A1:
A carbon fiber is a long, thin strand of material about 0.0002-0.0004 in (0.005-0.010 mm) in diameter and composed mostly of carbon atoms. The carbon atoms are bonded together in microscopic crystals that are more or less aligned parallel to the long axis of the fiber. The crystal alignment makes the fiber incredibly strong for its size. Several thousand carbon fibers are twisted together to form a yarn, which may be used by itself or woven into a fabric. The yarn or fabric is combined with epoxy and wound or molded into shape to form various composite materials. Carbon fiber-reinforced composite materials are used to make aircraft and spacecraft parts, racing car bodies, golf club shafts, bicycle frames, fishing rods, automobile springs, sailboat masts, and many other components where light weight and high strength are needed.

Carbon fibers are classified by the tensile modulus of the fiber. Tensile modulus is a measure of how much pulling force a certain diameter fiber can exert without breaking. The English unit of measurement is pounds of force per square inch of cross-sectional area, or psi. Carbon fibers classified as "low modulus" have a tensile modulus below 34.8 million psi (240 million kPa). Other classifications, in ascending order of tensile modulus, include "standard modulus," "intermediate modulus," "high modulus," and "ultrahigh modulus." Ultrahigh modulus carbon fibers have a tensile modulus of 72.5 -145.0 million psi (500 million-1.0 billion kPa). As a comparison, steel has a tensile modulus of about 29 million psi (200 million kPa). Thus, the strongest carbon fibers are ten times stronger than steel and eight times that of aluminum, not to mention much lighter than both materials, 5 and 1.5 times, respectively. Additionally, their fatigue properties are superior to all known metallic structures, and they are one of the most corrosion-resistant materials available, when coupled with the proper resins.

Below are comparisons of the characteristics of Carbon Fiber, Glass Fiber, Aluminum, and Steel:
　　
　　　　

Q2: How it is Made?

A2:
The raw material used to make carbon fiber is called the precursor. About 90% of the carbon fibers produced are made from polyacrylonitrile (PAN). The remaining 10% are made from rayon or petroleum pitch. All of these materials are organic polymers, characterized by long strings of molecules bound together by carbon atoms. The exact composition of each precursor varies from one company to another and is generally considered a trade secret.

During the manufacturing process, a variety of gases and liquids are used. Some of these materials are designed to react with the fiber to achieve a specific effect. Other materials are designed not to react or to prevent certain reactions with the fiber. As with the precursors, the exact compositions of many of these process materials are considered trade secrets.

The process for making carbon fibers is part chemical and part mechanical. The precursor is drawn into long strands or fibers and then heated to a very high temperature with-out allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms in the fiber to vibrate violently until most of the non-carbon atoms are expelled. This process is called carbonization and leaves a fiber composed of long, tightly inter-locked chains of carbon atoms with only a few non-carbon atoms remaining.

Here is a typical sequence of operations used to form carbon fibers from polyacrylonitrile (PAN):

Spinning
Acrylonitrile plastic powder is mixed with another plastic, like methyl acrylate or methyl methacrylate, and is reacted with a catalyst in a conventional suspension or solution polymerization process to form a polyacrylonitrile plastic.
The plastic is then spun into fibers using one of several different methods. In some methods, the plastic is mixed with certain chemicals and pumped through tiny jets into a chemical bath or quench chamber where the plastic coagulates and solidifies into fibers. This is similar to the process used to form polyacrylic textile fibers. In other methods, the plastic mixture is heated and pumped through tiny jets into a chamber where the solvents evaporate leaving a solid fiber. The spinning step is important because the internal atomic structure of the fiber is formed during this process.
The fibers are then washed and stretched to the desired fiber diameter. The stretching helps align the molecules within the fiber and provide the basis for the formation of the tightly bonded carbon crystals after carbonization.

Stabilizing
Before the fibers are carbonized, they need to be chemically altered to convert their linear atomic bonding to a more thermally stable ladder bonding. This is accomplished by heating the fibers in air to about 390-590° F (200-300° C) for 30-120 minutes. This causes the fibers to pick up oxygen molecules from the air and rearrange their atomic bonding pattern. The stabilizing chemical reactions are complex and involve several steps, some of which occur simultaneously. They also generate their own heat, which must be controlled to avoid overheating the fibers. Commercially, the stabilization process uses a variety of equipment and techniques. In some processes, the fibers are drawn through a series of heated chambers. In others, the fibers pass over hot rollers and through beds of loose materials held in suspension by a flow of hot air. Some processes use heated air mixed with certain gases that chemically accelerate the stabilization.

Carbonizing
Once the fibers are stabilized, they are heated to a temperature of about 1,830-5,500° F (1,000-3,000° C) for several minutes in a furnace filled with a gas mixture that does not contain oxygen. The lack of oxygen prevents the fibers from burning in the very high temperatures. The gas pressure inside the furnace is kept higher than the outside air pressure and the points where the fibers enter and exit the furnace are sealed to keep oxygen from entering. As the fibers are heated, they begin to lose their non-carbon atoms, plus a few carbon atoms, in the form of various gases including water vapor, ammonia, carbon monoxide, carbon dioxide, hydrogen, nitrogen, and others. As the non-carbon atoms are expelled, the remaining carbon atoms form tightly bonded carbon crystals that are aligned more or less parallel to the long axis of the fiber. In some processes, two furnaces operating at two different temperatures are used to better control the rate of heating during carbonization.
Treating the surfaceAfter carbonizing, the fibers have a surface that does not bond well with the epoxies and other materials used in composite materials. To give the fibers better bonding properties, their surface is slightly oxidized. The addition of oxygen atoms to the surface provides better chemical bonding properties and also etches and roughens the surface for better mechanical bonding properties. Oxidation can be achieved by immersing the fibers in various gases such as air, carbon dioxide, or ozone; or in various liquids such as sodium hypochlorite or nitric acid. The fibers can also be coated electrolytically by making the fibers the positive terminal in a bath filled with various electrically conductive materials. The surface treatment process must be carefully controlled to avoid forming tiny surface defects, such as pits, which could cause fiber failure.

Sizing
After the surface treatment, the fibers are coated to protect them from damage during winding or weaving. This process is called sizing. Coating materials are chosen to be compatible with the adhesive used to form composite materials. Typical coating materials include epoxy, polyester, nylon, urethane, and others.
The coated fibers are wound onto cylinders called bobbins. The bobbins are loaded into a spinning machine and the fibers are twisted into yarns of various sizes.

Q3: Why Carbon Fiber is light and strong?

A3:
Carbon Fiber is composed of carbon atoms by more than 90%. These carbon atoms are lighter than any metal atoms. Beautifully shining diamonds, Bincho-Charcoal, activated charcoal and graphite belong to the group of materials composed of carbon atoms, and in fact Carbon Fiber also belongs to this group and has similar properties to diamond. Diamond is the hardest and strongest material in the world, whereas it is known that graphite is soft and slippery. The reason for this difference is said to come from difference in crystal structure. Bincho-Charcoal and the like are soft and slippery because of their complicated irregular structure, whereas diamonds are very strong because of regular and orderly structure. Diamonds have 3D structure of carbon atoms, and Carbon Fibers have 2D mesh structure (like meshes of henhouses) with carbon atoms orderly lined up to fiber direction and entwine together. Furthermore, we can say that they are strong as they are manufactured very carefully lest they should have foreign materials or defects with them.

Q4: Please explain about difference between Carbon Fiber and other carbon materials (Charcoal,Graphite).

A4:
Both Carbon Fiber and Charcoal belong to carbon material group. Both have crystal structure of graphite structure but are quite different in regularity and way of line up from each other. In case of Carbon Fiber, carbon atoms line up regularly and orderly, form mesh structure with layers piling up and entwining each other, whereas in case of Charcoal, this layer structure is amorphous and irregular without strong entwining each other, resulting in fragility and low strength.
Graphite belongs to the same group having advanced regular graphite structure. It is known that graphite is soft and slippery. Natural graphite is produced in China, India and other countries.

