Composite Materials
Composite Materials
Particle-reinforced composites
Composite materials, or shortened to composites, are microscopic or macroscopic combinations of two or more distinct engineered materials (those with different physical and/or chemical properties) with a recognizable interface between them in the finished product. For structural applications, the definition can be restricted to include those materials that consist of a reinforcing phase such as fibers or particles supported by a binder or matrix phase. Wood composites are commonly seen examples of composite materials.
Other features of composites include the following: (1) The distribution of materials in the composite is controlled by mechanical means. (2) The term composite is usually reserved for materials in which distinct phases are separated on a scale larger than atomic,and in which the composite’s mechanical properties are significantly altered from those of the constituent components. (3) The composite can be regarded as a combination of two or more materials that are used in combination to rectify a weakness in one material by a strength in another. (4) A recently developed concept of composites is that the composite should not only be a combination of two materials, but the combination should have its own distinctive properties. In terms of strength, heat resistance, or some other desired characteristic, the composite must be better than either component alone.
Composites were developed because no single, homogeneous structural material could be found that had all of the desired characteristics for a given application. Fiber-reinforced composites were first developed to replace aluminum alloys, which provide high strength and fairly high stiffness at low weight but are subject to corrosion and fatigue.
An example of a composite material is a glass-reinforced plastic fishing rod in which glass fibers are placed in an epoxy matrix. Fine individual glass fibers are characterized by their high tensile stiffnesses and very high tensile strengths, but because of theirsmall diameters, have very small bending stiffnesses. If the rod were made only of epoxy plastic, it would have good bending stiffness, but poor tensile properties. When the fibers are placed in the epoxy plastic, however, the resultant structure has high tensile stiffness, high tensile strength, and high bending stiffness.
The discontinuous filler phase in a composite is usually stiffer or stronger than the binder phase. There must be a substantial volume fraction of the reinforcing phase (about10%) present to provide reinforcement. Examples do exist, however, of composites where the discontinuous phase is more compliant and ductile than the matrix.
Natural composites include wood and bone. Wood is a composite of cellulose and lignin. Cellulose fibers are strong in tension and are flexible. Lignin cements these fibers together to make them stiff. Bone is a composite of strong but soft collagen (a protein) and hard but brittle apatite (a mineral).
Particle-reinforced composites
A particle has no long dimension. Particle composites consist of particles of one material dispersed in a matrix of a second material. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral, or irregular in shape. They may be added to a liquid matrix that later solidifies; grown in place by a reaction such as age-hardening; or they may be pressed together and then interdiffused via a powder process. Theparticles may be treated to be made compatible with the matrix, or they may be incorporated without such treatment. Particles are most often used to extend the strength or other properties of inexpensive materials by the addition of other materials.
Fiber-reinforced composites
A fiber has one long dimension. Fiber-reinforced materials are typified by fiberglass in which there are three components: glass filaments (for mechanical strength), a polymer matrix (to encapsulate the filaments); and a bonding agent (to bind the glass to the polymer). Other fibers include metal, ceramics, and polymers. The fibers can be used as continuous lengths, in staple-fiber form, or as whiskers(short, fine, perfect, or nearly perfect single crystals). Fiber-reinforcement depends as much on fabrication procedure as on materials.
Laminar composites
Platelets or lamina have two long dimensions. Laminar composites include plywood, which is a laminated composite of thin layers of wood in which successive layers have different grain or fiber orientations. The result is a more-or-less isotropic composite sheet that is weaker in any direction than it would be if the fibers were all aligned in one direction. The stainless steel in a cooking vessel with a copper-clad bottom provides corrosion resistance while the copper provides better heat distribution over the base of the vessel.
Mechanical properties
The mechanical properties of composite materials usually depend on structure. Thus these properties typically depend on the shape of inhomogenities, the volume fraction occupied by inhomogenities, and the interfaces between the components. The strength of composites depends on such factors as the brittleness or ductility of the inclusions and matrix.
For example, failure mechanisms in fiber-filled composites include fracture of the fibers; shear failure of the matrix along the fibers; fracture of the matrix in tension normal to the fibers or failure of the fiber-matrix interface. The mechanism responsible for failure depends on the angle between the fibers and the specimen’s axis.
