular: A braided fabric with an over two, under two weave pattern (2 x 2).
Braid, Square: A braided pattern in which the yarns are formed into a square pattern.
Braid, Two-Dimensional: Braided fabric with no braiding yarns in the through thickness
direction.
Braid, Three-Dimensional: Braided fabric with one or more braiding yarns in the through
thickness direction.
Braid, Tri-axial: A biaxial braided fabric with laid in yarns running in the axis of braiding.
Braiding: A textile process where two or more strands, yarns or tapes are intertwined in the bias
direction to form an integrated structure.
Breakout: Separation or breakage of fibers when the edges of a composite part are drilled or cut.
Breather: A loosely woven material that does not come in contact with the resin but serves as a
continuous vacuum path over a part in production.
Breathing: It is the opening and closing of a mold to allow gas to escape early in the molding
cycle. It is also called degassing.
Bridging: Fabric reinforcement extended over a curved edge that does not come into contact
with the rest of the composite.
Broadgoods: A term loosely applied to prepreg material greater than about 12 inches in width,
usually furnished by suppliers in continuous rolls. The term is currently used to designate both
collimated uniaxial tape and woven fabric prepregs.
Buckling (Composite): This is a mode of structural response characterized by an out-of-plane
material deflection due to compressive action on the structural element involved. In advanced
composites, buckling may take the form not only of conventional general instability and local
instability but also micro-instability of individual fibers.
Bulk Density: The density of the molding material in loose form expressed as a ratio of weight
to volume.
Bulk Modulus: It is the ratio of the hydrostatic pressure to the volume strain.

Bulk Molding Compound: Thermosetting resin mixed with strand reinforcement, fillers, etc.
into a viscous compound for compression or injection molding.
Bundle: It is a general term used for a collection of essentially parallel filaments or fibres. It is a
general term for a collection of essentially parallel filaments or fibers.
Burst Strength:
(1) Hydraulic pressure required to burst a vessel of given thickness. It is 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.
Butt Joint: A joint in which parts are joined with no overlap.
C-Glass: A glass with a soda-lime-borosilicate composition that is used for its chemical
stability in corrosive environment.
C-Scan: The back and forth scanning of a specimen with ultrasonic. A non- destructive testing
technique for finding voids, delamination, defects in fibre distribution.
C-Stage: The final stage of the curing reaction of a thermosetting resin in which the material has
become practically infusable and insoluble (Normally considered fully cured and sometimes
referred to as resite).
CAD/CAM: Computer-aided design/computer-aided manufacturing.
Capstan: A friction type take-up device which moves braided fabric away from the fell. The
speed of which determines the braid angle.
Carbon-Carbon: A composite material consisting of carbon or graphite fibres in a carbon or
graphite matrix.
Carbon Fibers: Fibers produced by the pyrolysis of organic precursor fibers such as rayon,
polyacrylonitrile (PAN), and pitch in an inert atmosphere. The term is often used
interchangeably with "graphite"; however, carbon fibers and graphite fibers differ in the
temperature at which the fibers are made and heat-treated, and the amount of carbon produced.
Carbon fibers typically are carbonized at about 2400°F (1300°C) and assay at 93 to 95% carbon,
while graphite fibers are graphitized at 3450 to 5450°F (1900 to 3000°C) and assay at more than
99% elemental carbon.
Caroxyl: The chemical group characteristic of organic acids, which are incorporated into the
polyester reaction process.

Carrier: A mechanism for carrying a package of yarn through the braid weaving motion. A
typical carrier consists of a bobbin, a track follower and a tensioning device.
Casting: The process of pouring a mixture of resin, fillers and/or fibers into a mold as opposed
to building up layers through lamination. This technique produces different physical properties
from laminating.
Cast Polymer: Non-reinforced composite (resin used without reinforcing fibers). It
includes/combines polymers, fillers and additives as composites to meet specific applications
requirements.
Catalyst: A substance which changes the rate of a chemical reaction without itself undergoing
permanent change in its composition; a substance which markedly speeds up the cure of a
compound when added in minor quantity as compared to the amounts of primary reactants.
Catalyst Injection: Used with spray equipment to catalyze the polyester at the spray gun,
therefore eliminating the need to clean the system within the gel time of the polyester. Internal
mix guns do require a solvent flush for cleaning the head. Injector: Atomizing air atomizes the
catalyst at the injector unit itself. Pot: Catalyst is supplied to the spray gun, then air is introduced
to atomize the catalyst.
Catastrophic Failures: Failures of a mechanical or unpredictable nature.
Catenary: A measure of the difference in length of the strands in a specified length of roving as
a result of unequal tension; the tendency of some strands in a taut horizontal roving to sag lower
than the others.
Carrier: A mechanism for carrying a package of yarn through the braid weaving motion. A
typical carrier consists of a bobbin spindle, a track follower, and a tensioning device.
Caul Plates: Smooth metal plates, free of surface defects, the same size and shape as a
composite lay-up, used immediately in contact with the lay-up during the curing process to
transmit normal pressure and to provide a smooth surface on the finished laminate.
Caul Sheet: Plate or sheet the same size and shape used in contact with a composite layup to
transmit normal pressure and temperature during cure.
Caulk: An elastic material used to protect joints or connections from external elements,
particularly moisture.
Cavity: Depression in mold; the space inside a mold wherein a resin is poured; the molded
article; which forms the outer surface of the molded article (often referred to as the die); also, the
space between matched molds. (Depending on number of such depressions, molds are designated
as Single-Cavity or Multiple-Cavity).
Cell: It is a single honeycomb unit, usually a hexagon.

Censoring: Data is right (left) censored at M, if, whenever an observation is less than or equal to
M (greater than or equal to M), the actual value of the observation is recorded. If the observation
exceeds (is less than) M, the observation is recorded as M.
Centipoise: Measurement of viscosity as compared to water (1 centipoise).
Ceramic: A rigid, frequently brittle material made from clay and other inorganic, non-metallic
substances and fabricated into articles by sintering i.e. cold molding followed by fusion of the
part at high temperature.
Ceramic-Matrix Composites (CMC): Materials consisting of a ceramic or carbon fiber
surrounded by a ceramic matrix, usually SiC (silicon carbide).
Cermet: -Composite materials consisting of two constituents, one is an oxide, carbide, boride or
similar inorganic compound and the other a metallic binder.
Chemical Vapor Deposition (CVD): A process in which the reinforcement material is
deposited from vapor phase onto a continuous core such as boron or tungsten.
Chain-Growth Polymerization: One of the two principal polymerization mechanisms. In chain
growth polymerization, the reactive groups are continuously regenerated during the growth
process. Once started, the polymer molecule grows rapidly by a chain of reactions emanating
from a particular reactive initiator which may be a free radical, cat-ion or anion.
Chalking: Dry, powder-like appearance or deposit on gel coat surface.
Chemical Vapor Deposition (CVD): A process in which desired reinforcement material is
deposited from vapor phase onto a continuous core; boron on tungsten, for example.
Chromatogram: A plot of detector response against peak volume of solution (Eluate) emerging
from the system for each of the constituents which have been separated.
Chopped Strand: It is a continuous strand yarn or roving cut up into uniform lengths, usually
from 1/32-2" long. Lengths up to 1/8" are called milled fibers.
Chopper Gun: It is 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.
Circuit: One complete traverse of the fiber feed mechanism of a winding machine; one complete
traverse of a winding band from one arbitrary point along the winding path to another point on a
plane through the starting point and perpendicular to the axis.
Circuit: One complete traverse of the fiber feed mechanism of a filament-winding machine.

Circumferential Winding: A type of filament winding in which the filaments are perpendicular
to the axis.
Clamping Pressure: In injection molding and transfer molding, the pressure that I is applied to
the mold to keep it closed in opposition to the fluid pressure of the compressed molding material.
Clean Core: This form of honeycomb core is clean and packed in such a way that it is ready for
bonding without vapour degreasing or cleaning.
Closure: It is the complete coverage of a mandrel with one layer (two plies) of fibre.
Cloth: A fine weaves of woven fiberglass.
Cobalt: Used as the accelerator for Methyl Ethyl Ketone Peroxide catalyzed polyesters.
Cocure: The process of curing a material and bonding it to another surface during the same cure
cycle.
Cocured: Cured and simultaneously bonded to another prepared surface.
Cocuring: The act of curing a composite laminate and simultaneously bonding it to some other
prepared surface during the same cure cycle (see Secondary Bonding).
Coefficient of Elasticity: The reciprocal of Young's modulus in a tension test.
Coefficient Of Expansion (COE): Measure of the change in length or volume of an object.
Coefficient of Linear Thermal Expansion: The change in length per unit length resulting from
a one-degree rise in temperature.
Coefficient of Thermal Expansion (CTE): A material's fractional change in length for a
given unit of change in temperature.
Coefficient of Variation: The ratio of the population (or sample) standard deviation to the
population (or sample) mean.
Cohesion: Tendency of a single substance to adhere to itself. It is also, the force holding a single
substance together.
Cohesive Failure: Failure of an adhesive joint occurring primarily in an adhesive layer.
Cohesive Strength: It is the intrinsic strength of an adhesive.
Coin Tap: Tapping a laminate with a coin in different spots to detect a change in sound,
indicating the presence of a defect that may require repair.

Coin Test: Using a coin to tap a laminate in different spots, listening for a change in sound,
which would indicate the presence of a defect.
Cold Press Molding: Compression molding without the heat – A technique for molding
thermoset plastics in which a part is shaped by placing the fiber and resin into an open mold
cavity, closing the mold, and applying heat and pressure until the material has cured or achieved
its final form.
Cold-setting Adhesive: A synthetic resin adhesive capable of hardening at normal room
temperature in the presence of a hardener.
Collet: The drive wheel that pulls glass fibres from the bushing.
Collimated: Rendered parallel.
Collimated Roving: It is the roving that has been made using a special process so that the
strands are more parallel than in standard roving.
Colloidal: A state of suspension in a liquid medium in which extremely small particles are
suspended and dispersed but not dissolved.
Color Stability: It is the ability of a surface coating or pigment to resist degradation due to
environmental exposure.
Commingled Yarn: A hybrid yarn made with two types of material intermingled in a single
yarn; for example, thermoplastic filaments intermingled with carbon filaments to form a single
yarn.
Compaction: The application of a temporary vacuum bag and vacuum to remove trapped air and
compact the lay-up.
Compatible: The ability of different resin systems to be processed in contact with each other
without degradation of end product properties. (See Compatible)
Composite: A homogeneous material created by the synthetic assembly of two or more
materials (selected filler or reinforcing elements and compatible matrix binder) to obtain specific
characteristics and properties. Composites are subdivided into classes on the basis of the form of
the structural constituents; Laminar: Composed of layer or laminar constituents; Particular: The
dispersed phase consists of small particles; Fibrous: The dispersed phase consists of fibers;
Flake: The dispersed phase consists of flat flakes; Skeletal: Composed of a continuous skeletal
matrix filled by a second material.
Composite Class: It is a major subdivision of composite construction in which the class is
defined by the fiber system and the matrix class, e.g., organic-matrix filamentary laminate.

Composite Material: Composites are considered to be combinations of materials differing in
composition or form on a macro-scale. The constituents retain their identities in the composite;
that is, they do not dissolve or otherwise merge completely into each other although they act in
concert. Normally, the components can be physically identified and exhibit an interface between
one another.
Compound: An intimate mixture of polymer or polymers with all the materials necessary for the
finished product.
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. See also Compression molding.
Compression Molding: A technique for molding thermoset plastics in which a part is shaped by
placing the fiber and resin into an open mold cavity, closing the mold, and applying heat and
pressure until the material has cured or achieved its final form.
Compressive Modulus: A mechanical property description which measures the compression of
a sample at a specified load. Described in ASTM D-695.
Compressive Strength: The ability of a material to resist a force that tends to crush; the
crushing load at the failure of a specimen divided by the original sectional area of the specimen.
Compressive Stress: The normal stress caused by forces directed towards the plane on which
they act.
Condensation: A polymerization reaction in which simple by-products(for example, water) are
formed.
Condensation Polymerization: This is a special type of step-growth polymerization
characterized by the formation of water or other simple molecules during the stepwise addition
of reactive groups.
Conditioning: Subjecting a material to a prescribed environmental and/or stress history before
testing.
Conductivity: Reciprocal of volume resistivity; the conductance of a unit cube of any material.
Confidence Coefficient: See Confidence Interval.
Confidence Interval: A confidence interval is defined by a statement of one of the following
forms:
(1).P{a < θ} ≤1−α (2).P{ θ < b} ≤1−α (3).P{a < θ < b} ≤1−α

where 1- α is called the confidence coefficient. A statement of type (1) or (2) is called a onesided
confidence interval and a statement of type (3) is called a two-sided confidence interval. In
(1) a is a lower confidence limit and in (2) b is an upper confidence limit. With probability at
least 1- α, the confidence interval will contain the parameter θ.
Connection: Where two panels are attached to each other or a panel is attached to the building.
Consolidation: A processing step that compresses fiber and matrix to reduce voids and achieve a
desired density.
Constituent: In general, an element of a larger grouping. In advanced composites, the principal
constituents are the fibers and the matrix.
Contact Adhesive: An adhesive that .is apparently dries to touch and that will adhere to itself
simultaneously upon contact.
Contact Molding: A process for molding reinforced plastics in which reinforcement and resin
are placed on a mold, cure is either at room temperature using a catalyst-promoter system or by
heat in an oven and no additional pressure is used.
Contact Pressure Molding: A method of molding in which the pressure, usually less than 69
kPa (10 psi), is usually necessary to hold the materials together.
Contact Pressure Resin: Liquid resins that thicken or polymerize on heating and when used for
bonding laminates, require little or no pressure.
Contaminant: Impurity or foreign substance that affects one or more properties of composite
material, particularly adhesion.
Continuous Filament: A yarn or strand in which the individual filaments are substantially the
same length as the strand.
Continuous-Filament Yarn: Yarn that is formed by twisting two or more continuous filaments
into a single continuous strand.
Continuous Heat Resistance: This is the maximum temperature the material should be
subjected to in a continuous application. Below this temperature the material is acceptable.
Above this temperature the material may decompose, melt, or otherwise fail in an application.
Units - degrees Fahrenheit ((F) - degrees Centigrade ((C). Higher numbers mean that the material
can be used continuously at higher temperatures.
Continuous Laminating: A process for forming panels and sheeting in which fabric or a mat is
passed through a resin dip, brought together between cellophane covering sheets, and passed
through a heating zone for cure. Squeeze rolls control thickness and resin content as the various
plies are brought together.

Continuous Roving: Parallel filaments coated with sizing, gathered together into single or
multiple strands, and wound into a cylindrical package. It may be used to provide continuous
reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding
compounds, or it may be used chopped.
Continuous Strand Roving: A bundle of glass filaments which are fed through a chopper gun
in the spray up process.
Copolymer: A large chemical chain composed of two or more dissimilar groups.
Core: The central component of a sandwich construction to which the sandwich faces or skins
are attached; also, part of a complex mold that forms undercut parts.
Core Angle: The angle between the plane of the core slice and the cell wall. In normal cases it
should be 90°.
Core Crush: Compression damage of the core.
Core Density: It is the weight of one cubic foot of core, expressed in pounds per cubic foot
(pcf).
Core Depression: A gouge or indentation in the core material.
Core Failure: It is the sandwich panel failure caused by the core failing in compression, shear or
tension.
Core-to-facing Bond: The connection of the facings to the core by an adhesive, resin, brazing,
etc.
Core Orientation: Used on a honeycomb core to line up the ribbon direction, thickness of the
cell depth, cell size and transverse direction.
Core Separation: A partial or complete breaking of the core node bond.
Core Splicing: Joining two core segments by bonding them together.
Cored Mold: It is a mold incorporating passages for electrical heating elements, steam or water.
Count: For fabric, number of warp and filling yarns per inch in woven cloth. The sine of the
yarn based on relation of length and weight.
Coupling Agent: Any chemical substance designed to react with both the reinforcement and
matrix phases of a composite material to form or promote a stronger bond at the interface.
Coupling agents are applied to the reinforcement phase from an aqueous or organic solution or
from a gas phase, or added to the matrix as an integral blend.

Coupon: A specimen for a specific test, usually as a tensile coupon.
Coverage: The measure of the fraction of surface area covered by the braid.
Cowoven Fabric: It is a reinforcement fabric woven with two different types of fibers in
individual yarns; for example, thermoplastic fibers woven side by side with carbon fibers.
Crack: Actual separation of molded material, visible on opposite surfaces of the part and
extending through the thickness.
Crazing: Apparent fine cracks at or under the surface of an organic matrix.
Creel: A framework arranged to hold tows, rovings, or yarns so that many ends can be
withdrawn smoothly and evenly without tangling.
Creep: The time dependent part of strain resulting from an applied stress.
Creep, Rate Of: The slope of the creep-time curve at a given time.
Crimp: The undulations induced into a braided fabric via the braiding process.
Crimp Angle: The maximum acute angle of a single braided yarn's direction measured from the
average axis of tow.
Crimp Exchange: The process by which a system of braided yarns reaches equilibrium when
put under tension or compression.
Critical Length: The minimum length of a fiber necessary for matrix shears loading to develop
fiber ultimate strength by matrix.
Critical Value(s): When testing a one-sided statistical hypothesis, a critical value is the value
such that, if the test statistic is greater than (less than) the critical value, the hypothesis is
rejected. When testing a two-sided statistical hypothesis, two critical values are determined. If
the test statistic is either less than the smaller critical value or greater than the larger critical
value, then the hypothesis is rejected. In both cases, the critical value chosen depends on the
desired risk (often 0.05) of rejecting the hypothesis when it is true.
Cross Laminated: Material laminated so that some of the layers are oriented at various angles to
the other layers with respect to the laminate grain. A cross-ply laminate usually has plies oriented
only at [0/90].
Cross-Linking: It is the process of bridging two polymer chains, which converts a thermoplastic
to a thermoset.
Cross ply: Any filamentary laminate which is not uniaxial. This is same as Angle-ply. In some
references, the term cross-ply is used to designate only those laminates in which the laminae are

at right angles to one another, while the term angle-ply is used for all others. In the handbook,
the two terms are used synonymously. The reservation of a separate terminology for only one of
several basic orientations is unwarranted because a laminate orientation code is used.
Cross-ply Laminate: A laminate with plies oriented at 00 and 900 only.
Crystallinity: The quality of having a molecular structure with atoms arranged in an orderly,
three-dimensional pattern.
Cure: To change the properties of a thermosetting resin irreversibly by chemical reaction, i.e.,
condensation, ring closure, or addition. Cure may be accomplished by addition of curing (crosslinking)
agents, with or without catalyst, and with or without heat. Cure may occur also by
addition, such as occurs with anhydride cures for epoxy resin systems.
Cure Cycle: The schedule of time periods at specified conditions to which a reacting
thermosetting material is subjected in order to reach a specified property level.
Cure Stress: A residual internal stress produced during the curing cycle of composite structures.
Normally, these stresses originate when different components of a lay-up have different thermal
coefficients of expansion.
Cure Time: The time required for the liquid resin to reach a cured or fully polymerized state
after the catalyst has been added.
Cure Stress: A residual internal stress produced during the curing cycle of composite structures.
Normally, these stresses originate when different components of a lay-up have different thermal
coefficients of expansion.
Curing Agent: A catalytic or reactive agent which when added to a resin causes polymerization,
synonymous with hardener. It is an agent which when added to a resin causes polymerization.
Curing Temperature: It is temperature at which a cast, molded, or extruded product, a resinimpregnated
reinforcement, an adhesive, etc., is subjected to curing.
Curing Time: It is the period of time during which a part is subjected to heat (or pressure), or
both, to cure the resin. It is an 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.)
Cyanate Ester Resin: Thermosetting resins that are derived from bisphenols or polyphenols and
are available as monomers, blend~ and solutions.
Cycle Time: In molding, the total time used to carry out a complete sequence of operations
making up the molding cycle.

Cycle: 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.
D-Glass: A high boron content glass made especially for laminates requiring a precisely
controlled dielectric constant.
Dam: Boundary support used to prevent excessive edge bleeding or resin run out of a laminate
and to prevent crowning of the bag during cure.
Damage Tolerance: It is a measure of the ability of structures to retain load-carrying capability
after exposure to sudden loads (for example, ballistic impact).
Damping: Diminishing the intensity of vibrations.
Daylight: The distance in the open position, between the moving and fixed tables or the platens
of a hydraulic press. In the case of a multi-daylight press, daylight is the distance between
adjacent platens.
Debond: A deliberate separation of a bonded joint or interface, usually for repair or rework
purposes. See Disbond, Unbond.
Debulking: Compacting of a thick laminate under moderate heat and pressure and/or vacuum to
remove most of the air, vapour and volatiles between the plies.
Deep-Draw Mold: A mold having a core which is long in relation to the wall thickness.
Deflection Temperature under Load: The temperature at which a simple beam has deflected a
given amount under load (formerly called heat distortion temperature).
Deformation: The change in shape of a specimen caused by the application of a load or force.
Deformation Under Load: The 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, see 'cold flow' or 'creep'.
Degassing: It is the opening and closing of a mold to allow gas to escape early in the molding
cycle. It is also called breathing.
Degradation: A deleterious change in chemical structure, physical properties or appearance.
Delamination: The separation of the layers of material in a laminate. This may be local or may
cover a large area of the laminate. It may occur at any time in the cure or subsequent life of the
laminate and may arise from a wide variety of causes.