　 The illustration shows a descriptive picture how the molecular structure of a single thread of carbon fiber is being generated step by step under respective processing conditions of carbonization and graphitization procedures. The picture gives you a clear idea in what manner carbon atoms become regularly lined up along the longitudinal direction of a filament by means of high temperature treatments and stretching.
(Source: A. R. Bunsell, Fibre Reinforcements for Composite Materials, Amsterdam, The Netherlands: Elsevier Science Publishers B.V., 1988, p. 120.)
　

Q5: Please explain about properties and application of PAN Type Carbon Fiber and Pitch Type Carbon Fiber.

A5:
PAN Type Carbon Fiber is an aggregation of continuous fiber (filaments), 5 – 7 micron meter in diameter with 1.74 - 1.95 g/cm3 of density, generally. Products with various filaments, such as 1K (1000 filaments), 3K (3000 filaments), 6K (6000 filaments), 12K (12000 filaments) and 24K (24000 filaments), referred to as “Regular Tow” or “Small Tow”, have been used in large quantity for aircrafts and sports/recreational fields, making good use of low density, high specific tensile strength and high specific elastic modulus. PAN fibers have been undertaken a role for market expansion of carbon fibers. Large Tow, extra-40K, even though slightly lower tensile strength is mainly used for industrial fields as a relatively inexpensive material, along with Regular Tow. PAN type carbon fibers are classified into Standard Elastic Modulus Type (- 240 GPa), Intermediate Elastic Modulus Type (- 300 GPa) and High Elastic Modulus Type (350 GPa -).

Regarding Pitch Type Carbon Fiber, there are continuous type and discontinuous type, based on respective spinning process. Pitch Type Carbon Fiber is also classified into Isotropic Type (hardly-graphitizability) and Un-isotropic Type (easy-graphitizability), based on respective raw pitch. Isotropic Pitch Type Carbon Fiber is commonly a discontinuous fiber of 12 - 18 micron meter in diameter with 1.6 g/cm3 of density and has the properties of low modulus (- 40 GPa), strength and thermal conductivity due to its weak structural orientation of carbon atoms and underdeveloped graphite crystallinity. With its competitive cost, Isotropic Pitch Type Carbon Fiber is extensively applied for industrial fields due to light weight, chemical stability, heat resistance and abrasion characteristic.

On the other hand, Un-isotropic Pitch Type Carbon Fiber is sometimes referred to as “Mesophase Pitch Type Carbon Fiber”. This fiber is commonly a continuous fiber (filaments) of 7 - 10 micron meter in diameter with 1.7 - 2.2 g/cm3 of density. There are 1K, 2K, 3K, 6K and 12K as filaments per tow and wide variety of elastic modulus grades from 6 GPa (the lowest) to 953 GPa (the highest), whereas PAN Type Carbon Fiber cannot achieve this range. High Elastic Modulus Type Carbon Fiber (350 GPa -) has excellent processability due to high tensile strength, more than 2.5 GPa, and has been extensively applied for industrial and sports/recreation fields utilizing higher stiffness than iron and light weight (50% or less than iron) as molded composite materials. Applications for Ultra High Elastic Modulus Type Carbon Fiber (600 GPa -) are expanding, utilizing excellent stiffness, equivalent or higher thermal conductivity to metals and lightness in weight.
Please refer to "The Future of Carbon Fiber ".

Q6: The History of Carbon Fibe?

A6:
The History of Carbon FiberDuring the 1970s, experimental work to find alternative raw materials led to the introduction of carbon fibers made from a petroleum pitch derived from oil processing. These fibers contained about 85% carbon and had excellent flexural strength. Unfortunately, they had only limited compression strength and were not widely accepted.

Today, carbon fibers are an important part of many products, and new applications are being developed every year. The United States, Japan, and Western Europe are the leading producers of carbon fibers.

Below is a chart depicting the progress of carbon fibers:

Q7: The Future of Carbon Fiber?

A7:
The future of Carbon Fiber is very bright, with vast potential in many different industries. Among them are:

Alternate Energy– Wind turbines, compressed natural gas storage and transportation, fuel cells
Fuel Efficient Automobiles– Currently used in small production, high performance automobiles, but moving toward large production series cars
Construction and Infrastructure– Light weight pre-cast concrete, earth quake protection
Oil Exploration– Deep Sea drilling platforms, buoyancy, umbilical, choke, and kill lines, drill pipes

In order to fully develop carbon fibers in these industries and others, carbon fiber manufacturers need to continue to increase their capacity and change their mindset so that they are committed to the commercialization concept. The ideal conditions that would allow the carbon fiber industry to reach its vast potential are if carbon manufacturers:

　Target new applications
　Develop new and lower cost technology
　Reinvest profits with long term objectives in mind – no small operators focusing on low volume, high price
　Fully understand supplier’s costs and future strategy
　Identify and focus on market driver’s
　Work to aggressively reduce costs
　Consolidate so that weaker players help strengthen the stronger ones
　Share incremental improvements to help support market growth
　Understand that the primary competitors to carbon fibers are other materials, not other carbon fiber manufacturers.

If carbon fiber manufacturers can stay focused and execute, the rewards will be enormous. Below are the future growth trends for the carbon fiber industry:

Terminology

A
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 - A chemical additive that hastens
　　　　　　cure or chemical reaction (see also catalyst).
　　　　　　Addition - Polymerization reaction in which no
　　　　　　byproducts are formed.
　　　　　　Additive - An ingredient mixed into resin to
　　　　　　improve properties (e.g., plasticizers, initiators, light
　　　　　　stabilizers and flame retardants).
　　　　　　Adhesive - Substance applied to mating surfaces
　　　　　　to bond them together by surface attachment.
　　　　　　Adhesive film - A thin plastic film onto which
　　　　　　premixed adhesives are cast.
　　　　　　Aliphatic - Designates a large class of organic
　　　　　　compounds having open-chain structures, (e.g., isopropyl
　　　　　　alcohol).
　　　　　　Amorphous - Describes polymers with no
　　　　　　crystalline component.
　　　　　　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 trademark.)
　　　　　　Areal weight - Weight of a fiber reinforcement
　　　　　　per unit area (width times length) of tape or fabric.
　　　　　　Aspect ratio - Ratio of the length to the
　　　　　　diameter of a fiber.
　　　　　　Autoclave - Closed vessel for applying fluid
　　　　　　pressure, with or without heat, to an enclosed object (see
　　　　　　consolidation).
　　　　　　Autoclave molding - Molding technique in which
　　　　　　an entire assembly (layup and tooling) is placed into an autoclave
　　　　　　and subjected to heat and elevated pressure for consolidation and/or
　　　　　　curing while removing entrapped air and volatiles.
　　　　　　Automated tape laying - Fabrication process in
　　　　　　which a structure is formed by laying prepreg material, typically
　　　　　　unidirectional tape, across the surface of a mold in multiple layers
　　　　　　and directions by using an automated tape-application machine.
　　　　　　Axial winding - Filament winding wherein the
　　　　　　filaments are parallel to or at a small angle to the axis of
　　　　　　rotation (x-axis).
B
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 whose edges are sealed, followed
　　　　　　by consolidation and/or curing with pressure applied by vacuum,
　　　　　　autoclave, press or inflation of the bag.
　　　　　　Balanced design - In filament winding, a winding
　　　　　　pattern designed so that the stresses in all filaments are equal.
　　　　　　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
　　　　　　orientation (e.g., a laminate with a principle axis of 0° combined
　　　　　　with an equal number of plies having -45° and +45° orientations.
　　　　　　Barcol hardness - A surface hardness value
　　　　　　obtained by measuring the penetration resistance of a given material
　　　　　　to a sharp steel point under a spring load. The Barcol Impressor is
　　　　　　an instrument that measures hardness on a 0-100 scale.
　　　　　　Basket weave - Woven reinforcement wherein two
　　　　　　or more warp threads go over and under two or more filling threads
　　　　　　in a repeating pattern; less stable than plain weave but produces a
　　　　　　flatter, stronger, more pliable fabric (see plain weave).
　　　　　　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, quadraxial 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.
　　　　　　Binder - The agent applied to glass mat or
　　　　　　preforms to bond the fibers prior to laminating or molding.
　　　　　　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 intended to become 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.
　　　　　　BMC - See Bulk molding compound.
　　　　　　BMI - See Bismaleimide.
　　　　　　Bond ply - Ply or fabric patch that comes in
　　　　　　contact with the honeycomb core during repair.
　　　　　　Bond strength - The adhesion between bonded
　　　　　　surfaces. As measured by load/bond area, the stress required to
　　　　　　separate a layer of material from another material to which it is
　　　　　　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 production.
　　　　　　Bridging - Fabric plies over a curved edge that
　　　　　　do not come in full contact with the core material. Also, excess
　　　　　　resin that has formed on edges during cure.
　　　　　　Broadgoods - General term for fibers woven into
　　　　　　fabrics that may or may not be impregnated with resin; usually
　　　　　　furnished in rolls.
　　　　　　Bromine - A fire retardant (halogen) used to
　　　　　　reduce or eliminate a resin"s tendency to burn.
　　　　　　Buckling - Failure mode usually characterized by
　　　　　　unstable lateral deflection, rather than breakage, under compressive
　　　　　　force.
　　　　　　Bundle - General term for a collection of
　　　　　　essentially parallel filaments.
　　　　　　Bulk molding compound (BMC) - A premixed blend
　　　　　　of thermosetting resin, reinforcements, catalysts and fillers for
　　　　　　use in compression-, transfer- or injection-molding processes.
C
C-stage- The final step in the curing process for thermoset resins, resulting in irreversible hardening and insolubility. (See also A-stage, B-stage.)
　　　　　　CAD/CAM - Computer-aided design/computer-aided
　　　　　　manufacturing.
　　　　　　Carbon fiber - Reinforcing fiber produced by the
　　　　　　pyrolysis of an organic precursor fiber, such as PAN
　　　　　　(polyacrylonitrile), rayon or pitch, in an inert atmosphere at
　　　　　　temperatures above 982°C/1800°F. The term carbon is often used
　　　　　　interchangeably with the term graphite, but the fibers differ.
　　　　　　Carbon fibers are typically carbonized at about 1315°C/2400°F and
　　　　　　contain 93 percent to 95 percent carbon. Carbon fibers can be
　　　　　　converted to graphite fibers by graphitization at 1900°C to 2480°C
　　　　　　(3450°F to 4500°F), after which they contain more than 99 percent
　　　　　　elemental carbon. Carbon fibers are known for their light weight,
　　　　　　high strength and high stiffness.
　　　　　　CFRP - Carbon Fiber-Reinforced Polymer.
　　　　　　Carbon/carbon - Composite of carbon fiber in a
　　　　　　carbon matrix.
　　　　　　Cast polymer - A nonreinforced composite (resin
　　　　　　used without reinforcing fibers) that combines polymers, fillers and
　　　　　　additives as composites to meet specific application
　　　　　　requirements.
　　　　　　Catalyst - Substance that promotes or controls
　　　　　　curing of a compound without being consumed in the reaction. (See
　　　　　　also hardener.)
　　　　　　Catalyzed resin - A resin mixture possibly still
　　　　　　in the workable state, after it has been mixed with catalyst or
　　　　　　hardener.
　　　　　　Catenary - Uniformity of strand length in a
　　　　　　specified length of roving stretched under tension. Poor catenary
　　　　　　means some strands in the roving length are longer than others.
　　　　　　Caul plate - Plate or sheet the same size and
　　　　　　shape as the composite layup with which it will be used. The caul
　　　　　　plate is placed in immediate contact with the layup during curing to
　　　　　　transmit normal pressure and provide a smooth surface on the
　　　　　　finished part.
　　　　　　Centipoise (cps) - A unit of measure used to
　　　　　　designate a fluid"s viscosity (at 21°C/70°F, water is 1 cps; peanut
　　　　　　butter is 250,000 cps).
　　　　　　Centrifugal casting - A processing technique for
　　　　　　fabricating cylindrical structures, in which the composite material
　　　　　　is positioned inside a hollow mandrel designed to be heated and
　　　　　　rotated as resin is cured.
　　　　　　Ceramic-matrix composites (CMC) - Materials
　　　　　　consisting of a ceramic or carbon fiber surrounded by a ceramic
　　　　　　matrix, primarily silicon carbide.
　　　　　　Charge pattern - The ply schedule used in parts
　　　　　　made from sheet molding compound (SMC); a pre-weighed number of SMC
　　　　　　plies cut from an SMC sheet and oriented in such a way that the
　　　　　　material will fill the mold cavity when placed in the mold and
　　　　　　compressed.
　　　　　　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.
　　　　　　Chopped strand - Continuous roving that is
　　　　　　chopped into short lengths for use in mats, spray up or molding
　　　　　　compounds.
　　　　　　Circumferential winding - Process of winding
　　　　　　fiber perpendicular to the axis during filament winding.
　　　　　　Cloth - See Fabric.
　　　　　　CMC - See Ceramic-matrix composite.
　　　　　　Cocured - Cured and simultaneously bonded to
　　　　　　another prepared surface.
　　　　　　Coefficient of expansion (COE) - A measure of
　　　　　　the change in length or volume of an object.
　　　　　　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 require repair.
　　　　　　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 yarn).
　　　　　　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 acting alone.
　　　　　　Compression molding - Technique for molding
　　　　　　thermoset composites in which the part is shaped and cured in the
　　　　　　same step. Layered reinforcing fibers and resin paste (typically
　　　　　　precombined in a leather-like, preimpregnated sheet) are placed into
　　　　　　an open two-part mold cavity. The mold is closed and, with the
　　　　　　application of both heat and pressure, the resin viscosity drops,
　　　　　　the material is forcibly distributed throughout the mold cavity to
　　　　　　take its final shape and the part is allowed to cure.
