About the Author F.C.Campbell's 38-year career at The Boeing Company (retired 2007)was closely divided equally between engineering and manufacturing.He worked in the engineering laboratories,manufacturing research and development,as well as engineering on four production aircraft programs,and in production operations.At the time of his retire- ment.he was a Senior Technical Fellow in the field of structural materials and manufac- turing technology.He is knowledgeable about a large number of materials,fabrication, and assembly processes for airframe structural materials.Previously,he was director of manufacturing process improvement(1995-2000),and from 1987-1995,he was direc- tor of manufacturing research engineering.Earlier in his career,he worked in materials and process development with responsibility for composite related research and devel- opment programs.He has also worked on the F-15,F/A-18,AV-8B,and C-17 aircraft programs,conducted manufacturing research on composite and metallic materials,and worked as a laboratory engineer doing process development on both metal matrix and organic matrix composite materials
About the Author F.C. Campbell’s 38-year career at The Boeing Company (retired 2007) was closely divided equally between engineering and manufacturing. He worked in the engineering laboratories, manufacturing research and development, as well as engineering on four production aircraft programs, and in production operations. At the time of his retirement, he was a Senior Technical Fellow in the field of structural materials and manufacturing technology. He is knowledgeable about a large number of materials, fabrication, and assembly processes for airframe structural materials. Previously, he was director of manufacturing process improvement (1995–2000), and from 1987–1995, he was director of manufacturing research engineering. Earlier in his career, he worked in materials and process development with responsibility for composite related research and development programs. He has also worked on the F-15, F/A-18, AV-8B, and C-17 aircraft programs, conducted manufacturing research on composite and metallic materials, and worked as a laboratory engineer doing process development on both metal matrix and organic matrix composite materials
Structural Composite Materials Copyright 2010,ASM International F.C.Campbell All rights reserved www.asminternational.org CHAPTER 1 Introduction to Composite Materials A COMPOSITE MATERIAL can be defined sheets of continuous fibers in different orienta- as a combination of two or more materials that tions to obtain the desired strength and stiffness results in better properties than those of the indi- properties with fiber volumes as high as 60 to vidual components used alone.In contrast to 70 percent.Fibers produce high-strength com- metallic alloys,each material retains its separate posites because of their small diameter;they con- chemical,physical,and mechanical properties. tain far fewer defects (normally surface defects) The two constituents are a reinforcement and a compared to the material produced in bulk.As a matrix.The main advantages of composite ma- general rule,the smaller the diameter of the fiber, terials are their high strength and stiffness,com- the higher its strength,but often the cost increases bined with low density,when compared with as the diameter becomes smaller.In addition. bulk materials,allowing for a weight reduction smaller-diameter high-strength fibers have greater in the finished part. flexibility and are more amenable to fabrication The reinforcing phase provides the strength processes such as weaving or forming over radii. and stiffness.In most cases.the reinforcement is Typical fibers include glass,aramid,and carbon, harder,stronger,and stiffer than the matrix.The which may be continuous or discontinuous. reinforcement is usually a fiber or a particulate. The continuous phase is the matrix,which is a Particulate composites have dimensions that are polymer,metal,or ceramic.Polymers have low approximately equal in all directions.They may strength and stiffness,metals have intermediate be spherical,platelets,or any other regular or ir- strength and stiffness but high ductility,and ce- regular geometry.Particulate composites tend to ramics have high strength and stiffness but are be much weaker and less stiff than continuous- brittle.The matrix (continuous phase)performs fiber composites,but they are usually much less several critical functions,including maintaining expensive.Particulate reinforced composites usu- the fibers in the proper orientation and spacing ally contain less reinforcement (up to 40 to 50 and protecting them from abrasion and the envi- volume percent)due to processing difficulties ronment.In polymer and metal matrix compos- and brittleness. ites that form