If a mechanical property depends on the composite material’s orientation, the property is said to be anisotropic. Anisotropic composites provide greater strength and stiffness than do isotropic materials. However, the material properties in one direction are gained at the expense of the properties in other directions. For example, silica fibersin a pure aluminum matrix produce a composite with a tensile strength of about 110, 000 psi (pounds per square inch) along the fiber direction, but a tensile strength of only about 14, 000 psi at right angles to the fiber axis. It therefore only makes sense to use anisotropic materials if the direction that they will be stressed is known in advance.
Isotropic material are materials properties independent of orientation. Stiff plateletinclusions are the most effective in creating a stiff composite, followed by fibers, and then by spherical particles.
High performance composites
High performance composites are composites that have better performance than conventional structural materials such as steel and aluminum alloys. They are almost all continuous fiber-reinforced composites, with organic (resin) matrices.
Fibers for high performance composites
In a high-performance, continuous fiber-reinforced composite, fibers provide virtually all of the load-carrying characteristics of the composite; i.e., strength and stiffness. The fibers, within such a composite, form bundles, or filaments. Consequently, even if several fibers break, the load is redistributed to other fibers, which avoids a catastrophic failure.
Glass fibers are used for nonstructural, low-performance applications such as panels in aircraft and appliances to high-performance applications such as rocket-motor cases and pressure vessels. However, the sensitivity of the glass fiber to attack by moisture poses problems for other applications. The most commonly used glass fiber is a calcium aluminoborosilicate glass (E-glass). High silica and quartz fibers are also used for specialized applications.
Carbon fibers are the best known and most widely used reinforcing fiber in advanced composites. The earliest carbon fibers were produced by thermal decomposition of rayon precursor materials. The starting material is now polyacrylonitrile.
Aramid fibers are aromatic polyamide fibers. The aramid fiber is technically a thermoplastic polymer like nylon, but it decomposes when heated before it reaches its projected melting point. When polymerized, it forms rigid, rodlike molecules that cannot be spun from a melt. Instead they have to be spun from a liquid crystalline solution. Early applications of aramid fibers included filament-wound motor cases, and gas pressure vessels. Aramid fibers have lower compressive strengths than do carbon fibers, but their high specific strengths, low densities, and toughness keep them in demand.
Boron fibers were the first high-performance reinforcement available for use in advance composites. They are, however, more expensive and less attractive for their mechanical properties than carbon fibers. Boron filaments are made by the decomposition of boron halides on a hot tungsten wire. Composites can also be made from whiskers dispersed in an appropriate matrix.
Continuous silicon carbide fibers are used for large-diameter monofilaments and fine multifilament yarns. Silicon carbide fibers are inherently more economical than boron fibers, and the properties of silicon carbide fibers are generally as good or better than those of boron.
Aluminum oxide (alumina) fibers are produced by dry spinning from various solutions. They are coated with silica to improve their contact properties with molten metal.
There is usually a size effect associated with strong filaments. Their strengths decrease as their diameter increases. It turns out that very high strength materials have diameters of about 1 micrometer. They are consequently not easy to handle.
Matrices for high performance composites
The matrix binds fibers together by virtue of its cohesive and adhesive characteristics. Its purpose is to transfer load to and between fibers, and to protect the fibers from hostile environments and handling. The matrix is the weak link in the composite, so when the composite experiences loading, the matrix may crack, debond from the fiber surface, or break down under far lower strains than are usually desired. However, matrices keep the reinforcing fibers in their proper orientation and position so that they can carry loads, distribute loads evenly among fibers, and provide resistance to crack propagation and damage. Limitations in the matrix generally determine the overall service temperature limitations of the composite.
Polyester and vinyl esterresins are the most widely used matrix materials in high performance continuous-fiber composites. They are used for chemically resistant piping and reactors, truck cabs and bodies, appliances, bathtubs and showers, automobile hoods, decks,and doors. These matrices are usually reinforced with glass fibers, as it has been difficult to adhere the matrix suitably to carbon and aramid fibers. Epoxies and other resins,though more expensive, find applications as replacements for polyester and vinyl ester resins in high performance sporting goods, piping for chemical processing plants, and printed circuit boards.
