A-stage - An early stage of
polymerization of thermosetting resins in which the
material is still soluble in certain liquids and
fusible. (See also B-stage, C-stage.)
Ablative - Describes a material
that absorbs heat through a decomposition process
called pyrolysis at or near the exposed surface.
Accelerator - Chemical additive
that hastens cure or chemical reaction.
Addition - Polymerization reaction
in which no byproducts are formed.
Additives - Ingredients mixed into
resin to improve properties.
Adhesive - Substance applied to
mating surfaces to bond them together by surface
Amorphous - Polymers with no
Angle-ply laminate - Any balanced
laminate consisting of plies at angles of plus and
minus theta, where theta is an acute angle with the
principal laminate axis.
Anisotropic - Not isotropic.
Exhibiting different properties when tested along
axes in different directions within the material.
Aramid - Aromatic polyamide
fibers. (Often referred to as Kevlar, DuPont’s
Areal weight - Weight of a fiber
reinforcement per unit area (width times length) of
tape or fabric.
Aspect ratio - Ratio of length to
diameter of a fiber.
Autoclave - Closed vessel for
applying fluid pressure, with or without heat, to an
Autoclave molding - Molding
technique in which an entire assembly (lay up and
tooling) is placed into an autoclave and subjected
to heat and elevated pressure for consolidation
and/or curing while removing entrapped air and
Automated tape laying -
Fabrication process in which prepreg material,
typically unidirectional tape, is laid across the
surface of a mold in multiple layers and directions
by an automated tape-application machine to form a
Axial winding - Filament winding
wherein the filaments are parallel or at a small
angle to the axis of rotation.
B-stage - Intermediate stage in
the polymerization reaction of some thermosets in
which the material softens with heat and is plastic
and fusible but does not entirely dissolve or fuse.
The resin of an uncured prepreg or premix is usually
in this state. (See also A-stage, C-stage.)
Bag molding - Molding technique in
which the composite structure is placed in a rigid
mold and covered with a flexible impermeable layer
of film and the edges sealed, followed by
consolidation and/or curing with pressure applied by
vacuum, autoclave, press or inflation of the bag.
Balanced laminate - Any laminate
that contains one ply of minus theta orientation
with respect to the principal axis of the laminate
for every identical ply with a plus theta
Basket weave - 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 - Material made by the same
process at the same time having identical
characteristics throughout. Same as lot.
Bias fabric - Fabric in which warp
and fill fibers are at an angle to the length.
Biaxial fabric - Fabric with two
non-interwoven layers - a unidirectional warp (0°)
layer and a unidirectional weft (90°) layer - which
are bonded together, usually by
through-the-thickness stitching, to form a single
sheet of fabric. (See also triaxial fabric,
Biaxial winding - Filament winding
wherein helical bands are laid in sequence, side by
side, with no fiber crossover.
Bidirectional laminate - Laminate
with fibers oriented in more than one direction on
the same plane.
Bismaleimide (BMI) - Type of
thermoset polyimide that cures by an additional
reaction, thus avoiding formation of volatiles.
Exhibits temperature capabilities between those of
epoxy and polyimide.
Bleeder cloth - Layer of woven or
nonwoven material, not a part of the composite, that
allows excess gas and resin to escape during cure.
Bleedout - Excess liquid resin
appearing at the surface of the composite structure,
particularly during filament winding.
BMI - See bismaleimide.
Bond ply - Ply or fabric patch
that comes in contact with the honeycomb core during
Bond strength - As measured by
load/bond area, the stress required to separate a
layer of material from another material to which it
is bonded; the amount of adhesion between bonded
Boron fiber - Fiber produced by
chemical vapor deposition of boron onto a core
material, usually a tungsten-filament. Because of
the deposition process, a boron fiber is of a fairly
large diameter, typically about 0.4 mils, and is
thus often referred to as a wire.
Braiding - Textile process that
intertwines into a pattern three or more strands,
yarns or tapes, typically into a tubular shape.
Breakout - Separation or breakage
of fibers when the edges of a composite part are
drilled or cut.
Breather - Loosely woven material
that does not come in contact with the resin but
serves as a continuous vacuum path over a part in
Broadgoods - Fibers woven into
fabrics that may or may not be impregnated with
resin; usually furnished in rolls.
Buckling - Failure mode usually
characterized by unstable lateral deflection rather
than breaking under compressive action.
