16 Mechanics of Composite Materials,Second Edition Reaction bonding occurs when atoms or molecules of the fiber and the matrix diffuse into each other at the interface.This interdiffusion often creates a distinct interfacial layer,called the interphase,with different properties from that of the fiber or the matrix.Although this thin interfacial layer helps to form a bond,it also forms micro- cracks in the fiber.These microcracks reduce the strength of the fiber and thus that of the composite. Weak or cracked interfaces can cause failure in composites and reduce the properties influenced by the matrix.They also allow environmental hazards such as hot gases and moisture to attack the fibers. Although a strong bond is a requirement in transferring loads from the matrix to the fiber,weak debonding of the fiber-matrix interface is used advantageously in ceramic matrix composites.Weak interfaces blunt matrix cracks and deflect them along the interface.This is the main source of improving toughness of such composites up to five times that of the mono- lithic ceramics. What is the world market of composites? The world market for composites is only 10 x 10 US dollars as compared to more than 450 x 109 US dollars for steel.The annual growth of composites is at a steady rate of 10%.Presently,composite shipments are about 3 x 109 lb annually.Figure 1.7 gives the relative market share of US composite shipments and shows transportation clearly leading in their use.Table 1.3 shows the market share of composites since 1990. 1.2 Classification How are composites classified? Composites are classified by the geometry of the reinforcement-partic- ulate,flake,and fibers(Figure 1.8)-or by the type of matrix-polymer, metal,ceramic,and carbon. Parficulate composites consist of particles immersed in matrices such as alloys and ceramics.They are usually isotropic because the par- ticles are added randomly.Particulate composites have advantages such as improved strength,increased operating temperature,oxida- tion resistance,etc.Typical examples include use of aluminum par- ticles in rubber;silicon carbide particles in aluminum;and gravel, sand,and cement to make concrete. Flake composites consist of flat reinforcements of matrices.Typical flake materials are glass,mica,aluminum,and silver.Flake compos- 2006 by Taylor Francis Group,LLC
16 Mechanics of Composite Materials, Second Edition • Reaction bonding occurs when atoms or molecules of the fiber and the matrix diffuse into each other at the interface. This interdiffusion often creates a distinct interfacial layer, called the interphase, with different properties from that of the fiber or the matrix. Although this thin interfacial layer helps to form a bond, it also forms microcracks in the fiber. These microcracks reduce the strength of the fiber and thus that of the composite. Weak or cracked interfaces can cause failure in composites and reduce the properties influenced by the matrix. They also allow environmental hazards such as hot gases and moisture to attack the fibers. Although a strong bond is a requirement in transferring loads from the matrix to the fiber, weak debonding of the fiber–matrix interface is used advantageously in ceramic matrix composites. Weak interfaces blunt matrix cracks and deflect them along the interface. This is the main source of improving toughness of such composites up to five times that of the monolithic ceramics. What is the world market of composites? The world market for composites is only 10 × 109 US dollars as compared to more than 450 × 109 US dollars for steel. The annual growth of composites is at a steady rate of 10%. Presently, composite shipments are about 3 × 109 lb annually. Figure 1.7 gives the relative market share of US composite shipments and shows transportation clearly leading in their use. Table 1.3 shows the market share of composites since 1990. 1.2 Classification How are composites classified? Composites are classified by the geometry of the reinforcement — particulate, flake, and fibers (Figure 1.8) — or by the type of matrix — polymer, metal, ceramic, and carbon. • Particulate composites consist of particles immersed in matrices such as alloys and ceramics. They are usually isotropic because the particles are added randomly. Particulate composites have advantages such as improved strength, increased operating temperature, oxidation resistance, etc. Typical examples include use of aluminum particles in rubber; silicon carbide particles in aluminum; and gravel, sand, and cement to make concrete. • Flake composites consist of flat reinforcements of matrices. Typical flake materials are glass, mica, aluminum, and silver. Flake compos- 1343_book.fm Page 16 Tuesday, September 27, 2005 11:53 AM © 2006 by Taylor & Francis Group, LLC
