18 Introduction their applications.Furthermore,there may be much new science on how to produce composites. The significant trends in structural composites point to the direction of low-temperature metal matrix,resin matrix,metal- resin matrix,rubber matrix,cement-ceramic matrix and elevated- temperature composites.Non-structural composites are increasingly being recognized for their unique opportunities in electric,mag- netic,superconducting and biomedical applications.A brief sum- mary of those trends follows (see Kelly 1987a). A major motivation behind the development of low-temperature metal matrix composites in the U.S.has been for the utilization of high-stiffness continuous fibers in a matrix material without the disadvantage of thermosetting resins of low thermal conductivity, high thermal expansion,dimensional instability,hygrothermal de- gradation,material loss in high vacuum,susceptibility to radiation damage,and lower temperature brittleness.The lighter metals do not possess these disadvantages;their low atomic number(Z)is important in a neutron-rich environment.It is useful to bear in mind that five out of the 13 lowest-Z solids are metals.Some of these metals,together with their atomic number and density,are listed below:lithium (Z=3,density =0.53 Mg m),sodium (11,0.97),potassium (19,0.86),calcium (20,1.55),magnesium (12,1.741),beryllium(4,1.85),and aluminum(13,2.7). Reinforcement of a light metal,e.g.aluminum and magnesium,is attractive in the automobile industry in reducing creep at moderate temperatures and improving wear resistance.Coating for carbon fibers is necessary for incorporation into aluminum and magnesium matrices. Thermoplastic resins have certain advantages over thermosets in their infinite shelf life,good resistance to water and solvents,and ductility.Thermoplastics are attractive particularly from the view- point of composites manufacture because they are rapidly proces- sable,and are better adapted to automated manufacturing.Also, they can be recycled and joined by welding. Laminates formed by bonding metal sheets to fiber-resin com- posites take advantage of the synergistic effects of hybrid compos- ites.For instance,the combination of aluminum foil with Kevlar/epoxy composite results in enhanced fatigue resistance and compressive strength. Rubber (elastomeric)matrix can be reinforced with short and continuous fibers and can provide the capability of large non-linear elastic deformation.Automobile tires and coated fabrics are ex- amples in this category
18 Introduction their applications. Furthermore, there may be much new science on how to produce composites. The significant trends in structural composites point to the direction of low-temperature metal matrix, resin matrix, metalresin matrix, rubber matrix, cement-ceramic matrix and elevatedtemperature composites. Non-structural composites are increasingly being recognized for their unique opportunities in electric, magnetic, superconducting and biomedical applications. A brief summary of those trends follows (see Kelly 1987a). A major motivation behind the development of low-temperature metal matrix composites in the U.S. has been for the utilization of high-stiffness continuous fibers in a matrix material without the disadvantage of thermosetting resins of low thermal conductivity, high thermal expansion, dimensional instability, hygrothermal degradation, material loss in high vacuum, susceptibility to radiation damage, and lower temperature brittleness. The lighter metals do not possess these disadvantages; their low atomic number (Z) is important in a neutron-rich environment. It is useful to bear in mind that five out of the 13 lowest-Z solids are metals. Some of these metals, together with their atomic number and density, are listed below: lithium (Z = 3, density = 0.53 Mgm~3), sodium (11,0.97), potassium (19,0.86), calcium (20,1.55), magnesium (12,1.741), beryllium (4,1.85), and aluminum (13, 2.7). Reinforcement of a light metal, e.g. aluminum and magnesium, is attractive in the automobile industry in reducing creep at moderate temperatures and improving wear resistance. Coating for carbon fibers is necessary for incorporation into aluminum and magnesium matrices. Thermoplastic resins have certain advantages over thermosets in their infinite shelf life, good resistance to water and solvents, and ductility. Thermoplastics are attractive particularly from the viewpoint of composites manufacture because they are rapidly processable, and are better adapted to automated manufacturing. Also, they can be recycled and joined by welding. Laminates formed by bonding metal sheets to fiber-resin composites take advantage of the synergistic effects of hybrid composites. For instance, the combination of aluminum foil with Kevlar/epoxy composite results in enhanced fatigue resistance and compressive strength. Rubber (elastomeric) matrix can be reinforced with short and continuous fibers and can provide the capability of large non-linear elastic deformation. Automobile tires and coated fabrics are examples in this category