Epoxy resins are used more than all other matrices in advanced composite materials forstructural aerospace applications. Epoxies aregenerally superior to polyesters in their resistance to moisture and other environmental influences.
Bismaleimide resins, like epoxies, are fairly easy to handle, relatively easily processed, and have excellent composite properties. They are able to withstand greater fluctuations in hot/wet conditionsthan are epoxies, but they have worse failure characteristics.
Polyimide resins release volatiles during curing, which produces voids in the resulting composite. However, these resins do withstand even greater hot/wet temperature extremes than bismaleimide matrices, and work has been underway to minimize the void problem.
The thermoplastic resins used as composite matrices such as polyether etherketone, polyphenylene sulfide, and polyetherimide are very different from the commodity thermoplastics such as polyethylene and polyvinyl chloride. Although used in limited quantities, they are attractive for applications requiring improved hot/wet properties and impact resistance.
Other composites
In addition to the examples already given, examples of composites materials also include: (1) Reinforced and pre-stressed concrete, which is a composite of steel and concrete. Concrete is itself a composite of rocks (coarse aggregate), sand (fine aggregate), hydrated Portland cement, and usually, voids. (2) Cutters for machining made of fine particles of tungsten carbide, which is extremely hard, are mixed with about 6% cobalt powder and sintered at high temperatures. (3) Ordinary grinding wheels, which are composites of an abrasive with a binder that may be plastic or metallic. (4) Walls for housing, which have been made of thin aluminum sheets epoxied to polyurethane foam. The foam provides excellent thermal insulation. This composite has a higher structural rigidity than aluminum sheets or polyurethane foam alone. The polyurethane foam is itself a composite of air and polyurethane. (5) Underground electrical cables composed of sodium metal enclosed in polyethylene. (6) Superconducting ribbons made of Nb3 Sn deposited on copper. (7) Synthetic hard superconductors made by forcing liquid lead under pressure into porous glass fibers. (8) Microelectronic circuits made from silicon, which are oxidized to form an insulating layer of SiO2 . This insulating layer is etched away with hydrofluoric acid, and phosphorous is diffused into the silicon to make a junction. Aluminum or another metal can be introduced as a microconductor between points. The microelectronic circuit is thus a tailored composite.(9) Ceramic fiber composites including graphite or pyrolytic carbon reinforced with graphite fibers; and
KEY TERMS
Fiber— In terms of composite fillers, a fiber is a filler with one long dimension. More specifically, a fiber is a complex morphological unit with an extremely high ratio of length to diameter (typically several hundred to one) and a relatively high tenacity.
Lamina (platelet)— In terms of composite fillers, a lamina is afiller with two long dimensions.
Matrix— The part of the composite that binds the filler by virtue of its cohesive and adhesive characteristics.
Particle— In terms of composite fillers, a particle is a fillerwith no long dimension. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral or irregular in shape.
borosilicate glass lithium aluminum silicate glass ceramics reinforced with silicon carbide fibers. It was possible to drive a tungsten carbide spike through such a composition without secondary cracking in much the same way that a nail can be driven through wood.
Resources
Books
Backman, Bjorn F. Composite Structures, Design, Safety, and Innovation. Amsterdam, Netherlands, and Boston, MA: Elsevier, 2005.
Delhaes, Pierre. Fibers and Composites. London, UK, and New York: Taylor & Francis, 2003.
Nethercot, David A. Composite Construction. London, UK, and New York: Taylor & Francis, 2003.
Sperling, Leslie H. Introduction to Physical Polymer Science. Hoboken, NJ: Wiley-Interscience, 2006.