Introduction to Composite Materials 17 Total shipments in 1995:3.176(10)Ib [1.441 (10)kgs] Appliance business Other equipment Consumer products Electrical Transportation electronics Corrosion-resistant equipment Marine Construction FIGURE 1.7 Approximate shipments of polymer-based composites in 1995.(Source:Data used in figure published with permission of the SPI,Inc.;http://www.socplas.org.) TABLE 1.3 U.S.Composites Shipment in 106 lb,Including Reinforced Thermoset and Thermoplastic Resin Composites,Reinforcements,and Fillers Markets 1990 1991 1992 1993 1994 1995 Aircraft/aerospace/military 39 38.7 32.3 25.4 24.2 24.0 Appliance/business equipment 153 135.2 143.2 147.5 160.7 166.5 Construction 468 420.0 483.0 530.0 596.9 626.9 Consumer products 165 148.7 162.2 165.7 174.8 183.6 Corrosion-resistant equipment 350 355.0 332.3 352.0 376.3 394.6 Electrical/electronic 241 231.1 260.0 274.9 299.3 315.1 Marine 375 275.0 304.4 319.3 363.5 375.1 Transportation 705 682.2 750.0 822.1 945.6 984.0 Other 79 73.8 83.4 893 101.8 106.6 TOTAL 2575 2360 2551 2726 3043.1 3176.4 Source:Published with permission of the SPI,Inc. 2006 by Taylor Francis Group,LLC
Introduction to Composite Materials 17 FIGURE 1.7 Approximate shipments of polymer-based composites in 1995. (Source: Data used in figure published with permission of the SPI, Inc.; http://www.socplas.org.) TABLE 1.3 U.S. Composites Shipment in 106 lb, Including Reinforced Thermoset and Thermoplastic Resin Composites, Reinforcements, and Fillers Markets 1990 1991 1992 1993 1994 1995 Aircraft/aerospace/military 39 38.7 32.3 25.4 24.2 24.0 Appliance/business equipment 153 135.2 143.2 147.5 160.7 166.5 Construction 468 420.0 483.0 530.0 596.9 626.9 Consumer products 165 148.7 162.2 165.7 174.8 183.6 Corrosion-resistant equipment 350 355.0 332.3 352.0 376.3 394.6 Electrical/electronic 241 231.1 260.0 274.9 299.3 315.1 Marine 375 275.0 304.4 319.3 363.5 375.1 Transportation 705 682.2 750.0 822.1 945.6 984.0 Other 79 73.8 83.4 89.3 101.8 106.6 TOTAL 2575 2360 2551 2726 3043.1 3176.4 Source: Published with permission of the SPI, Inc. Appliance & business equipment Consumer products Electrical & electronics Corrosion-resistant equipment Marine Construction Transportation Other Total shipments in 1995: 3.176 (109)lb [1.441 (109) kgs] 1343_book.fm Page 17 Tuesday, September 27, 2005 11:53 AM © 2006 by Taylor & Francis Group, LLC
18 Mechanics of Composite Materials,Second Edition 444 O】 Particulate composites Flake composites 6 t 0 Fiber composites FIGURE 1.8 Types of composites based on reinforcement shape. ites provide advantages such as high out-of-plane flexural modulus,* higher strength,and low cost.However,flakes cannot be oriented easily and only a limited number of materials are available for use. Fiber composites consist of matrices reinforced by short(discontin- uous)or long(continuous)fibers.Fibers are generally anisotropict and examples include carbon and aramids.Examples of matrices are resins such as epoxy,metals such as aluminum,and ceramics such as calcium-alumino silicate.Continuous fiber composites are emphasized in this book and are further discussed in this chapter by the types of matrices:polymer,metal,ceramic,and carbon.The fundamental units of continuous fiber matrix composite are unidi- rectional or woven fiber laminas.Laminas are stacked on top of each other at various angles to form a multidirectional laminate. Nanocomposites consist of materials that are of the scale of nanome- ters(10m).The accepted range to be classified as a nanocomposite is that one of the constituents is less than 100 nm.At this scale,the Out of plane flexural stiffness is the resistance to deflection under bending that is out of the plane,such as bending caused by a heavy stone placed on a simply supported plate. t Anisotropic materials are the opposite of isotropic materials like steel and aluminum;they have different properties in different directions.For example,the Young's modulus of a piece of wood is higher(different)in the direction of the grain than in the direction perpendicular to the grain.In comparison,a piece of steel has the same Young's modulus in all directions. 2006 by Taylor Francis Group,LLC
18 Mechanics of Composite Materials, Second Edition ites provide advantages such as high out-of-plane flexural modulus,* higher strength, and low cost. However, flakes cannot be oriented easily and only a limited number of materials are available for use. • Fiber composites consist of matrices reinforced by short (discontinuous) or long (continuous) fibers. Fibers are generally anisotropic† and examples include carbon and aramids. Examples of matrices are resins such as epoxy, metals such as aluminum, and ceramics such as calcium–alumino silicate. Continuous fiber composites are emphasized in this book and are further discussed in this chapter by the types of matrices: polymer, metal, ceramic, and carbon. The fundamental units of continuous