Composite Material Structure and Processing 1.1 Introduction Composite materials are multiphase materials obtained through the artificial com- bination of different materials in order to attain properties that the individual com- ponents by themselves cannot attain.They are not multiphase materials in which the different phases are formed naturally by reactions,phase transformations, or other phenomena.An example is carbon fiber reinforced polymer.Composite materials should be distinguished from alloys,which can comprise two more com- ponents but are formed naturally through processes such as casting.Composite materials can be tailored for various properties by appropriately choosing their components,their proportions,their distributions,their morphologies,their de- grees of crystallinity,their crystallographic textures,as well as the structure and composition of the interface between components.Due to this strong tailorability, composite materials can be designed to satisfy the needs of technologies relat- ing to the aerospace,automobile,electronics,construction,energy,biomedical and other industries.As a result,composite materials constitute most commercial engineering materials. An example of a composite material is a lightweight structural composite that is obtained by embedding continuous carbon fibers in one or more orientations in a polymer matrix.The fibers provide the strength and stiffness,while the polymer serves as the binder.In particular,carbon fiber polymer-matrix composites have the following attractive properties: Low density(lower than aluminum) High strength (as strong as high-strength steels) High stiffness(stiffer than titanium,yet much lower in density) Good fatigue resistance Good creep resistance Low friction coefficient and good wear resistance Toughness and damage tolerance(as enabled by using appropriate fiber orien- tations) Chemical resistance(chemical resistance controlled by the polymer matrix) 1
1 Composite Material Structure and Processing 1.1 Introduction Composite materials are multiphase materials obtained through the artificial combination of different materials in order to attain properties that the individual components by themselves cannot attain. They are not multiphase materials in which the different phases are formed naturally by reactions, phase transformations, or other phenomena. An example is carbon fiber reinforced polymer. Composite materials should be distinguished from alloys, which can comprise two more components but are formed naturally through processes such as casting. Composite materials can be tailored for various properties by appropriately choosing their components, their proportions, their distributions, their morphologies, their degrees of crystallinity, their crystallographic textures, as well as the structure and composition of the interface between components. Due to this strong tailorability, composite materials can be designed to satisfy the needs of technologies relating to the aerospace, automobile, electronics, construction, energy, biomedical and other industries. As a result, composite materials constitute most commercial engineering materials. An example of a composite material is a lightweight structural composite that is obtained by embedding continuous carbon fibers in one or more orientations in a polymer matrix. The fibers provide the strength and stiffness, while the polymer serves as the binder. In particular, carbon fiber polymer-matrix composites have the following attractive properties: Low density (lower than aluminum) High strength (as strong as high-strength steels) High stiffness (stiffer than titanium, yet much lower in density) Good fatigue resistance Good creep resistance Low friction coefficient and good wear resistance Toughness and damage tolerance (as enabled by using appropriate fiber orientations) Chemical resistance (chemical resistance controlled by the polymer matrix) 1
2 1 Composite Material Structure and Processing Corrosion resistance Dimensional stability (can be designed for zero CTE) Vibration damping ability Low electrical resistivity High electromagnetic interference(EMI)shielding effectiveness High thermal conductivity. Another example of a composite is concrete,which is a structural composite obtained by combining (through mixing)cement (the matrix,ie.,the binder, obtained by a reaction-known as hydration-between cement and water),sand (fine aggregate),gravel(coarse aggregate),and optionally other ingredients that are known as admixtures.Short fibers and silica fume(a fine SiO2 particulate)are examples of admixtures.In general,composites are classified according to their matrix material.The main classes of composites are polymer-matrix,cement- matrix,metal-matrix,carbon-matrix and ceramic-matrix composites. Polymer-matrix and cement-matrix composites are the most common,due to the low cost of fabrication.Polymer-matrix composites are used for lightweight structures(aircraft,sporting goods,wheel chairs,etc.),in addition to vibration damping,electronic enclosures,asphalt(composite with pitch,a polymer,as the matrix),solder replacement,etc.Cement-matrix composites in the form of con- crete(with fine and coarse aggregates),steel-reinforced concrete,mortar(with fine aggregate,but no coarse aggregate)or cement paste(without any aggregate)are used for civil structures,prefabricated housing,architectural precasts,masonry, landfill cover,thermal insulation,sound absorption,etc. Carbon-matrix composites are important for lightweight