Bundle - General term for a
collection of essentially parallel filaments.
C-stage - Final step in the cure
of a thermoset resin, resulting in irreversible
hardening and insolubility. (See also A-stage and
CAD/CAM - Computer-aided
Carbon fiber - Reinforcing fiber
produced by the pyrolysis of an organic precursor
fiber, such as PAN, rayon or pitch, in an inert
environment at temperatures above 1,800°F. The term
carbon is often used interchangeably with the term
graphite, but the fibers differ. Carbon fibers are
typically carbonized at about 2,400°F and contain
93 percent to 95 percent carbon. Carbon fibers can
be converted to graphite fibers by graphitization at
3,450°F to 4,500°F, after which they contain more
than 99 percent elemental carbon. Carbon fibers are
known for their light weight, high strength and high
Carbon/carbon - Composite of
carbon fiber in a carbon matrix.
Catalyst - Substance that promotes
or controls curing of a compound without being
consumed in the reaction. (See also hardener.)
Caul plate - Plate or sheet the
same size and shape as the composite lay-up with
which it will be used. The caul plate is placed in
immediate contact with the lay-up during curing to
transmit normal pressure and provide a smooth
surface on the finished part.
Ceramic-matrix composites (CMC) -
Materials consisting of a ceramic or carbon fiber
surrounded by a ceramic matrix, primarily silicon
Chemical vapor deposition (CVD) -
Process in which the reinforcement material is
deposited from the vapor phase onto a continuous
core such as boron or tungsten.
Circumferential winding - Process
of winding fiber perpendicular to the axis during
Cloth - See fabric.
CMC - Ceramic-matrix composite.
Cocured - Cured and simultaneously
bonded to another prepared surface.
Coefficient of thermal expansion (CTE)
- A material’s fractional change in length for a
given unit change of temperature.
Cohesion - Tendency of a single
substance to adhere to itself. Also, the force
holding a single substance together.
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
Commingled yarn - 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
Composite - Three-dimensional
combination of at least two materials differing in
form or composition, with a distinct interface
separating the components. Composite materials are
usually manmade and created to obtain properties
that cannot be achieved by any of the components
Compression molding - 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 - Resistance
to a crushing or buckling force, the maximum
compressive load a specimen sustains divided by its
original cross-sectional area.
Condensation - Polymerization
reaction in which simple by-products (for example,
water) are released.
Consolidation - Processing step
that compresses fiber and matrix to reduce voids and
achieve a particular density.
Contaminant - Impurity or foreign
substance that affects one or more properties of
composite material, particularly adhesion.
Continuous filament - Individual,
small-diameter reinforcement that is flexible and
indefinite in length.
Continuous roving - Large bundle
of parallel filaments coated with sizing, gathered
together into single or multiple strands, and wound
into a cylindrical package. May be used to provide
continuous reinforcement in woven roving, filament
winding, pultrusion, prepregs, or high-strength
molding compounds (may also be used chopped).
Coordinate axes - See laminate
Core - In sandwich construction,
the central component to which inner and outer skins
are attached; also refers to a section of a complex
mold that forms undercut parts.
Core crush - Compression damage of
Core depression - Gouge or
indentation in the core material.
Core orientation - Used on a
honeycomb core to line up the ribbon direction,
thickness of the cell depth, cell size and
Core splicing - Joining of two
core segments by bonding them together.
Cowoven fabric - Reinforcement
fabric woven with two different types of fibers in
individual yarns (for example, thermoplastic fibers
woven side by side with carbon fibers).
Crazing - Region of ultrafine
cracks that may develop on or under a resin surface.
Creep - Time-dependent dimensional
change in a material under physical load.
Crimp - Degree of waviness of a
fiber, which determines its capacity to cohere.
Critical length - Minimum length
of a fiber necessary for matrix shear loading to
develop ultimate fiber strength.
Cross-laminated - Laminated with
some of the layers oriented at one or more angles to
the other layers with respect to the principal
Crossply laminate - Laminate
having plies oriented only at 0° and 90°. May or
may not be symmetric.
Crosslinking - Polymerization
reactions that branch out from the main molecular
chain to form a networked pattern of chemical links.
Crystalline - Having a molecular
structure in which the atoms are arranged in an
orderly, three-dimensional pattern.