Delaminate: To split a laminated plastic material along the plane of its layers. It is the physical
separation or loss of bond between laminate plies.
Demold: To remove a part from a tool, or a tool from an intermediate model.
Denier: A direct numbering system for expressing linear density, equal to the mass in grams per
9000 meters of yarn, filament, fiber, or other textile strand.
Density: The mass per unit volume.
Density, Fibers: Mass per unit volume of the solid matter of which a fiber is composed,
measured under specified conditions.
Design Allowable: A limiting value for a material property that can be used to design a
structural or mechanical system to a specified level of success with 95 percent statistical
confidence. B-basis allowable: material property exceeds the design allowable 90 times out of
100. A-basis allowable: material property exceeds the design allowable 99 times out of 100.
Desorption: A process in which an absorbed or adsorbed material is released from another
material. Desorption is the reverse of absorption, adsorption, or both.
Deviation: Variation from a specified dimension or requirement, usually defining the upper and
lower limits.
Diallylphthalate (DAP): In reinforced plastics, a monomer which will polymerize with heat and
catalyst into clear, hard polymer.
Dibuthyl Phthalate: A lubricant for spray equipment.
Dielectric: A nonconductor of electricity. It is the ability of a material to resist the flow of an
electric current.
Dielectric Constant: The ratio of the capacity of a condenser having a dielectric constant
between the plates to that of the same condenser when the dielectric is replaced by a vacuum; a
measure of the electrical charge stored per unit volume at unit potential.
Dielectric Strength:
1) This is an electrical property and gives an indication of how well the 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 - Volts per mil of
thickness (volts/mil). Higher numbers indicate materials which are better insulators. C
means that the material conducts electricity and therefore has no dielectric strength.
2) The average potential per unit thickness at which failure of the dielectric material occurs.
Diethylaniline (DEA): Accelerator used in conjunction with BPO catalyst or as a co-promoter
for cobalt/MEKP system.

Diluent: Diluting (reducing or thinning) agent.
Dimensional Stability: Capability of an item to maintain its constructed dimensions without
distorting.
Dimethyl Aniline (DMA): Accelerator used in conjunction with BPO catalyst. It is more
effective than DEA.
Dimples: 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.
Disbond: An area within a bonded interface between two adherents in which an adhesion failure
or separation has occurred. It may occur at any time during the life of the structure and may arise
from a wide variety of causes. Also, colloquially, an area of separation between two laminae in
the finished laminate (in this case the term "delamination" is normally preferred.) (See Debond,
Unbond, Delamination.)
Discontinuous Fibre Reinforced Composite: A ceramic-matrix composite material I reinforced
by chopped fibres.
Dispersion: The means of incorporating pigments into polyester.
Displacement Angle: -In filament winding, the advancement distance of the winding ribbon
after one complete circuit.
Distortion: A wavy gel coat image often found in conjunction with print through. It is
commonly caused by a problem in the laminating system.
Distribution: A formula which gives the probability that a value will fall within prescribed
limits (See Normal, Weibull, and Lognormal Distributions).
Doctor Roll; Doctor Bar: A device for regulating the amount of liquid material on the rollers of
a spreader.
Dolly: In filament winding, the planar reinforcement applied to a local area between windings to
provide extra strength in an area where a cut-out is to be made.
Dome: In filament winding, the portion of a cylindrical container that forms the spherical or
elliptical shell.

Doubler: Localized area of extra layer reinforcement, usually to provide stiffness or strength for
fastening or other abrupt load transfers.
Draft: The tape or slope of the vertical surfaces of a mold designed to facilitate removal of
molded parts.
Draft Angle: A mandrel's taper or angle for ease of part removal.
Drain Out: The leaking, sagging and puddling of the laminating resin from the laminate.
Drape: The ability of pre impregnated broad goods to conform to an irregular shape; textile
conformity.
Drawn Fibre: It is fibre with a certain amount of orientation imparted by the drawing process by
which it was formed.
Drilled Core: Honeycomb that has holes drilled in the cell walls to allow gases or fluids to
escape from the cells.
Dry: A material condition of moisture equilibrium with a surrounding environment at 5% or
lower relative humidity.
Dry Spot: Area of incomplete surface film on laminated plastics; in laminated glass, an area
over which the interlayer and the glass have not become bonded.
Dry Chop: Chopping a roving without the simultaneous admixture of resin and catalyst.
Dry Fiber Area: Area of fiber not totally encapsulated by resin.
Dry Laminate: The laminate containing insufficient resin.
Dry Lay-up: Construction of a laminate by the layering of pre-impregnating reinforcement in a
female mold or on a male mold usually followed by bag molding or autoclave molding.
Dry Winding: A type of filament winding in which pre-impregnated roving is used.
Ductility: The ability of a material to deform plastically before fracturing.
Duplication Mold: A mold made by casting over or duplicating another article.
Durometer of Hardness: The hardness of a material determined by Shore durometer.
Dwell Point: A lightning attachment point.
Dwell Time: The time that the lightning channel remains attached to a single spot Ion the
aircraft.

Ease of Rollout: The relative facility with which a sprayed admixture of chopped glass
and resin can be compacted by rolling.
E-Glass: "Electrical glass" A borosilicate glass; the type most used for glass fibers for reinforced
plastics; -suitable for electrical laminates because of its high resistivity. It is also called
‘electrical glasses.
Edge Joint: It is a joint made by bonding the edge faces of the two adherends.
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 material under stress that recovers upon
removal of the stress.
Elastic Limit: The greatest stress which a material is capable of sustaining without permanent
strain remaining upon 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 non-recoverable, deformation.
Elasticity: The property of a material which allows it to recover its original size and shape
immediately after removal of the force causing deformation.
Elastomer: A polymeric material which has a high modulus of elasticity and toughness.
Urethane elastomer is an example.
Elongation: The increase in gage length or extension of a specimen during a tension test, usually
expressed as a percentage of the original gage length.
Elongation at Break: Elongation recorded at the moment of rupture of the specimen, often
expressed as a percentage of the original length.
Eluate: The liquid emerging from a column (in liquid chromatography).
Eluent: The mobile phase used to sweep or elute the sample (solute) components into, through,
and out of the column.
Encapsulating: Enclosing an article in a closed envelope of plastics by immersion. Milled fibers
or short chopped strands are often poured with the catalyzed resin into open molds for casting
electrical components.

End: A single fiber, strand, roving or yarn being or already incorporated into a product. An end
may be an individual warp yarn or cord in a woven fabric. In referring to aramid and glass fibers,
an end is usually an untwisted bundle of continuous filaments.
End Count: An exact number of ends supplied on a ball or roving.
Engineering Plastics: These materials are made by specific design. These made through the use
of particular monomers and monomer sequences to produce a plastic with desired properties,
possibly for a specific application.
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 a component.
Environmental Stress Cracking: The susceptibility of a thermoplastic resin to crack or craze in
the presence of surface-active agents or other environments.
Epoxy: A chemical compound containing a resin with epoxide groups and a hardener, which
forms a durable, solid thermoset material.
Epoxy Equivalent Weight: The number of grams of resin which contain one chemical
equivalent of the epoxy group.
Epoxy Plastics: Plastics based on resins made by the reaction of epoxides or oxiranes with other
materials such as amines, alcohols, phenols, carboxylic acids, acid anhydrides and unsaturated
compounds.
Epoxy Resin: Resins which may be of widely different structures but are characterized by the
presence of the epoxy group. (The epoxy or epoxide group is usually present as glycidyl ether,
glycidyl amine, or as part of an aliphatic ring system. The aromatic type epoxy resins are
normally used in composites.)
Epoxide: A reactive group in which an oxygen atom is joined to each of two carbon atoms,
which are already united in some other way.
Euler-Bernoulli Beam Equation:
The out-of-plane displacement ‘w’ of a beam is governed by the Euler-Bernoulli Beam Equation,

where ‘p’ is the distributed loading (force per unit length) acting in the same direction as y (and
w), ‘E’ is the Young's modulus of the beam, and I is the area moment of inertia of the beam's
cross section.
If ‘E’ and ‘I’ do not vary with ‘x’ along the length of the beam, then the beam equation
simplifies to,
Exothermic: Accompanied by generation of heat.
Exotherm: The liberation or evolution of heat during the curing of a plastic product.
Exothermic Heat: Internally developed heat accompanying a chemical reaction.
Extenders: Low cost materials used to dilute or extend high cost resins without much lessening
of properties.
Extensometer: A device for measuring linear strain.
Extrusion: A process normally used with a compound. the compound is forced through a die,
producing a profiled shape.
Fabric: A material made of woven fibres or filaments.
Fabric Fill Face: That side of the woven fabric where the greatest number of yams are
perpendicular to the selvage.
Fabric, Nonwoven: A textile structure produced by bonding or interlocking of fibers, or both,
accomplished by mechanical, chemical, thermal, or solvent means, and combinations thereof.
Fabric Warp Face: That side of the woven fabric where the greatest numbers of yarns are
parallel to the selvage.
Fabric, Woven: A generic material construction consisting of interlaced yarns or fibers, usually
a planar structure. Specifically, as used in this handbook, a cloth woven in an established weave
pattern from advanced fiber yarns and used as the fibrous constituent in an advanced composite
lamina. In a fabric lamina, the warp direction is considered the longitudinal direction, analogous
to the filament direction in a filamentary lamina.

Fabrication: The process of making a composite part or tool.
Fabricator: Manufacturer of reinforced plastic products.
Face Dimpling: The compressive buckling of the facing in between the honeycomb cell walls
caused by thin facings or large cell core.
Face Wrinkling: The local compressive instability of facing; similar to plate buckling on an
elastic foundation.
Facing: It is one of the two outer layers, which has been bonded to the core of a sandwich.
Facing Failure: It is the sandwich panel failure caused by the facing failing in compression,
tension or local buckling.
Fading: It is loss of color in the gel coat.
Fairing: 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.
Fatigue: The failure or decay of mechanical properties after repeated applications of stress.
Fatigue Life: The number of cycles of deformation required to bring about failure of the test
specimen under a given set of oscillating conditions.
Fatigue Limit: It is the stress below which a material can be stressed cyclically for an infinite
number of times without failure.
Fatigue Ratio: It is the ratio of fatigue strength to tensile strength.
Fatigue Strength: It is the maximum cyclic stress a material can withstand for a given number
of cycles before failure occurs; the residual strength after being subjected to fatigue.
Faying Surfaces: These are the surfaces of materials in contact with each other and joined or
about to be joined together.
FEA: Finite element analysis, a process of selecting the optimum combination of materials in a
composite based on software analysis.
Feathered Edge: In reference to chopped strand mat, this is a mat whose edge was not trimmed
during production. Consequently, the edge tapers down in weight instead of abruptly ending.
Fell: The point of braid formation, which is defined as the point at which the yarns in a braid
system cease movement relative to each other.

Felt: a fibrous material made up of interlocking fibers by mechanical or chemical reaction,
pressure or heat. Felts may be made of cotton, glass, or other fibers.
Female Mold: A concave mold used to precisely define the convex surface of a molded part.
Fiber: A general term used to refer to filamentary materials. Often, fiber is used synonymously
with filament. It is a general term for a filament of finite length. This is a unit of matter, either
natural or man-made, which forms the basic element of fabrics and other textile structures.
Fiber Architecture: The design of a fibrous preform or part in which the fibers are arranged
(braided, stitched, woven, etc.) in a particular way in order to achieve the desired result.
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 Diameter: The measurement (expressed in hundred-thousandths) of the diameter of
individual filaments.
Fiber Direction: The orientation or alignment of the longitudinal axis of the fiber with respect to
a stated reference axis.
Fiber Glass: Fibers similar to wool or cotton fibers, but made from glass; sometimes called
fibrous glass. Glass fiber forms include cloth, yarn, mat, milled fibers, chopped strands, roving,
woven roving.
Fibre Glass Reinforcement: Major material used to reinforce plastic. These are available as
mat, roving, fabric, etc. It is incorporated into both thermoset and thermoplastics.
Fiber Orientation: Fiber alignment in a non-woven or a mat laminate where the majority of
fibers are in the same direction, resulting in a higher strength in that direction.
Fibre Pattern: Visible fibres on the surface of laminates or molding.
Fiber Placement: A continuous process for fabricating composite shapes with complex contours
and/or cutouts by means of a device that lays pre-impregnated fibers (in tow form) onto a nonuniform
mandrel or tool. It differs from filament winding (below) in several ways: there is no
limit on fiber angles; compaction takes place on-line via heat, pressure, or both; and fibers can be
added and dropped as necessary. The process produces more complex shapes and permits a
faster put-down rate than filament winding.
Fiber Reinforced Plastics (FRP): Term used to designate mid-range, glass fiber reinforced
composites.

Fiber System: The type and arrangement of fibrous material which comprises the fiber
constituent of an advanced composite. Examples of fiber systems are collimated filaments or
filament yarns, woven fabric, randomly oriented short-fiber ribbons, random fiber mats,
whiskers, etc.
Fiber Volume (Fraction): See fiber content.
Filament: The smallest unit of a fibrous material. The basic units formed during spinning and
which are gathered into strands of fiber, (for use in composites). Filaments usually are of
extreme length and of very small diameter. Filaments normally 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 previously 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 right numbers of layers
are applied the wound form is cured and the mandrel removed.
Filaments: Individual glass fibers of indefinite length, usually as pulled from a stream of molten
glass flowing through an orifice of the bushing. In the operation, a number are gathered together
to make a strand or end of roving or yarn.
Filament Yarn: A yarn composed of continuous filaments assembled with or without twists.
Filamentary Composite: A composite material reinforced with continuous fibers. It is a major
form of advanced composites in which the fiber constituent consists of continuous filaments.
Specifically, a filamentary composite is a laminate comprised of a number of laminae, each of
which consists of a nonwoven, parallel, uniaxial, planar array of filaments (or filament yarns)
embedded in the selected matrix material. Individual laminae are directionally oriented and
combined into specific multiaxial laminates for application to specific envelopes of strength and
stiffness requirements.
Filament winding: See Winding.
Filament Wound: Pertaining to an object created by the filament winding method of fabrication.
Fill (Filling): In a woven fabric, the yarn running from selvage to selvage at right angles to the
warp.
Filler: A relatively inert substance added to a material to alter its physical, mechanical, thermal,
electrical, and other properties or to lower cost. Sometimes the term is used specifically to mean
particulate additives.
Fill Threads: Also known as the weft. These are the crosswise fibers woven at 90 degrees to the
warp fibers.

Filler: A relatively inert material added to a plastic mixture to reduce cost, to modify mechanical
properties, to serve as a base for color effects, or to improve the surface texture.
Filler Ply: Additional patch to fill in a depression in repair or to build up an edge.
Fillet: A rounded filing of the internal angle between two surfaces of a plastic molding.
Filling Yarn: It is the transverse threads (or fibres) in a woven fabric. These are the fibres,
which running perpendicular to the warp.
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.
Final Entry Point: The spot where the lightning flash channel last enters the aircraft (usually a
trailing edge).
Final Exit Point: The spot where the lightning flash channel last exits .the aircraft (usually a
trailing edge).
Finish (or Size System): A material, with which filaments are treated, which contains a coupling
agent to improve the bond between the filament surface and the resin matrix in a composite
material. In addition, finishes often contain ingredients which provide lubricity to the filament
surface, preventing abrasive damage during handling, and a binder which promotes strand
integrity and facilitates packing of the filaments.
First Return Stroke: The high current surge that occurs when the leader completes the
connection between the two charge centers. The current serge has a high peak current, high rate
of change of current with respect to time and a high action integral.
Fixed Effect: A systematic shift in a measured quantity due to a particular level change of a
treatment or condition
Finite Element Analysis (FEA): A process of selecting the optimum combination of materials
in a composite based on software analysis.
Fire Point: The lowest temperature at which a liquid in an open container will give off enough
vapors to continue to burn once ignited. Fire point generally is only slightly higher than flash
point.
Fish Eye: It is a circular separation in a gel coat film generally caused by contamination such as
silicone, oil, dust, and/or water.
Flame Resistance: It is ability of a material to extinguish flame once the source of heat is
removed.

Flame Retardents: It is a certain chemicals that are used to reduce or eliminate a resins
tendency to burn.
Flame Retardent Resin: It is a resin which is compounded with certain chemicals to reduce or
eliminate its tendency to burn.
Flammability: A measure of how fast a material will burn under controlled conditions. ASTM
D-635/UL E-84 tests.
Flange: An extension around the perimeter of a mold or part for the purpose of demolding,
stiffening or connecting two components.
Flash: Excess material which forms at the parting line of a mold or die, or which is extruded
from a closed mold.
Flashover: This term is used when the arc produced by a gap breakdown passes over or close to
a dielectric surface without puncture.
Former Plate: A die attached to a braiding machine which helps to locate the fell.
Flash Point: The lowest temperature at which a substance gives off enough vapors to form a
flammable or ignitable mixture with air near the surface of the substance being tested.
Flexural Modulus: This is a number associated with the stiffness of materials. It is used to
calculate how far a bar will bend when a bending load is applied to it. Units are normally
millions of pounds per square inch. (106 psi) - Giga Pascals (gPa). Higher numbers for materials
mean that they are more resistant to deflection when equal thickness are being compared.
Flexural Strength: The resistance of a material to being broken by bending stresses; the strength
of a material in bending, expressed as the tensile stress of the outermost fibers of a bent test
sample at the instant of failure. (With plastics, this value is usually higher than the straight tensile
strength.)
Flow: The movement of resin under pressure, allowing it to fill all parts of a mold; flow or creep
- the gradual but continuous distortion of a material under continued load, usually at high
temperature.
Flow Line: A mark on a molded piece made by the meeting of two flow fronts during molding.
Also see, 'striae', or 'weld-mark,' or 'weld-line.')
Flow Meter: An instrument designed to measure the flow of a liquid. Normally used with
catalyst injection equipment.
Foam: A lightweight, cellular plastic material containing gas-filled voids. Typical foams
include urethane, PVC and polyester.

Foamed Plastic: Resins in sponge form, flexible or rigid, with cells closed or interconnected and
density over a range from that of the solid parent resin to 0.030 g/cm3.
Foaming Agent: Chemicals added to plastics and rubbers that generate inert gases on heating,
causing the resin to assume a cellular structure.
Foam-In-Place: The process of creating foam by the combination of two liquid polymers. See
In-Situ.
FPF: First ply failure.
Fracture: A rupture of the surface of a laminate because of external or internal forces, with or
without complete separation.
Fracture Ductility: The true plastic strain at fracture.
Fracture Stress: The true, normal stress on the minimum cross-sectional area at the beginning
of fracture.
Fracture Toughness: A measure of the damage tolerance of a material containing initial flaws
or cracks.
Free Radicals: Highly reactive molecular fragments capable of initiating chemical reactions,
such as polymerization of polyester resins.
FRP: Fibrous-glass reinforced plastic; a general term covering any type of plastic reinforced
cloth, mat, strands, or any other form of fibrous glass.
Fumed Silica (Aerosil, Cabosil): A thickening agent (having very low weight) used in
polyesters or epoxies to increase thixotropic qualities.
Fuzz: Accumulation of short broken filaments over a contact point.
Gap:
(a) In filament winding, the space between successive windings in which windings are
usually intended to lay next to each other.
(b) Separation between fibres within a filament winding band.
(c) The distance between adjacent plies in a lay-up of unidirectional tape materials.
Gauge Length: the original length of that portion of the specimen over which strain or change of
length is determined.
Gel: The initial jelly-like solid phase that develops during formation of a resin from a liquid.
Also, a semi-solid system consisting of a network of solid aggregates in which liquid is held.

Gel Coat: A quick-setting resin used in molding processes to provide an improved surface for
the composite; it is the first resin applied to the mold after the mold-release agent.
Gel Permeation Chromatography (GPC): A form of liquid chromatography in which the
polymer molecules are separated by their ability or inability to penetrate the material in the
separation column.
Gel Point: The stage at which a liquid begins to exhibit pseudo-elastic properties. (This can be
seen from the inflection point on a viscosity-time plot.)
Gelation Time: The time under application of load for a resin to reach a solid state.
Gelation: The formation of a gel.
Gel Time: The period of time from a pre-determined starting point to the onset of gelation (gel
point) as defined by a specific test method.
GFRP: Glass fiber-reinforced plastic, polymer or polyester.
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
interval of gel formation
Glass: An inorganic product of fusion which has cooled to a rigid condition without
crystallizing. In the handbook, all reference to glass will be to the fibrous form as used in
filaments, woven fabric, yarns, mats, chopped fibers, etc.
Glass Cloth: Conventionally-woven glass fiber material (see Scrim).
Glass Fibers: A fiber spun from an inorganic product of fusion which has cooled to a rigid
condition without crystallizing.
Glass Filament: A form of glass that has been drawn to a small diameter and I extreme length.
Most filaments are less than 0.15 mm in diameter.
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, Percent by Volume: The product of the specific gravity of a laminate and the percent
glass by weight, divided by the specific gravity of the glass.
Glass (/Resin) Ratio: The amount of glass by weight compared to the amount of resin by weight
in a finished laminate or molding.
Glass Rich: An area of molded part that has an overabundance of glass reinforcement. The
reinforcement may appear dry and unwet with the resin.

Glass Transition: The reversible change in an amorphous polymer or in amorphous regions of a
partially crystalline polymer from (or to) a viscous or rubbery condition to (or from) a hard and
relatively brittle one.
Glass Transition Temperature ( T g ): The approximate midpoint of the temperature range over
which the glass transition takes place.
Glint: A visual defect in a fiberglass 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.
Graphite: A form of carbon. Structural graphite is used in composite structures due to its
strength and stiffness.
Graphite Fibers: See Carbon Fibers.
Graphitization: The process of pyrolization at very high temperatures (as high as 5400(F) that
converts carbon to its crystalline allotropic form.
Greige: Fabric that has received no finish.
GRP: Glass-reinforced plastic, polymer, polyester. this derivation is commonly used in Europe.
Guide Pin: A pin, which guides mold halves into alignment on closing.
Hand Lay-up: A process in which components are applied either to a mold or a working
surface, and the successive plies are built up and worked by hand.
Handling Life: The out of refrigeration time over which, a material retains its handleability.
Hard Tool: A tool made of metallics or any "hard" material that is generally impervious to
damage during normal use.
Hardener: A substance or mixture added to a plastic composition to promote or control the
curing action.
Hardness: Resistance to deformation; usually measured by indention. Types of standard tests
include Brinell, Rockwell, Knoop, and Vickers.
Heat Cleaned: It is a Glass or other fibers which have been exposed to elevated temperatures to
remove preliminary sizing or binders which are not compatible with the resin system to be
applied.

