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Monday, August 10, 2009

Common Terminology in Composite Industry

Below are SOME terminology used in composite industry:

Accelerator - A chemical additive that fastens cure or chemical reaction (see also catalyst).

Additive - An ingredient mixed into resin to improve properties (e.g., plasticizers, initiators, light stabilizers and flame retardants).

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.)

Aspect ratio - Ratio of the length to the diameter of a fiber.

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).

Braiding - Textile process that intertwines into a pattern three or more strands, yarns or tapes, typically into a tubular shape

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

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.

Ceramic-matrix composites (CMC) - Materials consisting of a ceramic or carbon fiber surrounded by a ceramic matrix, primarily silicon carbide.

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

Crosslinking - Polymerization reactions that branch out from the main molecular chain to form a networked pattern of chemical links

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

E-glass - Abbreviation for "electrical glass," borosilicate glass fibers, which have high electrical resistivity. Most often used in conventional polymer matrix composites.

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.

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.

Fiber - One or more filaments in an ordered assemblage

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-reinforced plastics (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 plastics.

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.

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.

Graphite fibers - Carbon fibers that have been graphitized by heating and stretching at temperatures above 1649°C/3000°F.

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.)

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.

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 reinforcements (see aramid).

Knit - Textile process that interlocks, in a specific pattern, loops of yarn by means of stitching process, using needles or wires.
Lamina - Subunit of a laminate consisting of one or more adjacent plies of the same material with identical orientation. (Plural: laminae.)

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.

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.

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.

Phenolic resin - A thermosetting resin produced by a condensation reaction of an aromatic alcohol with an aldehyde (usually phenol with formaldehyde).

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

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.

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.

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.

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.

Promoter - A chemical which hastens the reaction between a catalyst and a resin (also known as an accelerator).

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.

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.

Roving - Large filament-count tow; a collection of continuous glass fiber filaments, either as untwisted strands or twisted yarn.

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.

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.

Stiffness - Measure of the resistance of a material to deformation. The ratio of applied stress to resulting strain for a particular material.

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.

Synthetic fiber - Fiber made of materials other than glass or carbon, such as polyester.

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

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).

Void - Any pocket of enclosed gas or air within a composite.

Warpage - Dimensional distortion in a composite.

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).

Woven roving - Heavy, coarse fabric produced by weaving continuous roving bundles.

Yarn - A 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 a material's proportional limit.

Saturday, August 8, 2009

Filament Winding : Type of Mandrel in Filament Winding

The mandrel, which determines accurate internal geometry for the component, is generally the only major tool. Low-cost mandrel materials such as cardboard or wood can often be used when winding low-cost routine parts.

For critical parts requiring close tolerances, expensive mandrels designed for long-term use may be required. For high-temperature cure 315°C (600"F), graphite mandrels with low thermal expansion may be advantageous, however some attention should be paid to the potential difficulties of mandrel removal. Gas containment pressure vessels often require metal liners because composites are porous; these metal liners can also serve as mandrels'*.

Mandrels are either removable or non-removable (remaining as a part of the wound structure). Removable mandrels are classified according to the removal technique as:

-entirely removed (for example, tubular mandrels with or without taper and with release agent);

-collapsible (the mandrel is disassembled or removed piece by piece)

-Breakable or soluble (plaster, sand or salts).

The selection of a mandrel involves several trade-offs. These include part size and complexity, size of openings, resin system and its curing cycle and the number of components to be fabricated. The basic requirements for a mandrel, whether it will be removed from the part after winding or remain as a part of the structure, are:

-It must be stiff and strong enough to support its own weight and the weight of the applied composite while resisting the fiber tension pressure from winding and curing.

-It must be dimensionally stable and should have a thermal coefficient of expansion greater than the transverse coefficient of the composite structure.

Components such as rocket motor cases or pressure vessels have closed or reduced-area end openings that require the use of dissolvable or fusible mandrel materials.

Tuesday, August 4, 2009

Composite Fabrication : Filament Winding

FILAMENT WINDING is a process for fabricating a composite structure in which continuous reinforcements (filament, wire, yarn, tape, or other), either previously impregnated with a matrix material or impregnated during winding, are placed over a rotating form or mandrel in a prescribed way to meet certain stress conditions. When the required number of layers is applied, the wound form is cured and the mandrel can be removed or left as part of the structure.

High-speed, precise lay-down of continuous reinforcement in predescribed patterns is the basis of filament winding. The filament-winding machine traverses the wind eye at speeds that are synchronized with the mandrel rotation, controlling the winding angle of the reinforcement and the fiber lay-down rate. The deposition can be controlled either by computer numerically controlled (CNC) machines or by simple mechanically controlled winders; the latter are less convenient, but require a lower capital investment.

Figure below shows the basic six axes of the CNC machines. Usually, the mechanical machines are limited to three axes or less, whereas the CNC machines can accommodate up to seven axes.

Thermoset resins, generally used as binders for reinforcements, can be applied to the dry roving at the time of winding, which is known as wet winding. They may also be applied prior to winding as a tow or tape prepreg and used promptly or refrigerated. Usually the cure of the filament-wound composite is conducted at elevated temperatures without vacuum bagging or autoclave compaction. Mandrel removal, trimming, and other finishing operations complete the process.

Six axes of filament-winding machine motion. Courtesy of McClean Anderson, Inc Axis 1- Spindle/ Mandrel rotation Axis 2-Horizontal Carriage Motion Axis 3-Cross or Radial Carriage Motion Axis 4- Axis 5-Eye Motion Axis 6-Yaw Motion

The mandrel can be cylindrical, spherical, or any other shape as long as it does not have reentrant (concave) curvature—although several manufacturers have been able to incorporate complex reentrant curves in filament wound structures (Fig. 4 and 5). Large or thick-walled structures, particularly structures of revolution such as cylinders or pressure vessels, are most easily wound.

A Complex Winding Pattern