CTE - See coefficient of thermal
Cure - To change the physical
properties of a material irreversibly by chemical
reaction via heat and/or catalysts, with or without
Cure temperature - Temperature at
which a material attains final cure.
Curing agent - Catalytic or
reactive agent that brings about polymerization when
added to a resin.
CVD - See chemical vapor
Damage tolerance - 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 - Deliberate separation of
a bonded joint or interface, usually for repair or
rework purposes. (See also disbond.)
Delamination - Separation of plies
in a laminate due to adhesive failure. This may be
local or may cover a large area. Also includes the
separation of layers of fabric from the core
Demold - To remove a part from a
tool, or a tool from an intermediate model.
Denier - Numbering system for
continuous yarn and continuous filaments in which
the yarn number is equal to the weight in grams per
9,000 meters of yarn; the finer the yarn, the lower
Design allowable - Limiting value
for a material property that can be used to design a
structural or mechanical system to a specified level
of success with a specific level of statistical
Dielectric - Nonconductor of
electricity; the ability of a material to resist the
flow of an electric current.
Disbond - Unplanned non-adhered or
unbonded area within a bonded interface. Can be
caused by adhesive or cohesive failure, may occur at
any time during the life of the structure and may
arise from a wide variety of causes. The term is
also sometimes used to describe a delamination.
Doubler - Extra layers of
reinforcement for added stiffness or strength in
laminate areas that incur abrupt load transfers.
Drape - The ability of prepreg to
conform to the shape of a contoured surface.
Dry winding - A filament-winding
operation in which resin is not used.
E-glass - Denotes "electrical
glass," so called because of its high
electrical resistivity. Refers to borosilicate glass
fibers most often used in conventional polymer
Elasticity - The property of
materials to recover immediately their original size
and shape when load is removed after deformation.
Elongation - The fractional
increase in length of a material loaded in tension.
When expressed as a percentage of the original
length, it is called percent elongation.
End - General term for a
continuous, ordered assembly of essentially
parallel, collimated filaments, with or without
Epoxy - Thermoset polymer
containing one or more epoxide groups, curable by
reaction with amines or other compounds.
Exotherm - Heat released during a
chemical reaction. Uncontrolled exotherm can lead to
extreme heat build up and possibly violent
Fabric - Planar textile. Also
known as cloth.
Fabric, nonwoven - Planar textile
constructed by bonding or interlocking, but not
interlacing, by mechanical, chemical, thermal or
Fabric, woven - Planar textile
constructed by interlacing in a weaving process.
Fabrication - Process of making a
composite part or tool.
Fatigue - Failure or deterioration
of a material’s mechanical properties as a result
of repeated cyclic loading or deformation over time.
Fatigue strength - Maximum
cyclical stress withstood for a given number of
cycles before a material fails. The residual
strength after being subjected to fatigue loading.
FEA - Finite-element analysis.
Fiber - One or more filaments in
an ordered assemblage.
Fiber architecture - Design of a
fibrous preform or part in which the fibers are
arranged (braided, stitched, woven, etc.) in a
particular way to achieve the desired result.
Fiber content - Amount of fiber
present in a composite expressed either as a percent
by weight or percent by volume. Also sometimes
stated as a fiber volume fraction.
Fiber orientation - Direction of
fiber alignment in a nonwoven or mat laminate
wherein most of the fibers are placed in the same
direction to afford greater strength in that
Fiber placement - 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. Differs from filament
winding 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.
Fiber-reinforced plastics (FRP) -
General term used for a polymer-matrix composite
that is reinforced with cloth, mat, strands or any
other fiber form. Often used to designate mid-range,
glass-fiber reinforced composites.
Fiber volume fraction - See fiber
Filament - Polycrystalline or
amorphous individual fiber unit with a
length-to-diameter ratio greater than one. The
minimum diameter of a filament is not limited, but
the maximum diameter may not exceed 0.010 inches.
Filaments greater than about 0.002 inches in
diameter are often referred to as wires.
Filament count - Number of
filaments in the cross-section of a fiber bundle.
Filament winding - Process of
fabricating composites in which continuous
reinforcing fibers, either preimpregnated with resin
or drawn through a resin bath, are wound under
controlled tension around a rotating form to make a
structure. (See also winding, mandrel.)
Fill - Fiber bundles in a woven
fabric that run transverse to the warp yarns; also
known as weft or woof.