fiber matrix composite are unidirectional or woven fiber laminas. Laminas are stacked on top of each other at various angles to form a multidirectional laminate. • Nanocomposites consist of materials that are of the scale of nanometers (10–9 m). The accepted range to be classified as a nanocomposite is that one of the constituents is less than 100 nm. At this scale, the FIGURE 1.8 Types of composites based on reinforcement shape. * Out of plane flexural stiffness is the resistance to deflection under bending that is out of the plane, such as bending caused by a heavy stone placed on a simply supported plate. † Anisotropic materials are the opposite of isotropic materials like steel and aluminum; they have different properties in different directions. For example, the Young’s modulus of a piece of wood is higher (different) in the direction of the grain than in the direction perpendicular to the grain. In comparison, a piece of steel has the same Young’s modulus in all directions. Particulate composites Flake composites Fiber composites 1343_book.fm Page 18 Tuesday, September 27, 2005 11:53 AM © 2006 by Taylor & Francis Group, LLC
Introduction to Composite Materials 19 properties of materials are different from those of the bulk material. Generally,advanced composite materials have constituents on the microscale(106m).By having materials at the nanometer scale,most of the properties of the resulting composite material are better than the ones at the microscale.Not all properties of nanocomposites are better;in some cases,toughness and impact strength can decrease. Applications of nanocomposites include packaging applications for the military in which nanocomposite films show improvement in properties such as elastic modulus,and transmission rates for water vapor,heat distortion,and oxygen.s Body side molding of the 2004 Chevrolet Impala is made of olefin- based nanocomposites.This reduced the weight of the molding by 7%and improved its surface quality.General MotorsTM currently uses 540,000 lb of nanocomposite materials per year. Rubber containing just a few parts per million of metal conducts electricity in harsh conditions just like solid metal.Called Metal Rubber it is fabricated molecule by molecule by a process called electrostatic self-assembly.Awaited applications of the Metal Rubber include artificial muscles,smart clothes,flexible wires,and circuits for portable electronics.10 1.2.1 Polymer Matrix Composites What are the most common advanced composites? The most common advanced composites are polymer matrix composites (PMCs)consisting of a polymer(e.g.,epoxy,polyester,urethane)reinforced by thin diameter fibers(e.g.,graphite,aramids,boron).For example,graphite/ epoxy composites are approximately five times stronger than steel on a weight- for-weight basis.The reasons why they are the most common composites include their low cost,high strength,and simple manufacturing principles. What are the drawbacks of polymer matrix composites? The main drawbacks of PMCs include low operating temperatures,high coefficients of thermal and moisture expansion,and low elastic properties in certain directions. What are the typical mechanical properties of some polymer matrix com- posites?Compare these properties with metals. Table 1.4 gives typical mechanical properties of common polymer matrix composites. *Some materials,such as polymers,absorb or deabsorb moisture that results in dimensional changes.The coefficient of moisture expansion is the change in length per unit length per unit mass of moisture absorbed per unit mass of the substance. 2006 by Taylor Francis Group,LLC
Introduction to Composite Materials 19 properties of materials are different from those of the bulk material. Generally, advanced composite materials have constituents on the microscale (10–6 m). By having materials at the nanometer scale, most of the properties of the resulting composite material are better than the ones at the microscale. Not all properties of nanocomposites are better; in some cases, toughness and impact strength can decrease. Applications of nanocomposites include packaging applications for the military in which nanocomposite films show improvement in properties such as elastic modulus, and transmission rates for water vapor, heat distortion, and oxygen.8 Body side molding of the 2004 Chevrolet Impala is made of olefinbased nanocomposites.9 This reduced the weight of the molding by 7% and improved its surface quality. General Motors™ currently uses 540,000 lb of nanocomposite materials per year. Rubber containing just a few parts per million of metal conducts electricity in harsh conditions just like solid metal. Called Metal Rubber®, it is fabricated molecule by molecule by a process called electrostatic self-assembly. Awaited applications of the Metal Rubber include artificial muscles, smart clothes, flexible