　　　　　　Compressive strength - Resistance to a crushing
　　　　　　or buckling force; the maximum compressive load a specimen sustains
　　　　　　divided by its original cross-sectional area.
　　　　　　Condensation - A polymerization reaction in
　　　　　　which simple byproducts (e.g., water) are released.
　　　　　　Consolidation - A processing step in which a
　　　　　　fiber and matrix are compressed to reduce voids and achieve a
　　　　　　particular density.
　　　　　　Contaminant - An impurity or foreign substance
　　　　　　that affects one or more properties of composite materials,
　　　　　　particularly adhesion.
　　　　　　Continuous filament - An individual, flexible,
　　　　　　small-diameter fiber of indefinite 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 coordinate axes.
　　　　　　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 (also see
　　　　　　honeycomb).
　　　　　　Core crush - Compression damage of the core.
　　　　　　Core depression - A 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 transverse direction.
　　　　　　Core separation - A breaking of honeycomb core cells.
　　　　　　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 (e.g.,
　　　　　　thermoplastic fibers woven side by side with carbon fibers).
　　　　　　Crazing - Region of ultrafine cracks that may
　　　　　　develop on or under a resin surface.
　　　　　　Creel - A device for holding the required number
　　　　　　of roving spools or other supply packages of reinforcement in the
　　　　　　desired unwinding position.
　　　　　　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 laminate axis. (See cross-ply laminate and
　　　　　　fiber architecture.)
　　　　　　Crosslinking - Polymerization reactions that
　　　　　　branch out from the main molecular chain to form a networked pattern
　　　　　　of chemical links.
　　　　　　Cross-ply laminate - A laminate having plies
　　　　　　oriented only at 0° and 90°. May or may not be symmetrical.
　　　　　　Crystalline - Having a molecular structure in
　　　　　　which the atoms are arranged in an orderly, three-dimensional
　　　　　　pattern.
　　　　　　CTE - See Coefficient of thermal
　　　　　　expansion.
　　　　　　Cure - Irreversible alteration of the molecular
　　　　　　structure and physical properties of a thermosetting resin by
　　　　　　chemical reaction, typically stimulated by heat and/or the presence
　　　　　　of catalysts, with or without applied pressure. However, see
　　　　　　ultraviolet (UV) cure.
　　　　　　Cure temperature - The temperature at which a
　　　　　　material attains final cure.
　　　　　　Curing agent - Catalytic or reactive agent that
　　　　　　brings about polymerization when added to a resin (also see
　　　　　　accelerator, catalyst and hardener).
　　　　　　CVD - See Chemical vapor deposition.
D
Damage tolerance - A measure of a structure"s ability 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 occur locally or involve a large
　　　　　　area. Also includes the separation of layers of fabric from the core
　　　　　　structure.
　　　　　　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 the denier.
　　　　　　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 performance with a
　　　　　　specific level of statistical confidence.
　　　　　　Dielectric - Electrically nonconductive; the
　　　　　　ability of a material to resist the flow of an electric current.
　　　　　　Dielectric strength - The voltage required to
　　　　　　penetrate insulating material. Material with high dielectric
　　　　　　strength offers excellent electrical insulating properties.
　　　　　　Disbond - Undesirable separation of bonded
　　　　　　surfaces at the bond interface, due to adhesive or cohesive failure,
　　　　　　occurring at any time during the life of the bonded structure and
　　　　　　arising from any of a wide variety of causes. The term is also
　　　　　　sometimes used to describe delamination. (Also see debond.)
　　　　　　Doubler - An extra layer of reinforcement,
　　　　　　applied to increase stiffness or strength in portions of a laminate
　　　　　　expected to incur abrupt load transfers.
　　　　　　Draft - The degree of taper designed into the
　　　　　　sides of a mold so the part can be removed.
　　　　　　Draft angle - A mandrel"s taper or angle for
　　　　　　ease of part removal.
　　　　　　Drape - The ability of fabric or prepreg to
　　　　　　conform to a contoured surface.
　　　　　　Dry winding - In filament-winding, when fiber
　　　　　　reinforcement is applied to the mandrel without first being wet out
　　　　　　with resin.
E
E-glass- Abbreviation for "electrical glass," borosilicate glass fibers, which have high electrical resistivity. Most often used in conventional polymer matrix composites.
　　　　　　Elastic limit - The greatest stress a material
　　　　　　is capable of sustaining without permanent strain remaining after
　　　　　　complete release of the stress (see stress and strain).
　　　　　　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 - A general term for a single strand of
　　　　　　roving, which is a continuous, ordered assembly of essentially
　　　　　　parallel, collimated filaments, with or without twist.
　　　　　　End count - The exact number of strands
　　　　　　contained in a particular roving.
　　　　　　Engineering plastics - A general term covering
　　　　　　all plastics, with or without fillers or reinforcements, that have
　　　　　　mechanical, chemical and thermal properties suited for use as
　　　　　　construction materials or in components for machines and chemical
　　　　　　processing equipment.
　　　　　　Epoxy - A thermosetting polymer containing one
　　　　　　or more epoxide or oxirane groups, curable by reaction with amines
　　　　　　or alcohols; used as a resin matrix in reinforced plastic products
　　　　　　and as the primary component in certain structural adhesives. Cured
　　　　　　epoxy resin is highly resistant to chemicals and water and its
　　　　　　performance properties are relatively unaffected by extreme
　　　　　　temperatures.
　　　　　　Exotherm - Heat released during a chemical
　　　　　　reaction. Uncontrolled exotherm during cure of a composite component
　　　　　　can lead to heat build up, which can result in part warpage and/or
　　　　　　mold damage and, in extreme cases, could produce an explosion.
F
Fabric- Planar textile. Also known as cloth.
　　　　　　Fabric, nonwoven - Planar textile constructed by
　　　　　　bonding or interlocking but not interlacing fibers or yarns by
　　　　　　mechanical, chemical, thermal or solvent means.
　　　　　　Fabric, woven - Planar textile constructed by
　　　　　　interlacing fibers or yarns, using 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 - See Finite-element analysis.
　　　　　　Fiber - One or more filaments in an ordered
　　　　　　assemblage.
　　　　　　Fiber architecture - The design of a fibrous
　　　　　　preform or part in which the fibers are arranged in a particular way
　　　　　　to achieve a desired result. Mats and braided, stitched and woven
　　　　　　fabrics are common forms of fiber architecture.
　　　　　　Fiber bridging - Reinforcing fiber material
　　　　　　bridging an inside radius of a pultruded product. The condition is
　　　　　　caused by shrinkage stresses around such a radius during cure.
　　　　　　Fiber content - The 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 or
　　　　　　expressed in ratio to the matrix content (e.g., a 60:40
　　　　　　fiber-to-resin ratio denotes a composite with 60 percent fiber
　　　　　　content and 40 percent resin content).
　　　　　　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
　　　　　　direction.
　　　　　　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 can produce shapes with greater
　　　　　　complexity and permits a faster putdown rate than filament
　　　　　　winding.
　　　　　　Fiber-reinforced polymer (FRP) - General term
　　　　　　for a polymer-matrix composite that is reinforced with cloth, mat,
　　　　　　strands or any other fiber form. However, in practice, the term is
　　　　　　most often used in reference to glass fiber-reinforced polymer.
　　　　　　Fiber wash - Dislocation or displacement of
　　　　　　reinforcing fibers placed within a mold caused by the force of the
　　　　　　resin flow, resulting in unintended fiber distribution within the
　　　　　　finished part.
　　　　　　Fiber volume fraction - See Fiber
　　　　　　content.
　　　　　　Fiberglass - Reinforcing fiber made by drawing
　　　　　　molten glass through bushings. The predominant reinforcement used
　　　　　　with polymer matrix composites, it is known for its good strength,
　　　　　　processability and low cost.
　　　　　　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 - An automated process 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 composite structure. (Also see
　　　　　　winding and mandrel.)
　　　　　　Fill - The fiber bundles in a woven fabric that
　　　　　　run transverse (at a 90° angle) to the warp yarns; also known as
　　　　　　weft or woof.
　　　　　　Filler - A solid constituent, usually inert,
　　　　　　added to a matrix to modify a composite"s properties (e.g., increase
　　　　　　viscosity, improve appearance or de-crease density) or to decrease
　　　　　　part material cost.
　　　　　　Filler ply - An additional patch used to fill in
　　　　　　a depression in a repair or build up an edge.
　　　　　　Film adhesive - Adhesive in the form of a thin,