Filler - Solid constituent,
usually inert, added to the matrix to modify the
composite properties - such as increase viscosity,
improve appearance or lower density - or to lower
Filler ply - Additional patch to
fill in a depression in repair or to build up an
Film adhesive - Adhesive in the
form of a thin, dry resin film with or without a
carrier; commonly used for adhesion between laminate
Finish - Material applied to
textiles to improve the bond between the fiber and
matrix; applied after sizing is removed.
Finite element analysis - Process
of selecting the optimum combination of materials in
a composite based on computational modeling and
Flexural modulus - Ratio, within
the elastic limit, of the applied stress on a test
sample in flexure to the corresponding strain in the
outermost fibers of the sample.
Flexural strength - Strength of a
material in bending, usually expressed in force per
unit area, as the stress of a bent test sample at
the instant of failure.
Fracture - Rupture of the surface
of a laminate due to external or internal forces;
may or may not result in complete separation.
Fracture toughness - Measure of
the damage tolerance of a material containing
initial flaws or cracks.
FRP - Fiber-reinforced plastic.
Gel time - Period of time from
initial mixing of liquid reactants to the point when
gelation occurs as defined by a specific test
Glass transition - Reversible
change in an amorphous polymer between a viscous
condition and a hard, relatively brittle condition.
Glass-transition temperature (Tg)
- 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 on the test method.
Graphitization - Process of
pyrolysis at very high temperatures (up to 5,400°F)
that converts carbon to its crystalline allotropic
Graphite fibers - Carbon fiber
that has been graphitized by heating and stretching
at temperatures above 3,000°F.
Hand layup - Fabrication method in
which reinforcement layers, preimpregnated or coated
afterwards, are placed in a mold by hand, prior to
cure to the formed shape.
Hard tool - Tool made of metal or
any "hard" material that is generally
impervious to damage during normal use.
Hardener - Substance used to
promote or control curing action by participating in
and being consumed by the cure reaction. (See also
Heat - Term used colloquially to
indicate any temperature above ambient (room)
temperature to which a part or material is or will
Heat-distortion temperature (HDT)
- Temperature at which deflection occurs under
specified temperature and stated load.
Helical - Ply laid onto a mandrel
at an angle, often at a 45° angle.
High-performance composites -
Composites offering properties better than
conventional structural metals, typically on a
strength-to-weight or stiffness-to-weight basis.
Such composites use continuous, oriented fibers in
polymer, metal or ceramic matrices to achieve their
Honeycomb - Resin-impregnated
material, most commonly manufactured in hexagonal
cells, that serves as a core in sandwich structure.
May also be a metal or a polymer in rigid, open-cell
Hoop - Ply laid onto a mandrel at
a 90° angle.
Hoop stress - Circumferential
stress in a cylindrically shaped part as a result of
internal or external pressure.
Hot-bond repair - Repair made on a
hot-patch bonding machine to cure and monitor
curing. Typically includes heat and vacuum source.
Hybrid composite - Composite
containing at least two distinct types of matrix or
reinforcement. The matrix or reinforcement types can
be distinct because of their physical properties,
mechanical properties, material form and/or chemical
Impact strength - A material’s
ability to withstand shock loading as measured by
fracturing a specimen.
Impregnate - To saturate the voids
and interstices of a reinforcement with a resin.
Impregnated fabric - See prepreg.
Inclusion - Physical and
mechanical discontinuity occurring within a material
Integral heating - 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 - Surface between two
materials: in glass fibers, for instance, the area
at which the glass and sizing meet; in a laminate,
the area at which the reinforcement and laminating
Interlaminar - Existing or
occurring between two or more adjacent laminae in a
Interlaminar shear - Shearing
force that produces displacement between two laminae
along the plane of their interface.
Intralaminar - Existing or
occurring within a single lamina in a laminate.
Isotropic - Fiber directionality
with uniform properties in all directions,
independent of the direction of applied load.
Kevlar - Trademark of DuPont for
high-performance para-aramid fibers used as
Knit - Textile process that
interlocks, in a specific pattern, loops of yarn by
means of needles or wires.
Lamina - Subunit of a laminate
consisting of one or more adjacent plies of the same
material with identical orientation.