Microstructure-performance relationships 19 Contrary to the large deformation of rubber type flexible com- posites,ceramic based composites offer the other extreme on the scale of deformation.The brittle nature of ceramic solids requires a new way of thinking in 'reinforcement'.Fibers are added for the purpose of improving toughness against fracture and ductility in terms of energy absorption and deformation range. Ceramic matrix composites,directionally solidified eutectics, intermetallic solids,certain types of metal based composites,and carbon-carbon composites are the candidate materials for elevated- temperature applications.Among these,carbon-carbon composites present the ultimate in high-temperature materials under reducing conditions.They have many tribological applications.Protection against oxidation and densification of the matrix are major chal- lenges to carbon-carbon composites. Finally,the potential of non-structural composites has not been fully explored.Kelly (1987a)cited the examples in making special devices.For example,a magnetoresistive device obtained by coupling a metal rod with a semiconductor matrix provides a contactless potentiometer or a fluxmeter,or coupling a piezoelectric and magnetostrictive material gives a magnetoelectric material.The potential for biomedical applications of flexible composites also exists (see Chou 1989). 1.5 Microstructure-performance relationships Chapters 2-9 examine the stiffness,strength and failure behavior of several types of composites:laminated composites composed of continuous fibers;composites reinforced with short fibers in biassed or random orientations;composites with two types of fibers in intermingled,interlaminated or interwoven forms; composites reinforced with textile preforms;and flexible composites exhibiting large deformations.The mathematical tools for analyzing their thermomechanical properties have been presented.Most significantly,an effort has been made to delineate the relationship between the behavior and these composites. In the following,a comparison is first made among the stress- strain behaviors of three composite systems.The purpose is to demonstrate the versatility in composite performance through the design of microstructure.This is followed by specific examples of tailoring the material performance through microstructural design.Lastly,the emerging field of 'intelligent composites'is introduced
Microstructure -performance relationships 19 Contrary to the large deformation of rubber type flexible composites, ceramic based composites offer the other extreme on the scale of deformation. The brittle nature of ceramic solids requires a new way of thinking in 'reinforcement'. Fibers are added for the purpose of improving toughness against fracture and ductility in terms of energy absorption and deformation range. Ceramic matrix composites, directionally solidified eutectics, intermetallic solids, certain types of metal based composites, and carbon-carbon composites are the candidate materials for elevatedtemperature applications. Among these, carbon-carbon composites present the ultimate in high-temperature materials under reducing conditions. They have many tribological applications. Protection against oxidation and densification of the matrix are major challenges to carbon-carbon composites. Finally, the potential of non-structural composites has not been fully explored. Kelly (1987a) cited the examples in making special devices. For example, a magnetoresistive device obtained by coupling a metal rod with a semiconductor matrix provides a contactless potentiometer or a fluxmeter, or coupling a piezoelectric and magnetostrictive material gives a magnetoelectric material. The potential for biomedical applications of flexible composites also exists (see Chou 1989). 1.5 Microstructure-performance relationships Chapters 2-9 examine the stiffness, strength and failure behavior of several types of composites: laminated composites composed of continuous fibers; composites reinforced with short fibers in biassed or random orientations; composites with two types of fibers in intermingled, interlaminated or interwoven forms; composites reinforced with textile preforms; and flexible composites exhibiting large deformations. The mathematical tools for analyzing their thermomechanical properties have been presented. Most significantly, an effort has been made to delineate the relationship between the behavior and these composites. In the following, a comparison is first made among the stressstrain behaviors of three composite systems. The purpose is to demonstrate the versatility in composite performance through the design of microstructure. This is followed by specific examples of tailoring the material performance through microstructural design. Lastly, the emerging field of 'intelligent composites' is introduced
20 Introduction 1.5.1 Versatility in performance For the purpose of demonstrating the versatility of the performance of composites,the stress-strain relationships of three types of composites are examined.Figure 1.10 shows the stress- strain curves of a unidirectional carbon fiber reinforced glass matrix composite (Nardone and Prewo 1988).The behavior is typical for brittle matrix composites based upon polymer and glass/ceramic matrices.The knee phenomenon of the stress-strain curve re- sembles the yield behavior of metallic alloys. Figure 1.11 gives the stress-strain curves of interlaminated carbon/glass hybrid composites.The ability of the low elongation phase (carbon)in developing multiple fractures enables the hybrid composites to sustain deformations at a level much higher than that of the all-low elongation fiber composite.The energy absorption capability as indicated by the area under the stress-strain curve is also much higher than that of the all-carbon fiber composite.The shape of this stress-strain curve resembles those of ductile metals with strain-hardening behavior. The stress-strain data of a flexible composite (Fig.1.12)show rapid increase in stress and stiffness at large deformation (Chou 1989).It resembles the behavior of certain biological materials such as soft animal tissues(Humphrey and Yin 1987;Gordon 1988). Fig.1.10.Tensile stress-strain curves of a carbon/borosilicate glass composite.(After Nardone and Prewo 1988). 900 130 Transverse strain 110 700 90 2 500 70 Longitudinal strain 50 300 30 100 E 10 -0.10 0.10 0.30 0.50 Strain (%