a strong bond between the fiber A fiber has a length that is much greater than and the matrix,the matrix transmits loads from its diameter.The length-to-diameter(l/d)ratio is the matrix to the fibers through shear loading at known as the aspect ratio and can vary greatly. the interface.In ceramic matrix composites,the Continuous fibers have long aspect ratios,while objective is often to increase the toughness rather discontinuous fibers have short aspect ratios. than the strength and stiffness;therefore,a low Continuous-fiber composites normally have a interfacial strength bond is desirable. preferred orientation,while discontinuous fibers The type and quantity of the reinforcement generally have a random orientation.Examples determine the final properties.Figure 1.2 shows of continuous reinforcements include unidirec- that the highest strength and modulus are ob- tional,woven cloth,and helical winding (Fig. tained with continuous-fiber composites.There is 1.la),while examples of discontinuous rein- a practical limit of about 70 volume percent rein- forcements are chopped fibers and random mat forcement that can be added to form a composite. (Fig.1.1b).Continuous-fiber composites are At higher percentages,there is too little matrix to often made into laminates by stacking single support the fibers effectively.The theoretical
Chapter 1 Introduction to Composite Materials Structural Composite Materials Copyright © 2010, ASM International® F.C. Campbell All rights reserved. www.asminternational.org Chapter 1 Introduction to Composite Materials A composite material can be defined as a combination of two or more materials that results in better properties than those of the individual components used alone. In contrast to metallic alloys, each material retains its separate chemical, physical, and mechanical properties. The two constituents are a reinforcement and a matrix. The main advantages of composite materials are their high strength and stiffness, combined with low density, when compared with bulk materials, allowing for a weight reduction in the finished part. The reinforcing phase provides the strength and stiffness. In most cases, the reinforcement is harder, stronger, and stiffer than the matrix. The reinforcement is usually a fiber or a particulate. Particulate composites have dimensions that are approximately equal in all directions. They may be spherical, platelets, or any other regular or irregular geometry. Particulate composites tend to be much weaker and less stiff than continuousfiber composites, but they are usually much less expensive. Particulate reinforced composites usually contain less reinforcement (up to 40 to 50 volume percent) due to processing difficulties and brittleness. A fiber has a length that is much greater than its diameter. The length-to-diameter (l/d) ratio is known as the aspect ratio and can vary greatly. Continuous fibers have long aspect ratios, while discontinuous fibers have short aspect ratios. Continuous-fiber composites normally have a preferred orientation, while discontinuous fibers generally have a random orientation. Examples of continuous reinforcements include unidirectional, woven cloth, and helical winding (Fig. 1.1a), while examples of discontinuous reinforcements are chopped fibers and random mat (Fig. 1.1b). Continuous-fiber composites are often made into laminates by stacking single sheets of continuous fibers in different orientations to obtain the desired strength and stiffness properties with fiber volumes as high as 60 to 70 percent. Fibers produce high-strength composites because of their small diameter; they contain far fewer defects (normally surface defects) compared to the material produced in bulk. As a general rule, the smaller the diameter of the fiber, the higher its strength, but often the cost increases as the diameter becomes smaller. In addition, smaller-diameter high-strength fibers have greater flexibility and are more amenable to fabrication processes such as weaving or forming over radii. Typical fibers include glass, aramid, and carbon, which may be continuous or discontinuous. The continuous phase is the matrix, which is a polymer, metal, or ceramic. Polymers have low strength and stiffness, metals have intermediate strength and stiffness but high ductility, and ceramics have high strength and stiffness but are brittle. The matrix (continuous phase) performs several critical functions, including maintaining the fibers in the proper orientation and spacing and protecting them from abrasion and the environment. In polymer and metal matrix composites that form a strong bond between the fiber and the matrix, the matrix transmits loads from the matrix to the fibers through shear loading at the interface. In ceramic matrix composites, the objective is often to increase the toughness rather than the strength and stiffness; therefore, a low interfacial strength bond is desirable. The type and quantity of the reinforcement determine the final properties. Figure 1.2 shows that the highest strength and modulus are obtained with continuous-fiber composites. There is a practical limit of about 70 volume percent reinforcement that can be added to form a composite. At higher percentages, there is too little matrix to support the fibers effectively. The theoretical