Lamina orientation - See ply
Laminate - Any fiber- or
fabric-reinforced composite consisting of laminae
with one or more orientations with respect to some
Laminate coordinate axes - Set of
coordinate axes, usually right-handed Cartesian,
used as a reference in describing the directional
properties and geometrical structure of the
laminate. Usually the x-axis and the y-axis lie in
the plane of the laminate and the x-axis is the
reference axis from which ply angle is measured. The
x-axis is often in the principal load direction of
the laminate and/or in the direction of the laminate
principal axis. (See also principal axis, off-axis
Layup - Process of placing layers
of reinforcing material placed in position in the
mold. The reinforcing materials placed in the mold.
Layup code - Designation system
for abbreviating the stacking sequence of laminated
Liquid-crystal polymers (LCP) -
High-performance melt-processible thermoplastics
that develop high orientation in the melt and after
molding, resulting in very high tensile strength and
Lot - See batch.
Mandrel - A form, fixture or male
mold used as the base for production of a part in
processes such as lay-up or filament winding.
Mat - An unwoven textile fabric
made of fibrous reinforcing material such as chopped
filaments (to produce chopped-strand mat) or swirled
filaments (to produce continuous-strand mat) with a
binder applied to maintain form. Available in
blankets of various widths, weights, thicknesses and
lengths. May be oriented.
Matrix - Material in which
reinforcing fiber of a composite is imbedded:
polymer, metal or ceramic.
Matrix content - Amount of matrix
present in a composite expressed either as a percent
by weight or percent by volume. For polymer-matrix
composites this is the resin content. (See also
Metal-matrix composites (MMC) -
Continuous carbon, silicon carbide, or ceramic
fibers embedded in a metallic matrix material.
Midplane - Plane that is
equidistant from both surfaces of the laminate.
Microcracking - Microscopic cracks
formed in composites when thermal stresses locally
exceed the strength of the matrix.
MMC - Metal-matrix composite.
Modulus - Measure of the ratio of
applied load (stress) to the resultant deformation
of a material. May be represented by a number or in
descriptive terms as low, intermediate, high or
ultrahigh. (See also stiffness, Young’s modulus.)
Moisture absorption - Pickup of
water vapor from the air by a material. Refers to
vapor withdrawn from the air only as distinguished
from water absorption, which is weight gain due to
the absorption of water by immersion.
Monomer - A single molecule that
reacts with like or unlike molecules to form a
Monofilament - Single continuous
filament strong enough to function as a fiber in
textile or other operations.
Multifilament - Yarn or tow
consisting of many continuous filaments.
NDE, NDI, NDT - Non-destructive
evaluation, non-destructive inspection,
Near-net shape - Part fabrication
resulting in final dimensions that require minimal
machining, cutting or other finishing.
Net shape - Part fabrication
resulting in final dimensions that do not require
machining or cutting.
Nomex - Trademark of DuPont for
moderate-performance meta-aramid material that is
often used in paper-form to make honeycomb core.
Nondestructive inspection (NDI) -
Determining material or part characteristics without
permanently altering the test object. Nondestructive
testing (NDT) and nondestructive evaluation (NDE)
are broadly considered synonymous with NDI.
Nonwoven roving - Reinforcement
composed of continuous rovings loosely gathered
Off-axis laminate - Laminate whose
principal axis is oriented at an angle theta other
than 0° or 90° with respect to a reference
direction, usually related to the principal load or
One-off - Fabrication process in
which a single part is fabricated.
One-part resin system - 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
Original equipment manufacturer
(OEM) - Companies that design and build products
bearing their name, such as Boeing 777 aircraft or
Prince tennis racquets.
Out time - Period of time in which
a prepreg remains handleable with properties intact
outside a specified storage environment (such as a
freezer, in the case of thermoset prepregs).
Outgassing - Release of solvents
and moisture from composite parts under the hard
vacuum of space.
PAN - Same as polyacrylonitrile.
Part consolidation - 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.
Peel ply - Layer of material
applied to a prepreg layup surface that is removed
from the cured laminate prior to bonding operations,
leaving a clean, resin-rich surface ready for
Peel strength - Strength of an
adhesive bond obtained by stress that is applied in
a "peeling" mode.
Phenolic resin - Thermosetting
resin produced by a condensation reaction of an
aromatic alcohol with an aldehyde (usually phenol
Pin holes - Small holes that
penetrate the surface of a cured part.