　　　　　　dry resin film, with or without a carrier; commonly used for
　　　　　　adhesion between laminate layers.
　　　　　　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
　　　　　　computer-based computational modeling and analysis.
　　　　　　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 - A rupture in the surface of a
　　　　　　laminate due to external or internal forces; may or may not result
　　　　　　in complete separation.
　　　　　　Fracture toughness - A measure of the damage
　　　　　　tolerance of a material containing initial flaws or cracks.
　　　　　　FRP - See Fiber-reinforced polymer.
G
Gel- To enter an initial jelly-like, semi-solid phase during a resin curing process.
　　　　　　Gel coat - An unreinforced, clear or pigmented
　　　　　　coating resin applied to the surface of a mold or part to provide a
　　　　　　smooth, more impervious finish on the part exterior.
　　　　　　Gel time - The period of time from initial
　　　　　　mixing of liquid reactants in a resin to the point when gelation
　　　　　　occurs as defined by a specific test method.
　　　　　　Glass fiber - see Fiberglass.
　　　　　　Glass transition - A 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 2982°C/5400°F) that converts carbon to its
　　　　　　crystalline allotropic form.
　　　　　　Graphite fibers - Carbon fibers that have been
　　　　　　graphitized by heating and stretching at temperatures above
　　　　　　1649°C/3000°F.
H
Halogenated resin - A resin combined with chlorine or bromine to increase fire retardancy.
　　　　　　Hand layup - A fabrication method in which
　　　　　　reinforcement layers, preimpregnated or coated afterwards, are
　　　　　　placed and arranged in a mold manually. (In contrast to sprayup or
　　　　　　automated methods, such as fiber placement.)
　　　　　　HAPs - see Hazardous air pollutants.
　　　　　　
　　　　　　Hard tool - A tool made of metal or any "hard"
　　　　　　material that is generally impervious to process-related damage
　　　　　　(e.g., exothermic distortion) during normal molding operations (in
　　　　　　contrast to soft tool). See tool.
　　　　　　Hardener - A substance that may be added to a
　　　　　　resin to promote and/or control the curing process by participating
　　　　　　in and being consumed by the cure reaction. (Also see accelerator,
　　　　　　curing agent and catalyst.)
　　　　　　Hazardous air pollutants (HAPS) - Potentially
　　　　　　airborne compounds determined to be hazardous to human health by the
　　　　　　U.S. Environmental Protection Agency (EPA).
　　　　　　Heat - The term used colloquially to indicate
　　　　　　any temperature above ambient (room) temperature to which a part or
　　　　　　material is or will be subjected.
　　　　　　Heat-deflection temperature (HDT) - The
　　　　　　temperature at which a standard plastic test bar deflects a
　　　　　　specified distance under a stated load.
　　　　　　Helical - Describes ply laid onto a rotating
　　　　　　mandrel at an angle, often at a 45° angle.
　　　　　　Helix angle - The angle at which continuous
　　　　　　filaments are wound in relation to the longitudinal mandrel axis in
　　　　　　the filament-winding process.
　　　　　　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 superior properties.
　　　　　　Honeycomb - A lightweight cellular structure
　　　　　　(typically hexagonal nested cells) used as core in composite
　　　　　　sandwich structures. May be made from either metallic (e.g.,
　　　　　　aluminum) or nonmetallic (e.g., resin-impregnated paper or woven
　　　　　　fabric) sheet materials. Rectangular sheets are adhesively bonded
　　　　　　together in stacks, by means of parallel stripes of adhesive placed
　　　　　　at regular intervals along one axis. Stacks are sliced across the
　　　　　　transverse axis, and each sliced stack is expanded to form a
　　　　　　honeycomb grid.
　　　　　　Hoop - Describes ply layed onto a rotating
　　　　　　mandrel at a 90° angle to the long axis of the mandrel.
　　　　　　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 distinguished by their physical
　　　　　　properties, mechanical properties, material form and/or chemical
　　　　　　composition.
　　　　　　Hygroscopy - A material"s readiness to absorb or
　　　　　　retain moisture.
I
Impact strength - A material"s ability to withstand shock loading as measured during a test in which a specimen is fractured.
　　　　　　Impregnate - To saturate the voids and
　　　　　　interstices of a reinforcement with resin.
　　　　　　Impregnated fabric - See Prepreg.
　　　　　　Inclusion - Physical and mechanical
　　　　　　discontinuity occurring within a material or part.
　　　　　　In situ - In the original position; in filament
　　　　　　winding, designates a mandrel that remains in place after winding,
　　　　　　as opposed to a mandrel that is removed after winding. In pipe
　　　　　　repair, a type of repair that does not require pipe excavation;
　　　　　　rather a composite sleeve is inserted into the existing pipe through
　　　　　　a manhole.
　　　　　　Inhibitor - A chemical additive that slows or
　　　　　　delays a cure cycle.
　　　　　　Injection molding - A method of forming a
　　　　　　plastic to the desired shape by forcibly injecting the polymer into
　　　　　　a mold.
　　　　　　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 - The plane formed when two material
　　　　　　surfaces make contact: 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 resin meet.
　　　　　　Interlaminar - Existing or occurring between two
　　　　　　or more adjacent laminae in a laminate.
　　　　　　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.
　　　　　　Intumescent - Capable of swelling or enlarging.
　　　　　　In reference to fire-retardants, describes a layer or coating of
　　　　　　material designed to swells or thicken in order to form a more
　　　　　　effective barrier to heat and/or flame when exposed to either.
　　　　　　Isotropic - Fiber directionality with uniform
　　　　　　properties in all directions, independent of the direction of
　　　　　　applied load.
　　　　　　Isotropic laminate - A laminate in which the
　　　　　　strength properties are equal in all directions, such as
　　　　　　contact-molded laminates or metals.
K
Kevlar- Trademark of DuPont for high-performance para-aramid fibers used as reinforcements (see aramid).
　　　　　　Knit - Textile process that interlocks, in a
　　　　　　specific pattern, loops of yarn by means of stitching process, using
　　　　　　needles or wires.
L
Lamina - Subunit of a laminate consisting of one or more adjacent plies of the same material with identical orientation. (Plural: laminae.)
　　　　　　Lamina orientation - See Ply
　　　　　　orientation.
　　　　　　Laminate - To unite or bond two or more layers
　　　　　　or laminae (often with the aid of pressure and/or heat). Any fiber-
　　　　　　or fabric-reinforced composite consisting of laminae with one or
　　　　　　more orientations with respect to some reference direction.
　　　　　　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 principal axis, off-axis laminate and
　　　　　　x-axis.)
　　　　　　Lap joint - A joint made by overlapping two
　　　　　　parts and bonding them together.
　　　　　　Layup - To place or the process of placing
　　　　　　layers of reinforcing material into position in or on a mold; also
　　　　　　used to refer to the reinforcing materials as placed in the mold
　　　　　　("the layup").
　　　　　　Layup code - Designation system for abbreviating
　　　　　　the stacking sequence of laminated composites.
　　　　　　Liner - The continuous, usually flexible,
　　　　　　reinforced resin barrier on the inside surface of a plastic or
　　　　　　thermoset laminate, used to protect the laminate from chemical
　　　　　　attack or to prevent leakage under stress.
　　　　　　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
　　　　　　high-temperature capability.
　　　　　　Lot- See Batch.
　　　　　　Low profile - Describes resin compounds
　　　　　　formulated for low-to-zero shrinkage during molding.
M
MACT - Maximum Achievable Control Technology. A technology-based air pollution control standard developed by the U.S. Environmental Protection Agency (EPA) aimed at reducing emissions of hazardous air pollutants (HAPs) from U.S. manufacturing operations.
　　　　　　Mandrel - A form, fixture or male mold used as
　　　　　　the base for production of a part in processes such as layup 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.
　　　　　　Matched metal molding - See compression
　　　　　　molding.
　　　　　　Matrix - Material in which reinforcing fiber of
　　　　　　a composite is embedded. Matrix materials include thermosetting and
　　　　　　thermoplastic polymers, metals and ceramic compounds.
　　　　　　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.
　　　　　　(Also see fiber content.)
　　　　　　Metal-matrix composite (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.
　　　　　　Mil - The unit used in measuring the diameter of
　　　　　　glass fiber strands, wire and so forth (1 mil = 0.0254 mm/0.001
　　　　　　inch).
　　　　　　Milled fiber - Continuous glass or carbon
　　　　　　strands hammer-milled into very short fibers.
　　　　　　MMC - See Metal-matrix composite.
　　　　　　Modulus - The physical measure of a material"s
　　　　　　stiffness, equal to the ratio of applied load (stress) to the