Randall Frost
Composite Materials
Composite materials
A composite material is a microscopic or macroscopic combination of two or more distinct materials with a recognizable interface between them. For structural applications, the definition can be restricted to include those materials that consist of a reinforcing phase such as fibers or particles supported by a binder or matrix phase. Other features of composites include the following: (1) The distribution of materials in the composite is controlled by mechanical means; (2) The term composite is usually reserved for materials in which distinct phases are separated on a scale larger than atomic, and in which the composite's mechanical properties are significantly altered from those of the constituent components; (3) The composite can be regarded as a combination of two or more materials that are used in combination to rectify a weakness in one material by a strength in another. (4) A recently developed concept of composites is that the composite should not only be a combination of two materials, but the combination should have its own distinctive properties. In terms of strength, heat resistance, or some other desired characteristic, the composite must be better than either component alone.
Composites were developed because no single, homogeneous structural material could be found that had all of the desired characteristics for a given application. Fiber-reinforced composites were first developed to replace aluminum alloys, which provide high strength and fairly high stiffness at low weight but are subject to corrosion and fatigue.
An example of a composite material is a glass-reinforced plastic fishing rod in which glass fibers are placed in an epoxy matrix. Fine individual glass fibers are characterized by their high tensile stiffnesses and a very high tensile strengths, but because of their small diameters, have very small bending stiffnesses. If the rod were made only of epoxy plastic, it would have good bending stiffness, but poor tensile properties. When the fibers are placed in the epoxy plastic, however, the resultant structure has high tensile stiffness, high tensile strength, and high bending stiffness.
The discontinuous filler phase in a composite is usually stiffer or stronger than the binder phase. There must be a substantial volume fraction of the reinforcing phase (~10%) present to provide reinforcement. Examples do exist, however, of composites where the discontinuous phase is more compliant and ductile than the matrix.
Natural composites include wood and bone. Wood is a composite of cellulose and lignin. Cellulose fibers are strong in tension and are flexible. Lignin cements these fibers together to make them stiff. Bone is a composite of strong but soft collagen (a protein) and hard but brittle apatite (a mineral).
Particle-reinforced composites
A particle has no long dimension. Particle composites consist of particles of one material dispersed in a matrix of a second material. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral, or irregular in shape. They may be added to a liquid matrix that later solidifies; grown in place by a reaction such as agehardening; or they may be pressed together and then inter-diffused via a powder process. The particles may be treated to be made compatible with the matrix, or they may be incorporated without such treatment. Particles are most often used to extend the strength or other properties of inexpensive materials by the addition of other materials.
Fiber-reinforced composites
A fiber has one long dimension. Fiber-reinforced materials are typified by fiberglass in which there are three components: glass filaments (for mechanical strength), a polymer matrix (to encapsulate the filaments); and a bonding agent (to bind the glass to the polymer). Other fibers include metal , ceramics , and polymers. The fibers can be used as continuous lengths, in staple-fiber form, or as whiskers (short, fine, perfect, or nearly perfect single crystals). Fiber-reinforcement depends as much on fabrication procedure as on materials.
Laminar composites
Platelets or lamina have two long dimensions. Laminar composites include plywood, which is a laminated composite of thin layers of wood in which successive layers have different grain or fiber orientations. The result is a more-or-less isotropic composite sheet that is weaker in any direction than it would be if the fibers were all aligned in one direction. The stainless steel in a cooking vessel with a copper-clad bottom provides corrosion resistance while the copper provides better heat distribution over the base of the vessel.
Mechanical properties
The mechanical properties of composite materials usually depend on structure. Thus these properties typically depend on the shape of inhomogenities, the volume fraction occupied by inhomogenities, and the interfaces between the components.The strength of composites depends on such factors as the brittleness or ductility of the inclusions and matrix.
For example, failure mechanisms in fiber-filled composites include fracture of the fibers; shear failure of the matrix along the fibers; fracture of the matrix in tension normal to the fibers or failure of the fiber-matrix interface. The mechanism responsible for failure depends on the angle between the fibers and the specimen's axis.
If a mechanical property depends on the composite material's orientation, the property is said to be anisotropic. Anisotropic composites provide greater strength and stiffness than do isotropic materials. But the material properties in one direction are gained at the expense of the properties in other directions. For example, silica fibers in a pure aluminum matrix produce a composite with a tensile strength of about 110,000 psi along the fiber direction, but a tensile strength of only about 14,000 psi at right angles to the fiber axis. It therefore only makes sense to use anisotropic materials if the direction that they will be stressed is known in advance.