Pitch - Residual petroleum product
used as a precursor in the manufacture of certain
Planar winding - Filament winding
in which the filament path lays on a plane that
intersects the winding surface.
Plied yarn - Two or more yarns
collected together with or without twist.
Ply - Constituent single layer
used in fabricating or occurring within a composite
structure. Also, the number of single yarns twisted
together to form a plied yarn.
Ply orientation - Acute angle
(theta) - including 90° - between a reference
direction and the ply principal axis. The ply
orientation is positive if measured counterclockwise
from the reference direction and negative if
Ply schedule - Layup of individual
plies or layers to form a laminate. Plies may be
arranged in alternating fiber orientation to produce
multidirectional strength in a part.
Polar winding - 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) - Base
material in the manufacture of some carbon fibers.
Polyimide - Highly heat-resistant
Polymer - Large organic molecule
formed by combining many smaller molecules
(monomers) in a regular pattern.
Polymerization - Chemical reaction
that links monomers to form polymers.
Porosity - Presence of visible
voids within a solid material into which either air
or liquids may pass.
Postcure - Additional exposure to
elevated temperature, often occurring without
tooling or pressure, that improves mechanical
Pot life - Length of time in which
a catalyzed thermosetting resin retains sufficiently
low viscosity for processing.
Precure - Full or partial setting
of a resin or adhesive before the clamping operation
is complete or before pressure is applied.
Precursor - Material from which
carbon fiber is made by pyrolysis. Common precursors
are polyacrylonitrile (PAN), rayon and pitch.
Preform - Pre-shaped fibrous
reinforcement, normally without matrix, but often
containing a binder to facilitate manufacture;
formed by distribution of fibers to the approximate
contour and thickness of the finished part,
typically on a mandrel or mock-up.
Prepreg - Admixture of fibrous
reinforcement and polymeric matrix used to fabricate
composite materials in a form that can be stored for
later use. It may be sheet, tape, tow or fabric. For
thermosetting matrices the resin is usually
partially cured or otherwise brought to a controlled
viscosity, called B-stage. Additives such as
catalysts, inhibitors and flame retardants can be
added to obtain specific end-use properties and
improve processing, storage and handling
Primary structure - An aerospace
critical load-bearing structure; if damaged the air-
or spacecraft cannot fly.
Prime contractors - Referred to as
"primes"; companies that are awarded
government contracts and usually work with
subcontractors (or "subs") who provide
individual and specific components or systems
relevant to the contract. Primes often team on
contracts, sharing portions of the contract funding.
Principal axis - Laminate
coordinate axis that coincides with the direction of
maximum inplane Young’s modulus. Within a ply, for
a balanced weave fabric either warp or fill
direction may be chosen. (See also laminate
coordinate axes and x-axis.)
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
Puckers - Local areas on prepreg
where material has blistered and pulled away from
the separator film or release paper.
Pultrusion - Continuous process
for manufacturing composites in rods, tubes and
structural shapes having constant cross sections.
After the reinforcement is passed through the
resin-impregnation 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.
Puncture - Break in composite skin
in sandwich structure that may or may not go through
to the core material or completely through the part.
Quadraxial fabric - Fabric with
four non-interwoven layers - +45°, -45°, 0° and
90°, - which are bonded together, usually by
through-the-thickness stitching, to form a single
sheet of fabric. (See also biannual fabric, triaxial
Quasi-isotropic laminate - A
laminate approximating isotropy by orientation of
plies in several or more directions.
Ramping - Gradual programmed
increase/ decrease in temperature or pressure to
control cure or cooling of composite parts.
Rate tools - Tools designed to be
used repeatedly in a production setting to fabricate
many parts rather than a single prototype or small
number of demonstration parts.
Reinforcement - Key element added
to the matrix to provide the required properties
(primarily strength); ranges from short fibers
through complex textile forms.
Release agent - Used to prevent
cured matrix material from bonding to tooling;
usually sprayed or painted on mold.
Release film - Impermeable film
layer that does not bond to the composite during
Resin - Solid polymeric material,
often of high molecular weight, which exhibits a
tendency to flow when subjected to stress, usually
has a softening or melting range, and usually
fractures conchoidally. As composite matrices,
resins bind together reinforcement fibers.
Resin content - See matrix
Resin-rich - Filled with excess
resin and thus departing from a consistent
Resin-starved - Lacking sufficient
resin for fiber wetout.