　　　　　　resulting deformation of a material. May be represented by a number
　　　　　　or in descriptive terms as low, intermediate, high or ultrahigh. A
　　　　　　higher modulus indicates greater stiffness. (See stiffness and
　　　　　　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.
　　　　　　Mold - An enclosed cavity or open form from
　　　　　　which a composite component takes its shape, size and exterior
　　　　　　surface appearance (also known as a tool).
　　　　　　Mold release agent - A lubricant used to prevent
　　　　　　a part from sticking to a mold surface.
　　　　　　Molding - The process of forming composite
　　　　　　materials into a solid mass of prescribed shape and size, using a
　　　　　　mold or tool.
　　　　　　Monomer - A single molecule that reacts with
　　　　　　like or unlike molecules to form a polymer.
　　　　　　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 (also see yarn and tow).
N
Naphtha - A petroleum distillate commonly used as a solvent for natural resins and rubber.
　　　　　　NDE, NDI, NDT - Nondestructive evaluation,
　　　　　　nondestructive inspection, nondestructive testing. (See
　　　　　　nondestructive inspection.)
　　　　　　Near-net shape - Describes a manufactured part
　　　　　　or reinforcement preform fabricated to final dimensions that require
　　　　　　minimal machining, cutting or other finishing.
　　　　　　Net shape - Fabricated to 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 - A form of fiber reinforcement
　　　　　　composed of continuous fiber strands loosely gathered together.
　　　　　　Nylon - The generic name, by common usage, for
　　　　　　all synthetic polyamides.
O
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 stress direction.
　　　　　　One-off - Denotes a fabrication process in which
　　　　　　a single part is produced.
　　　　　　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 process.
　　　　　　Original equipment manufacturer (OEM) -
　　　　　　Describes a company that designs and builds products bearing its
　　　　　　name; for example, Boeing 777 aircraft or Prince tennis racquets.
　　　　　　Out-time - Period of time in which a prepreg
　　　　　　retains desirable handling characteristics and performance
　　　　　　properties outside a specified storage environment (such as a
　　　　　　freezer, in the case of thermoset prepregs).
　　　　　　Outgassing - The release of solvents and
　　　　　　moisture from composite parts under a vacuum.
P
PAN- See Polyacrylonitrile.
　　　　　　Part consolidation - A design-and-fabrication
　　　　　　process in which a number of previously discrete parts are combined
　　　　　　in a single component to reduce or eliminate assembly operations and
　　　　　　associated costs.
　　　　　　Parting film - A layer of thin plastic that
　　　　　　prevents bagging materials from sticking to a part. It may be
　　　　　　perforated to vent excess resin. It is removed after cure.
　　　　　　PBO - See Poly
　　　　　　p-phenylene-2,6-bensobisoxazale.
　　　　　　Peel ply - A layer of material that, when
　　　　　　applied to a layup surface, can be removed from the cured laminate
　　　　　　prior to bonding operations, leaving a clean, resin-rich surface
　　　　　　suitable for bonding.
　　　　　　Peel strength - Strength of an adhesive bond
　　　　　　between sheet materials; determined by applying parting stress at a
　　　　　　right angle (perpendicular) to the plane of the adhesive
　　　　　　interface.
　　　　　　Phenolic resin - A thermosetting resin produced
　　　　　　by a condensation reaction of an aromatic alcohol with an aldehyde
　　　　　　(usually phenol with formaldehyde).
　　　　　　Pin holes - Small voids open to and visible on
　　　　　　the surface of a cured composite part.
　　　　　　Pitch - Residual petroleum product used as a
　　　　　　precursor in the manufacture of certain carbon fibers.
　　　　　　Planar winding - Filament winding method in
　　　　　　which the filament path lays on a plane that intersects the winding
　　　　　　surface.
　　　　　　Plastic - General term for a range of
　　　　　　high-molecular-weight thermoplastic or thermosetting polymers that
　　　　　　have characteristics and properties that make them suitable for use
　　　　　　in molding, casting, extruding or laminating processes.
　　　　　　Plied yarn - Two or more yarns collected
　　　　　　together, with or without twist.
　　　　　　Ply - A single layer (or lamina) used to
　　　　　　fabricate a laminate. 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 measured clockwise.
　　　　　　Ply schedule - A prescribed sequence for laying
　　　　　　up individual plies or layers to form a laminate, indicating the
　　　　　　arrangement of plies by material type and other characteristics,
　　　　　　such as fiber orientation.
　　　　　　Poisson"s ratio - When a material is stretched,
　　　　　　its cross-sectional area changes as well as its length. Poisson"s
　　　　　　ratio is the constant relating these changes in dimensions, and is
　　　　　　defined as the ratio of the change in width per unit width to the
　　　　　　change in length per unit length.
　　　　　　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) - Polymer base material
　　　　　　that is spun into a fiber form and used as a precursor in the
　　　　　　manufacture of certain carbon fibers.
　　　　　　Polyester - Thermosetting resins produced by
　　　　　　dissolving unsaturated, generally linear, alkyd resins in a
　　　　　　vinyl-type active monomer, such as styrene. The resins are usually
　　　　　　furnished in solution form, but powdered solids are also
　　　　　　available.
　　　　　　Polyetherimide (PEI) - A high-performance
　　　　　　thermoplastic resin with repeating aromatic imide and ether
　　　　　　molecular units. Characterized by high strength and rigidity over a
　　　　　　wide range of temperatures, as well as long-term heat resistance,
　　　　　　highly stable dimensional properties and broad chemical
　　　　　　resistance.
　　　　　　Polyimide (PI) - Highly heat-resistant
　　　　　　thermoplastic polymer resin.
　　　　　　Polymer - Large organic molecule formed by
　　　　　　combining many smaller molecules (monomers) in a regular pattern.
　　　　　　Polymer alloy (or polymer blend) - A blend of
　　　　　　polymers, copolymers or elastomers.
　　　　　　Polymerization - Chemical reaction that links
　　　　　　monomers to form polymers.
　　　　　　Poly p-phenylene-2,6-bensobisoxazale (PBO) - A
　　　　　　relatively new polymer fiber, with a modulus and tensile strength
　　　　　　almost double that of aramid fiber and a decomposition temperature
　　　　　　almost 100°C/212°F higher.
　　　　　　Porosity - The presence of voids open to the
　　　　　　surface of a solid material into which air or liquids may pass.
　　　　　　Postcure - Exposure of a molded component to
　　　　　　elevated temperature after initial in-mold curing, performed for the
　　　　　　purpose of improving the component"s mechanical properties. Postcure
　　　　　　may occur after demolding and is often done without the use of
　　　　　　pressure.
　　　　　　Pot life - Length of time in which a catalyzed
　　　　　　thermosetting resin retains sufficiently low viscosity for
　　　　　　processing.
　　　　　　Precure - Full or partial hardening of a resin
　　　　　　or adhesive before pressure is applied.
　　　　　　Precursor - Material from which carbon fibers
　　　　　　are made by pyrolysis. Common precursors are polyacrylonitrile
　　　　　　(PAN), rayon and pitch.
　　　　　　Preform - Pre-shaped fibrous reinforcement,
　　　　　　supplied without matrix, but often containing a binder to facilitate
　　　　　　manufacture and maintain shape. A preform"s fiber components are
　　　　　　distributed or arranged, typically on a mandrel or mock-up, to
　　　　　　approximate the contours and thickness of the finished part, saving
　　　　　　time and labor during the molding process.
　　　　　　Prepreg - Fibrous reinforcement (sheet, tape,
　　　　　　tow, fabric or mat) preimpregnated with resin and capable of storage
　　　　　　for later use. For thermosetting matrices the resin is usually
　　　　　　partially cured or otherwise brought to a controlled viscosity,
　　　　　　called B-stage. Additives (e.g., catalysts, inhibitors and flame
　　　　　　retardants) are used to obtain specific end-use properties and/or
　　　　　　improve processing, storage and handling characteristics.
　　　　　　Primary structure - An aerospace critical
　　　　　　load-bearing structure; if damaged the aircraft or space vehicle
　　　　　　cannot operate safely.
　　　　　　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.)
　　　　　　Promoter - A chemical which hastens the reaction
　　　　　　between a catalyst and a resin (also known as an accelerator).
　　　　　　Prototype - A test part not intended for
　　　　　　commercial release, which establishes design, material and
　　　　　　fabrication parameters for a new product. Also, to fabricate such a
　　　　　　test part (a process that can 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.
　　　　　　Pultrusion - Continuous process for
　　　　　　manufacturing composite rods, tubes and other linear structures that
　　　　　　have constant cross-sections. The process involves drawing
　　　　　　continuous reinforcement through a resin-impregnation bath (or an
　　　　　　alternative resin-impregnation method is used), then pulling the