Isotropic material are materials properties independent of orientation. Stiff platelet inclusions are the most effective in creating a stiff composite, followed by fibers, and then by spherical particles.
High performance composites
High performance composites are composites that have better performance than conventional structural materials such as steel and aluminum alloys. They are almost all continuous fiber-reinforced composites, with organic (resin) matrices.
Fibers for high performance composites
In a high-performance, continuous fiber-reinforced composite, fibers provide virtually all of the load-carrying characteristics of the composite, i.e., strength and stiffness. The fibers in such a composite form bundles, or filaments. Consequently, even if several fibers break, the load is redistributed to other fibers, which avoids a catastrophic failure.
Glass fibers are used for nonstructural, low-performance applications such as panels in aircraft and appliances to high-performance applications such as rocket-motor cases and pressure vessels. But the sensitivity of the glass fiber to attack by moisture poses problems for other applications. The most commonly used glass fiber is a calcium aluminoborosilicate glass (E-glass). High silica and quartz fibers are also used for specialized applications.
Carbon fibers are the best known and most widely used reinforcing fiber in advanced composites.The earliest carbon fibers were produced by thermal decomposition of rayon precursor materials. The starting material is now polyacrylonitrile.
Aramid fibers are aromatic polyamide fibers. The aramid fiber is technically a thermoplastic polymer like nylon, but it decomposes when heated before it reaches its projected melting point. When polymerized, it forms rigid, rod-like molecules that cannot be spun from a melt. Instead they have to be spun from a liquid crystalline solution . Early applications of aramid fibers included filament-wound motor cases, and gas pressure vessels. Aramid fibers have lower compressive strengths than do carbon fibers, but their high specific strengths, low densities, and toughness keep them in demand.
Boron fibers were the first high-performance reinforcement available for use in advance composites. They are, however, more expensive and less attractive for their mechanical properties than carbon fibers. Boron filaments are made by the decomposition of boron halides on a hot tungsten wire. Composites can also be made from whiskers dispersed in an appropriate matrix.
Continuous silicon carbide fibers are used for large-diameter monofilaments and fine multifilament yarns. Silicon carbide fibers are inherently more economical than boron fibers, and the properties of silicon carbide fibers are generally as good or better than those of boron.
Aluminum oxide (alumina) fibers are produced by dry spinning from various solutions. They are coated with silica to improve their contact properties with molten metal.
There is usually a size effect associated with strong filaments. Their strengths decrease as their diameter increases. It turns out that very high strength materials have diameters of about 1 micrometer. They are consequently not easy to handle.
Matrices for high performance composites
The matrix binds fibers together by virtue of its cohesive and adhesive characteristics. Its purpose is to transfer load to and between fibers, and to protect the fibers from hostile environments and handling. The matrix is the weak link in the composite, so when the composite experiences loading, the matrix may crack, debond from the fiber surface, or break down under far lower strains than are usually desired. But matrices keep the reinforcing fibers in their proper orientation and position so that they can carry loads, distribute loads evenly among fibers, and provide resistance to crack propagation and damage. Limitations in the matrix generally determine the overall service temperature limitations of the composite.
Polyester and vinyl esterresins are the most widely used matrix materials in high performance continuous-fiber composites. They are used for chemically resistant piping and reactors, truck cabs and bodies, appliances, bathtubs and showers, automobile hoods, decks, and doors. These matrices are usually reinforced with glass fibers, as it has been difficult to adhere the matrix suitably to carbon and aramid fibers. Epoxies and other resins , though more expensive, find applications as replacements for polyester and vinyl ester resins in high performance sporting goods, piping for chemical processing plants, and printed circuit boards.
Epoxy resins are used more than all other matrices in advanced composite materials for structural aerospace applications. Epoxies are generally superior to polyesters in their resistance to moisture and other environmental influences.
Bismaleimide resins, like epoxies, are fairly easy to handle, relatively easily processed, and have excellent composite properties. They are able to withstand greater fluctuations in hot/wet conditions than are epoxies, but they have worse failure characteristics.