Resin transfer molding (RTM) -
Molding process in which catalyzed resin is
transferred into an enclosed mold into which a
fibrous reinforcement has been placed. The mold
and/or resin may or may not be heated. RTM combines
relatively low tooling and equipment costs with the
ability to consolidate large structural parts.
Ribbon direction - On a honeycomb
core, the way the honeycomb can be separated. The
direction of one continuous ribbon.
Roving - Large filament-count tow.
RTM - See resin-transfer molding.
S-glass - Denotes "structural
glass" a magnesia/ alumina/silicate glass
reinforcement designed to provide very high tensile
strength. Used in high-performance composites.
Sandwich structure - Composite
composed of lightweight core material (usually
honeycomb or foam) to which two relatively thin,
dense, high-strength, functional or decorative skins
Scrim - Low-cost, woven
reinforcing fabric in an open mesh construction.
Sealant - Paste or liquid applied
to a joint that hardens in place to form a seal.
Secondary structure - Aerospace
structure that is not critical to flight safety.
Shear - Action or stress resulting
from applied forces that causes or tends to cause
two contiguous parts of a body to slide relative to
Shelf life - Length of time in
which a material can be stored and continue to meet
specification requirements, remaining suitable for
its intended use. (See also storage life.)
Silicon carbide fiber -
Reinforcing fiber with high strength and modulus;
density is equal to that of aluminum. May be formed
as wires by chemical vapor deposition onto a
carbon-filament core, or as filaments. Used in both
organic and metal-matrix composites.
Size - Material applied to
textiles to facilitate subsequent operations such as
weaving or braiding. Sizes may be used to bind
together and stiffen warp yarns during weaving
and/or to minimize abrasion and wear. Sizes are
usually removed and replaced with finish before
matrix application. Also called sizing.
Skin - Layer of relatively dense
material used on the surface of the core of a
Soft tool - Tool made of
composites or a similar "soft" material
that is vulnerable to damage during use, storage or
Solvent - Liquid used to dissolve
Spec - Specification 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
Specific gravity - Density (mass
per unit volume) of a material divided by that of
water at a standard temperature.
Stacking sequence - Arrangement of
ply orientations and material components in a
laminate specified with respect to some reference
Staple - Collection of short
filaments of spinnable length.
Stiffness - Measure of the
resistance of a material to deformation. The ratio
of applied stress to resulting strain for a
Storage life - Amount of time a
material can be stored and retain specific
properties. (See also shelf life.)
Strain - Deformation resulting
from applied stress. Measured as the change in
length per unit of length in a given direction, and
expressed in percentage or as inches per inch.
Strand - See tow.
Stress - Internal resistance to
change in size or shape, expressed in units of force
(load) per unit area.
Stress concentration -
Magnification of applied stress in the region of a
notch, void, hole or inclusion.
Stress crack - External or
internal cracks in a composite caused by tensile
stresses; cracking may be present internally,
externally or in combination.
Structural adhesive - Adhesive
used to transfer loads between adherents.
Structural bond - Bond joining
load-bearing components of an assembly.
Structural repair manual (SRM) -
Document prepared by an OEM that designates original
structural materials (both composite and metal) used
for a 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, including whether autoclave cure is
required. Updated periodically by OEMs based on
input from repair technicians.
Substrate - Material upon the
surface of which an adhesive-containing substance is
spread for any purpose, such as bonding or coating.
Symmetric laminate - Laminate in
which the stacking sequence for the plies located on
one side of the geometric midplane are the mirror
image of the stacking sequence on the other side of
Tack - Stickiness of an uncured
Tape - Thin unidirectional prepreg
in widths up to 12 inches for carbon fiber.
Tensile strength - Maximum tensile
stress sustained by a specimen before it fails in a
Tg - Glass-transition temperature.
Thermal conductivity - Ability to
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.
Thermoplastic - Class of plastics
that can be repeatedly softened by heating and
hardened by cooling through a temperature range
characteristic of the plastic, and that in the
softened state can be shaped by flow into articles
by molding or extrusion.
Thermoset - Class of plastics
that, when cured using heat, chemical or other
means, changes into a substantially infusible and
insoluble material. Once cured, a thermoset cannot
be returned to the uncured state.