　　　　　　wetout material through a heated shaping die, where cure takes
　　　　　　place, securing the desired cross-section before the laminate
　　　　　　departs from the die.
　　　　　　Puncture - A break in the composite skin of a
　　　　　　sandwich structure that may or may not go through to the core
　　　　　　material or completely through the part thickness.
　　　　　　Pyrolysis - The decomposition or other
　　　　　　transformation of a compound caused by exposure to heat.
Q
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 biaxial fabric, triaxial fabric.)
　　　　　　Quasi-isotropic - Approximates isotropy by
　　　　　　orientation of plies in multiple directions.
R
Ramping - A programmed gradual increase/ decrease in temperature and/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.
　　　　　　Reagent - A substance used in a chemical
　　　　　　reaction to produce other substances.
　　　　　　Regrind - Scrap composites (thermoset or
　　　　　　thermoplastic) collected in-plant or from post-consumer sources and
　　　　　　reground into pellets or fine powder for reuse in molding new parts,
　　　　　　either as a new base material or in combination with virgin
　　　　　　materials.
　　　　　　Reinforced reaction injection molding (RRIM) - A
　　　　　　closed molding process that mixes two highly reactive resin
　　　　　　components for cure. Reinforcement, generally flake glass or milled
　　　　　　fibers, is added to one of the resin components to add strength and
　　　　　　reduce thermal expansion.
　　　　　　Reinforcement - The key element added to a
　　　　　　matrix to provide the required properties (primarily strength).
　　　　　　Reinforcement forms range from individual short fibers to complex
　　　　　　braided, woven or stitched textile forms.
　　　　　　Release agent - An specially formulated material
　　　　　　placed between the mold and uncured resin/fiber (usually sprayed or
　　　　　　painted on the mold surface) to prevent permanent bonding between
　　　　　　the two during cure and facilitates demolding after cure.
　　　　　　Release film - A release agent made from an
　　　　　　impermeable film that does not form a bond with the composite
　　　　　　material during cure.
　　　　　　Resin - A solid or pseudo-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 and work with them to
　　　　　　produce specified performance properties.
　　　　　　Resin content - See matrix content.
　　　　　　Resin-rich - Describes a localized buildup of
　　　　　　resin in excess of the expected resin/fiber ratio in a composite.
　　　　　　Resin-starved - Describes an area in a composite
　　　　　　that lacks sufficient resin to achieve thorough fiber wetout.
　　　　　　Resin transfer molding (RTM) - A closed molding
　　　　　　process in which catalyzed resin is transferred into an enclosed
　　　　　　mold cavity to impregnate a pre-positioned fibrous reinforcement
　　　　　　(see preform). The mold and/or resin may or may not be heated. RTM
　　　　　　involves relatively low tooling and equipment costs and enables
　　　　　　fabricators to consolidate large parts.
　　　　　　Resin viscosity - Describes a resin system"s
　　　　　　solid-to-liquid transition resistance to flow, which can be altered
　　　　　　by temperature and pressure to achieve desired flow characteristics.
　　　　　　(Also see viscosity.)
　　　　　　Ribbon direction - On a honeycomb core, the
　　　　　　length of the core splice; the direction perpendicular to the
　　　　　　direction of cell expansion (w-direction). The direction of one
　　　　　　continuous ribbon. (See honeycomb.)
　　　　　　Reaction injection molding (RIM) - A process
　　　　　　involving high-pressure mixing of two highly reactive resin
　　　　　　components to promote fast cure; primarily used in the molding of
　　　　　　parts with polyurethane matrices.
　　　　　　Reinforced reaction injection molding (RRIM) -
　　　　　　Reaction injection molding process in which one of the two mixed
　　　　　　components is reinforced, usually with flake glass or milled fibers,
　　　　　　to stiffen the part and reduce thermal expansion (see previous
　　　　　　entry).
　　　　　　Roving - Large filament-count tow; a collection
　　　　　　of continuous glass fiber filaments, either as untwisted strands or
　　　　　　twisted yarn.
　　　　　　RTM - See [b]Resin transfer molding.
S
S-glass- The standard abbreviation for "structural glass," which is a magnesia/alumina/silicate glass fiber reinforcement designed to provide the very high tensile strength required in high-performance composites.
　　　　　　Sandwichstructure - A composite
　　　　　　component featuring a lightweight core material (usually honeycomb,
　　　　　　foam or balsa wood) placed between (hence the term "sandwich") two
　　　　　　relatively thin, dense, high-strength, functional and/or decorative
　　　　　　skins. (Also see core.)
　　　　　　Scrim - Low-cost, woven reinforcing fabric in an
　　　　　　open mesh construction.
　　　　　　Sealant - A paste or liquid that, when applied
　　　　　　to a joint, hardens in place to form a seal.
　　　　　　Secondary bonding - The joining, by means of
　　　　　　adhesive, of two or more already cured composite parts.
　　　　　　Secondary structure - Aerospace structure that
　　　　　　is not critical to flight safety. (In contrast to primary
　　　　　　structure.)
　　　　　　Separator - A permeable layer that separates and
　　　　　　also acts as a release film (e.g., porous, Teflon-coated
　　　　　　fiberglass). Often placed between lay up and bleeder to facilitate
　　　　　　bleeder systems" removal from laminate after cure.
　　　　　　Shear - An action or stress resulting from force
　　　　　　applied in a direction parallel to the plane of adhesion between the
　　　　　　surfaces of two adjacent components or layers, causing or tending to
　　　　　　cause one to slide relative to the other.
　　　　　　Shear strength - The maximum shear stress that a
　　　　　　material is capable of sustaining.
　　　　　　Sheet molding compound (SMC) - A ready-to-mold,
　　　　　　glass fiber-reinforced polyester material primarily used in
　　　　　　compression molding.
　　　　　　Shelf life - Length of time a material can be
　　　　　　stored and continue to meet specification requirements, remaining
　　　　　　suitable for its intended use. (Also see storage life.)
　　　　　　Shot - One complete cycle on an
　　　　　　injection-molding machine.
　　　　　　Shot weight - The measured amount of compound
　　　　　　required to completely fill the mold in injection or transfer
　　　　　　molding processes.
　　　　　　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.
　　　　　　Sizing - A chemical solution used to coat fiber
　　　　　　filaments, facilitating operations such as weaving or braiding.
　　　　　　Sizing protects the filament from water absorption and abrasion (to
　　　　　　minimize fiber wear) and also can be used to bind together and
　　　　　　stiffen warp yarns during weaving. Sizing is usually removed and
　　　　　　replaced with finish before matrix application. Also called size.
　　　　　　Skin - The relatively dense laminate adhered to
　　　　　　the outer surfaces of the core material in a sandwich structure.
　　　　　　Soft tool - Tool made of composites or a similar
　　　　　　"soft" material that is vulnerable to damage during use, storage or
　　　　　　transportation. (In contrast to hard tool.)
　　　　　　Solvent - A liquid capable of dissolving another
　　　　　　substance. Certain solvents find application as evaporative diluents
　　　　　　in paints or coatings and/or as cleaning solutions in maintenance
　　　　　　operations.
　　　　　　Spec - Colloquial abbreviation for
　　　　　　"specification"; describes the required properties and
　　　　　　characteristics a particular material or part must have in order to
　　　　　　be acceptable to a potential user.
　　　　　　Specific gravity - The density (mass per unit of
　　　　　　volume) of a material divided by the density of water at a standard
　　　　　　temperature.
　　　　　　Sprayup - A technique in which continuous strand
　　　　　　roving is fed into a chopper gun, which chops the roving into
　　　　　　predetermined lengths and sprays the chopped fiber, along with a
　　　　　　measured amount of resin and catalyst, onto an open mold.
　　　　　　Stacking sequence - Arrangement of ply
　　　　　　orientations and material components in a laminate specified with
　　　　　　respect to some reference direction (also see ply schedule).
　　　　　　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 particular material.
　　　　　　Storage life - The length of time a material can
　　　　　　be stored and retain specific properties. (Also see shelf life.)
　　　　　　Strain - Deformation resulting from applied
　　　　　　stress. Measured as the change in length per unit of length in a
　　　　　　given direction; expressed as a percentage or in 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 - A 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 that alone would not have
　　　　　　caused corrosion.
　　　　　　Stress crack - External or internal cracks in a
　　　　　　composite caused by tensile stresses. Cracking may be present
　　　　　　internally, externally or in combination.
　　　　　　Structural adhesive - An adhesive used to
　　　　　　transfer loads between adhesively bonded surfaces.
　　　　　　Structural bond - A bond that joins load-bearing