Heat-Convertible Resin: A thermosetting resin convertible by heat to an infusible and insoluble
mass.
Heat-Distortion Temperature: Temperature at which a test bar deflects a certain amount under
specified temperature and a sated load.
Heat Resistance: The property or ability of plastics and Elastomer to resist the deteriorating
effects of elevated temperatures.
Heat-Pressure Laminants: Laminants molded and cured at pressures not lower than 1000 psi.
Heat Sink: A device for the absorption or transfer of heat away from a critical element or part.
Helical Winding: In filament winding, a winding in which a filament band advances along a
helical path.
Helical: Ply lay onto a mandrel at an angle, often a 45-degree angle.
Het-Acid Resin: Polyester resin with exceptional fire qualities.
Heterogeneous: Descriptive term for a material consisting of dissimilar constituents separately
identifiable; a medium consisting of regions of unlike properties separated by internal
boundaries. (Note that all non-homogeneous materials are not necessarily heterogeneous).
High Pressure Laminate: -Laminate molded and cured at pressures not lower than 6.9 MPa
(1000 psi) and in the range of 6.9-14 MPa (1000-2000 psi).
High Pressure Molding: A molding process wherein pressure is in excess of 6.9 MPa (1000
psi).
HOBE: Honeycomb before Expansion, the solid block of bonded sheets.
Homogeneous: Descriptive term for a material of uniform composition throughout; a medium
which has no internal physical boundaries; a material whose properties are constant at every
point, in other words, constant with respect to spatial coordinates (but not necessarily with
respect to directional coordinates).
Honeycomb: Resin-impregnated material manufactured in, usually, hexagonal cells that serve as
a core material in sandwich constructions. Honeycomb may also be metallic or polymer
materials in a rigid, open structure.
Honeycomb Core: Strips of paper, plastic metal, etc., joined together to form a honeycomb
pattern. Used as a lightweight core in sandwich moldings.
Hoop: Ply lay onto a mandrel at a 90-degree angle.

Hoop Stress: It is a circumferential stress in a cylindrically shaped part as a result of internal or
external pressure.
Horizontal Shear: Sometimes used to indicate inter-laminar shear. This is not an approved term
for use in this handbook.
Hot Bond Repair: Repair made on a hot-patch bonding machine to cure and monitor curing.
Typically includes heat and vacuum source.
Hot Bond Repair: Repair made on a hot patch bonding machine to cure and monitor curing.
Typically includes heat and vacuum source.
Hot Melt Adhesive: An adhesive that is applied in a molten state and forms a bond after cooling
to a solid state.
Hot Pot: Catalyst is mixed with the gel coat or resin in the material container prior to spraying,
as opposed to internal or external gun mixing.
Hot Setting Adhesive: An adhesive that requires a temperature at or above 100°C (212°F) to
set.
Hot Working Any form of mechanical deformation process carried out on a metal or alloy
above its re-crystallization temperature but below its melting point.
Humidity: Moisture content of the air.
Humidity, Relative: The ratio of the pressure of water vapor present to the pressure of saturated
water vapor at the same temperature.
Hybrid: A composite laminate comprised of laminae of two or more composite material
systems. Or, a combination of two or more different fibers such as carbon and glass or carbon
and aramid into a structure (tapes, fabrics and other forms may be combined).
Hybrid Composite: A composite with two or more types of reinforcing fibers.
Hybrid Resin: What we call a modified resin. For example, we sometimes add a rubber
modifier to an epoxy resin in order to achieve cured properties with a higher impact resistance.
Hydraulic Press: A press in which the molding force is created by the pressure exerted on a
fluid.
Hydro-mechanical Pressure: A press in which the molding forces are created partly by the
mechanical system and partly by the hydraulic system
Hydrophilic: Capable of absorbing water.

Hydrophobic: Moisture resistant capability, moisture repelling.
Hygroscopic: Capable of absorbing and retaining atmospheric moisture.
Hygrothermal Effect: Change in properties due to moisture absorption and temperature change.
Hysteresis: The energy absorbed in a complete cycle of loading and unloading.
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 in
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.
Impregnate: In reinforced plastics, the saturation of the reinforcement with a resin.
Impregnated Fabric: Also see Prepreg.
Inclusion: A physical and mechanical discontinuity occurring within a material or part, usually
consisting of solid, encapsulated foreign material. Inclusions are often capable of transmitting
some structural stresses and energy fields, but in a noticeably different manner from the parent
material.
Inhibitor: A substance which retards a chemical reaction; used in certain types of monomers and
resins to prolong storage life.
Initial Entry Point: The spot where the lightning flash channel first enters the aircraft (usually
an extremity).
Initial Exit Point: The spot where the lightning flash channel first exits the aircraft (usually an
extremity).
Initiator: A substance that initiates the curing reaction at elevated temperatures.
Injection Molding: Method of forming a plastic to the designed shape by forcing the heat
softened plastic into a relatively cool cavity under pressure.
Inorganic: Designating or pertaining to the chemistry of all elements and compounds not
classified as organic.
Inorganic Pigments: Natural or synthetic metallic oxides, sulphides and other salts that impart
heat and light stability, weathering resistance, colour and migration resistance to plastics.

Insert: An integral part of a plastic molding consisting of metal or other material that may be
molded or pressed into position after the molding is completed.
Instron: An instrument utilized to determine the tensile and compressive properties of materials.
Integral Composite Structure: Composite structure in which several structural elements, which
would conventionally be assembled by bonding or with mechanical fasteners after separate
fabrication, are instead laid up and cured as a single, complex, continuous structure; e.g., spars,
ribs, and one stiffened cover of a wing box fabricated as a single integral part. The term is
sometimes applied more loosely to any composite structure not assembled by mechanical
fasteners.
Integral Heating: A system in which heating elements are built into a tool, forming part of the
tool and usually eliminating the need for an oven or autoclave as a heat source.
Interface: The boundary between the individual, physically distinguishable constituents of a
composite.
Interference Fit: A joint or mating of two parts in which the male part has an external
dimension larger than the internal dimension of the mating female part. Interlaminar- Existing
between two or more adjacent laminae.
Interlaminar: Between the laminae of a laminate.
Discussion: describing objects (e.g., voids), events (e.g., fracture), or fields (e.g., stress).
Interlaminar Shear: Shearing force tending to produce a relative displacement between two
laminae in a laminate along the plane of their interface.
Interlaminar Shear Strength: The maximum shear stress existing between layers of a
laminated material.
Intermediate Bearing Stress: The bearing stress at the point on the bearing load-deformation
curve where the tangent is equal to the bearing stress divided by a designated percentage (usually
4%) of the original hole diameter.
Intermediate Temperature Setting Adhesive: An adhesive that sets in the temperature range
from 30 to 100°C (87 to 212°F).
Inter-phase: The boundary region between a bulk resin or polymer and an adherend in which
the polymer has a high degree of orientation to the adherend on a molecular basis.
Inter-ply Cracks: Through-cracking of individual layers within a composite lay-up,
perpendicular to the ply interfaces.
Inter-ply Hybrid: A composite in which adjacent laminae are composed of different materials.

Intra-laminar: Within the laminae of a laminate.
Discussion: describing objects (for example, voids), event (for example, fracture), or
fields (for example, stress).
Intra-ply Hybrid: A composite in which different materials are used within a specific layer or
band.
Intumescent: A fire-retardant technology which causes an otherwise flammable material to
foam, forming an insulating barrier when exposed to heat.
In-Situ: In the position which it will finally occupy, e.g. molding or forming foam.
Irreversible: Chemical reactions that proceed in a single direction and are not capable of
reversal (As applied to thermosetting resins).
Isocyanate Plastics: Plastics based on resins made by the condensation of organic isocyanates
with other compounds.
ISO: Abbreviated terminology for the isophthalic acid type resins and gel coats.
Isotropic -- Having uniform properties in all directions. The measured properties of an isotropic
material are independent of the axis of testing.
Isophthalic: A polyester resin based on isophthalic acid, generally higher in properties that a
general purpose or orthothatic polyester resin.
Izod Impact Test: A destructive test designed to determine the resistance of a plastic to the
impact of a suddenly applied force.
Jackstrawing: Prominence of fiberglass pattern having turned white in the Laminants
because the glass has separated from the resin due to excessive exothermic heat usually
associated with thick, resin rich Laminants.
Jig: Any fixture for holding parts in position, while joining them together or to maintain their
shape.
Joint: A line or distinction formed when two panels are connected. It is also referred to as a
seam.
Jammed State: It is the state of a braided fabric under tension or compression where the
deformation of the fabric is dominated by the deformation properties of the yarn.
Kerf: The width of a cut made by a saw blade, torch, water jet, laser beam, etc.
Kevlar: Trademark of DuPont for aramid fibers used as a reinforcement fiber.

Knitting: A method of constructing fabric by interlocking series of loops of one or more yarns.
Knuckle Area: The area of transition between sections of different geometry in a filament
wound part.
K-Sample Data: A collection of data consisting of values observed when sampling from k
batches.
L-Cell Pitch: The distance between the centers of nodes in the L direction in the expanded
honeycomb.
L-Direction: The core's ribbon direction; the direction of continuous sheets. . Lamina -A single
ply or layer, in a series of layers.
Laid-In Yarns: It is a system of longitudinal yarns in a triaxial braid which are inserted between
the bias yarns.
Lamina: A single ply or layer in a laminate.
Discussion: For filament winding, a lamina is a layer.
Laminae: Plural of lamina.
Laminate: for fiber-reinforced composites, a consolidated collection of laminae (plies) with one
or more orientations with respect to some reference direction.
Laminate (noun): Layers of a composite consisting of a resin and a reinforcement, bonded
together to form a single structure.
Laminate (verb): The act of processing resin and reinforcement into a bonded structure.
Saturating glass reinforcement and rolling out air voids is to laminate.
Laminate: To unite sheets of material by a bonding material usually with pressure and heat
(normally used with reference to flat sheets); a product made by so bonding.
Laminant: The product.
Laminate Coordinates: A coordinate system (used to describe the properties of a laminate),
generally in the direction of principal axes, when they exist.
Laminate Orientation: The configuration of a cross-plied composite laminate with regard to the
angles of cross-plying, the number of laminae at each angle, and the exact sequence of the
lamina lay-up.
Lattice Pattern: It is a pattern of filament winding with a fixed arrangement of open voids.

Laminated Plastics: Material consisting of superimposed layers of a synthetic resinimpregnated
or coated filler that has been bonded together usually by means of heat and
pressure, to form a single piece.
Laminate Ply: One layer of a laminated product.
Land: The portion of a mold which provides the separation or cutoff of the flash from the
molded article; in the screw of an extruder, the bearing surface along the top of the flights; in an
extrusion die, the surface parallel to the flow of material; in a semi-positive or flash mold, the
horizontal bearing surface; in a two-piece mold, a platform build up to the split line.
Lap: In filament winding, the amount of overlay between successive windings, usually intend to
minimize gapping.
Lap Joint: A joint made by bonding overlapped portions of two adherents.
Lay: In glass fiber, the spacing of the roving bands on the roving package expressed in the
number of bands per inch; in filament winding, the orientation of the ribbon with some reference,
usually the axis of rotation.
Lay-up: A process of fabrication involving the assembly of successive layers of resinimpregnated
material.
Layer: A single ply of lay up or laminate.
Leader: The low luminosity, low current precursor of a lightning return stroke, .accompanied by
an intense electric field.
Lengthwise Direction: Refers to the cutting of specimens and to the application of loads. For
rods and tubes, lengthwise is the direction of the long axis. For the other shapes of materials that
are stronger in one direction than in the other, lengthwise is the direction that is stronger. For
materials that are equally strong in both directions, lengthwise is an arbitrarily designated
direction that may be with the grain, direction of flow in manufacture, longer direction, etc.
Lightning Attachment Point: Any spot where the lightning flash attaches to the aircraft.
Lightning Channel: The ionized path through the air along which the lightning current pulse
passes.
Lightning Flash: The total lightning event. It may occur within a cloud, between clouds or
between a cloud and ground. It can consist of one or more return strokes, plus intermediate or
continuing currents.
Lightning Strike: Any attachment of the lightning flash to the aircraft.

Lightning Strike Zones: Aircraft surface areas and structures classified according to the
possibility of lightning attachment dwell time and current conduction.
Linear Expansion: The increase of a given dimension, measured by the expansion or
contraction of a specimen subject to a thermal gradient or changing temperature.
Liner: In a filament wound pressure vessel, the continuous coating on the inside .surface of the
vessel, used to protect the laminate from chemical attack or to prevent leakage under stress.
Liquid Adhesive: An adhesive that is in the liquid state when applied.
Liquid-Crystal Polymers: A newer type of thermoplastic, melt processible, with high
orientation in molding, improved tensile strength, and high-temperature capability.
Load-Deflection Curve: A curve in which the increasing flexural loads are plotted on the
ordinate axis and the deflections caused by those loads are plotted on the abcissae axis.
Loft: The height of the glass lay-down either dry (as in chopping SMC glass) or wet (as in the
height of the glass/resin mixture in a spray-up on the mold before roll-out).
Lognormal Distribution: A probability distribution for which the probability that an
observation selected at random from this population falls between ‘a’ and ‘b’ (0 < a < b < B) is
given by the area under the normal distribution between log ‘a’ and ‘log b’. The common (base
10) or the natural (base e) logarithm may be used.
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.
Lower Confidence Bound: See Confidence Interval.
Low Profile Additives: Low profile additives are thermoplastics that are added to the
formulation in 2-5% (by weight) of the final product to control the shrinkage of the cured
composites.
Low Profile: Low shrink resins, which reduce surface distortion.
Low-Pressure Laminants: In general, laminates molded and cured in the range of pressures
from 400 psi down to an including pressure obtained by the mere contact of the plies.
Low-Pressure Molding: The distribution of relatively uniform low pressure (200 psi or less)
over a resin-bearing fibrous assembly of cellulose, glass, asbestos, or other material, with or
without application of heat from external source, to form a structure possessing definite physical
properties.
LPF: Last Ply Failure.

L/D Ratio: A term used to define an extrusion screw, which denotes the ratio of the screw length
to the screw diameter.
Lubricant: A material added to most sizing to improve the handling and processing properties
of textile strands, especially during weaving.
M-Glass: A high beryllia (BeO2) content glass designed especially for high modulus of
elasticity.
Macro: In relation to composites, denotes the gross properties of a composite as a structural
element but does not consider the individual properties or identity of the constituents.
Macro-strain: The mean strain over any finite gage length of measurement which is large in
comparison to the material's inter-atomic distance.
Male Mold: A convex mold where the concave surface of the part is precisely defined by the
mold surface.
Mandrel: A form fixture or male mold used for the base in the production of a part by lay-up,
filament winding or braiding.
Mass: The quantity of matter contained in a specific body. In reference to polyesters, mass is
measured in mils or inches thickness of a film or casting.
Master (Plug): The permanent tool used to build molds for the manufacture of fiberglass parts.
Mat: A fibrous material consisting of randomly oriented chopped or swirled filaments loosely
held together with a binder.
Mat Binder: Resin applied to glass fiber and cured during the manufacture of mat, to hold the
fibers in place and maintain the shape of the mat.
Matched Die Molding: The technique for producing long runs of identical parts with two
finished sides.
Matched Metal Molding: A reinforced plastics manufacturing process in which matching male
and female metal molds are used to form the part, with time, pressure and heat.
Matched Molds: Two or more tools arranged in a set as a male and female mold. Normally
used in a press.
Matched Metal Molding: A reinforced plastics manufacturing process in which matching male
and female metal molds are used ( similar to compression molding) to form the part - as opposed
to low pressure laminating or spray-up.
Material Acceptance: The testing of incoming material to ensure that it meets requirements.

Material Qualification: The procedures used to accept a material by a company or organization
for production use.
Material System: A specific composite material made from specifically identified constituents
in specific geometric proportions and arrangements and possessed of numerically defined
properties.
Material System Class: As used in this handbook, a group consisting of material systems
categorized by the same generic constituent materials, but without defining the constituents
uniquely; e.g., the carbon/epoxy class.
Material Variability: A source of variability due to the spatial and consistency variations of the
material itself and due to variation in its processing.
Matrix: The essentially homogeneous material in which the fiber system of a composite is
embedded.
Matrix Content: The amount of matrix present in a composite expressed either as percent by
weight or percent by volume. Discussion: For polymer matrix composites this is called resin
content, which is usually expressed as percent by weight
Mean: See Sample Mean and Population Mean.
Mechanical Adhesion: Adhesion between surfaces in which the adhesive holds the parts
together by interlocking action.
Mechanical Properties: The properties of a material which are associated with elastic and
inelastic reaction when force is applied, or the properties involving the relationship between
stress and strain.
Median: See Sample Median and Population Median.
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.
MEK Solvent: Abbreviation for Methyl Ethyl Ketone; a colorless, flammable liquid commonly
used in gun clean up procedures.
Melt: A charge of molten metal or plastic.
Metal-Matrix Composites (MMC): Materials in which continuous carbon, silicon carbide, or
ceramic fibers are embedded in a metallic matrix material.
Micro: In relation to composites, denotes the properties of the constituents, i.e., matrix and
reinforcement and interface only, as well as their effects on the composite properties.

Micro-balloons: Microscopic bubbles of glass, ceramic or Phenolic, used as a filler or to create
syntactic foam or putty mixtures.
Microcracking: Microscopic cracks formed in composites when thermal stresses locally exceed
the strength of the matrix.
Micro-mechanics: Analysis of the structural behaviour of composites on a constituent (matrix,
reinforcement, interface) level.
Micron: One micron = .001 millimeter = .00003937 inch.
Microspheres: Microscopic bubbles of glass, ceramic or phenolic, used as a filler or to create
syntactic foam or putty mixtures.
Micro-strain: The strain over a gage length comparable to the material's inter-atomic distance.
Microstructure: A structure with heterogeneities that can be seen through a microscope.
Mil: The unit used in measuring film thickness and the diameter of glass fiber strands, wire, etc.
(one mil = .001")
Mil Thickness: Mils - The unit used in measuring film thickness. One mil equals one
thousandth of an inch. (1mil = .001).
Milled Fibers: Crushed glass used generally for making glass filled putty.
Modulus: A number which expresses a measure of some property of a material: modulus of
elasticity, shear modulus, etc.; a coefficient of numerical measurement of a property. (Note: The
use of the word without modifying terms may be confusing; and such use should not be
encouraged.)
Modulus, Chord: The slope of the chord drawn between any two specified points on the stressstrain
curve.
Modulus, initial: The slope of the initial straight portion of a stress-strain curve.
Modulus of Elasticity: The ratio of the stress or load applied to the strain or deformation
produced in a material that is elastically deformed. If a tensile strength of 2000 pounds per
square inch results in an elongation of one percent, the modulus of elasticity is 2000 divided by
0.01, or 200,000 pounds per square inch (Young's Modulus).
Modulus, Secant: The slope of the secant drawn from the origin to any specified point on the
stress-strain curve.

Modulus, Tangent: The ratio of change in stress to change in strain derived from the tangent to
any point on a stress-strain curve.
Modulus, Young's: The ratio of change in stress to change in strain below the elastic limit of a
material (Applicable to tension and compression).
Modulus of Resilience: The energy that can be absorbed per unit volume without creating a
permanent distortion.
Modulus of Rigidity (also Shear Modulus or Torsional Modulus): The ratio of stress to strain
below the proportional limit for shear or torsional stress.
Modulus of Rupture, in Bending: The maximum tensile or compressive stress (whichever
causes failure) value in the extreme fiber of a beam loaded to failure in bending. The value is
computed from the flexure equation:
F
M
=
I
b c
where,
M = maximum bending moment computed from the maximum load and the original moment
arm,
c = initial distance from the neutral axis to the extreme fiber where failure occurs,
I = the initial moment of inertia of the cross section about its neutral axis.
Modulus of Rupture, in Torsion: The maximum shear stress in the extreme fiber of a member
of circular cross section loaded to failure in torsion calculated from the equation:
F
T
=
J
s r
where,
T = maximum twisting moment,
r = original outer radius,
J = polar moment of inertia of the original cross section.
Moisture Absorption: The pick-up 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 condition
and expressed as a percentage of the mass of the moist specimen, i.e., the mass of the dry
substance plus the moisture present.