Thixotropic - Substance that is
gel-like at rest, but fluid when agitated, and thus
can be applied easily but clings to a vertical
surface. Thixotropic substances have high static
shear strength and low dynamic shear strength at the
same time, and lose viscosity under stress.
Tool - The mold, either one- or
two-sided and either open or closed, in or upon
which composite material is placed in order to make
Tooling resins - Plastic resins,
chiefly epoxy and silicone, that are used as tooling
Toughness - Measure of the ability
of a material to absorb energy.
Tow - Continuous, ordered assembly
of essentially parallel, collimated filaments,
normally continuous filaments without twist. Same as
Tow size - Designation indicating
the number of filaments in a tow, usually a number
followed by K, indicating multiplication by 1,000
(for example, 12K tow has 12,000 filaments).
Triaxial fabric - Fabric with
three non-interwoven layers - +45°, - 45° and
either 0° or 90° - which are bonded together,
usually by through-the-thickness stitching, to form
a single sheet of fabric. (See also biaxial fabric,
Twist - Measure of the number of
turns per unit length that a fiber bundle makes
around its axis. "Z"-twist denotes a
right-handed twist, while "S"-twist
denotes a left-handed twist. "U" is often
used to represent no twist and "N" means
Unidirectional (UD) - Referring to
fibers that are oriented in the same direction, such
as unidirectional fabric, tape or laminate.
Vacuum-bag molding - Molding
technique wherein the part is cured inside a layer
of film from which entrapped air is removed by
Viscosity - Tendency of a material
to resist flow. Viscosity is measured in comparison
with water. The higher the number, the less flow.
Void - Any pockets of enclosed gas
or air within a composite.
Volatiles - Materials, such as
water and alcohol, in a sizing or resin formulation
that can be vaporized at room or slightly elevated
Warp - Fiber bundles in a woven
fabric that run parallel to the length of the loom,
lengthwise along the long-dimension of the fabric.
Warpage - Dimensional distortion
in a composite part.
Water absorption - Ratio of weight
of water absorbed by a material to the weight of dry
Water jet - High-pressure water
stream used for cutting polymer composite parts.
Weave - Fabric pattern formed from
interlacing yarns. In plain weave, warp and fill
fibers alternate to make both fabric faces
identical. A satin weave pattern is produced by a
warp tow over several fill tows and under one fill
tow (for example, eight-harness satin would have one
warp tow over seven fill tows and under the eighth).
Wet layup - Application of a resin
to a dry reinforcement in the mold.
Wet winding - Filament winding
wherein fiber strands are impregnated with resin
immediately before contact with the mandrel.
Wetout - Saturation with resin of
all voids between strands and filaments.
Wetting agent - Surface-active
agent that promotes wetting by decreasing the
cohesion within a liquid.
Wind angle - Measure in degrees
between the direction parallel to the filaments and
an established reference point.
Winding - Process in which
continuous material is applied under controlled
tension to a form in a predetermined geometric
relationship to make a structure. A matrix material
to bind the fibers together may be added before,
during or after winding. Filament winding is the
most common type.
Winding pattern - In filament
winding, recurring pattern of the filament path
after a certain number of mandrel revolutions.
Wire - Large diameter (greater
than about 2 mils) high-performance fiber, such as
boron or silicon carbide, usually made by chemical
vapor deposition onto a filamentary substrate.
Wire mesh - Fine wire screen used
to dissipate the electrical charge from lighting.
Woof - Same as fill.
Woven roving - Heavy, coarse
fabric produced by weaving continuous roving
Wrinkle - Imperfection in the
surface of a laminate that looks like a crease in
one of the outer layers. This occurs in vacuum-bag
molding when the bag is improperly placed.
X-axis - Usually, the axis in the
plane of the laminate used as 0° reference.
Typically, the y-axis is the axis in the plane of
the laminate perpendicular to the x-axis, and the
z-axis is the reference axis normal to the laminate
plane in the composite laminate. (See also laminate
coordinate axes, off-axis laminate and principal
Y-axis - See x-axis.
Yarn - Continuous, ordered
assembly of essentially parallel, collimated
filaments, usually with a twist.
Young's modulus - Ratio of normal
stress to the corresponding strain for tensile or
compressive stresses less than the proportional
limit of the material.
Z-axis - See x-axis.
Zero bleed - Laminate fabrication
procedure that does not allow loss of resin during
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