wires, and circuits for portable electronics.10 1.2.1 Polymer Matrix Composites What are the most common advanced composites? The most common advanced composites are polymer matrix composites (PMCs) consisting of a polymer (e.g., epoxy, polyester, urethane) reinforced by thin diameter fibers (e.g., graphite, aramids, boron). For example, graphite/ epoxy composites are approximately five times stronger than steel on a weightfor-weight basis. The reasons why they are the most common composites include their low cost, high strength, and simple manufacturing principles. What are the drawbacks of polymer matrix composites? The main drawbacks of PMCs include low operating temperatures, high coefficients of thermal and moisture expansion,* and low elastic properties in certain directions. What are the typical mechanical properties of some polymer matrix composites? Compare these properties with metals. Table 1.4 gives typical mechanical properties of common polymer matrix composites. * Some materials, such as polymers, absorb or deabsorb moisture that results in dimensional changes. The coefficient of moisture expansion is the change in length per unit length per unit mass of moisture absorbed per unit mass of the substance. 1343_book.fm Page 19 Tuesday, September 27, 2005 11:53 AM © 2006 by Taylor & Francis Group, LLC
20 Mechanics of Composite Materials,Second Edition TABLE 1.4 Typical Mechanical Properties of Polymer Matrix Composites and Monolithic Materials Graphite/ Glass/ Property Units epoxy epoxy Steel Aluminum System of units:USCS Specific gravity 1.6 1.8 7.8 2.6 Young's modulus Msi 26.25 5.598 30.0 10.0 Ultimate tensile strength ksi 217.6 154.0 94.0 40.0 Coefficient of thermal expansion uin./in./℉ 0.01111 4.778 6.5 12.8 System of units:SI Specific gravity 1.6 1.8 7.8 2.6 Young's modulus GPa 181.0 38.6 206.8 68.95 Ultimate tensile strength MPa 150.0 1062 648.1 275.8 Coefficient of thermal expansion μm/m/C 0.02 8.6 11.7 23 TABLE 1.5 Typical Mechanical Properties of Fibers Used in Polymer Matrix Composites Property Units Graphite Aramid Glass Steel Aluminum System of units:UISCS Specific gravity 1.8 1.4 2.5 78 2.6 Young's modulus Msi 33.35 17.98 12.33 30 10.0 Ultimate tensile strength ksi 299.8 200.0 224.8 94 40.0 Axial coefficient of uin./in./F -0.722 -2.778 2.778 6.5 12.8 thermal expansion System of units:SI Specific gravity 1.8 1.4 2.5 7.8 2.6 Young's modulus GPa 230 124 85 206.8 68.95 Ultimate tensile strength MPa 2067 1379 1550 648.1 275.8 Axial coefficient of um/m/℃ -1.3 -5 5 11.7 23 thermal expansion Give names of various fibers used in advanced polymer composites. The most common fibers used are glass,graphite,and Kevlar.Typical properties of these fibers compared with bulk steel and aluminum are given in Table 1.5. Give a description of the glass fiber. Glass is the most common fiber used in polymer matrix composites.Its advantages include its high strength,low cost,high chemical resistance,and good insulating properties.The drawbacks include low elastic modulus, 2006 by Taylor Francis Group,LLC
20 Mechanics of Composite Materials, Second Edition Give names of various fibers used in advanced polymer composites. The most common fibers used are glass, graphite, and Kevlar. Typical properties of these fibers compared with bulk steel and aluminum are given in Table 1.5. Give a description of the glass fiber. Glass is the most common fiber used in polymer matrix composites. Its advantages include its high strength, low cost, high chemical resistance, and good insulating properties. The drawbacks include low elastic modulus, TABLE 1.4 Typical Mechanical Properties of Polymer Matrix Composites and Monolithic Materials Property Units Graphite/ epoxy Glass/ epoxy Steel Aluminum System of units: USCS Specific gravity Young’s modulus Ultimate tensile strength Coefficient of thermal expansion — Msi ksi μin./in./°F 1.6 26.25 217.6 0.01111 1.8 5.598 154.0 4.778 7.8 30.0 94.0 6.5 2.6 10.0 40.0 12.8 System of units: SI Specific gravity Young’s modulus Ultimate tensile strength Coefficient of thermal expansion — GPa MPa μm/m/°C 1.6 181.0 150.0 0.02 1.8 38.6 1062 8.6 7.8 206.8 648.1 11.7 2.6 68.95 275.8 23 TABLE 1.5 Typical Mechanical Properties of Fibers Used in Polymer Matrix Composites Property Units Graphite Aramid Glass Steel Aluminum System of units: USCS Specific gravity Young’s modulus Ultimate tensile strength Axial coefficient of thermal expansion — Msi ksi μin./in./°F 1.8 33.35 299.8 –0.722 1.4 17.98 200.0 –2.778 2.5 12.33 224.8 2.778 7.8 30 94 6.5 2.6 10.0 40.0 12.8 System of units: SI Specific gravity Young’s modulus Ultimate tensile strength Axial coefficient of thermal expansion — GPa MPa μm/m/°C 1.8 230 2067 –1.3 1.4 124 1379 –5 2.5 85 1550 5 7.8 206.8 648.1 11.7 2.6 68.95 275.8 23 1343_book.fm Page 20 Tuesday, September 27, 2005 11:53 AM © 2006 by Taylor & Francis Group, LLC