Moisture Equilibrium: The condition reached by a sample when it no longer takes up moisture
from, or gives up moisture to, the surrounding environment.
Moisture Vapour Transmission: A rate at which water vapour passes through a material at a
specified temperature and relative humidity.
Mold: The cavity or matrix into or on which the plastic composition is placed and from which it
takes form; to shape plastic parts or finished articles by heat and pressure; the assembly of all the
parts that function collectively in the molding process.
Mold Coat: A coat of resin over the bare mold. Used to seal the mold and make a smooth
surface on which to mold parts. This is essentially the same as a gel coat.
Mold Release Agent: A lubricant applied to mold surfaces to facilitate release of the molded
article.
Mold Shrinkage: The immediate shrinkage which a molded part undergoes when it is removed
from a mold and cooled to room temperature; the difference in dimensions, expressed in inches
per inch between a molding and the mold cavity in which it was molded (at normal temperature
measurement); the incremental difference between the dimensions of the molding and the mold
from which it was made, expressed as a percentage of the dimensions of the mold.
Mold Surface: The side of a laminate that faced the mold (tool) during cure in an autoclave.
Molded Edge: It is an edge which is not physically altered after molding for use in final form
and particularly one which does not have fiber ends along its length.
Molding: The forming of a polymer or composite into a solid mass of prescribed shape and size
by the application of pressure and heat.
Molding Compounds: Plastics in a wide range of forms to meet specific processing
requirements. Granules or pellets are popular forms.
Molding Cycle: It is the period of time occupied by the complete sequence of operations on a
molding press requisite for the production of one set of moldings; the operations necessary to
produce a set of moldings without reference to the time taken.
Molding powder or Compound: It is the Plastic material in varying stages of pallets or
granulation. It consisting of resin, filler, pigments, reinforcements and other ingredients ready for
use in the molding operation.
Molding Pressure: It is the pressure applied to the ram of an injection machine (or press) to
force the softened plastic completely to fill the mold cavities.
Molding Pressure, Compression: The unit pressure applied to the molding material in the
mold. The area is calculated from the projected area taken at right angles to the direction of

applied force and includes all areas under pressure during complete closing of the mold. The unit
pressure is calculated by dividing the total force applied by this projected area, and is expressed
in pounds per square inch.
Mold Release: A wax or polymer compound that is applied to the mold surface which acts as a
barrier between the mold and the part, thus preventing the part from the bonding to the mold.
Molecules: Chemical units composed of one or more atoms.
Monofilament: A single filament of indefinite length. Monofilaments are generally produced by
extrusion.
Monolayer: The basic laminate unit from which cross-plied or other laminates are constructed.
Monomer: A compound consisting of molecules each of which can provide one or more
constitutional units.
Multifilament Yarn: A large number (500 to 2000) of fine, continuous filaments (often 5 to 100
individual filaments), usually with some twist in the yam to facilitate handling.
Multifilament: A yarn consisting of many continuous filaments.
Multiple-Cavity Mold: A mold with two or more mold impressions; that is, a mold which
produces more than one molding per molding cycle.
Mylar: A trade name for a polyester film used as a release sheet in adhesive and composite
bonding.
Nanometer: Abbreviated (nm) and equal to one millimicron or one billionth meter (10-9),
used to measure wavelengths of light.
NDE: Nondestructive evaluation. It is broadly considered synonymous with NDI.
NDI: Nondestructive inspection. It is a process or procedure for determining the quality or
characteristics of a material, part, or assembly without permanently altering the subject or its
properties.
NDT: Nondestructive testing. It is a broadly considered synonymous with NDI.
Near-Net Shape: A characteristic of a part whose fabricated dimensions are so close to final
dimensions that machining or cutting to final shape is not required.
Neat Resin: Resin to which nothing (additives, reinforcements, etc.) has been added.
Necking: A localized reduction in cross-sectional area which may occur in a material under
tensile stress.

Negatively Skewed: A distribution is said to be negatively skewed if the distribution is not
symmetric and the longest tail is on the left.
NEO: Abbreviation for the neopentyl glycol type of gel coats.
Nesting: In reinforced plastics, the placing of plies of fabric so that the yarns of one ply lie in the
valleys between the yams of the adjacent ply (nested cloth).
Net Resin Content Prepreg: A prepreg product form that contains the final desired resin
content and does not require resin bleeding during the cure process.
Net Shape: Part fabrication resulting in final dimensions that do not require machining or
cutting.
Node: The bonded portion of adjacent ribbon sheets, double sheets.
Node Adhesive: The adhesive used to bond the honeycomb nodes together.
Nol Ring: A parallel filament wound test specimen used for measuring various mechanical
strength properties of the material by testing the entire ring, or segments of it.
Nomex: Trademark of DuPont for nylon paper-treated material that is commonly made into
honeycomb core.
Nominal Specimen Thickness: The nominal ply thickness multiplied by the number of plies.
Nominal Value: A value assigned for the purpose of a convenient designation. A nominal value
exists in name only.
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-perforated Core: The standard aluminium honeycomb with no small holes put in the
aluminium foil.
Non-Volatile Material: Portion remaining as solid under specific conditions short of
decomposition.
Nondestructive Inspection (NDI): A process or procedure for determining material or part
characteristics without permanently altering the test subject. Non-destructive testing (NDT) and
non-destructive evaluation (NDE) are broadly considered synonyms with NDI.
Non-Woven Fabric: A textile structure produced by bonding or interlocking of fibers, or both,
accomplished by mechanical, chemical, thermal, or solvent means and combinations thereof.

Nonwoven Roving: Reinforcement composed of continuous rovings loosely gathered together.
Normal Distribution: A two parameter (μ, σ) family of probability distributions for which the
probability that an observation will fall between ‘a’ and ‘b’ is given by the area under the curve
between ‘a’ and ‘b’.
2
1 ⎡ (x − μ)
⎤
f(x) = exp⎢−
2 ⎥
σ 2π ⎢⎣ 2σ
⎥⎦
Normalization: A mathematical procedure for adjusting raw test values for fiber-dominated
properties to a single (specified) fiber volume content.
Normalized Stress: Stress value adjusted to specified fiber volume content by multiplying the
measured stress value by the ratio of specimen fiber volume to the specified fiber volume. This
ratio may be obtained directly by experimentally measuring fiber volume, or indirectly by
calculation using specimen thickness and fiber aerial weight.
NOT: Non-destructive testing. It is broadly considered synonymous with NOI.
Notch Sensitivity: The extent to which the sensitivity of a material to fracture is increased by the
presence of a surface non-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.
NPG Gel Coat: Neopentyl glycol gel coat has enhanced weatherability compared to non-NPG
gel coat.
Nylon: The generic name for all synthetic polyamides.
Nylon Plastic: Plastics based on a resin composed principally of a long chain synthetic
polymeric amide that has recurring amide groups as an integral part of the main polymer chain.
Observed Significance Level (OSL): The probability of observing a more extreme
value of the test-statistic when the null hypothesis is true.
Off-axis Laminate: A laminate whose principal axis is oriented at an angle (8) other than 00 or
900 with respect to a reference direction, usually related to principal load or stress direction.
Offset Shear Strength: (from valid execution of a material property shear response test) the
value of shear stress at the intersection between a line parallel to the shear chord modulus of
elasticity and the shear stress/strain curve, where the line has been offset along the shear strain
axis from the origin by a specified strain offset value.
Oligomer: A polymer consisting of only a few monomer units such as a dimer, trimer, etc., or
their mixtures.

One-Off: A fabrication process in which a single part is fabricated.
One-Part Resin System: A resin system (often used in resin-transfer molding) in which the neat
resin and catalyst are mixed together by the materials supplier as part of the resin production
operation.
One-Sided Tolerance Limit Factor: See Tolerance Limit Factor.
Orange Peel: Backside of the gel coated surface that takes on the rough wavy texture of an
orange peel.
Organic: Matter originating in plant or animal life or composed of chemicals of hydrocarbon
origin, either natural or synthetic.
Orientation: The alignment of the crystalline structure in polymeric materials in order to
produce a highly aligned structure.
Oriented Materials: Composites whose constituents are aligned in a particular way.
Orifice: An opening generally referred to regarding spray tip size.
Original Equipment Manufacturer (OEM): Companies that design and build products bearing
their name, such as Boeing 777 aircraft of Prince tennis racquets.
Ortho: Abbreviation for orthophthalic acid type resins and gel coats.
Orthophthalic Or Ortho Resin: A polyester resin based on orthophthalic acid, also known as a
general-purpose resin (GP).
Orthotropic: Having three mutually perpendicular planes of elastic symmetry.
Outgassing: It is the release of solvents and moisture from composite parts under the hard
vacuum of space.
Out-Time (Out-Life): The period of time a prepreg material remains in a handleable form and
with properties intact outside of the specified storage environment; for example, out of the
freezer in the case of thermoset prepregs.
Oven Dry: The condition of a material that has been heated under prescribed conditions of
temperature and humidity until there is no further significant change in its mass.
Overlay Mat: A tissue of staple fibers lightly bonded with a medium, which insures good
draping and wetting out in contact resins. It is particularly valuable in masking the veining of
strands on the surface of matched metal die moldings.

Overlay Sheet: A non-woven fibrous mat (of glass, synthetic fibre, etc.) used as the top layer in
a cloth or mat lay-up to provide a smooth finish.
Over-expanded Core: The standard hexagon cell honeycomb pulled out into a rectangle cell
shape.
Oxidation: In carbon/graphite fibre processing, the step of reacting the precursor polymer
(rayon, pitch or PAN) with oxygen, resulting in stabilization of the Structure for the hot
stretching operation.
PAN: See Polyacrlonitrile.
PAN Fibers: Reinforcement fiber derived from the controlled pyrolysis of poly (acrylonitrile)
fiber.
Parallel Laminate: A laminate of woven fabric in which the plies are aligned in the same
position as originally aligned in the fabric roll.
Parallel Wound: A term used to describe yarn or other material wound into a flanged spool.
Parting Agent: Also, see Mold Release.
Part Consolidation: A process of composites fabrication in which multiple discrete parts are
designed and fabricated together into a single part, thus reducing the number of fabricated parts
and the need to join those parts together.
Parting Line: It is a mark on a molded piece where the sections of a mold have met in closing.
Pattern: The initial model for making fiberglass molds.
Peel Ply: Layer of material applied to a lay-up surface that is removed from the cured laminate
prior to bonding operations and leaves a clean resin-rich surface ready for bonding. A layer of
resin free material used to protect a laminate for later secondary bonding.
Peel Strength: Strength of an adhesive bond obtained by stress that is applied "in a peeling
mode."
Peroxides: Category of compounds containing an unstable O - O Group: Oxygen to Oxygen
atoms.
pH: A measure of acidity or alkalinity of a solution, with neutrality represented by a value of 7,
with increasing acidity corresponding to progressively smaller values, and increasing alkalinity
corresponding to progressively higher values.
Phenolic Resin: Thermosetting resin produced by condensation of an aromatic alcohol with an
aldehyde, particularily phenol with formaldehyde.

Pick Count: The number of filling yarns per inch or per centimeter of woven fabric.
Pigment: the ingredient used in gel coats to impart color.
Pigment Separation: A mottled (varied colors) appearance of the gel coat surface.
Pinhole: A tiny hole in the surface of, or through, a plastic material; usually occurring in
multiples.
Pit: Small regular or irregular crater in the surface of a plastic, usually with width approximately
of the same order of magnitude as its depth.
Pitch: A residual petroleum product used in the manufacture of certain carbon fibers.
Pitch Fibers: Reinforcement fiber derived from petroleum or coal tar pitch.
Planar Winding: A type of filament winding in which the filament path lies on a plane that
intersects the winding surface.
Plastic: A material that contains one or more organic polymers of large molecular weight, is
solid in its finished state, and, at some state in its manufacture or processing into finished
articles, can be shaped by flow.
Plastic Deformation
Change in dimensions of an object under load that is not recovered when the load is removed;
opposed to elastic deformation.
Plasticizer: A material of lower molecular weight added to a polymer to separate the molecular
chains. These results in a depression of the glass transition temperature, reduced stiffness and
brittleness, and improved processability. (Note, many polymeric materials do not need a
plasticizer.)
Plastic Tooling: Tools (mostly for the metal forming trades) constructed of plastics, generally
laminates or casting materials.
Platens: The mounting plates of a press, to which the entire mold assembly is bolted.
Plied Yarn: It is a yarn formed by twisting together two or more single yarns in one operation.
Plug: See Master - Both terms used to imply the same meaning.
Ply: The number of single yarns twisted together to form a plied yarn; one of the layers that
make up a stack or laminate.

Ply Schedule: Lay-up of individual plies or layers to form a laminate. Plies may be arranged in
alternating fiber orientation to produce multi-directional strength in a part.
Poisson's Ratio: The absolute value of the ratio of transverse strain to the corresponding axial
strain resulting from uniformly distributed axial stress below the proportional limit of the
material.
Polar Winding: A type of filament winding in which the filament path passes tangent to the
polar opening at one end of the chamber and tangent to the opposite side of the polar opening at
the other end of the chamber.
Polyacrylonitrile (PAN): A product used as a base material in the manufacture of certain carbon
fibers.
Polyamide: A polymer in which the structural units are linked by amide or thioamide groupings.
Many polyamides are fiber-forming.
Polyesters: Thermosetting resins, produced by dissolving unsaturated, generally linear, alkyd
resins in a vinyl-type active monomer such as styrene, methyl styrene, and diallyl phthalate. Cure
is affected through vinyl polymerization using peroxide catalysts and promoters, or heat, to
accelerate the reaction. The resins are usually furnished in solution form, but powdered solids are
also available.
Polyester Resing (Unsaturated): The 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: An organic material composed of molecules characterized by the repetition of one or
more types of monomeric units.
Polymerization: A chemical reaction in which the molecules of monomers are linked together to
form polymers via two principal reaction mechanisms. Addition polymerizations precede by
chain growth and most condensation polymerizations through step growth.
Polymerize: To unite molecules of the same kind to form a compound having the elements in
the same proportion but possessing much higher molecular weight and different physical
properties.
Polyvinyl Alcohol (PVA): A liquid water soluble release agent.
Population: The set of measurements about which inferences are to be made or the totality of
possible measurements which might be obtained in a given testing situation. For example, "all
possible ultimate tensile strength measurements for carbon/epoxy system A, conditioned at 95%
relative humidity and room temperature". In order to make inferences about a population, it is
often necessary to make assumptions about its distributional form. The assumed distributional
form may also be referred to as the population.

Population Mean: The average of all potential measurements in a given population weighted by
their relative frequencies in the population.
Population Median: That value in the population such that the probability of exceeding it is 0.5
and the probability of being less than it is 0.5.
Population Variance: A measure of dispersion in the population.
Porosity: A condition of trapped pockets of air, gas, or vacuum within a solid material, usually
expressed as a percentage of the total nonsolid volume to the total volume (solid plus nonsolid)
of a unit quantity of material.
Positive Mold: A mold designed to apply pressure to a piece being molded with no escape of
material.
Positively Skewed: A distribution is said to be positively skewed if the distribution is not
symmetric and the longest tail is on the right.
Post Bake: See Post Cure.
Post-cure: Additional elevated temperature cure, usually without pressure, to increase the glass
transition temperature, to improve final properties, or to complete the cure.
Pot Life: It is the period of time during which a reacting thermosetting composition remains
suitable for its intended processing after mixing with a reaction initiating agent.
Potting: Similar to encapsulating, except that steps are taken to insure complete penetration of
all the voids in the object before the resin polymerizes.
Precision: The degree of agreement within a set of observations or test results obtained.
Precision involves repeatability and reproducibility.
Precure: The full or partial setting of a resin or adhesive before the clamping operation is
complete or before pressure is applied.
Precursor (for Carbon or Graphite Fiber): Either the PAN or pitch fibers from which carbon and
graphite fibers are derived.
Preform: An assembly of dry fabric and fibers which has been prepared for one of several
different wet resin injection processes. A preform may be stitched or stabilized in some other
way to hold it’s a shape. A commingled preform may contain thermoplastic fibers and may be
consolidated by elevated temperature and pressure without resin injection.
Preform Mat: Fiber reinforced mat shaped like the mold in which it will be used. It eliminates
the need for overlapping the corners in molding.

Pregel: An unintentional extra layer of cured resin on part of the surface of a reinforced plastic.
(Not relating to "gel coat.")
Preheating: The heating of compound prior to molding or casting in order to facilitate the
operation or to reduce the molding cycle.
Premix: A molding compound prepared prior to and apart from the molding operations and
containing all components required for molding: resin, reinforcement, fillers, catalysts, release
agents, and other compounds.
Pre-ply: Layers of prepreg material, which have been assembled according to a user specified
stacking sequence.
Prepreg: Ready to mold or cure material in sheet form which may be tow, tape, cloth, or mat
impregnated with resin. It may be stored before 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.
Pre-Release: The premature release of the gel coat or laminate from the mold. Pre-release
causes cosmetic or dimensional problems.
Pressure Bag: A membrane which conforms to the inside of a laminate laid up on a mold. The
membrane or bag is then inflated applying pressure, which consolidates and densifies the
laminate.
Pressure: The force or load per unit area.
Pressure-Bag Molding: A molding technique in which a flexible bag is placed over the contact
layup in the mold, sealed, and clamped in place, and pressure applied by compressed air, which
forces the bag against the part while the part cures.
Primary Laminate: Laminate applied after the skin coat has cured. Generally thicker than the
skin coat.
Print Through: "Telegraphing" of the image of glass strands through the gel coat film.
Promoter: See Accelerator.
Proportional Limit: The maximum stress that a material is capable of sustaining without any
deviation from the proportionality of stress to strain (also known as Hooke's law).
Prototype: Process of creating a test part not intended for commercial release that establishes
design, material and fabrication parameters for a new product. May entail multiple iterations to
arrive at final/commercial part design.

Puckers: Local areas on prepreg where material has blistered and pulled away from the
separator film or release paper.
Puncture: Break in composite skin in sandwich structure that may or may not go through to the
core material or completely through the part.
Pultrusion: Reversed "extrusion" of resin-impregnated roving in the manufacture of rods, tubes
and structural shapes of a permanent cross-section. The roving, after passing through the resin
dip tank, is drawn through a die to form the desired cross-section.
Putty: A thickened mixture of resin made by adding fillers, thixotrophs and reinforcing fibers.
Quasi-Isotropic Laminate: A balanced and symmetric laminate for which a
constitutive property of interest, at a given point, displays isotropic behavior in the plane of the
laminate.
Discussion: Common quasi-isotropic laminates are [ 0/ ± 60] and[ 0/ ± 45/60] .
S
S
Ramping: A gradual, programmed increase/decrease in temperature or pressure to control
the cure or cooling of composite parts.
Random Effect: A shift in a measured quantity due to a particular level change of an external,
usually uncontrollable, factor.
Random Error: That part of the data variation that is due to unknown or uncontrolled factors
and that affects each observation independently and unpredictably.
Rate Tools: Tools designed to be used repeatedly in a production
Reduction of Area: The difference between the original cross sectional area of a tension test
specimen and the area of its smallest cross section, usually expressed as a percentage of the
original area.
Refractive Index: The ratio of the velocity of light (of specified wavelength) in air to its
velocity in the substance under examination. It is also defined as the sine of the angle of
incidence divided by the sine of the angle of refraction as light passes from air into the substance.
Reinforced Molding Compound: Compound supplied by raw material producer in the form of
ready-to-use material, as distinguished from premix.
Reinforced Plastic: A plastic with relatively high stiffness or very high strength fibers
embedded in the composition. This improves some mechanical properties over that of the base
resin.
Reinforcement: A strong inert material bonded into a plastic to improve its strength, stiffness,
and impact resistance. Reinforcements are usually long fibers of glass, asbestos, sisal, cotton,

etc., in woven or non-woven form. To be effective, the reinforcing material must form a strong
adhesive bond with the resin. ("Reinforcement" should not be used synonymously with "filler").
Reinforcing Cord: Loosely twisted cord made up from rovings and designed for incorporation
in moldings where edge reinforcement and high strength ribs are necessary.
Release Agent: A material which is applied in a thin film to the surface of a mold to keep the
resin from bonding to the mold. See Mold Release Agent.
Release Film: An impermeable film layer that does not bond to the composite during cure.
Resilience: A property of a material which is able to do work against restraining forces during
return from a deformed condition.
Resin: An organic polymer or pre-polymer used as a matrix to contain the fibrous reinforcement
in a composite material or as an adhesive. This organic matrix may be a thermoset or a
thermoplastic, and may contain a wide variety of components or additives to influence; handleability,
processing behavior and ultimate properties.
Resin Content: See Matrix content.
Resin Starved Area: Area of composite part where the resin has a non-continuous smooth
coverage of the fiber.
Resin System: A mixture of resin, with ingredients such as catalyst, initiator, diluents, etc.
required for the intended processing and final product.
Resin-Rich Area: Space which is filled with resin and lacking reinforcing material.
Resin-Starved: Areas of insufficient resin, usually identified by low gloss, dry spots or fiber
show.
Resin Tearing: Separation of pigments from the polyester, usually seen as black wavy lines.
Resin Transfer Molding (RTM): A molding process in which catalyzed resin is transferred into
an enclosed mold into which the fiber reinforcement has been placed; cure normally is
accomplished without external heat. RTM combines relatively low tooling and equipment costs
with the ability to mold large structural parts.
Room Temperature Ambient (RTA): 1) an environmental condition of 73±5°F (23±3°C) at
ambient laboratory relative humidity; 2) a material condition where, immediately following
consolidation/cure, the material is stored at 73±5°F (23±3°C) and at a maximum relative
humidity of 60%.

Resin Viscosity: Viscous property of a resin system or solid to liquid transition resistance to
flow, which can be altered by temperature and pressure as necessary to achieve desired flow
characteristics.
Reverse Impact Test: In which one side of a sheet of material is struck by a pendulum or falling
object and the reverse side is inspected for damage.
Reverse Sandwich Lay-Up: A laminate arrangement in which the reinforced mat is laminated
between outer layers of other materials.
RIB: A reinforcing member of a fabricated or molded part.
Ribbon Direction: On a honeycomb core, the way the honeycomb can be separated. The
direction of one continuous ribbon.
Roll-Out: A process used in spray-up and hand lay-up procedures where the resin and glass
reinforcement are compacted to a uniform laminate with rollers.
Room Temperature Curing Adhesives: Adhesives that set (to handling strength) within an
hour at temperatures from 68 to 86 F, and later reach full strength without heating.
Roving: A number of strands, tows, or ends collected into a parallel bundle with little or no
twist. In spun yarn production, it is an intermediate state between sliver and yarn.
Roving (filament winding): The term roving is used to designate a collection of bundles of
continuous filaments either as untwisted strands or as twisted yarns. Roving may be lightly
twisted, but for filament winding they are generally wound as bands or tapes with as little twist
as possible. Glass rovings are predominantly used in filament winding.
Roving Ball: A term used to describe the supply package offered to the winder. It consists of a
number of ends or strands wound to a given outside diameter onto a length of cardboard tube.
RP: Reinforced plastic, polymer, or polyester.
RTA (Room Temperature Ambient): An environmental condition 73(5( (23(3(C) at ambient
laboratory relative humidity; 2) a material condition where, immediately following
consolidation/cure, the material is stored at 73(5( (23(3(C) and at a maximum relative humidity
of 60%.
RTP: Sometimes used to distinguish reinforced thermoplastic from reinforced thermosetting
plastic.
Rubber Plunger Molding: A variation of the matched-die molding process which uses a heated
metal female mold (or outer half) and a rubber plunger male mold. Applicable for relatively
small molds with modest undercuts where low pressures are involved.