2 Structural Composite Materials Continuous Unidirectional (UD) Cloth Roving 0000000 9oopo 0 0°/90°(Noven) ±30°Helical Filament Wound (a) Discontinuous Chopped Mat \1 八1 (b) Fig.1.1 Typical reinforcement types strength of discontinuous-fiber composites can a low-viscosity resin that reacts and cures during approach that of continuous-fiber composites processing,forming an intractable solid.A ther- if their aspect ratios are great enough and they moplastic is a high-viscosity resin that is pro- are aligned,but it is difficult in practice to main- cessed by heating it above its melting tempera- tain good alignment with discontinuous fibers. ture.Because a thermoset resin sets up and cures Discontinuous-fiber composites are normally during processing,it cannot be reprocessed by somewhat random in alignment,which dramati- reheating.By comparison,a thermoplastic can cally reduces their strength and modulus.How- be reheated above its melting temperature for ad- ever,discontinuous-fiber composites are gen- ditional processing.There are processes for both erally much less costly than continuous-fiber classes of resins that are more amenable to dis- composites.Therefore,continuous-fiber com- continuous fibers and others that are more ame- posites are used where higher strength and stiff- nable to continuous fibers.In general,because ness are required (but at a higher cost),and metal and ceramic matrix composites require discontinuous-fiber composites are used where very high temperatures and sometimes high pres- cost is the main driver and strength and stiffness sures for processing,they are normally much are less important. more expensive than polymer matrix composites Both the reinforcement type and the matrix af- However,they have much better thermal stabil- fect processing.The major processing routes for ity,a requirement in applications where the com- polymer matrix composites are shown in Fig.1.3. posite is exposed to high temperatures. Two types of polymer matrices are shown:ther- This book will deal with both continuous and mosets and thermoplastics.A thermoset starts as discontinuous polymer,metal,and ceramic matrix
2 / Structural Composite Materials strength of discontinuous-fiber composites can approach that of continuous-fiber composites if their aspect ratios are great enough and they are aligned, but it is difficult in practice to maintain good alignment with discontinuous fibers. Discontinuous-fiber composites are normally somewhat random in alignment, which dramatically reduces their strength and modulus. However, discontinuous-fiber composites are gen erally much less costly than continuous-fiber composites. Therefore, continuous-fiber composites are used where higher strength and stiffness are required (but at a higher cost), and discontinuous-fiber composites are used where cost is the main driver and strength and stiffness are less important. Both the reinforcement type and the matrix affect processing. The major processing routes for polymer matrix composites are shown in Fig. 1.3. Two types of polymer matrices are shown: thermosets and thermoplastics. A thermoset starts as a low-viscosity resin that reacts and cures during processing, forming an intractable solid. A thermoplastic is a high-viscosity resin that is processed by heating it above its melting temperature. Because a thermoset resin sets up and cures during processing, it cannot be reprocessed by reheating. By comparison, a thermoplastic can be reheated above its melting temperature for additional processing. There are processes for both classes of resins that are more amenable to discontinuous fibers and others that are more amenable to continuous fibers. In general, because metal and ceramic matrix composites require very high temperatures and sometimes high pressures for processing, they are normally much more expensive than polymer matrix composites. However, they have much better thermal stability, a requirement in applications where the composite is exposed to high temperatures. This book will deal with both continuous and discontinuous polymer, metal, and ceramic matrix Fig. 1.1 Typical reinforcement types
