8 Introduction in concert.A prerequisite for adhesion is that the matrix,in its fluid form,be capable of wetting the fibers.Fibers that would otherwise not be wetted by their matrix can be given a coating that fosters contact by interacting with both the fibers and the matrix.In some cases varying the matrix composition can also promote the process. Once the matrix has wetted the fibers thoroughly,intermolecular forces or chemical reactions can establish a bond. The properties of an advanced composite are shaped not only by the kind of matrix and reinforcing materials it contains but also by a factor that is distinct from composition:the geometry of the reinforcement.Reinforcing geometries of composites can be grouped roughly by the shape of the reinforcing elements:particles, continuous fibers or short fibers (Fig.1.3).Sets of parallel con- tinuous fibers are often embedded in thin composite layers,which are assembled into a laminate.Alternatively,each ply in a laminate can be reinforced with continuous fibers woven or knitted into a textile 'preform'.Recently developed geometries dispense with lamination:the fibers are woven or braided in three dimensions (Fig.1.4),a strategy that in some cases enables the final shape of the composite to be formed directly. Progress toward managing the many variables of composite design has encouraged investigators to contemplate new com- plexities.An ordinary composite reinforced with stiff,straight fibers usually displays a nearly constant value of stiffness.New composites designed to display specific non-linear relations of strain and stress are now attracting interest.One such example,a flexible composite consisting of undulating fibers in an elastomeric matrix,can Fig.1.3.Particle-and fiber-reinforced composites.(From 'Composites' Chou,McCullough and Pipes.)Copyright C (1986)by Scientific American,Inc.All rights reserved p●Pp Particles Short fibers Continuous fibers
8 Introduction in concert. A prerequisite for adhesion is that the matrix, in its fluid form, be capable of wetting the fibers. Fibers that would otherwise not be wetted by their matrix can be given a coating that fosters contact by interacting with both the fibers and the matrix. In some cases varying the matrix composition can also promote the process. Once the matrix has wetted the fibers thoroughly, intermolecular forces or chemical reactions can establish a bond. The properties of an advanced composite are shaped not only by the kind of matrix and reinforcing materials it contains but also by a factor that is distinct from composition: the geometry of the reinforcement. Reinforcing geometries of composites can be grouped roughly by the shape of the reinforcing elements: particles, continuous fibers or short fibers (Fig. 1.3). Sets of parallel continuous fibers are often embedded in thin composite layers, which are assembled into a laminate. Alternatively, each ply in a laminate can be reinforced with continuous fibers woven or knitted into a textile 'preform'. Recently developed geometries dispense with lamination: the fibers are woven or braided in three dimensions (Fig. 1.4), a strategy that in some cases enables the final shape of the composite to be formed directly. Progress toward managing the many variables of composite design has encouraged investigators to contemplate new complexities. An ordinary composite reinforced with stiff, straight fibers usually displays a nearly constant value of stiffness. New composites designed to display specific non-linear relations of strain and stress are now attracting interest. One such example, a flexible composite consisting of undulating fibers in an elastomeric matrix, can Fig. 1.3. Particle- and fiber-reinforced composites. (From 'Composites' Chou, McCullough and Pipes.) Copyright © (1986) by Scientific American, Inc. All rights reserved. Particles Short fibers Continuous fibers
Fig.1.4.Preforms of textile structural composites.(From 'Composites'Chou,McCullough and Pipes.) Copyright C(1986)by Scientific American,Inc.All rights reserved. Bi-axial weave Tri-axial weave Knit Multi-axial multi-layer warp knit Three-dimensional Three-dimensional Angle-interlock construction cylindrical construction Three-dimensional braiding orthogonal fabric
Fig. 1.4. Preforms of textile structural composites. (From 'Composites' Chou, McCullough and Pipes ) Copyright © (1986) by Scientific American, Inc. All rights reserved. Bi-axial weave Tri-axial weave Knit Multi-axial multi-layer warp knit Three-dimensional cylindrical construction Three-dimensional braiding Three-dimensional orthogonal fabric Angle-interlock construction
10 Introduction elongate readily at low stresses but stiffens when the fibers become fully extended.A hybrid composite strengthened with two kinds of fibers,some of them brittle and inextensible and the others ductile and tough,can display the opposite behavior.The stiff fibers cause stress to increase very sharply at low strains,but when the strain is sufficient to break the stiff,brittle fibers,the curve of stress over strain flattens.The ductile fibers come into play,and as a result the composite becomes more extensible.The hybrid design can yield a material that combines much of the stiffness of an ordinary composite containing only stiff fibers with increased toughness. Overall,the opportunity in the engineering of fiber composites is the potential to control the composition as well as internal geometry of the materials for optimized performance. 