Polyimide resins release volatiles during curing, which produces voids in the resulting composite. However, these resins do withstand even greater hot/wet temperature extremes than bismaleimide matrices, and work has been underway to minimize the void problem.
The thermoplastic resins used as composite matrices such as polyether etherketone, polyphenylene sulfide, and polyetherimide are very different from the commodity thermoplastics such as polyethylene and polyvinyl chloride. Although used in limited quantities, they are attractive for applications requiring improved hot/wet properties and impact resistance.
Other composites
In addition to the examples already given, examples of composites materials also include: (1) Reinforced and prestressed concrete , which is a composite of steel and concrete. Concrete is itself a composite of rocks (coarse aggregate), sand (fine aggregate), hydrated Portland cement, and usually, voids. (2) Cutters for machining made of fine particles of tungsten carbide, which is extremely hard, are mixed with about 6% cobalt powder and sintered at high temperatures. (3) Ordinary grinding wheels, which are composites of an abrasive with a binder that may be plastic or metallic. (4) Walls for housing, which have been made of thin aluminum sheets epoxied to polyurethane foam. The foam provides excellent thermal insulation. This composite has a higher structural rigidity than aluminum sheets or polyurethane foam alone. The polyurethane foam is itself a composite of air and polyurethane. (5) Underground electrical cables composed of sodium metal enclosed in polyethylene. (6) Superconducting ribbons made of Nb3Sn deposited on copper. (7) Synthetic hard superconductors made by forcing liquid lead under pressure into porous glass fibers. (8) Micro-electronic circuits made from silicon, which are oxidized to form an insulating layer of SiO2. This insulating layer is etched away with hydrofluoric acid, and phosphorous is diffused into the silicon to make a junction. Aluminum or another metal can be introduced as a microconductor between points. The microelectronic circuit is thus a tailored composite. (9) Ceramic fiber composites including graphite or pyrolytic carbon reinforced with graphite fibers; and borosilicate glass lithium aluminum silicate glass ceramics reinforced with silicon carbide fibers. It was possible to drive a tungsten carbide spike through such a composition without secondary cracking in much the same way that a nail can be driven through wood.
Resources
books
Reinhart, Theodore J. "Introduction to Composites." In Engineered Materials Handbook Vol. 1. Metals Park, OH: ASM International, 1987.
Smith, Charles O. The Science of Engineering Materials. Englewood Cliffs, NJ: Prentice-Hall, Inc. 1969.
Sperling, L.H. Introduction to Physical Polymer Science. New York, John Wiley & Sons, Inc. 1992.
Randall Frost
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Fiber
—In terms of composite fillers, a fiber is a filler with one long dimension. More specifically, a fiber is a complex morphological unit with an extremely high ratio of length to diameter (typically several hundred to one) and a relatively high tenacity.
- Lamina (platelet)
—In terms of composite fillers, a lamina is a filler with two long dimensions.
- Matrix
—The part of the composite that binds the filler by virtue of its cohesive and adhesive characteristics.
- Particle
—In terms of composite fillers, a particle is a filler with no long dimension. Particles may have any shape or size, but are generally spherical, ellipsoidal, polyhedral, or irregular in shape.
Composite Materials
Composite materials
A composite material (or just composite) is a mixture of two or more materials with properties superior to the materials of which it is made. Many common examples of composite materials can be found in the world around us. Wood and bone are examples of natural composites. Wood consists of cellulose fibers embedded in a compound called lignin. The cellulose fibers give wood its ability to bend without breaking, while the lignin makes wood stiff. Bone is a combination of a soft form of protein known as collagen and a strong but brittle mineral called apatite.
Traditional composites
Humans have been using composite materials for centuries, long before they fully understood the structures of such composites. The important building material concrete, for example, is a mixture of rocks, sand, and Portland cement. Concrete is a valuable building material because it is much stronger than any one of the individual components of which it is made. Interestingly enough, two of those components are themselves natural composites. Rock is a mixture of stony materials of various sizes, and sand is a composite of small-grained materials.
Reinforced concrete is a composite developed to further improve the strength of concrete. Steel rods embedded in concrete add both strength and flexibility to the concrete.