Rule-Of-Mixtures: When combined, the properties of the composite material is some
combination of the properties of the two constituent materials. The composite property equals the
amount of the fiber property multiplied by the volume percentage of fiber, plus the amount of
matrix property multiplied by the volume percentage of the matrix.
S-Basis (or S-Value): The mechanical property value which is usually the specified
minimum value of the appropriate government specification or SAE Aerospace Material
Specification for this material.
S-Glass: A family of magnesium-alumina-silicate glasses with a certified chemical composition
which conforms to an applicable material specification and which produces high mechanical
strength. (ASTM D578-90)
S-2 Glass Fiber: The Owens Corning trademarked brand of high tensile strength "S" glass
fibers.
Sags/Runs: Sag - slumping of the gel coat or resin film. Run - draining of the gel coat film or
laminating resin.
Sample: A small portion of a material or product intended to be representative of the whole.
Statistically, a sample is the collection of measurements taken from a specified population.
Sample Mean: It is the arithmetic average of the measurements in a sample. The sample mean is
an estimator of the population mean.
Sample Median: Order the observation from smallest to largest. Then the sample median is the
value of the middle observation if the sample size is odd; the average of the two central
observations if ‘n’ is even. If the population is symmetric about its mean, the sample median is
also an estimator of the population mean.
Sample Standard Deviation: The square root of the sample variance.
Sample Variance: The sum of the squared deviations from the sample mean, divided by n-1.
Sandwich Construction: A structural panel concept consisting in its simplest form of two
relatively thin, parallel sheets of structural material bonded to, and separated by, a relatively
thick, light-weight core. It is defined as “A composite composed of a lightweight core material
(usually honeycomb or foamed plastic) to which two relatively thin, dense, high-strength,
functional, or decorative skins (also called faces) are adhered”.
Sandwich Lay-Up: A lamination composed of two outside layers of reinforced material such as
glass mat and an inside layer or layers of honeycomb, glass cloth, or other light-weight core
material.
Saturation: An equilibrium condition in which the net rate of absorption under prescribed
conditions falls essentially to zero.

Scarf Joint: A bonded joint in which similar segments of adherends are cut away, with cut areas
overlapped and bonded.
Scrim (also called Glass Cloth, Carrier): A low cost fabric woven into an open mesh
construction, used in the processing of tape or other B-stage material to facilitate handling.
Scrimp: A patented variation of the RTM process.
Sealant: Applied to a joint in paste or liquid from that hardens in place to form a seal.
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: Aircraft or Aerospace structure that is not critical to flight safety.
Self Extinguishing: Ceases to burn when the source of flame is removed.
Selvage or Selvedge: The woven edge portion of a fabric parallel to the warp.
Set: The strain remaining after complete release of the force producing the deformation.
Set-Up: To harden, as in curing.
Shear: An action or stress resulting from applied forces which 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. Inter laminar Shear (ILS). The plane of contact is composed of resin only.
Shear Edge: The cut-off edge of the mold.
Shear Fracture (for crystalline type materials): A mode of fracture resulting from translation
along slip planes which are preferentially oriented in the direction of the shearing stress.
Sheet Molding Compound (SMC): A ready-to-mold glass fiber reinforced polyester material
primarily used in compression molding.
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.
Shore Hardness: A measure of the resistance of a material to indentation by a spring loaded
indenter. A higher number indicates greater resistance.
Short Beam Strength (SBS): a test result from valid execution of ASTM test method D 2344.

Shrinkage: The relative change in dimension between the length measured on the mold when it
is cold and the length on the molded object 24 hours after it has been taken out of the mold.
Significant: Statistically, the value of a test statistic is significant if the probability of a value at
least as extreme is less than or equal to a predetermined number called the significance level of
the test.
Significant Digit: Any digit that is necessary to define a value or quantity.
Silicon Carbide Fiber: A reinforcing fiber with high strength and modulus; density equal to that
of aluminum. It is used in organic metal-matrix composites.
Sink Mark: A shallow depression or dimple on the surface of an injection molded part due to
collapsing of the surface following local internal shrinkage after the gate seals; an incipient short
shot.
Sisal: A white fiber produced from the leaves of the agave plant. Used as a reinforcing filler, in
short chopped lengths, to impart moderate impact resistance.
Size System: See Finish.
Sizing: A generic term for compounds which are applied to yarns to bind the fiber together and
stiffen the yarn to provide abrasion-resistance during weaving. Starch, gelatin, oil, wax, and
man-made polymers such as polyvinyl alcohol, polystyrene, poly-acrylic acid, and polyacetatates
are employed.
Skewness: See Positively Skewed, Negatively Skewed.
Skein: A continuous filament, strand, yarn, roving, etc., wound up to some measurable length,
and usually used to measure various physical properties.
Skin: A layer of relatively dense material used in sandwich construction on the surface of the
core.
Skin Coat: The first layer of laminate next to the gel coat, generally one ply of chopped strand
mat.
Slave Pump: A catalyst pump, which is driven by the resin pump through a pair of level arms.
Sleeving: It is a common name for tubular braided fabric.
Slenderness Ratio: The unsupported effective length of a uniform column divided by the least
radius of gyration of the cross-sectional area.
Sliver: A continuous strand of loosely assembled fiber that is approximately uniform in crosssectional
area and has no twist.

Soft Tool: A tool made of composites or a similar "soft" material which is vulnerable to damage
during use, storage, or transportation.
Solvent Resistance: The non-swelling of a material and, of course, the impossibility for it to be
dissolved by the solvent in question.
Solid: The amount of sizing on the glass expressed as a percentage of the total weight.
Solute: The dissolved material.
Solvent: A liquid used to dissolve and clean materials.
SPEC: Specifications of the properties, characteristics, or requirements a particular material or
part must have in order to be acceptable to a potential user of the material or part.
Specific Gravity: The ratio of the weight of any volume of a substance to the weight of an equal
volume of another substance taken as standard at a constant or stated temperature. Solids and
liquids are usually compared with water at 39°F (4°C).
Specific Heat: The quantity of heat required to raise the temperature of a unit mass of a
substance one degree under specified conditions. Units = BTUs per pound per degree Fahrenheit
(BTA/lb/ (F); = Joules/Kilogram Kelvin (J/Kg-K). Higher numbers means that it takes more
input heat energy to raise the temperature of a material.
Specimen: A piece or portion of a sample or other material taken to be tested. Specimens
normally are prepared to conform to the applicable test method.
Spindle: A slender upright rotation rod on a spinning frame, roving frame, twister or similar
machine.
Splice: The joining of two ends of glass fiber yarn or strand, usually by means of an air drying
glue.
Split Mold: A mold in which the cavity is formed of two or more components held together by
an outer chase. The components are known as splits.
Spray-Up: Techniques in which a spray gun is used as the processing 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 which is directed at the
mold by either of two spray systems. In foamed plastics, very 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.
Stabilizer: And additive for polyesters to maintain liquid characteristics such as; gel time,
viscosity and liquidity.

Standard Deviation: See Sample Standard Deviation.
Staple: Either naturally occurring fibers or lengths cut from filaments.
Starved Area: It is 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.
Starved Joint: It is a joint that does not have the proper amount of adhesive because of
insufficient spread or excessive pressure.
Step-Growth Polymerization: One of the two principal polymerization mechanisms. In sepgrowth
polymerization, the reaction grows by combination of monomer, oligomer, or polymer
molecules through the consumption of reactive groups. Since average molecular weight increases
with monomer consumption, high molecular weight polymers are formed only at high degrees of
conversion.
Stiffness: The relationship of load and deformation; a term often used when the relationship of
stress to strain does not conform to
the definition of Young's modulus.
Stop Motion: A device on a creel that quickly stops the winder when a strand breaks so that the
strand can be repaired /replaced for further winding.
Storage Life: The period of time during which a liquid resin or packaged adhesive can be stored
under specified temperature conditions and remain suitable for use. Also see "shelf life”.
Strain: the per unit change, due to force, in the size or shape of a body referred to its original
size or shape. Strain is a non-dimensional quantity, but it is frequently expressed in inches per
inch, meters per meter, or percent.
Strand: Normally an untwisted bundle or assembly of continuous filaments used as a unit,
including slivers, tow, ends, yarn, etc. Sometimes a single fiber or filament is called a strand.
Strand Count: The number of strands in a plied yarn; the number of strands in a roving.
Strand Integrity: The degree to which the individual filaments making up the strand or end are
held together by the sizing applied.
Strength, Flexural: 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 prior to failure by
bending, usually expressed in pounds per square inch.
Strength: the maximum stress which a material is capable of sustaining.

Stress: The intensity at a point in a body of the forces or components of forces that act on a
given plane through the point. Stress is expressed in force per unit area (pounds-force per square
inch, megapascals, etc.).
Stress Concentration: The magnification of 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 Crack: External or internal cracks in a composite caused by tensile stresses; cracking can
be present internally, externally or in combination.
Stress Relaxation: The time dependent decrease in stress in a solid under given constraint
conditions.
Stress-Strain Curve (Diagram): A graphical representation showing the relationship between
the change in dimension of the specimen in the direction of the externally applied stress and the
magnitude of the applied stress. Values of stress usually are plotted as ordinates (vertically) and
strain values as abscissa (horizontally).
Stress-Strain: Stiffness, expressed in pounds per square inch or kilograms per square
centimeter, at a given strain.
Structural Adhesive: An adhesive used for transferring loads between adherends.
Structural Bond: A bond joining load-bearing components of an assembly.
Structural Element: a generic element of a more complex structural member (for example,
skin, stringer, shear panels, sandwich panels, joints, or splices).
Structural Glass
A magnesia/alumina/silicate glass reinforcement designed to provide very high tensile strength.
Structural Repair Manual (SRM)
Document prepared by an OEM that designates original structural materials (both composite and
metal) used for specific aircraft. It usually includes schematics for all parts and listings of
fastener types and adhesives. It also suggests general repair methodology so that structural
integrity can be maintained.
Styrene Monomer: An unsaturated hydrocarbon, used in plastics. In polyester, it is a co-reactant
diluent.
Substrate: A material upon the surface of which an adhesive-containing substance is spread for
any purpose, such as bonding or coating.

Surfacing Agent: It is a material, which allows the surface of polyesters to cure. It limits
adhesion of another coat of resin if the first is thoroughly cured. It may be removed by sanding
or rubbing with steel wool.
Surfacing Mat: It is a thin mat of fine fibers used primarily to produce a smooth surface on an
organic matrix composite.
Surfacing Veil: Used with other reinforcing mats and fabrics to enhance the quality of the
surface finish. Designed to block out the fiber patterns of the underlying reinforcements; also
called surfacing mat.
Surfactant: Chemicals used to modify or change the surface of a layer of resin or polymer.
Usually used to form a film on a curing resin, producing a tack-free surface.
Symmetrical Laminate: A composite laminate in which the sequence of plies below the
laminate mid-plane is a mirror image of the stacking sequence above the mid-plane.
Syntactic Foam: A foam made by mixing microspheres with a resin.
Tack: Stickiness of the prepreg. Stickiness of an adhesive or filament reinforced resin prepreg
material.
Tangent Modulus: The slope of the line at any point on a static stress-strain curve expressed in
psi per unit strain. This is the tangent modulus at that point in shear, extension, or compression as
the case may be.
Tape: Prepreg fabricated in widths up to 12 inches wide for carbon and 3 inches for boron. Cross
stitched carbon tapes up to 60 inches wide are available commercially in some cases.
Tape Laying: It is an automated fabrication process in which prepreg tape is laid side by side or
overlapped to form a structure.
TBPB: Abbreviation for tertiary-butyl peroctoate used in high speed, heated cures of polyester
resin systems.
TBPO: Abbreviation for tertiary-butyl peroctoate used in high speed, heated cures of polyester
resin systems.
Tenacity: The tensile stress expressed as force per unit linear density of the unstrained specimen
i.e., grams-force per denier or grams-force per Tex. The term generally used in yarn manufacture
and textile engineering to denote the strength of a yarn or of a filament of a given size.
Numerically it is the 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: An engineering term referring to the amount of stretch a sample
experiences during tensile strain. ASTM D-638.
Tensile Load: A dulling load applied to opposite ends of a given sample.
Tensile Modulus: When a bar is pulled in tension, it has to get longer. The tensile modulus is
used to calculate how much longer it will get when a certain load is applied to it. Units are
normally millions of pounds per square inch. (10 6 psi) - Giga Pascals (gPa). Higher numbers
indicate materials, which will not elongate as much as others when they are being compared
under equal tensile loading conditions.
Tensile Strength or Stress: The maximum tensile load per unit area of original cross section,
within the gage boundaries, sustained by the specimen during a tension test. It is expressed as
psi. Tensile load is interpreted to mean the maximum tensile load sustained by the specimen
during the test, whether or not this coincides with the tensile load at the moment of rupture.
Tex: A unit for expressing linear density equal to the mass or weight in grams of 1000 meters of
filament, fiber, yarn or other textile strand.
Thermal Coefficient of Expansion: Measures dimensional change of a material when heated or
cooled. It is measured in inches per inch per degree.
Thermal Conductivity: Ability of a material to conduct heat. It is the physical constant for
quantity of heat that passes through unit cube of a substance in unit time when the difference in
temperature of two faces is one degree.
Thermoplastic: A plastic that repeatedly can be softened by heating and hardened by cooling
through a temperature range characteristic of the plastic, and when in the softened stage, can be
shaped by flow into articles by molding or extrusion.
Thermal Stress Cracking: Crazing and cracking of some thermoplastic resins from
overexposure to elevated temperatures.
Thermocouple: Wire assembly used with a control device to sense temperature readings.
Thermoset: A class of polymers that, when cured using heat, chemical, or other means, changes
into a substantially infusible and insoluble material.
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.
Thixotropic Index (T.I.): A measurement of thixotropy nature determination, using a
Brookfield viscometer, as the low speed viscosity divided by the high-speed viscosity.
Tolerance: The total amount by which a quantity is allowed to vary.

Tolerance Limit: A lower (upper) confidence limit on a specified percentile of a distribution.
For example, the B-basis value is a 95% lower confidence limit on the tenth percentile of a
distribution.
Tolerance Limit Factor: The factor which is multiplied by the estimate of variability in
computing the tolerance limit.
Tool: The mold, either one- or two-sided or either open or closed, in or upon which composite
material is placed in order to make a part.
Tooling Gel Coat/Laminating Resin: It is special polyesters designed to give ultimate strengths
and cure values for moldmaking.
Tooling Resins: Plastic resins, chiefly epoxy and silicon that are used as tooling aids.
Toughness: A measure of a material's ability to absorb work, or the actual work per unit volume
or unit mass of material that is required to rupture it. Toughness is proportional to the area under
the load-elongation curve from the origin to the breaking point.
Tow: An untwisted bundle of continuous filaments. It is commonly used in referring to manmade
fibers, particularly carbon and graphite fibers, in the composites industry.
Transformation: A transformation of data values is a change in the units of measurement
accomplished by applying a mathematical function to all data values. For example, if the data is
given by x, then y = x + 1, x, 1/x, log x, and cos(x) are transformations.
Transition, First Order: A change of state associated with crystallization or melting in a
polymer.
Translucent: Permits a percentage of light to pass but not optically clear like window glass.
Transversely Isotropic: Descriptive term for a material exhibiting a special case of orthotropy
in which properties are identical in two orthotropic dimensions, but not the third; having
identical properties in both transverse directions but not the longitudinal direction.
Traveller: A small piece of the same product (panel, tube, etc.) as the test specimen, used for
example to measure moisture content as a result of conditioning.
Twist: The number of turns about its axis per unit of length in a yarn or other textile strand. It
may be expressed as turns per inch (tpi) or turns per centimeter (tpcm).
Twist, Direction of: The direction of twist in yarns and other textile strands is indicated by the
capital letters S and Z. Yarn has S twist if, when held in a vertical position, the visible spirals or
helices around its central axis are in the direction of slope of the central portion of the letter S,
and Z twist is in the other direction.

Twist multiplier: The ratio of turns per inch to the square root of the cotton count.
Typical Basis: A typical property value is a sample mean. Note that the typical value is defined
as the simple arithmetic mean which has a statistical connotation of 50% reliability with a 50%
confidence.
Ultimate Tensile Strength: The ultimate or final stress sustained by a specimen in a
tension test; the stress at moment of rupture.
Unbond: An area within a bonded interface between two adherends in which the intended
bonding action failed to take place. Also used to denote specific areas deliberately prevented
from bonding in order to simulate a defective bond, such as in the generation of quality standards
specimens (See Disbond, Debond).
Undercut: Having a protuberance or indentation that impedes the withdrawal from a two-piece,
rigid mold; any such protuberance or indentation, depending on the design of the mold (that is,
tilting a model in design of a mold for that model may eliminate an apparent "undercut").
Unidirectional: Refers to fibers that are oriented in the same direction, such as unidirectional
fabric, tape, or laminate, often called UD.
Unidirectional Fiber-Reinforced Composite: Any fiber-reinforced composite with all fibers
aligned in a single direction.
Unit Cell: The term applied to the path of a yarn in a braided fabric representing a unit cell of a
repeating geometric pattern. It is the smallest element representative of the braided structure.
Upper Confidence Limit: See Confidence Interval.
UV Stabilizer: A chemical compound which improves resistance to degradation from ultraviolet
radiation.
Vacuum-Assisted Resin Transfer Molding (VARTM): An infusion process
where a vacuum draws resin into a one-sided mold. A cover, either rigid or flexible, is placed
over the top to form a vacuum-tight seal.
Vacuum Bag Molding: A process in which the lay-up is cured under pressure generated by
drawing a vacuum in the space between the lay-up and a flexible sheet placed over it and sealed
at the edges.
Vacuum Infusion Processing: A process where the reinforcement is laid-up in the mold dry,
then vacuum bagged and the resin is pulled into the mold at lower then atmospheric pressure.
Vapor Barrier: A material through which water vapor will not pass readily or at all.

Variance: See Sample Variance.
Veil: An ultra thin mat similar to a surface mat, often composed of organic fibers as well as glass
fibers.
Viscosity: The property of resistance to flow exhibited within the body of a material.
Vinylester: A laminating resin which is processed much like polyester, but has cured properties
similar to some epoxy resins.
Void: Any pocket of enclosed gas or near-vacuum within a composite.
Void Content: The percentage of voids in a laminate.
Volatiles: Materials in a sizing or a resin formulation that can be vaporized at room or slightly
elevated temperature.
Volatile Organic Compounds (VOC): Carbon-containing chemical compounds (e.g., solvents
or liquids) that evaporate readily at ambient or process temperatures. Environmental, safety and
health regulations often limit exposure to these compounds, making low VOC content is
preferable.
Volatile Portions: Those portions vaporizing under specific conditions short of decomposition.
Non-volatiles remain.
Void Free: A laminate containing no entrapped air cavities, blisters, or voids.
Warp: The longitudinally oriented yarn in a woven fabric (see Fill); a group of yarns in long
lengths and approximately parallel.
Warpage: Dimensional distortion in a composite part.
Water Absorption: It is a ratio of weight of water absorbed by a material to the weight of dry
material. It is also called moisture content.
Water Jet: It is a high-pressure stream of water used for cutting organic composites.
Wax: A mold release agent.
Weave: The particular manner in which a fabric is formed by interlacing yarns, and usually
assigned a style number.
Weft: The yarns running perpendicular to the warp in a woven fabric; also called "woof".
Wet Flexural Strength (WFS): It is the flexural strength after water immersion; usually after
boiling the test specimen for two hours in water.

Weibull Distribution (Two - Parameter): A probability distribution for which the probability
that a randomly selected observation from this population lies between ‘a’ and ‘b’ (0 < a< b

Winding Pattern: The regularly recurring pattern of the filament winding after a certain number
of mandrel revolutions.
Winding: It is a process in which continuous material is applied under controlled tension to a
form in a predetermined geometric relationship to make a structure.
Discussion: A matrix material to bind the fibers together may be added before, during or
after winding. Filament winding is the most common type.
Work Life: The period during which a compound, after mixing with a catalyst, solvent, or other
compounding ingredient, remains suitable for its intended use.
Woven Fabric Composite: It is a major form of advanced composites in which the fiber
constituent consists of woven fabric. A woven fabric composite normally is a laminate
comprised of a number of laminae, each of which consists of one layer of fabric embedded in the
selected matrix material. Individual fabric laminae are directionally oriented and combined into
specific multi-axial laminates for application to specific envelopes of strength and stiffness
requirements.
Woven Roving: It is a heavy glass fiber fabric made by the weaving of roving.
Woven Roving Fabric: Heavy fabrics woven from continuous filament in roving form. They
drape well, are quickly impregnated, and intermediate in price between mats and yarn cloths.
Woven Tape: Tape of various thicknesses woven from continuous filament yarns.
Wrinkle: An imperfection in the surface of a laminate that looks like a crease in one of the outer
layers; it occurs in a vacuum bag molding due to improper placement of the bag.
X-Axis: In composite laminates, an axis in the plane of the laminate which is used as the 0
degree reference for designating the angle of a lamina.
X-Y Plane: In composite laminates, the reference plane parallel to the plane of the laminate.
Y-Axis: In composite laminates, the axis in the plane of the laminate which is perpendicular
to the x-axis.
Yarn: A generic term for strands or bundles of continuous filaments or fibers, usually twisted
and suitable for making textile fabric.
Yarn, Plied: Yarns made by collecting two or more single yarns together. Normally, the yarns
are twisted together though sometimes they are collected without twist.
Yield Strength: The stress at which a material exhibits a specified limiting deviation from the
proportionality of stress to strain; the lowest stress at which a material undergoes plastic
deformation. Below this stress, the material is elastic; above it, viscous.