1.3 Why composites? The question of Why composites?'was raised in the 1975 text by Vinson and Chou (1975).The rationale provided then focussed on (a)the limitations in strength and ductility for metallic alloys from the viewpoints of theoretical cohesive strength of solids and the arrangement of crystalline defects, (b)the need of a balanced pursuit in strength and ductility and the potential of achieving both in fiber composites,and (c)the strength limitation of metallic alloys at elevated tem- peratures and the potential of carbon-carbon composites and refractory metal wire reinforced super-alloys. The field of fiber composites has witnessed drastic changes and advancement since the mid-1970s because of the availability of several ceramic fibers,high-temperature thermoplastics,glass- ceramic matrices,and intermetallic solids for composites.Although the fundamental physical principles governing the synergism of the component phases in composites should not change,the advance- ment in materials technology coupled with that in processing, surface science and instrumentation has greatly changed the per- spective of composite technology.In the following,the answer to the question of 'why composites?'is re-examined from both economic and technological points of view. 1.3.1 Economic aspect For the discussion of the economic aspect of advanced materials in general and fiber composites in particular,it is
10 Introduction elongate readily at low stresses but stiffens when the fibers become fully extended. A hybrid composite strengthened with two kinds of fibers, some of them brittle and inextensible and the others ductile and tough, can display the opposite behavior. The stiff fibers cause stress to increase very sharply at low strains, but when the strain is sufficient to break the stiff, brittle fibers, the curve of stress over strain flattens. The ductile fibers come into play, and as a result the composite becomes more extensible. The hybrid design can yield a material that combines much of the stiffness of an ordinary composite containing only stiff fibers with increased toughness. Overall, the opportunity in the engineering of fiber composites is the potential to control the composition as well as internal geometry of the materials for optimized performance. 1.3 Why composites? The question of 'Why composites?' was raised in the 1975 text by Vinson and Chou (1975). The rationale provided then focussed on (a) the limitations in strength and ductility for metallic alloys from the viewpoints of theoretical cohesive strength of solids and the arrangement of crystalline defects, (b) the need of a balanced pursuit in strength and ductility and the potential of achieving both in fiber composites, and (c) the strength limitation of metallic alloys at elevated temperatures and the potential of carbon-carbon composites and refractory metal wire reinforced super-alloys. The field of fiber composites has witnessed drastic changes and advancement since the mid-1970s because of the availability of several ceramic fibers, high-temperature thermoplastics, glassceramic matrices, and intermetallic solids for composites. Although the fundamental physical principles governing the synergism of the component phases in composites should not change, the advancement in materials technology coupled with that in processing, surface science and instrumentation has greatly changed the perspective of composite technology. In the following, the answer to the question of 'why composites?' is re-examined from both economic and technological points of view. 1.3.1 Economic aspect For the discussion of the economic aspect of advanced materials in general and fiber composites in particular, it is
Why composites? 11 worthwhile referring to a recent survey entitled Problems and Opportunities in Metals and Materials:An Integrated Perspective by the U.S.Department of the Interior (Sousa 1988).The report asserts that the future growth prospects seem best not in tonnage commodities but rather in materials that are more technology- intensive and more high-value-added.As the economy grows and matures,the rate of growth in consumption of tonnage metals first exceeds,eventually parallels,and finally trails that of the economy as a whole. Figure 1.5 shows the estimated current relative market maturity of the major metals and other materials.The vertical dimension indicates intensity-of-use(amount/GNP).The potential of polymer, metal and ceramic based composites is obvious.This figure also demonstrates a hard fact of life that eventually catches up with virtually any product-that of market saturation and,as the inexorable evolution of technology proceeds,eventual displacement and decline. By incorporating different materials into composites,the synthe- tic class of materials can thus draw on the essential characteristics of diverse materials:the high strength,ductility,thermal-electrical conductivity and formability of metals,the low cost fabrication, light weight and corrosion resistance of polymers,and the strength corrosion resistance and high-temperature performance of ceramics. Fig.1.5.Relative market maturity of materials.(After Sousa (1988).) Commodity plastics Aluminum Stainless steel- Copper Super-alloy-> -Carbon steel Specialty metals Traditional engineering plastics-> High-performance engineering plastics- Engineering plastics.alloys and blends Fiber optics-→ Advanced polymer> matrix composites Advanced metal matrix composites Structural ceramics-> Heavy R&D Rapid growth Growth Growth Imaturing <GNP Growth =GNP
Why composites? 11 worthwhile referring to a recent survey entitled Problems and Opportunities in Metals and Materials: An Integrated Perspective by the U.S. Department of the Interior (Sousa 1988). The report asserts that the future growth prospects seem best not in tonnage commodities but rather in materials that are more technologyintensive and more high-value-added. As the economy grows and matures, the rate of growth in consumption of tonnage metals first exceeds, eventually parallels, and finally trails that of the economy as a whole. Figure 1.5 shows the estimated current relative market maturity of the major metals and other materials. The vertical dimension indicates intensity-of-use (amount/GNP). The potential of polymer, metal and ceramic based composites is obvious. This figure also demonstrates a hard fact of life that eventually catches up with virtually any product-that of market saturation and, as the inexorable evolution of technology proceeds, eventual displacement and decline. By incorporating different materials into composites, the synthetic class of materials can thus draw on the essential characteristics of diverse materials: the high strength, ductility, thermal-electrical conductivity and formability of metals, the low cost fabrication, light weight and corrosion resistance of polymers, and the strength, corrosion resistance and high-temperature performance of ceramics. Fig. 1.5. Relative market maturity of materials. (After Sousa (1988).) Commodity plastics - Stainless steel Super-alloy - Specialty metals Traditional engineering plastics - High-performance engineering plastics - Engineering plastics, alloys and blends — Fiber optics - Advanced polymer . matrix composites Advanced metal matrix composites Structural ceramics - Aluminum Copper Carbon steel Growth maturing Growth = GNP Growth <GNP