Cutting wheels designed for use with very hard materials are also composites. They are made by combining fine particles of tungsten carbide with cobalt powder. Tungsten carbide is one of the hardest materials known, so the composite formed by this method can be used to cut through almost any natural or synthetic material.
Words to Know
Fiber: In terms of composite fillers, a fiber is a filler with one long dimension.
Matrix: The part of the composite that binds the filler.
Particle: In terms of composite fillers, a particle is a filler with no long dimension.
Some forms of aluminum siding used in homes are also composite materials. Thin sheets of aluminum metal are attached to polyurethane foam. The polyurethane foam is itself a composite consisting of air mixed with polyurethane. Joining the polyurethane foam to the aluminum makes the aluminum more rigid and provides excellent insulation, an important property for the walls of a house.
In general, composites are developed because no single structural material can be found that has all of the desired characteristics for a given application. Fiber-reinforced composites, for example, were first developed to replace aluminum alloys (mixtures), which provide high strength and fairly high stiffness at low weight but corrode rather easily and can break under stress.
Composite structure
Composites consist of two parts: the reinforcing phase and the binder, or matrix. In reinforced concrete, for example, the steel rods are the reinforcing phase; the concrete in which the rods are embedded are the binder or matrix.
In general, the reinforcing phase can exist in one of three forms: particles, fibers, or flat sheets. In the cutting wheels described above, for example, the reinforcing phase consists of tiny particles of cobalt metal in a binder of tungsten carbide. A plastic fishing rod is an example of a composite in which the reinforcing phase is a fiber. In this case, the fiber is made of threadlike strips of glass placed in an epoxy matrix. (Epoxy is a strong kind of plastic.) An example of a flat sheet reinforcing phase is plywood. Plywood is made by gluing together thin layers of wood so that the wood grain runs in different directions.
The binder or matrix in each of these cases is the material that supports and holds in place the reinforcing material. It is the tungsten carbide in the cutting wheel, the epoxy plastic in the fishing rod, or the glue used to hold the sheets of wood together.
High-performance composites
High-performance composites are composites that perform better than conventional structural materials such as steel and aluminum alloys. They are almost all fiber-reinforced composites with polymer (plasticlike) matrices.
The fibers used in high-performance composites are made of a wide variety of materials, including glass, carbon, boron, silicon carbide, aluminum oxide, and certain types of polymers. These fibers are generally interwoven to form larger filaments or bundles. Thus, if one fiber or a few individual fibers break, the structural unit as a whole—the filament or bundle—remains intact. Fibers usually provide composites with the special properties, such as strength and stiffness, for which they are designed.
In contrast, the purpose of the matrix in a high-performance composite is to hold the fibers together and protect them from damage from the outside environment (such as heat or moisture) and from rough handling. The matrix also transfers the load placed on a composite from one fiber bundle to the next.
Most matrices consist of polymers such as polyesters, epoxy vinyl, and bismaleimide and polyimide resins. The physical properties of any given matrix determine the ultimate uses of the composite itself. For example, if the matrix melts or cracks at a low temperature, the composite can be used for applications only at temperatures less than that melting or cracking point.
composite
com·pos·ite / kəmˈpäzət; käm-/ • adj. 1. made up of various parts or elements. ∎ (esp. of a constructional material) made up of recognizable constituents. ∎ (of a railroad car) having compartments of more than one class or function. ∎ Math. (of an integer) being the product of two or more factors greater than one; not prime.2. (Composite) relating to or denoting a classical order of architecture consisting of elements of the Ionic and Corinthian orders.3. Bot. relating to or denoting plants of the daisy family (Compositae).• n. 1. a thing made up of several parts or elements. ∎ a composite photograph. ∎ a composite constructional material.2. Bot. a plant of the daisy family (Compositae).3. (Composite) the Composite order of architecture.• v. [tr.] [usu. as n.] (compositing) combine (two or more images) to make a single picture, esp. electronically: photographic compositing by computer. ∎ ∎ amalgamate (two or more similar resolutions).DERIVATIVES: com·pos·ite·ly adv.com·pos·ite·ness n.