Young's Modulus: The ratio of tensile stress to tensile strain below the proportional limit.
Yield Strength: The stress at which a material exhibits a specified limiting deviation from the
proportionality of stress to strain. (The deviation is expressed in terms of strain such as 0.2
percent for the Offset Method or 0.5 percent for the Total Extension under Load Method.)
Z-Axis: In composite laminates, the reference axis normal to the plane of the laminate.
Zero Bleed: Laminate fabrication procedure that does not allow loss of resin during cure.
(From) http://www.composites-by-design.com/glossary.htm
A-stage: An early stage of polymerization of thermosetting resins in which the material is still
soluble in certain liquids and is fusible. It is also called resole. See also B-stage and C-stage.
Ablative: A material that absorbs heat through a decomposition process called pyrolysis at or
near the exposed surface.
Addition: A polymerization reaction in which no by-products are formed.
Additives: Ingredients mixed into resin to improve properties.
Adhesive failure: A rupture of adhesive bond that appears to be a separation at the adhesiveadherend
interface.
Amorphous: Polymers that have no order to their molecules, thus no crystalline component.
Anisotropic: The tendency of a material to exhibit different properties in response to stresses
applied along axes in different directions.
Aramid: Aromatic polyamide fibers characterized by excellent high-temperature, flameresistance,
and electrical properties. Aramid fibers are used to achieve high-strength, highmodulus
reinforcement in plastic composites.
Arial weight: The weight of fiber per unit area (width time’s length) of tape or fabric.
Aspect ratio: The ratio of length to diameter of a fiber.
Autoclave: A closed vessel that permits application of pressure and heat used for curing
composites.

Autoclave molding: A molding technique in which an entire assembly (lay-up) is placed into an
autoclave at 50 to 100 psi, in order to consolidate layers of the part by removing entrapped air
and volatiles. Usually includes application of heat to trigger curing of the resin.
Axial winding: A type of filament winding in which the filaments are parallel to the axis.
B-stage: Intermediate stage in the polymerization reaction of thermosets, following which
material will soften with heat and is plastic and fusible. It is also called resistal. The resin of an
uncured prepreg or premix is usually in B-stage. See A-Stage and B-Stage.
Bag molding: A technique in which the composite material is placed in a rigid mold and covered
with a flexible bag, with pressure applied by vacuum, autoclave, press, or by inflating the bag.
Balanced laminate: All lamina except those at 0/90 are placed in plus-minus pairs (not
necessarily adjacent) symmetrically about the lay-up centerline.
Bearing strength: It is the maximum bearing stress that will not cause a composite to fail when
applied through a cylindrical fastener surface.
Bearing stress: Applied load divided by bearing area (hole diameter times thickness).
Bias fabric: A fabric in which warp and fill fibers are at an angle to the length.
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 in which the fibers are oriented in more
than one direction in the plane of the laminate.
Bismaleimide: A type of polyimide that cures by an addition reaction, avoiding formation of
volatiles, and has temperature capabilities between those of epoxy and polyimide.
Bleeder cloth: A layer of woven or non-woven material, not a part of the composite, that allows
excess gas and resin to escape during cure.
Bleed-out: The excess liquid resin appearing at the surface, primarily during filament winding.
Bond strength: The amount of adhesion between bonded surfaces. As measured by load/bond
area, the stress required to separate a layer of material from that to which it is bonded.
Boron fiber: A fiber usually of a tungsten-filament core with elemental boron vapor deposited
on it to impart strength and stiffness.
Braiding: Weaving fibers into a tubular shape.

Breather: A loosely woven material that does not come in contact with the resin but serves as a
continuous vacuum path over a part in production.
Broadgoods: Fibers woven into fabrics that may or may not be impregnated with resin, usually
furnished in rolls.
Buckling (composite): It is a failure usually characterized by fiber deflection rather than
breaking because of compressive action.
Butt joint: A joint in which parts are joined with no overlap.
C-stage: The final step in the cure of a thermoset resin; results in essentially irreversible
hardening and insolubility.
Carbon/carbon: A composite of carbon fiber in a carbon matrix.
Carbon fiber: An important reinforcing fiber known for its light weight, high strength, and high
stiffness that is produced by pyrolysis of an organic precursor fiber in an inert atmosphere at
temperatures above 1,800° F. The material may also be graphitized by heat treating above 3,000°
F.
Catalyst: A substance used in small quantities to promote or control the curing of a compound
without being consumed in the reaction.
Caul sheet: Plate or sheet the same size and shape used in contact with a composite lay-up to
transmit normal pressure and temperature during cure.
Ceramic-matrix composites: Materials consisting of a ceramic or carbon fiber surrounded by a
ceramic matrix, usually SiC (silicon carbide).
Chemical vapor deposition (CVD): A process in which desired reinforcement material is
deposited from vapor phase onto a continuous core; boron on tungsten, for example.
Circuit: One complete traverse of the fiber feed mechanism of a filament-winding machine.
Circumferential winding: A type of filament winding in which the filaments are perpendicular
to the axis.
Co-cured: Cured and simultaneously bonded to another prepared surface.
Coefficient of thermal expansion: A material's fractional change in length for a given unit
change of temperature.
Commingled yarn: A hybrid yarn made with two types of materials intermingled in a single
yarn; for example, thermoplastic filaments intermingled with carbon filaments to form a single
yarn.

Composite: A material created from a fiber (or reinforcement) and an appropriate matrix
material in order to maximize specific performance properties. The constituents do not dissolve
or merge completely but retain their identities as they act in concert.
Compression molding: A technique for molding thermoset plastics in which a part is shaped by
placing the fiber and resin into an open mold cavity, closing the mold, and applying heat and
pressure until the material has cured or achieved its final form.
Compressive strength: A material's ability to resist a force that tends to crush or buckle;
maximum compressive load a specimen sustains divided by the specimen's original crosssectional
area.
Condensation: A polymerization reaction in which simple by-products (for example, water) are
formed.
Consolidation: A processing step that compresses fiber and matrix to reduce voids and achieve a
desired density.
Contact molding: A technique in which reinforcement and resin are placed in a mold, with cure
taking place at room temperature with a catalyst/promoter system, or in a heated oven. No
additional pressure is used.
Continuous filament: An individual, small-diameter reinforcement that is flexible and indefinite
in length.
Continuous-filament yarn: Yarn that is formed by twisting two or more continuous filaments
into a single continuous strand.
Continuous roving: Parallel filaments coated with sizing, gathered together into single or
multiple strands, and wound into a cylindrical package. It may be used to provide continuous
reinforcement in woven roving, filament winding, pultrusion, prepregs, or high-strength molding
compounds, or it may be used chopped.
Core: The central component of a sandwich construction to which the sandwich faces or skins
are attached; also, part of a complex mold that forms undercut parts.
Co-woven fabric: It is a reinforcement fabric woven with two different types of fibers in
individual yarns; for example, thermoplastic fibers woven side by side with carbon fibers.
Creep: The dimensional change in a material under physical load over time beyond
instantaneous elastic deformation.
Crimp: A fiber's waviness, which determines the capacity of the fiber to cohere.

Critical length: The minimum lengths of a fiber necessary for matrix shear loading to develop
fiber ultimate strength by a matrix.
Cross laminated: Material laminated so that some of the layers are oriented at various angles to
the other layers with respect to the laminate grain. A cross-ply laminate usually has plies oriented
only at 0°/90°.
Crystallinity: The quality of having a molecular structure with atoms arranged in an orderly,
three-dimensional pattern.
Cure: To change the physical properties of a material irreversibly by chemical reaction via heat
and catalysts, alone or in combination, with or without pressure.
Cure temperature: The temperature at which a material attains final cure.
Curing agent: It is a catalytic or reactive agent that brings about polymerization when it is
added to a resin.
Damage tolerance: A measure of the ability of structures to retain load-carrying capability after
exposure to sudden loads (for example, ballistic impact).
Damping: Diminishing the intensity of vibrations.
Debond: An unplanned non-adhered or unbonded region in an assembly.
Delamination: The separation of a laminated plastic material along the plane of its layers.
Denier: A numbering system for yarn and filament in which yarn number is equal to weight in
grams of 9000 meters of yarn.
Design allowable: A limiting value for a material property that can be used to design a structural
or mechanical system to a specified level of success with 95% statistical confidence. B-basis
allowable: material property exceeds the design allowable 90 times out 100. A-basis allowable:
material property exceeds the design allowable 99 times out of 100.
Doubler: Localized area of extra layers of reinforcement, usually to provide stiffness or strength
for fastening or other abrupt load transfers.
Draft angle: A mandrel's taper or angle for ease of part removal.
Drape: The ability of prepreg to conform to the shape of a contoured surface.
Dry winding: A type of filament winding in which preimpregnated roving is used.
E-glass: "Electrical glass"; the borosilicate glass most often used for the glass fibers in
conventional reinforced plastics.

Fabric, non-woven: A material formed from fibers or yarns without interlacing.
Fabric, woven: A material constructed of interlaced yarns, fibers, or filaments.
Fatigue: The failure of a material's mechanical properties as a result of repeated stress.
Fatigue strength: Maximum cyclical stress withstood for a given number of cycles before a
material fails.
Fiber orientation: The fiber alignment in a non-woven or a mat laminate in which most of the
fibers are in the same direction, thereby affording higher strength in that direction.
Fiber placement: A continuous process for fabricating composite shapes with complex contours
and/or cutouts by means of a device that lays preimpregnated fibers (in tow form) onto a nonuniform
mandrel or tool. It differs from filament winding (below) in several ways: there is no
limit on fiber angles; compaction takes place online via heat, pressure, or both; and fibers can be
added and dropped as necessary. The process produces more complex shapes and permits a
faster putdown rate than filament winding.
Filament winding: A process for fabricating composites in which continuous reinforcing fibers,
either preimpregnated with resin or drawn through a resin bath, are wound around a rotating,
removable mandrel.
Filaments: Individual fibers of indefinite length used in tows, yarns, or roving.
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.
Finish: Material applied to fibers, after sizing is removed, to improve matrix-to-fiber coupling.
Fracture: A rupture of the surface of a laminate because of external or internal forces, with or
without complete separation.
Fracture toughness: A measure of the damage tolerance of a material containing initial flaws or
cracks.
Glass transition: The reversible change in an amorphous polymer between a viscous or rubbery
condition and a hard, relatively brittle one.
Glass-transition temperature (Tg): The approximate temperature at which increased molecular
mobility results in significant changes in properties of a cured resin. The measured value of Tg
can vary depending upon the test method.
Hand lay-up: A fabrication method in which reinforcement layers, preimpregnated or coated
afterwards, are placed in a mold by hand, then cured to the formed shape.

Hardener: A substance used to promote or control curing action by taking part in it; as opposed
to catalyst.
Heat-distortion temperature: Temperature at which a test bar deflects a certain amount under
specified temperature and a stated load.
Honeycomb: Resin-impregnated material manufactured in, usually, hexagonal cells that serves
as a core material in sandwich constructions. Honeycomb may also be metallic or polymer
materials in a rigid, open-cell structure.
Hoop stress: Circumferential stress in a cylindrically shaped part as a result of internal or
external pressure.
Hybrid composite: A composite with two or more reinforcing fibers.
Impact strength: A material's ability to withstand shock loading as measured by the work done
in fracturing a specimen.
Impregnate: To saturate the voids and interstices of reinforcement with a resin.
Impregnated fabric: See Prepreg.
Interface: The surface between two different materials: in fibers, the area at which the glass and
sizing meet; in a laminate, the area at which the reinforcement and the laminating resin meet.
Interlaminar: Existing or occurring between two or more adjacent laminae.
Interlaminar shear: The shearing force tending to produce displacement between two laminae
along the plane of their interface; usually the weakest element of a composite.
Isotropic: Having uniform properties in all directions independent of the direction of load
application.
Laminate ply: One layer of a laminated product.
Lap joint: A joint made by bonding overlapped portions of two adherends.
Lay-up: The placement of layers of reinforcement in a mold.
Liquid-crystal polymers: A newer type of thermoplastic, melt processible, with high orientation
in molding, improved tensile strength, and high-temperature capability.
Mandrel: The form around which resin-impregnated fiber or tape is wound to form structural
shapes or tubes.

Mat: A fibrous reinforcing material comprised of chopped filaments (for chopped-strand mat) or
swirled filaments (for continuous-strand mat) with a binder to maintain form; available in
blankets of various widths, weights, and lengths.
Matrix: A material in which the fiber of a composite is imbedded; it can be plastic, metal,
ceramic, or glass.
Metal-matrix composites: Materials in which continuous carbon, silicon carbide, or ceramic
fibers are embedded in a metallic matrix material.
Modulus: A measure of the ratio of load (stress) applied to the resultant deformation of a
material, such as elasticity or shear.
Multifilament: A yarn consisting of many continuous filaments.
Nondestructive inspection (NDI): A process or procedure for determining material or part
characteristics without permanently altering the test subject. Nondestructive testing (NDT) is
broadly considered synonymous with NDI.
Nonwoven roving A reinforcement composed of continuous rovings loosely gathered together.
Oriented materials: Composites whose constituents are aligned in a particular way.
Out-life: The period of time a prepreg material remains in a handleable form and with properties
intact outside of the specified storage environment; for example, out of the freezer in the case of
thermoset prepregs.
PAN: See Polyacrylonitrile.
Peel ply: Layer of material applied to a prepreg lay-up surface that is removed from the cured
laminate prior to bonding operations and leaves a clean resin-rich surface ready for bonding.
Peel strength: Strength of an adhesive bond obtained by stress applied "in a peeling mode."
Pitch: A residual petroleum product used in the manufacture of certain carbon fibers.
Planar winding: A type of filament winding in which the filament path lies on a plane that
intersects the winding surface.
Ply: The number of single yarns twisted together to form a plied yarn; one of the layers that
make up a stack or laminate.
Polar winding: A type of filament winding in which the filament path passes tangent to the
polar opening at one end of the chamber and tangent to the opposite side of the polar opening at
the other end of the chamber.

Polyacrylonitrile: A product used as a base material in the manufacture of certain carbon fibers.
Polymer: A very large molecule formed by combining a large number of smaller molecules,
called monomers, in a regular pattern.
Polymerization: A chemical reaction in which the molecules of monomers are linked together to
form polymers.
Post-cure: An additional elevated-temperature exposure that is performed often without tooling
or pressure to improve elevated-temperature mechanical properties, for example.
Pot life: The length of time a catalyzed thermosetting resin system retains a viscosity low
enough for it to be suitable for processing.
Pre-cure: The full or partial setting of a resin or adhesive before the clamping operation is
complete or before pressure is applied.
Precursor: For carbon fibers, the rayon, PAN, or pitch fibers from which carbon fibers are
made.
Preform: A fibrous reinforcement pre-shaped to approximate contour and thickness desired in
the finished part.
Prepreg: Resin-impregnated cloth, mat, or filaments in flat form that can be stored for later use.
The resin is often partially cured to a tack-free state called "B-staging." Catalysts, inhibitors,
flame retardants, and other additives may be included to obtain specific end-use properties and
improve processing, storage, and handling characteristics.
Pressure-bag molding: A molding technique in which a flexible bag is placed over the contact
lay-up in the mold, sealed, and clamped in place, and pressure applied by compressed air, which
forces the bag against the part while the part cures.
Pultrusion: A continuous process for manufacturing composites in rods, tubes, and structural
shapes having a constant cross-section. After the reinforcement is passed through the resinimpregnation
bath, it is drawn through a shaping die to form the desired cross-section; curing
takes place before the laminate can depart from that cross-section.
Quasi-isotropic: Approximating isotropy by orientation of plies in several directions.
Ramping: A gradual, programmed increase or decrease in temperature or pressure to control the
cure or cooling of composite parts.
Reinforcement: A material added to the matrix to provide the required properties; ranges from
short fibers through complex textile forms.

Release agents: Materials that are used to prevent cured matrix material from bonding to
tooling.
Release film: An impermeable film layer that does not bond to the composite during cure.
Resin: A material, generally a polymer, which has an indefinite and often high molecular weight
and a softening or melting range and exhibits a tendency to flow when it is subjected to stress.
Resins are used as the matrices to bind together the reinforcement material in composites.
Resin rich Localized area filled with resin but lacking reinforcement fiber.
Resin starved: Localized area lacking sufficient resin for wet-out of the fibers.
Resin-transfer molding (RTM): A molding process in which catalyzed resin is transferred into
an enclosed mold into which the fiber reinforcement has been placed; cure normally is
accomplished without external heat. RTM combines relatively low tooling and equipment costs
with the ability to mold large structural parts.
Roving: It is a collection of bundles of continuous filaments either as untwisted strands or as
twisted yarns.
S-glass: Structural glass; a magnesia/alumina/silicate glass reinforcement designed to provide
very high tensile strength.
Sandwich construction: A composite composed of a lightweight core material (usually
honeycomb or foamed plastic) to which two relatively thin, dense, high-strength, functional, or
decorative skins (also called faces) are adhered.
Scarf joint: A bonded joint in which similar segments of adherends are cut away, with cut areas
overlapped and bonded.
Selvage: The narrow edge of woven fabric that runs parallel to the warp. It is made with stronger
yarns in a tighter construction than the body of the fabric to prevent raveling.
Shelf life: The length of time a material can be stored and continue to meet specification
requirements and remain suitable for its intended use.
Silicon carbide fiber: A reinforcing fiber with high strength and modulus; density is equal to
that of aluminum. It is used in organic metal-matrix composites.
Sizing: A compound that binds together and stiffens warp yarn to provide resistance to abrasion
during weaving; normally removed and replaced with finish before matrix application.
Skin: A layer of relatively dense material used in a sandwich construction on the surface of the
core.

Specific gravity: The density (mass per unit volume) of a material divided by that of water at a
standard temperature.
Starved joint: A joint that does not have the proper amount of adhesive because of insufficient
spread or excessive pressure.
Stiffness: The relationship of load to deformation for a particular material.
Storage life: The amount of time a material can be stored and remain suitable for use.
Strain: The elastic deformation of a material as a result of stress.
Stress: The internal force that resists change in size or shape, expressed in force per unit area.
Structural adhesive: An adhesive used for transferring loads between adherends.
Structural bond: A bond joining load-bearing components of an assembly.
Tack: The stickiness of a prepreg.
Tape A unidirectional woven prepreg, in widths up to 12 inches for carbon fiber, for example.
Tape laying: A fabrication process in which prepreg tape is laid side by side or overlapped to
form a structure.
Tensile strength: The maximum tensile stress sustained by a plastic specimen before it fails in a
tension test.
Thermal conductivity: The ability of a material to conduct heat.
Thermoplastic: A plastic material that is capable of being repeatedly softened by application of
heat and repeatedly hardened by cooling.
Thermoset: A plastic material that is capable of being cured by heat or catalyst into an infusible
and insoluble material. Once cured, a thermoset cannot be returned to the uncured state.
Tooling resins: Plastic resins, chiefly epoxy and silicone, that are used as tooling aids.
Toughness: Tendency of a material to absorb work.
Tow: An untwisted bundle of continuous filaments, usually designated by a number followed by
"K," indicating multiplication by 1,000; for example, 12K tow has 12,000 filaments.

Unbond: Area of a bonded surface in which bonding of adherends has failed to occur, or where
two prepreg layers of a composite fail to adhere to each other; also denotes areas where bonding
is deliberately prevented to simulate a defective bond.
Unidirectional: Refers to fibers that are oriented in the same direction, such as unidirectional
fabric, tape, or laminate, often called UD.
Vacuum bag molding: A molding technique in which the part is cured inside a layer of film,
from which entrapped air is removed by vacuum.
Viscosity: The tendency of a material to resist flow.
Voids: Pockets of entrapped gas that have been cured into a laminate.
Volatiles: Materials in a sizing or a resin formulation that can be vaporized at room or slightly
elevated temperature.
Warp: The yarns running lengthwise and parallel to the selvage in a woven fabric.
Water jet: A high-pressure stream of water used for cutting organic composites.
Weave: The pattern by which a fabric is formed from interlacing yarns. In plain weave, the warp
and fill fibers alternate to make both fabric faces identical; in satin weave, the pattern produces a
satin appearance, with the warp tow over several fill tows and under the next one (for example,
eight-harness satin would have warp tow over seven fill tows and under the eighth).
Weft: The yarns running perpendicular to the warp in a woven fabric; also called "woof."
Wet lay-up: The application of resin to dry reinforcement in the mold.
Wet winding: A type of filament winding in which the fiber strand is impregnated with resin
immediately before it contacts the mandrel.
Wet-out: The saturation of all voids between strands and filaments of porous materials with
resin.
Wetting agent: A surface-active agent that promotes wetting by decreasing the cohesion within
a liquid.
Winding pattern: The regularly recurring pattern of the filament path in a filament winding
after a certain number of mandrel revolutions.
Woven roving: A heavy, coarse fabric produced by the weaving of continuous roving bundles.
Wrinkle: An imperfection in the surface of a laminate that looks like a crease or fold in one of
the outer layers; it occurs in vacuum bag molding due to improper placement of the bag.

X axis: 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.
Yarn: Continuously twisted fibers or strands suitable for use in weaving into fabrics.
Young's modulus: The ratio of normal stress to the corresponding strain for tensile or
compressive stresses less than the proportional limit of the material.
Source: Information herein was obtained from various government agencies. None of the text
within this section is copyrighted material to the best knowledge of Composites-By-Design.
ABS: Acrylonitrile Butadiene Styrene (Thermoplastic Resin).
Ablative Plastic: A material that absorbs heat (with low material loss and char rate) through a
decomposition process (pyrolysis). Absorption takes place at or near the surface exposed to the
heat.
Abrasion: Wearing away by friction. Glass is highly resistant to abrasion from other materials,
but can be damaged through contact with itself. Lubrication during processing and fabrication
helps prevent abrasion.
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.
Activator: See Accelerator.
Acoustic Emission: The amount of integrity in a material after it has been stressed, as
determined by sound emission. Ideally, emissions can be correlated with defects and/or early
failure.
Acrylic: Thermoplastic polymer made by the polymerization of esters of acrylic acid and its
derivatives.
Actual End Count: The number of bundles or splits that are actually counted in one doff of
roving. (This is less than the theoretical end count due to splitting efficiencies of less than
100%).