20 Introduction 1.5.1 Versatility in performance For the purpose of demonstrating the versatility of the performance of composites, the stress-strain relationships of three types of composites are examined. Figure 1.10 shows the stressstrain curves of a unidirectional carbon fiber reinforced glass matrix composite (Nardone and Prewo 1988). The behavior is typical for brittle matrix composites based upon polymer and glass/ceramic matrices. The knee phenomenon of the stress-strain curve resembles the yield behavior of metallic alloys. Figure 1.11 gives the stress-strain curves of interlaminated carbon/glass hybrid composites. The ability of the low elongation phase (carbon) in developing multiple fractures enables the hybrid composites to sustain deformations at a level much higher than that of the all-low elongation fiber composite. The energy absorption capability as indicated by the area under the stress-strain curve is also much higher than that of the all-carbon fiber composite. The shape of this stress-strain curve resembles those of ductile metals with strain-hardening behavior. The stress-strain data of a flexible composite (Fig. 1.12) show rapid increase in stress and stiffness at large deformation (Chou 1989). It resembles the behavior of certain biological materials such as soft animal tissues (Humphrey and Yin 1987; Gordon 1988). Fig. 1.10. Tensile stress—strain curves of a carbon/borosilicate glass composite. (After Nardone and Prewo 1988). 130 110 - 90 - 70 50 30 - 10 700 500 300 100 - - \ - \ - ^ \ Transverse strain / E 1 yf ^^Longitudinal « * ^ strain - 1 I -0.10 0.10 -0.10 Strain (%) 0.50
【oad(kg) 三 堂 Stress (MPa) 3 岂 19..) 8 Strain ( Experiments ◆o Fig.1.12.Tensile stress-strain data of a flexible composite.(After Chou Strain (% 0 ated composite.(After Bunsell and Harris 1974). ● Fig.1.11.Tensile stress-strain curves of a carbon/glass/epoxy interlamin- Microstructure-performance relationships 1000 Acoustic emission rate (counts) 2
Loa d (kg ) Stres s (MPa ) C/ J 3 • 1 >o 1 1 o 1 1 o • 1 o »Experiments • 3 • oo 55?sa8 Acousti c emissio n rat e (counts )
22 Introduction It is interesting to note that through the selection of fiber and matrix materials,as well as their geometric arrangements,a broad spectrum of material performance can be accomplished.It is feasible to design the physical and mechanical properties of composites which not only duplicate the performance of some existing materials but also fulfil the most demanding structural roles not envisioned before. 1.5.2 Tailoring of performance The structure-performance relationships of the various types of fiber composites are further demonstrated in this section. First,for continuous fiber composites,the problem of edge de- lamination is used as an example.Next the variation of composite electric properties with the configuration of reinforcements is demonstrated. Consider the [45/03/903ls laminate.The effect of fiber orienta- tion on the deformation of each individual lamina is highly anisotropic (Fig.1.13).The compatibility of displacements among the laminae induces interlaminar stresses through the thickness direction of the laminate.Sun (1989)has demonstrated that the opening mode of delamination can be minimized through fiber hybridization,stitching,the use of adhesive layers,ply termination, and modification of edge geometry. Figure 1.14 shows the free-edge interlaminar normal stresses in the all-carbon composite and the hybrid composite formed by replacing 90 plies with a glass/epoxy composite.A significant Fig.1.13.Effect of fiber orientation on the deformation of composite laminae.(After Sun 1989.) 45
22 Introduction It is interesting to note that through the selection of fiber and matrix materials, as well as their geometric arrangements, a broad spectrum of material performance can be accomplished. It is feasible to design the physical and mechanical properties of composites which not only duplicate the performance of some existing materials but also fulfil the most demanding structural roles not envisioned before. 1.5.2 Tailoring of performance The structure-performance relationships of the various types of fiber composites are further demonstrated in this section. First, for continuous fiber composites, the problem of edge delamination is used as an example. Next the variation of composite electric properties with the configuration of reinforcements is demonstrated. Consider the [±45°/0?/902]s laminate. The effect of fiber orientation on the deformation of each individual lamina is highly anisotropic (Fig. 1.13). The compatibility of displacements among the laminae induces interlaminar stresses through the thickness direction of the laminate. Sun (1989) has demonstrated that the opening mode of delamination can be minimized through fiber hybridization, stitching, the use of adhesive layers, ply termination, and modification of edge geometry. Figure 1.14 shows the free-edge interlaminar normal stresses in the all-carbon composite and the hybrid composite formed by replacing 90° plies with a glass/epoxy composite. A significant Fig. 1.13. Effect of fiber orientation on the deformation of composite laminae. (After Sun 1989.) 1 \ 45° I I 90°