　　　　　　components in an assembly.
　　　　　　Structural Reaction Injection Molding (SRIM) - A
　　　　　　closed molding process employing a fiber reinforced preform or mat
　　　　　　that is injected with a reactive resin to impregnate the fibers and
　　　　　　cure quickly.
　　　　　　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 methodologies and curing
　　　　　　parameters (e..g., autoclave requirements) that will maintain
　　　　　　structural integrity. Updated periodically by OEMs based on input
　　　　　　from repair technicians.
　　　　　　Substrate - Material that provides the surface
　　　　　　on which an adhesive-containing substance is applied for any
　　　　　　purpose, such as bonding or coating.
　　　　　　Surfacing veil - A reinforcing fabric
　　　　　　specifically designed to block out the fiber patterns of underlying
　　　　　　reinforcements. It often adds ultraviolet protection to the
　　　　　　structure as well. (Also see veil.)
　　　　　　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 the midplane.
　　　　　　Synthetic fiber - Fiber made of materials other
　　　　　　than glass or carbon, such as polyester.
T
Tack- Stickiness of an uncured prepreg.
　　　　　　Tape - Thin, unidirectional prepreg, available
　　　　　　in up to 12-inch widths in carbon fiber. (Also see
　　　　　　unidirectional.)
　　　　　　Tape laying - An automated fabrication process
　　　　　　in which preimpregnated tape is laid side by side and/or overlapped
　　　　　　to form a structure.
　　　　　　Tensile strength - The maximum tensile stress
　　　　　　sustained by a test specimen before failure during a tension
　　　　　　test.
　　　　　　Tex- A unit of linear density equal to the
　　　　　　weight in grams of 1,000m of filament fibers, yarns or strands.
　　　　　　Tg - See Glass-transition temperature.
　　　　　　
　　　　　　Thermal conductivity - The ability to conduct
　　　　　　heat.
　　　　　　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.
　　　　　　Thermoplastics - A 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 reshaped by means of molding or extrusion.
　　　　　　Thermosets - A class of plastics that, when
　　　　　　cured by thermal and/or chemical or other means, become
　　　　　　substantially infusible and insoluble. Once cured, a thermoset
　　　　　　cannot be returned to the uncured state.
　　　　　　Thixotropic - Describes substances that have
　　　　　　high static shear strength and low dynamic shear strength, which
　　　　　　results in a predictable, time-dependent loss of viscosity under
　　　　　　shear (e.g., when mixed, sprayed or otherwise subjected to force)
　　　　　　and subsequent substantial to complete return to the higher at-rest
　　　　　　viscosity when shear force is removed. Highly thixotropic resins,
　　　　　　for example, may be applied easily with spray equipment, yet
　　　　　　immediately afterward resist running on a vertical surface.
　　　　　　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 a part.
　　　　　　Tooling resin - A plastic resin, typically epoxy
　　　　　　or silicone, used to make a tool.
　　　　　　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 strand but used when the reference
　　　　　　is specific to carbon fiber.
　　　　　　Tow size - Designates the number of filaments in
　　　　　　a tow, denoted by a number followed by K, indicating multiplication
　　　　　　by 1,000 (for example, 12K tow has 12,000 filaments).
　　　　　　Tracer - A visually different or distinctive
　　　　　　fiber, tow, or yarn added to a prepreg to verify fiber alignment or
　　　　　　to distinguish warp fibers frfill fibers.
　　　　　　Triaxial fabric - Fabric with three
　　　　　　non-interwoven layers - oriented at +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, quadraxial 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 never
　　　　　　twisted.
U
Ultrahigh molecular weight (UHMW) polyethylene - A polyethylene (PE) resin with very high molecular weight and very high abrasion resistance and impact strength.
　　　　　　Unidirectional (UD) - General term denoting
　　　　　　orientation of fibers in one direction.
　　　　　　Ultraviolet (UV) cure - The process of curing
　　　　　　resins and adhesives with ultraviolet light.
V
Vacuum-bag molding - Molding technique wherein a part layed up on an open mold is cured under a layer of sealed film from which entrapped air has been removed by vacuum. The technique more effectively consolidates the laminate and reduces void content, compared to conventional open molding.
　　　　　　Vacuum-assisted resin transfer molding (VARTM) -
　　　　　　An infusion process by which a vacuum draws resin into a one-sided
　　　　　　mold; a cover, either rigid or flexible, is placed over the laminate
　　　　　　and taped or otherwise fixed to form a vacuum-tight seal. (See
　　　　　　previous entry.)
　　　　　　Veil - An ultrathin, nonwoven mat often composed
　　　　　　of organic fibers as well as glass fibers and used primarily as a
　　　　　　corrosion barrier.
　　　　　　Vinyl esters - A class of thermosetting resins
　　　　　　containing ester of acrylic and/or methacrylic acids.
　　　　　　Viscosity - Describes the tendency of a material
　　　　　　to resist flow. Viscosity is measured in comparison with water, and
　　　　　　computed in centipoise (cps). The higher the number, the greater the
　　　　　　resistance to flow.
　　　　　　Void - Any pocket 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
　　　　　　ambient or slightly elevated temperatures.
　　　　　　Volatile content - The percent of volatiles that
　　　　　　are driven off as a vapor from a plastic or an impregnated
　　　　　　reinforcement during cure.
　　　　　　Volatile organic compounds (VOCs) - Chemical
　　　　　　substances, such as solvents, that readily evaporate or volatilize
　　　　　　into the air. Many VOCs also are considered hazardous air pollutants
　　　　　　(HAPs) because of potential health concerns.
W
Warp - Fiber bundles in a woven fabric that run parallel to the length of the loom and lengthwise along the longer dimension of the finished fabric.
　　　　　　Warpage - Dimensional distortion in a
　　　　　　composite.
　　　　　　Water absorption - Ratio of the weight of water
　　　　　　ab-sorbed by a material to the weight of dry material.
　　　　　　Waterjet - High-pressure water stream used for
　　　　　　cutting polymer composite parts.
　　　　　　Weave - To interlace fibers in a pattern, often
　　　　　　based on a 0°/90° grid; the fabric pattern formed by interlacing
　　　　　　yarns. Interlacing patterns vary. In plain weave, for instance, 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 (e.g., eight-harness satin features one warp
　　　　　　tow over seven fill tows and under the eighth).
　　　　　　Weft - See Fill.
　　　　　　Wet layup - Application of a resin to dry
　　　　　　reinforcements in the mold.
　　　　　　Wet winding - A filament winding technique that
　　　　　　impregnates fiber strands with resin immediately before they contact
　　　　　　the mandrel.
　　　　　　Wetout - Saturation with resin of all voids
　　　　　　between reinforcement strands and filaments.
　　　　　　Wetting agent - A surface-active agent that
　　　　　　promotes wetting by decreasing a liquid"s cohesion.
　　　　　　Whisker - A short, single crystal fiber or
　　　　　　filament used as a reinforcement in a matrix.
　　　　　　Wind angle - Measure in degrees between the
　　　　　　direction parallel to the filaments and an established reference
　　　　　　point.
　　　　　　Winding - Any process in which continuous
　　　　　　material is applied under controlled tension to a rotating form
　　　　　　(mandrel) in a predetermined geometric relationship to make a
　　　　　　structure. (See filament winding.)
　　　　　　Winding pattern - In filament winding, the
　　　　　　recurring pattern of the filament path after a certain number of
　　　　　　mandrel revolutions.
　　　　　　Wire - Large diameter (greater than about 2
　　　　　　mils) high-performance fiber (e.g., see boron fiber or silicon
　　　　　　carbide fiber). In contrast, see filament and fiber.
　　　　　　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 bundles.
　　　　　　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
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 axis.)

Y
Y-axis- The axis in the plane of a laminate perpendicular to the x-axis.
　　　　　　Yarn - A continuous, ordered assembly of
　　　　　　essentially parallel, collimated filaments, usually with a twist.
　　　　　　Yield point - The first stress in a material,
　　　　　　less than the maximum rate attainable stress, at which the strain
　　　　　　increases at a higher rate than the stress. The point at which
　　　　　　permanent deformation of a stressed specimen begins to take place.
　　　　　　(Also see stress and strain.)
　　　　　　Young"s modulus - Ratio of normal stress to the
　　　　　　corresponding strain for tensile or compressive stresses less than a
　　　　　　material"s proportional limit.
Z
Z-axis- The axis perpendicular to the plane formed by the x and y axes. In a sheet laminate, if the x and y axes are parallel to the length and width, respectively, the z-axis would indicate sheet thickness. (See x-axis and y-axis.)
　　　　　　Zero bleed - Laminate fabrication procedure that
　　　　　　does not allow loss of resin during cure.