Addition Polymerization: Chemical reaction in which simple molecules (monomers) are added
to each other to form long-chain molecules (polymers) without byproducts.
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.
Admixture: Addition and homogeneous dispersion of discrete components, before cure.
Aggregate: Hard, coarse material usually of mineral origins used in composite tools. Also used
in flooring or as a surface medium.
Aging: The process of exposing materials to an environment for an interval of time.
Air-Bubble: A localized, non-interconnected, spherically shaped entrapment of air within and
between the plies of reinforcement.
Air Splice: The coupling made by a jet of air to entwine/snarl two roving doffs together. The air
splice is used instead of a knot.
Air Vent: It is a small outlet to prevent entrapment of gases in a molding or tooling fixture.
Alkyd Plastics: Resin composed principally of polymeric esters, in which the recurring ester
groups are an integral part of the main polymer chain or are part of the cross links present
between chains.
Alligatoring: Visible cosmetic defect in exposed gel coat resembling wrinkled or alligator-like
skin.
Alloy: In plastics, a blend of polymers or copolymers blended with other polymers or elastomers
under select conditions.
Ambient: Surrounding environmental conditions, such as pressure, temperature, or relative
humidity.
Amine Resins: A synthetic resin derived from the reaction of urea, thiourea, melamine or allied
compounds with aldehydes, particularly formaldehyde.
An Isotropic: Exhibiting different properties when tested along axes in different directions. See
An Isotropic laminate
An Isotropic Laminate: A laminate whose properties are different in different directions.
Angle-Ply Laminate: A laminate with fibers of adjacent plies oriented at alternating angles.

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.
Arimid: A highly oriented organic material derived from polyamide with an aromatic ring
structure that is used primarily as a high-strength, high-modulus fiber. Examples include Kevlar
and Nomex.
Arc Resistance: Ability to withstand exposure to an electric voltage. It is also, the total time in
seconds that an intermittent arc may play across a plastic surface without rendering it conductive.
Ash Content: Proportion of solid residue remaining after a reinforcing substance has been
incinerated (charred or intensely heated).
Aspect Ratio: Ratio of length to diameter of a fiber.
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. It is 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.
Autosprue: Automatic gate at the inlet sprue of the mold that eliminates the need to remove
the resin supply during injection.
Axial Winding: Filaments that are wound parallel or at a small angle to the axis (0° helix angle).
Bagging: Applying an impermeable layer of film over an uncured part and sealing the edges so
that a vacuum can be drawn.
Bag Molding: It is the 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.
Ball: See Roving
Band Width: In filament winding, the width of the reinforcement as it is applied to the mandrel.
Barcol-Shore Rockwell Hardness: A material’s ability to be indented. The Rockwell method
measures the amount of penetration caused when a steel point is forced into the material. The
suffix (alphabetic letter) in the Rockwell reading describes the shape of the point and units
describe how much load was applied during the test. The letter and number cannot be separated.
Higher numbers with the same letter indicate harder materials with greater resistance to
penetration by another substance.
Bare Glass: Glass in fiber form as it flows from the bushing before a binder or sizing is applied.
Batch Oven: Large temperature-controlled oven used to heat-clean rolls of glass fiber fabric.
Batt: Felted fabrics or structures built by the interlocking action of compressing fibers, without
spinning, weaving, or knitting.
Beam: A spool on which parallel ends of single or plied yarns are wound for use in weaving or
similar processing operations.
Beaming: Operation in which many ends of yarn from a creel are combined on a section beam.
Bearing Strength: It is the maximum amount of stress that can be sustained by bearing. 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: It is 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.
Bed: The mat of chopped glass fibers deposited over a layer of resin mix on carrier film
following a chopping operation.
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. It is 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.
Birdnest: A large, tangled up collection of continuous glass bundles unable to run through the
guide eye into roving creel. In the field, it can also be a large tangled collection of roving which
does not run through the tube or guide eyes to the chopper.
Bismaleimide (BMI): A polyamide that cures through an addition rather than a condensation
reaction, thus avoiding problems with volatiles forming. It is produced by a vinyl-type
polymerization of a pre-polymer terminated with two maleimide groups. BMI has an
Intermediate temperature capability (between epoxy and polyamide).
Bisphenol A: A condensation product formed by the reaction of two molecules of phenol with
acetone (A). This polyhydric phenol is a standard resin intermediate along with epichlorohydrin
in the production of epoxy resins.
Blade Packing: Glass bundles or chopper fuzz which build up and pack between the blades of a
chopper. Blade packing can result in poor choppability. If it falls off, it usually does not wetthrough,
causing blisters or porosity.
Blanket: Fiber or fabric plies that have been laid up in a complete assembly and placed on or in
the mold all at one time (flexible bag process). It is also, the form of bag in which the edges are
sealed against the mold.
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.
BMC: Bulk Molding Compound (Thermoset).
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.
Boron Fiber: Fiber produced by vapor deposition of elemental boron, usually onto a tungsten
filament core, to impart strength and stiffness.

Braid/Braider: A narrow tubular or flat fabric produced by intertwining a single set of yarns
according to a definite pattern.
Breakup: see Dispersion
Breathing: Opening and closing a mold so that gas can escape early in the molding cycle. Also
called "degassing"; sometimes called "bumping" in Phenolic molding.
Bridging: It is the condition in which fibers do not move into or conform to radii and corners
during molding, resulting in voids and dimensional control problems.
Broad Strand: see Wides or Matchsticks
Broken Strand: see Broken End
Broken End: In the roving operation, a severed strand (bundle), this causes the forming cake to
stop running.
Broken Fibers: see Fuzz
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. It is also
called "resistol" or "resitol." Resin in an uncured prepreg or premix is usually in this stage.
Buckling (Composite): It is a failure mode generally characterized by an unstable lateral
material deflection due to compressive action on the structural element involved.
Buildup: Glass bundles or chopper fuzz that collect on the chopper, cot, static bars or machine
frame.
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.
Bundle: A discrete collection of many parallel glass filaments. It is a collection of individual
filaments; a sub-strand.
Burst Strength:
• Hydraulic pressure required to burst a vessel of given thickness. It is commonly used in
testing filament-wound composite structures.
• Pressure required to break a fabric by expanding a flexible diaphragm or pushing a
smooth spherical surface against a securely held circular area of fabric. The Mullen
expanding diaphragm and Scott ball burst machine are examples of equipment used for
this purpose.
Bushing: Plate with holes through which molten glass is pulled to produce glass fibers.

Bushing Tip: It is a small tapered protrusion on the bottom of bushings, each containing an
orifice through which molten glass flows and from which continuous filaments are drawn.
Cabled Yarn: The yarn that is plied more than once or made by plying two or more previously
plied yarns.
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.
Carding: It the process of untangling and partially straightening fibers by passing them between
two closely spaced surfaces which are moving at different speeds of which one or more is
covered with sharp points, thus converting a tangled mass of fibers into a filmy web.
Casting: Process of pouring resin, fillers and/or fibers into a mold vs. building up layers through
lamination. Casting results in physical properties that are different than those resulting from
lamination.
Catalyst (Hardener): A substance that markedly speeds up the cure of a compound by
decomposing in the presence of a promoter to release active oxygen radical. Catalyst content can
vary from 0.2% to 2.0% with higher catalyst levels giving faster gel times. Examples are methyl
ethyl ketone peroxide, benzoyl peroxide.
Catastrophic Failures: Totally unpredictable failures of a mechanical, thermal, or electrical
nature.
Catenary: A consolidated group of individual strands which, when checked between two fixed
points of length, exhibit strands of different lengths. The resulting curve between fixed points of
the inextensionable material is defined as catenary. The amount of catenary is measured by the
distance between the topmost and lowermost strands when stretched between two fixed points.
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.
Cavity: Space inside a mold in which a resin or molding compound is poured or injected. It is
the female portion of the 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 a
single cavity or multiple cavities.

Cell: The vertical plane of doffs on a pallet. A pallet may be constructed for 4- or 12-end run-out
depending on application.
C-Glass: Glass with a soda-lime-borosilicate composition that maintains chemical stability in
corrosive environments.
Chalking: It is the surface phenomenon indicating degradation of a cosmetic surface. Chalking
is a powdery film that appears lighter than the original color.
Chemical Size: A surface finish applied to the fiber that contains some chemical constituents
other than water.
Choppability: The ease of chopping/cutting the glass fibers to a uniform length.
Clamping Pressure: In injection molding and transfer molding, the pressure that is applied to
the mold to keep it closed in opposition to the fluid pressure of the compressed molding material.
Cloth: Fiberglass reinforcement made by weaving strands of glass fiber yarns.
Clump: A group of chopped bundles of glass fibers that has collected on the SMC machine and
then fallen into the bed of glass. The clump produces areas of high glass content which may not
wet-through.
Coefficient of Thermal Expansion (CTE): How much a material’s shape will change per each
degree of temperature fluctuation.
Cold Flow: Distortion that occurs in a material under continuous load within its working
temperature range and without a phase or chemical change.
Collet: A spool on which the gathered strands from the bushing are wound for further
processing.
Compaction: Applying a temporary vacuum bag and vacuum to remove trapped air and
compact the lay-up; also removing air in SMC machines prior to roll-up.
Compatibility: The ability of two or more substances to be combined in order to form a
homogeneous composition of useful plastic properties; for example, the suitability of a sizing or
finish for use with certain general resin types.
Composite: Chemical or mechanical bonding of dissimilar materials such as glass fiber and
polyester resin, whose cumulative properties are superior to the individual materials.
Composite Material: A combination of two or more materials (reinforcing elements, fillers, and
composite matrix binder). Although they act in concert, the constituents retain their identities;
that is, they do not dissolve or merge completely into one another. Normally, the components can
be physically identified and exhibit an interface between one another.

Compounder: Manufacturer who mixes a polymer, fillers, additives and glass fibers, and sells
the resulting pellets for injection molding.
Compression Molding: An open molding process in which material is introduced and shaped by
the pressure of closing 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.
Compressive Strength: The amount of nonmoving load that a bar can take before it is crushed.
Units are normally thousands of pounds per square inch. (103 psi) - Mega Pascals (MPa). Higher
numbers indicate stronger materials that can withstand a heavier load before they break.
Condensation Polymerization: A chemical reaction in which two or more molecules combine,
with the separation of water or some other simple substance.
Conductivity: Reciprocal of volume resistivity. It is the electrical or thermal conductance of a
unit cube of any material (conductivity per unit volume).
Conformability: A mat’s ability to conform to difficult shapes without wrinkling or leaving
excessively resin-rich or glass-rich radii, which may craze.
Contact Molding: Refers to the use of a single or open mold onto which layers of polymer and
reinforcement materials can be applied. Contact molding is characterized by one finished
cosmetic side. Cure is either at room temperature using a catalyst-promoter system or by heating
in an oven, without additional pressure.
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 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 copolymerization. A process in which unlike molecules are
arranged in alternate sequence in a chain.
Core: A low-density material used between two FRP skins. Examples are end-grain balsa wood,
urethane foam, PVC foam and various honeycomb materials. The central member, usually foam
or honeycomb, has a sandwich construction to which the faces of the sandwich are attached or
bonded. It is the central member of a plywood assembly. A channel in a mold for circulation of
heat transfer media. A device on which prepreg is wound.
Coronizing: Continuous heat cleaning and weave setting.
Corrosion Resistance: A material’s ability to withstand ambient natural factors or those of a
particular artificially created atmosphere, without degrading or changing in properties. For
metals, this could be pitting or rusting; for or organic materials, it could be crazing.
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: It is the cracking of gel coat or resin due to stress. It is the region of ultra-fine cracks,
which may extend in a network on or under the surface of a resin or plastic material. It 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.
Composite Fabrication: The area in which pallets of roving are placed and "threaded up"
through metal tubes or guide eyes to a chopper.
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. It is deflection with
time under a given static load.
Cross-linking: Chemical links between molecule chains in thermosetting resins. Styrene
monomer is a cross-linking agent in polyester resins.
C-Stage: Final stage in the reaction of certain thermosetting resins in which the material is
practically insoluble and infusible.
CTE: see Coefficient of Thermal Expansion
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, resinimpregnated
reinforcement, adhesive or other material is subjected to curing.
Cure Time: The time required for liquid resin to reach a cured or fully polymerized state after
catalyst has been added.
Curing Agent: A catalytic or reactive agent that, when added to a resin, causes polymerization.
It is also called hardener.
Cut Ends on Doff: Severed ends generally caused by abrasion during shipping or by careless
use of a knife when the package is removed from the pallet.
Cycle: The complete, repeating sequence of operations in a process or part of a process. In
molding, cycle time is the period (or elapsed time) between a certain point in one cycle and the
same point in the next.
DAP: Diallyl Phthalate (Thermoset Resin).
Damping: The decay with time of the amplitude of free vibrations of a specimen.
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
plastic molding.
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. It can occur
in the cure or subsequent life.
Denier: A direct numbering system for expressing linear density, equal to mass in grams per
9000m of yarn, filament, fiber, or other textile strand.

Dielectric: A nonconductor of electricity. It is the 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
means that the material conducts electricity and therefore has no dielectric strength.
Dimensional Stability: A plastic part’s ability to retain the precise shape to which it was
molded, cast, or otherwise fabricated.
Direct-Sized Yarn: Specially formulated sizing on textile yarns that allows them to be resin
compatible.
Dispersion: Degree to which 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.
Distortion: Change in shape from that which is intended. Symptomatic of laminating
difficulties, curing problems, tooling problems or resin shrinkage.
Doctor Blade or Bar: A straight piece of material used to spread resin, as applying a thin film of
resin for use in hot melt prepreg or as an adhesive film. It is also called paste metering blade.
Doff: see Roving Doff
Doff Collapse: the failures of the roving doff to maintain its shape and stability during run-out or
storage. Doff collapse generally occurs when there is only a 1/2" to 1/4" ring of roving left from
the original doff.
Draft: The 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.
Dry Loft: Height of the bed of chopped fibers.
Dry Spot: Area of incomplete surface film on laminated plastics; in laminated glass, an area
over which the interlayer and the glass have not become bonded.

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.
E Glass: A family of calcia-alumina-silicate glasses with a certified chemical composition used
for general purpose and most electrical applications (ASTM D578-90).
Elasticity: A material’s ability to recover its original size and shape after the force deforming it
has been removed.
Ejection (De-molding): 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.
Elongation: As mentioned under tensile modulus, when a bar 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.
End: A single bundle of filaments.
End count: An exact number of ends supplied on a ball of roving.
Environment: The aggregate of all conditions (such as contamination, temperature, humidity,
radiation, magnetic and electric fields, shock, and vibration) that externally influence the
performance of an item.
Epoxy: A polymerizable thermoset polymer containing one or more epoxide groups cured by its
reaction with amines, alcohols, phenols, carboxylic acids, acid anhydrides, and mercaptans. It is
an important matrix resin in composites and structural adhesive. Epoxies generally have higher
physical properties than polyester resins. They are also more costly and difficult to process, and
less able to withstand sunlight.
Even Tension: Applying the same amount of tension to each end of roving in a ball.
Exotherm: The liberation or evolution of heat during the curing of a plastic product.

Exothermic Heat: Heat given off during polymerization by chemical ingredients as they react
and the resin cures.
Extend: To add fillers or low-cost materials in an economy producing endeavor. To add inert
materials to improve void-filling characteristics and reduce crazing.
Extenders: Low-cost materials used to dilute or extend high-cost resins without extensive
lessening of properties.
Extruder: Machine that pushes molten plastic through small holes to form fibers.
Fabrication: The process of glass fiber production during which forming cakes are put into
creels and "roved" or fabricated onto doffs.
Fabricator: Manufacturer of reinforced plastic products.
Fall: Shape or pattern of chopped fibers as they drop from the chopper to the bed.
Fan or Curtain: Chopped bundles that fall or are thrown off the chopper and cot.
Fatigue: Failure or decay of mechanical properties after repeated applications of stress. Fatigue
tests indicate a material’s ability to resist cracking, which eventually causes failure due to a large
number of cycles.
Fatigue Life: How many cycles of deformation it takes before a test specimen will fail under a
given set of oscillating conditions (stresses and strains).
Fatigue Limit: It is 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. It is 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 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 Diameter Letter Designation: Fibers are generally classified in hundred thousandths, i.e.,
a "K" fiber has a mean average diameter of 50+ to 55 heights.
Fiber Direction: The orientation or alignment of the longitudinal axis of the fiber with respect to
a stated reference axis.

Fiber Glass: Primarily means glass in fiber form. It is also used to describe composite
processing and applications, for example, fiber glass molding plant, fiber glass car.
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 Orientation: Fiber alignment in a non-woven or a mat laminate where the majority of
fibers are in the same direction, resulting in greater strength in that direction.
Fiber Pattern: Visible fibers on the surface laminate or molding. The thread size and weave of
glass cloth.
Fiber Reinforced Plastic (FRP): A general term for a composite that is reinforced with cloth,
mat, strands, or any other fiber form.
Filament: A single, threadlike fiber of glass.
Filament Winding: Process in which a resin-saturated strand of glass filament is wound around
a rotating mandrel.
Filament Yarn: A yarn composed of continuous filaments assembled with or without twist.
Fill: The system of yarns running crosswise in a fabric (short for filling). It is also known as
weft. see Warp
Filler: An inexpensive substance that is added to plastic resins to extend volume, improve
properties, and lower cost.
Fillet: A rounded filing of the internal angle between two surfaces of a plastic molding.
Fines: Bundles that have been split apart into smaller bundles composed of only a few or single
filaments. Fuzz is usually made of fines.
Finish: see Size
Fire Retardants: These are the chemicals that reduce a resin’s tendency to burn.
Fish Eye: It is the effect of surface contamination causing a circular separation of paint or gel
coat.
Flammability: How fast a plastic material will burn when subjected to a particular ASTM test.
In this test, a flame is applied to one end of a strip of material. When the material starts burning
the flame is removed and the time to consume a given amount of material is measured. Units are
measured in inches per minute (in/min.). Higher numbers indicate that the material will burn
faster under this particular test’s conditions. S.E. means self extinguishing; S.E. classified
materials stop burning when the flame is taken away.

Flash: Portion of the charge that flows or is extruded from the mold cavity during the molding.
Also refers to extra plastic attached to a molding along the parting line that 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 number referring to a material’s stiffness. It is used to calculate how far a
bar will bend when a bending load is applied. Units are normally millions of pounds per square
inch. (106 psi) - Giga Pascals (GPa). In two materials of equal thickness, the one with a higher
number is more resistant to deflection.
Flexural Strength: Also known as bending strength. Describes how much nonmoving load can
be applied to a bar before it yields or breaks. Units are normally thousands of pounds per square
inch. (103 psi) - Mega Pascals (MPa). Higher numbers indicate stronger materials that can
withstand a heavier load.
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.
Flow Line: A mark on a molded piece made by the meeting of two flow fronts during molding.
It is also called striae, 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.
Fly: Fibers that fly out into the atmosphere during handling and processing.
Foam: Lightweight, cellular plastic material containing glass-filled voids. Typical foams include
urethane, PVC, and polyester.
Force: The male half of the mold that enters the cavity, exerting pressure on the resin and
causing it to flow. It is also called punch.
Forming: It is the process of glass fiber production during which fibers are drawn, attenuated
from molten glass and collected in forming cakes.
Forming Cakes or "Cakes": Package of glass fibers produced during forming. This package is
generally found on a tube placed on a forming carrier and sent through a drying/curing oven.
"Cakes" are subsequently put into a roving creel and collected together into a roving doff.

Fracture: When a surface ruptures without the laminate completely separating, or where there is
complete separation of a body because of external or internal forces.
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.
Fuzz: Creel Fuzz - In glass fiber manufacturing, the broken filaments found around and on a
roving creel. Chopper Fuzz - In Composite Fabrication, the broken filaments found around the
glasscutter or chopper. See Fines. In the field, the broken filaments found around a roving pallet.
Fuzz Plug: Small, broken, compacted filaments of glass that collect inside the guide eye tubes
feeding the chopper, preventing glass from running through.
Gate: Point at which molten thermoplastic enters the injection molding tool cavity.
Gel: A partial cure of plastic resins; a semisolid, jelly-like state similar to gelatin in consistency.
Gel: The initial jelly-like solid phase that develops during a resin’s formation from a liquid state.
It is a semisolid network of solid aggregates in which liquid is held.
Gelation: The point during resin cure when viscosity has increased so much that resin barely
moves when probed with a sharp instrument.
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.
Gel Time: Time required to change a flowable liquid resin into a non-flowing gel.
GFRP: Glass fiber-reinforced plastic, polymer or polyester.
Glass Blends: When several different fiber types, i.e. different lengths and diameters, are
blended in the fiber slurry.
Glass Content: Percentage of glass in the compound.
Glass Fiber Wet-Process: Process of forming a glass mat on modified papermaking equipment.