12 Introduction The survey of the U.S.Department of the Interior forecasts the total demand for advanced materials in the U.S.in the year 2000 to be approximately $55 billion annually,roughly the same magnitude as the current U.S.steel market.By comparison,a Japanese Ministry of International Trade and Industry report showed that the Japanese annual demand for advanced materials is expected to be about $34 billion.The breakdown of the market in terms of material categories is (1)advanced polymer composites:22% (U.S.),7.6%(Japan);(2)advanced metal alloys and composites: 35%(U.S.),28.3%(Japan);(3)advanced ceramics:30%(U.S.), 35.9%(Japan);(4)engineering plastics:13%(U.S.),28.3% (Japan).Although the rudimentary nature of such forecasts cannot be overemphasized,the transition from a metals economy to a materials economy,and the importance of composite materials to the economy of advanced materials,is unmistakable. 1.3.2 Technological aspect From the technological viewpoint,advanced composite materials can offer a competitive edge in many products,including aircraft,automobile,industrial machinery and sporting goods, provided their overall production costs can be reduced and their performance improved.According to the study New Structural Materials Technologies made by the Congress of the United States, Office of Technology Assessment (1988),the broader use of advanced structural materials requires not only solutions to techni- cal problems but also changes in attitudes among researchers and end-users.The traditional approach based upon discrete design and manufacturing steps for conventional structural materials needs to be replaced by an integrated design and manufacturing process which necessitates a closer relationship among researchers,design- ers,and production personnel as well as a new approach to the concept of material costs.A fully integrated design process capable of balancing all of the relevant design and manufacturing variables requires an extensive database on matrix and fiber properties, the ability to model fabrication processes,and three-dimensional analysis of the properties and behavior of the resulting structure. Knowledge of the relationships among the constituent properties, microstructure and macroscopic behavior of the composite is basic to the development of an integrated design methodology. To further understand the impacts of advanced structural mate- rials on manufacturing,this report examines the following two possibilities:substitution by direct replacement of metal com-
12 Introduction The survey of the U.S. Department of the Interior forecasts the total demand for advanced materials in the U.S. in the year 2000 to be approximately $55 billion annually, roughly the same magnitude as the current U.S. steel market. By comparison, a Japanese Ministry of International Trade and Industry report showed that the Japanese annual demand for advanced materials is expected to be about $34 billion. The breakdown of the market in terms of material categories is (1) advanced polymer composites: 22% (U.S.), 7.6% (Japan); (2) advanced metal alloys and composites: 35% (U.S.), 28.3% (Japan); (3) advanced ceramics: 30% (U.S.), 35.9% (Japan); (4) engineering plastics: 13% (U.S.), 28.3% (Japan). Although the rudimentary nature of such forecasts cannot be overemphasized, the transition from a metals economy to a materials economy, and the importance of composite materials to the economy of advanced materials, is unmistakable. 1.3.2 Technological aspect From the technological viewpoint, advanced composite materials can offer a competitive edge in many products, including aircraft, automobile, industrial machinery and sporting goods, provided their overall production costs can be reduced and their performance improved. According to the study New Structural Materials Technologies made by the Congress of the United States, Office of Technology Assessment (1988), the broader use of advanced structural materials requires not only solutions to technical problems but also changes in attitudes among researchers and end-users. The traditional approach based upon discrete design and manufacturing steps for conventional structural materials needs to be replaced by an integrated design and manufacturing process which necessitates a closer relationship among researchers, designers, and production personnel as well as a new approach to the concept of material costs. A fully integrated design process capable of balancing all of the relevant design and manufacturing variables requires an extensive database on matrix and fiber properties, the ability to model fabrication processes, and three-dimensional analysis of the properties and behavior of the resulting structure. Knowledge of the relationships among the constituent properties, microstructure and macroscopic behavior of the composite is basic to the development of an integrated design methodology. To further understand the impacts of advanced structural materials on manufacturing, this report examines the following two possibilities: substitution by direct replacement of metal com-