Chapter 1:Introduction to Composite Materials/3 Continuous aligned Practical cutoff 'sninpow 'y6uans Random aligned Woven cloth Random short Random long 20 40 60 80 80 Fiber volume, Fig.1.2 Influence of reinforcement type and quantity on composite performance Composites Processing Thermoset Composites Thermoplastic Composites Processing Processing Short-Fiber Continuous-Fiber Short-Fiber Continuous-Fiber Composites Composites Composites Composites Injection Molding Lay-Up Injection Molding Lay-Up Compression Molding Filament Winding Compression Molding Thermoforming Liquid Molding Liquid Molding Compression Molding Spray-Up Pultrusion Fig.1.3 Major polymer matrix composite fabrication processes
Chapter 1: Introduction to Composite Materials / 3 Fig. 1.2 Influence of reinforcement type and quantity on composite performance Fig. 1.3 Major polymer matrix composite fabrication processes
4 Structural Composite Materials composites,with an emphasis on continuous- material is anisotropic (for example,the compos- fiber,high-performance polymer composites. ite ply shown in Fig.1.5),it has properties that vary with direction within the material.In this example,the moduli are different in each direc- tion(Eoe≠E4so≠Ego).While the modulus of 1.1 Isotropic,Anisotropic,and elasticity is used in the example,the same depen- Orthotropic Materials dence on direction can occur for other material properties,such as ultimate strength,Poisson's Materials can be classified as either isotropic ratio,and thermal expansion coefficient. or anisotropic.Isotropic materials have the same Bulk materials,such as metals and polymers, material properties in all directions,and normal are normally treated as isotropic materials,while loads create only normal strains.By compari- composites are treated as anisotropic.However, son,anisotropic materials have different mate- even bulk materials such as metals can become rial properties in all directions at a point in the anisotropic-for example,if they are highly cold body.There are no material planes of symmetry, worked to produce grain alignment in a certain and normal loads create both normal strains and direction. shear strains.A material is isotropic if the prop- Consider the unidirectional fiber-reinforced erties are independent of direction within the composite ply (also known as a lamina)shown material. in Fig.1.6.The coordinate system used to de- For example,consider the element of an iso- scribe the ply is labeled the 1-2-3 axes.In this tropic material shown in Fig.1.4.If the material case,the 1-axis is defined to be parallel to the is loaded along its0°，45°，and90°directions, fibers(0),the 2-axis is defined to lie within the the modulus of elasticity (E)is the same in each plane of the plate and is perpendicular to the fi- direction (Eo Ease E2o).However,if the bers(90),and the 3-axis is defined to be normal Eo=E45=E90=E E。 0g0 O Fig.1.4 Element of isotropic material under stress
4 / Structural Composite Materials composites, with an emphasis on continuousfiber, high-performance polymer composites. 1.1 Isotropic, Anisotropic, and Orthotropic Materials Materials can be classified as either isotropic or anisotropic. Isotropic materials have the same material properties in all directions, and normal loads create only normal strains. By comparison, anisotropic materials have different material properties in all directions at a point in the body. There are no material planes of symmetry, and normal loads create both normal strains and shear strains. A material is isotropic if the properties are independent of direction within the material. For example, consider the element of an isotropic material shown in Fig. 1.4. If the material is loaded along its 0°, 45°, and 90° directions, the modulus of elasticity (E) is the same in each direction (E0° = E45° = E90°). However, if the material is anisotropic (for example, the composite ply shown in Fig. 1.5), it has properties that vary with direction within the material. In this example, the moduli are different in each direction (E0° ≠ E45° ≠ E90°). While the modulus of elasticity is used in the example, the same dependence on direction can occur for other material properties, such as ultimate strength, Poisson’s ratio, and thermal expansion coefficient. Bulk materials, such as metals and polymers, are normally treated as isotropic materials, while composites are treated as anisotropic. However, even bulk materials such as metals can become anisotropic––for example, if they are highly cold worked to produce grain alignment in a certain direction. Consider the unidirectional fiber-reinforced composite ply (also known as a lamina) shown in Fig. 1.6. The coordinate system used to describe the ply is labeled the 1-2-3 axes. In this case, the 1-axis is defined to be parallel to the fibers (0°), the 2-axis is defined to lie within the plane of the plate and is perpendicular to the fibers (90°), and the 3-axis is defined to be normal Fig. 1.4 Element of isotropic material under stress