Glass Transition: Reversible change in an amorphous polymer or amorphous regions of a
partially crystalline polymer from or to a viscous, rubbery, or hard and relatively brittle
condition.
Glass Transition Temperature (Tg): The appropriate midpoint of the temperature range over
which glass transition takes place.
Good Side: Side of molding in contact with a mold surface.
Graphite Fiber: Fiber made from a precursor by oxidation, carbonization and graphitization
process (which provides a graphitic structure).
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.
Glint: A visual defect in a fiberglass reinforced cured organic (usually corrosion- resistant resin)
panel. The defect appears as many small visible unwet or foreign substances similar to a saltand-pepper
effect. The defect is not visible before cure but appears at the panel’s exotherm.
Guide Pin: A pin which guides mold halves into alignment on closing.
Hand: Fabric softness as determined by touch (individual judgment).
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. It is a 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 plastic composition to promote or control curing
by taking part in it.
Hard Glass: A roving product that is not very soluble in acetone or styrene, causing individual
bundles to retain their integrity (hardness) and not filamentize in the matrix resin. Hard glass is
often stiff and brash. The size is generally less than 40% soluble in acetone. see Soft Glass

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 Cleaning: It is a batch and continuous processes in which organic yarn binder is removed
from glass fabrics.
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 Distortion Point: Temperature at which a material’s strength begins to degrade. It is also
called deflection temperature.
Heat Resistance: It is the ability of plastics and elastomers to resist deterioration due to elevated
temperatures.
Heat Sink: A contrivance for the absorption or transfer of heat away from a critical element or
part. Bulk graphite is often used as a heat sink.
Helical Winding: In filament wound items, a winding in which a filament band advances along
a helical path, but not necessarily at a constant angle (except on cylinders).
Het-Acid Resin: Polyester resin with exceptional fire qualities.
High-Pressure Laminates: Laminates molded and cured at pressures not lower than 6.9 MP
(1.0 ksi), and more commonly between 8.3 to 13.9 Mpa (1.2 to 2.0 ksi).
Homogeneous: Describes a material with a uniform composition.
Homopolymer: A compound produced by polymerization.
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.
Hoop Stress: The circumferential stress in a material of cylindrical form subjected to internal or
external pressure.
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 Strength-Izod: A moving load is one that is moving when it strikes a bar. The effect of
such a load is denoted by the work "impact". The impact strength of a material is a measure of
how much energy is absorbed by the bar when it is broken by a moving weight. Izod is one of
many different test methods for measuring impact. Units are measured in foot pounds per inch of
width; sometimes given as foot pounds per inch of notch. Joules/Meter (J/M). Higher numbers
mean that the material will absorb more energy before it is broken by a moving weight.
Impact Test: It is the measure of the energy necessary to fracture a standard sample by an
impulse load.
Impregnate: In reinforced plastics, to saturate 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: Instrument for determining the tensile and compressive properties of materials.
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 laminae.
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.
Irreversible: Not capable of re-dissolving or re-melting. It is a chemical reaction that proceeds
in a single direction and are not capable of reversal (as applied to thermosetting resins).
Isocyanate Plastics: Plastics based on resins made by the reaction or organic isocyanates with
other compounds.
Isophthalic: Polyester resin based on isophthalic acid, generally higher in properties than a
general purpose or orthothatic polyester resin.
Isotropic: Having uniform properties in all directions.
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 composed of aromatic polyamides having a para-type orientation
(parallel chain extending bonds from each aromatic nucleus).
Knitted Fabric: It is a textile structure produced by interlooping one or more ends of yarn or
comparable material.
Knot: The means of joining strands of two doffs of roving. The knot is generally a reduced triple
loop surgeon's knot, square knot or overhand knot.
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.
Land: Portion of a mold which provides the separation or cutoff of the flash from the molded
article; in the screw of an extruder, the bearing surface along the top of the flights; in an
extrusion die, the surface parallel to the flow of material; in a semi-positive or flash mold, the
horizontal bearing surface; in a two-piece mold, a platform build up to the split line.

Lay: In glass fiber, the spacing of the roving bands on the roving package expressed in the
number of bands per inch; in filament winding, the orientation of the ribbon with some reference,
usually the axis of rotation.
Lay-down: The degree of uniformity in thickness that a bed of chopped glass fibers exhibits
across the width of the chopped pattern.
Layer: The horizontal plane of doffs on a pallet. A pallet usually has four layers of twelve doffs
each. also see Cell
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.
L/D Ratio: A term used to define an extrusion screw, which denotes the ratio of the screw length
to the screw diameter.
Lengthwise Direction: Refers to the cutting of specimens and application of loads. For rods and
tubes, lengthwise is the direction of the long axis. For other shapes of materials that are stronger
in one direction than in the other, lengthwise is the direction that is stronger. For materials that
are equally strong in both directions, lengthwise is an arbitrarily designated direction that may be
with the grain, direction of flow in manufacture, longer direction, etc.
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.
Load-Deflection Curve: A curve in which the increasing tension or compressions of flexural
load are plotted on the ordinate axis and the deflections caused by those loads are plotted on an
abscissa axis.
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.
Loom Beam: A large, flanged cylinder onto which all warp yarns are wound and from which
yarns enter the loom.
Loop: Small open place in the strands due to excessive length of one or more strands.
Loss on Ignition: Weight loss, usually expressed as a percent of the total, after burning off an
organic sizing from glass fibers, or an organic resin from a glass fiber laminate.
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.

Low-Pressure Molding: The distribution of relatively uniform low pressure (200 psi or less)
over a resin-bearing fibrous assembly of cellulose, glass, asbestos, or other material, with or
without application of heat from an external source, to form a structure possessing definite
physical properties.
Lubricant: A material added to most sizing to improve the handling and processing properties
of textile strands.
Mandrel: The core tool around which resin-impregnated paper, fabric or fiber is wound to form
pipes, tubes or structural shell shapes.
Manipulator: Highly efficient system eliminating the need for a press screw driven frame
structure that opens and closes the mold in a controlled line of draw to prevent tool damage.
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.
Matrix: The resin component of a polymer composite. Both thermoplastic and thermoset resins
may be used, as well as metals, ceramic and glasses. (Plural: matrices.)
Mat Strength: The mat’s ability to resist being pulled apart under tension during impregnation
and molding.
Matchsticks: Strand-to-strand adhesion. A matchstick is a wide bundle that has 3 to 4 times as
many filaments than the majority of bundles in the bed.
Mechanical Properties: A material’s properties, such as compressive and tensile strength and
modulus, those are associated with elastic and inelastic reaction when force is applied. These are
individual relationship between stress and strain.
Melamine: Thermoset resin.
Metallic Fiber: Manufactured fiber composed of metal, plastic-coated metal, metal-coated
plastic or core completely covered by metal.
Microballoons: Microscopic bubbles of glass, ceramic or Phenolic, used as a filler or to create
syntactic foam or putty mixtures.

Microcracking: Crack formed in composites when thermal stresses locally exceed the strength
of the matrix.
Micron: 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. It is useful as
inexpensive filler or anti-crazing reinforcing filler for adhesives.
M.I.T: see Multiple Insert Tooling
MPG: see Mold Protection Guard
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.
Moisture: The amount of volatiles on the glass expressed as a percentage of the total weight.
Moisture Content: The amount of moisture 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.
Molded Edge: It is an edge that is not physically altered after molding for use in final form, and
particularly one that does not have fiber ends along its length.
Molding: The forming of a polymer or composite into a solid mass of prescribed shape and size.
Molding Compounds: Plastics available in a wide range of forms meeting specific processing
requirements. Granules or pellets are popular forms.
Molding Cycle: The period of time required for the complete sequence of operations on a
molding press to produce one set of moldings.
Molding Pressure: Pressure applied to the ram of an injection machine or compression or
transfer press forcing softened plastic to completely fill mold cavities.
Molding Pressure, Compression: Unit pressure applied to material in the mold. The area is
calculated from the projected area taken at right angles to the direction of applied force and

includes all areas under pressure during complete closing of the mold. Unit pressure is calculated
by dividing the total force applied by the projected area, expressed in pounds per square inch.
Mold Protection Guard: Prevents over pressurization of the RTM mold during the injection
process.
Mold-Release Agent: Lubricant, liquid or powder (often silicone oils and waxes) that prevents
molded articles from sticking in the cavity.
Mold Shrinkage: Immediate shrinkage that occurs when a part is removed from a mold and
cooled to room temperature; the difference in dimensions, expressed in inches between a
molding and the mold cavity in which it was molded (at normal temperature measurement); the
incremental difference between the dimensions of the molding and the mold from which it was
made, expressed as a percentage of the dimensions of the mold.
Monomer: A simple molecule capable of reacting with like or unlike molecules to form a
polymer; the smallest repeating structure of a polymer; for addition polymers, this represents the
original unpolymerized compound.
Multiple-Cavity Mold: A mold with two or more mold impressions; that is, a mold producing
more than one molding per molding cycle.
Multiple Insert Tooling: Low-risk, high-output composite tooling breakthrough. MIT tooling
technology offers the composite molder quick-change multiple mold surfaces without multiple
tooling costs. Each surface is an exact clone of its counterpart, guaranteeing mold cavity
accuracy and replication of molded parts.
Nesting: In reinforced plastics, placing of plies of fabric so that the yarns of one ply lie in the
valleys between the yarns of the adjacent ply (nested cloth).
NOL Ring: Parallel filament wound test specimen used in whole or in part to measure a
material’s mechanical strength properties.
Non-Air-Inhibited Resin: Resin in which the surface cure will not be inhibited or stopped by
the presence of air.
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 flow of polymer in the mold.
Orthophthalic Resin: Polyester resin based on Orthophthalic acid, also known as a general
purpose resin (GP).
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. It is also called surfacing
mat.
Overspray: A specially formulated binder applied to texturized yarn that helps retain the yarn
bulk after texturizing.
Parting Line: It is a mark on a molded piece where mold sections have met in closing.
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.
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. It is made of
plastic. 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. These are
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.
Plied Yarn: It is yarn formed by twisting two or more single yarns together in one operation.
(Synonyms: folded yarn, formed yarn)
Plug: A composite industry term for a pattern or model.
Ply: The number of single yarns twisted together to form a plied yarn; also the number of plied
yarns twisted together to form a cord. It is the individual yarn in a plied yarn or in a cord. It is
one of several layers of fabric.
Poisson's Ratio: The ratio of transverse strain to axial strain during axial load.
Polyamide: A polymer in which the structural units are linked by amide or thioamide groupings.
Many polyamides are fiber-forming.
Polyester Combination Yarn: A polyester/fiber glass hybrid yarn.
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.
Positive Mold: Mold that applies pressure to the piece being molded without material escaping.
Post Bake: see Post Cure
Post Cure: Heat cycle that a roving doffs goes through after fabrication. Generally used to help
"set" the ribbon on the outside of doff, improving doff's stability and resistance to package
collapse.
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: Pre-shaped 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 pre-shaped 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.
Pregel: An unintentional extra layer of cured resin on part of a reinforced plastic’s surface. (not
related to gel coat)
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.
Pressure Bag: A membrane conforming to the inside of a laminate laid up on a mold. The
membrane or bag is then inflated, applying pressure that consolidates and densifies the laminate.
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.
Putty: Thickened mixture of resin made by adding fillers and reinforcing fibers.
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).
Re-Chop: Bundles that have clung to the chopper or cot and are chopped again into shorter
lengths. Re-chop causes excessive chopper fuzz as the strands are cut and mashed into smaller
bundles.
Reinforced Molding Compound: Compound consisting of a polymer and reinforcement fiber
or filler, supplied by a raw material producer as ready-to-use materials.

Reinforced Plastics: It is 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
Reinforcement: Strong material bonded into a matrix to improve its mechanical properties.
Reinforcements are usually long fibers, chopped fibers, whiskers and particulates. The term
should not be used synonymously with filler.
Release Agent: Compound used to reduce surface tension or adhesion between a mold and a
part.
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. See Matrix and Polymer
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.
Ribbon: The propensity of glass bundles to "stick" together and act as a strand or end.
RIM: see Reaction Injection Molding.
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: A collection of untwisted strands wound together into a doff (ball). It is also, another
name for the fabrication process step.
Roving Ball: A term used to describe the supply package offered to the winder. It consists of a
number of ends or strands wound to a given outside diameter onto a length of cardboard tube.

Roving Doff or "Doff": The final product sold to the customer. It is 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.
RRIM: see Reinforced Reaction Injection Molding
RTM: see Resin Transfer Molding
RTM Light: see Vacuum Molding
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.
Run-Out: The process of pulling glass from doff to the chopper.
Runner: The channel through which thermoplastic material moves through a mold.
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.
Section Beam: Flanged cylinder onto which yarn is drawn and accumulated from yarn bobbins
or packages.
Serving: Wrapping of yarn around a product in one or more layers to form a protective covering.
Set-Up: To harden, as in curing.
Sewing Thread: Flexible small diameter yarn or strand, usually treated with a surface coating
and/or lubricant, used to stitch one or more pieces of material together or stitch an object to a
material.

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.
Shear Edge: Cut-off edge of the mold.
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.
Shrinkage: The relative change in dimension between the length measured on the mold when it
is cold and the length of the molded object 24 hours after it has been taken out of the mold.
Single Yarn: The simplest strand of textile material suitable for weaving, knitting and other
operations.
Sink Mark: A shallow depression or dimple on the surface of an injection molded part due to its
surface collapsing from local internal shrinkage after the gate seals; an incipient short shot.
Size: Treatment applied to glass fiber that allows resin and glass to adhere to one another. Also
allows glass fiber to be conveniently handled.
Skein: A loose coil of roving or strands generally used as a Quality Control sample.
Skin Coat: It is the first layer of laminate next to the gel coat.
Slashing: Applying size to a width of warp yarns on a continuous basis.
Slink: Glass bundles thrown off the chopper or cot during chopping.
Sliver: A term describing the geometry of fibrous glass reinforcement in the forming operation.
For example, 2K37 S/2 means a configuration that makes a nominal fiber diameter in the "K"
range (3700 yards to a pound) and is split into two discrete bundles in the forming cake.
Sluffing: see Doff Collapse
SMC: see Sheet Molding Compound
Snarl: see Birdnest

Soft Glass: A roving product whose sizing is moderately soluble in acetone or styrene, resulting
in bundles that tend to open readily or filamentize 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.
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 heavier materials.
Specific Heat (Thermal Capacity): The amount of heat needed to raise the temperature of one
pound of material one degree Fahrenheit (F). Units are measured in BTUs per pound per degree
(BTA/LB/°F) - Joules/Kilogram Kelvin (J/KgK). 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.
Splice: Joining two ends of yarn by intertwining, knotting, overlapping or adhering them
together.
Split: One bundle.
Split Mold: A mold whose cavity is formed of two or more components held together by an
outer chase. The components are known as splits.
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.
Sprue: Connector between the runner and nozzle in an injection-molding machine.
Splitting Efficiency: Ratio of the actual number of ends divided by the theoretical number of
ends in a roving doff, expressed as a percentage.
Staple: Filaments produced in short lengths from the bushing (usually less than 17 inches)
gathered into strands or sliver. See Continuous Filament
Starved Area: It is a portion of a plastic part without sufficient resin to completely wet out the
reinforcement. It is usually due to improper wetting, impregnation or excessive molding
pressure.

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 inch.
Sticker: see Trapped End
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.
Strand: In the roving process or shop, a primary group of bundles gathered together in a creel. A
strand is that which is pulled out of a doff; also a plurality of drawn and elongated individual
filaments combined together to form an individual strand. Strands are held together and protected
by sizing.
Strand Count: According to the U.S. Yardage System, the length in hundreds of yards of a
single strand having a mass of one pound. In the European TEX System, the mass in grams of a
strand 1000 meters in length.
Strand Integrity: The size’s ability to keep all filaments in a bundle stuck together during
chopping. Good strand integrity is required for good flow in or wet-through and wet-out on the
mold.
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.
Stress-Strain: Stiffness, expressed in pounds per square inch or kilograms per square
centimeter, at a given strain.
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/Acetone Solubility: Percentage of sizing on glass fiber dissolved off the strand in
styrene or acetone after soaking in the solvent.
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 180 to 510 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 surface of a layer of resin or polymer. It is
usually used to form a film on a curing resin, producing a tack-free surface.
Tack: Surface stickiness.
Tack Free: Surface which is not sticky after cure.
Tangent Modulus: Slope of the line at a predefined point on a static stress-strain curve,
expressed in force per unit area per unit strain. This is the tangent modulus at that point in shear,
tension or compression, as the case may be.
Tape: A narrow fabric whose mass per unit area is less than 0.5 kg/m 2 (0.1 LB/ft2) for each
25.4 mm (1 in.) of width; used primarily for utilitarian purposes.
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 strain.
Tensile Load: Lo ad applied away from and to opposite ends of a given sample.
Tensile Modulus: When a bar is pulled in tension, it gets longer. Tensile modulus calculates
how much longer it will get when a certain load is applied. Units are normally millions of pounds
per square inch. (10 6 psi) - Giga Pascals (GPa). Higher numbers indicate materials that do not
elongate as much as others under equal tensile loading conditions.
Tensile Strength: The amount of nonmoving load a bar can withstand before it breaks due to
elongation. Units are normally thousands of pounds per square inch. (103 psi) - Mega Pascals
(MPa). Higher numbers indicate materials that can withstand a stronger pull before breaking.
Tensile Stress: Normal stress caused by forces directed away from the plane on which they act.
Tension Device: A mechanical or magnetic device that controls tension.
TEX: A unit for expressing linear density, equal to the mass in grams of 1 km of yarn, filament,
fiber or other textile strand.
Texturized Glass Yarn: Yarn processed from continuous filaments that have been disoriented,
adding bulk.
Thermal Coefficient of Expansion: Measures how much the length of a material will change
when it is heated or cooled. The value given is based on the inch as a unit. The number given

shows how much one-inch of material will increase if its temperature is raised one degree
Fahrenheit. Units are expressed in inches per inch per degree Fahrenheit (in/in/°F) -
Meters/Meter/°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): The amount of heat transferred by conduction; i.e., how
much heat is transferred from one side of a plate to the other. 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. Units are
expressed as BTU/hr/sq/°F/inch. - Watt/(Meter Deg Kelven) W/MK. Higher numbers mean that
the material will absorb more energy before it is broken by a moving weight.
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 diallyl phthalate.
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 builtup
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 dially phthalate).
Theoretical End Count: The maximum number of bundles in a roving doff; for example, a
roving doff made with 18 forming cakes in the creel that were "split out" 4 ways in forming will
have 64 theoretical ends.
Thickeners: Material added to resin to thicken it or raise its viscosity index so that it will not
flow as readily.

Thixotropic: The property of becoming a gel at rest, but liquefying again on agitation.
Tooling Gel Coat: It is a gel coat formulated for mold surfaces.
Transfer: The smooth and successful transition from one roving doff to another during
processing.
Translucent: Permits a percentage of light to pass but not optically clear like window glass.
Trapped End: A loop embedded into doff during the roving process that gets stuck during runout
with such tenacity that it prevents the entire strand from running freely to the chopper.
Tube Stoppage or Plug: The failure of glass to run through metal tubes or guide-eyes from the
creel to the chopper. It is usually caused by a large knot or small birdnest becoming stuck inside
the tube or guide-eye.
Turn: One 360° revolution of the components around the axis of the strand.
Turnaround: The portion of the roving doff where the roving changes direction when it is
pulled out of doff.
Twist and Ply Frames: Machines used to twist and ply glass yarns.
Ultimate Tensile Strength: The ultimate or final stress sustained by a specimen in a tension
test; the stress at moment of rupture.
Undercut: Having a protuberance or indentation that impedes withdrawal from a rigid two-piece
mold; any such protuberance or indentation, depending on the design of the mold (tilting a model
in the mold may eliminate an apparent "undercut").
Unidirectional: Strength lying mainly in one direction. It is a glass reinforcement in which the
fiber is oriented in one direction.
Untied Ends: The lack of a knot or splice between two doffs, one on top of the other, which
prevents successful transfer from the top doff to the bottom doff in a creel par.
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. It is also
called bag molding.
Vacuum Molding: Low cost entry method into RTM which uses a rigid cavity mold half and a
semi-rigid upper mold half, both made of FRP. Capable of producing 3-4 times as many
moldings as that produced through open mold, with acceptable repeatability (but not equal to that
of RTM.)
Veil: Ultra thin mat similar to a surface mat.
Vinyl-Coated Glass Yarn: Continuous glass filament yarn coated with pigment and plasticized
vinyl chloride resin.
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 in a laminate.
VM: See Vacuum Molding.
Warp: Yarn running lengthwise in a woven fabric. A group of yarns in long lengths and
approximately parallel, put on beams or warp reels for further textile processing including
weaving, knitting, twisting, dyeing, etc.
Warp Size: Chemicals applied to the warp yarn to improve strand integrity, strength and
smoothness in order to withstand rigors of weaving.
Weave: Particular manner in which a fabric is formed by interlacing yarns; usually assigned a
style number.
Weft: The system of yarns running crosswise in a fabric. It is also known as fill.
Wet Flexural Strength (WFS): It is the flexural strength after water immersion, usually after
boiling the test specimen for two hours in water.
Wet Lay-Up: Reinforced plastic with liquid resin applied after the reinforcement is laid up. It is
opposite of "dry lay-up", "prepreg".
Wet-Out Rate: Time required for a plastic to fill the interstices of a reinforcement material and
wet the surface of the reinforcement fibers; usually determined by optical or light transmission
means.

Wet-Process: A process for forming a non-woven web from water slurry on "papermaking’
equipment. It is also known as "wet-laid" or "wet-formed".
Wet Strength: It is strength of paper when saturated with water, especially in discussions of
processes whereby the strength of paper is increased by the addition, in manufacture, or plastic
resins; the strength of an adhesive joint determined immediately after removal from a liquid in
which it has been immersed under specified conditions of time, temperature and pressure.
Wet-Through: The degree and/or rate of encapsulation of sized glass fiber bundles in a
laminate. Also, the rate and/or degree of which the polymer matrix resin system can flow
through a bed of sized glass bundles or strands and encapsulate each bundle of filaments.
Wides: Term describing bundles of roving that are wider than most of the other bundles in a bed
of chopped glass fibers. They usually contain 3 to 4 times more filaments than most of the other
bundles in the roving (See Matchstick).
Woven Roving Fabric: Heavy fabrics woven from continuous filament in roving form. It is
usually in weights between 18-30 per square yard.
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.
Yardage: Similar to Yield, but used to describe the linear density of "bare glass" or an un-sized
product. Yardage specifies the number of yards of glass required to weigh one pound, measured
in hundreds. For example, K18 is a K fiber diameter that has 180yards in one pound of glass.
Yarn: Generic term for a continuous strand of textile fibers, filaments or material in a form
suitable for knitting, weaving or intertwining to form a textile fabric.
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
increases at a higher rate than stress. It is 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. It is a stress at which a material exhibits a specified
limiting deviation from the proportionality of stress to strain. It is the lowest stress at which a
material undergoes plastic deformation. Below this stress, material is elastic; above it, material is
viscous. Often it is defined as the stress needed to produce a specified amount of plastic
deformation (usually a 0.2% change in length).
Young's Modulus: Ratio of normal stress to corresponding strain for tensile or compressive
stresses less than the proportional limit of the material.