structures (e.g.,Space Shuttles)and components(e.g.,aircraft brakes)that need to withstand high tem- peratures,but they are relatively expensive due to the high cost of fabrication. Carbon-matrix composites suffer from their tendency to be oxidized(2C +O2 2CO),thereby becoming vapor. Carbon fiber carbon-matrix composites,also called carbon-carbon composites, are the most advanced form of carbon,as the carbon fiber reinforcement makes them stronger,tougher,and more resistant to thermal shock than conventional graphite.With the low density of carbon,the specific strength(strength/density), specific modulus(modulus/density)and specific thermal conductivity(thermal conductivity/density)of carbon-carbon composites are the highest among com- posites.Furthermore,the coefficient of thermal expansion is near zero. Ceramic-matrix composites are superior to carbon-matrix composites in terms of oxidation resistance,but they are not as well developed as carbon-matrix composites.Metal-matrix composites with aluminum as the matrix are used for lightweight structures and low-thermal-expansion electronic enclosures,but their applications are limited by the high cost of fabrication and by galvanic corrosion. Metal-matrix composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion (CTE)and increase the strength and modulus.If a relatively graphitic kind of carbon fiber is used,the thermal conductivity can also be enhanced.The combination of low CTE and high
2 1 Composite Material Structure and Processing Corrosion resistance Dimensional stability (can be designed for zero CTE) Vibration damping ability Low electrical resistivity High electromagnetic interference (EMI) shielding effectiveness High thermal conductivity. Another example of a composite is concrete, which is a structural composite obtained by combining (through mixing) cement (the matrix, i.e., the binder, obtained by a reaction – known as hydration – between cement and water), sand (fine aggregate), gravel (coarse aggregate), and optionally other ingredients that are known as admixtures. Short fibers and silica fume (a fine SiO2 particulate) are examples of admixtures. In general, composites are classified according to their matrix material. The main classes of composites are polymer-matrix, cementmatrix, metal-matrix, carbon-matrix and ceramic-matrix composites. Polymer-matrix and cement-matrix composites are the most common, due to the low cost of fabrication. Polymer-matrix composites are used for lightweight structures (aircraft, sporting goods, wheel chairs, etc.), in addition to vibration damping, electronic enclosures, asphalt (composite with pitch, a polymer, as the matrix), solder replacement, etc. Cement-matrix composites in the form of concrete (with fine and coarse aggregates), steel-reinforced concrete, mortar (with fine aggregate, but no coarse aggregate) or cement paste (without any aggregate) are used for civil structures, prefabricated housing, architectural precasts, masonry, landfill cover, thermal insulation, sound absorption, etc. Carbon-matrix composites are important for lightweight structures (e.g., Space Shuttles) and components (e.g., aircraft brakes) that need to withstand high temperatures, but they are relatively expensive due to the high cost of fabrication. Carbon-matrix composites suffer from their tendency to be oxidized (2C + O2 → 2CO), thereby becoming vapor. Carbon fiber carbon-matrix composites, also called carbon-carbon composites, are the most advanced form of carbon, as the carbon fiber reinforcement makes them stronger, tougher, and more resistant to thermal shock than conventional graphite. With the low density of carbon, the specific strength (strength/density), specific modulus (modulus/density) and specific thermal conductivity (thermal conductivity/density) of carbon-carbon composites are the highest among composites. Furthermore, the coefficient of thermal expansion is near zero. Ceramic-matrix composites are superior to carbon-matrix composites in terms of oxidation resistance, but they are not as well developed as carbon-matrix composites. Metal-matrix composites with aluminum as the matrix are used for lightweight structures and low-thermal-expansion electronic enclosures, but their applications are limited by the high cost of fabrication and by galvanic corrosion. Metal-matrix composites are gaining importance because the reinforcement serves to reduce the coefficient of thermal expansion (CTE) and increase the strength and modulus. If a relatively graphitic kind of carbon fiber is used, the thermal conductivity can also be enhanced. The combination of low CTE and high
1.1 Introduction 3 thermal conductivity makes them very attractive for electronic packaging (e.g., heat sinks).Besides good thermal properties,their low density makes them par- ticularly desirable for aerospace electronics and orbiting space structures;orbiters are thermally cycled by moving through the Earth's shadow. Compared to the metal itself,a carbon fiber metal-matrix composite is char- acterized by a higher strength-to-density ratio (i.e.,specific strength),a higher modulus-to-density ratio (i.e.,specific modulus),better fatigue resistance,bet- ter high-temperature mechanical properties(a higher strength and a lower creep rate),a lower CTE,and better wear resistance. Compared to carbon fiber polymer-matrix composites,a carbon fiber metal- matrix composite is characterized by higher temperature resistance,higher fire resistance,higher transverse strength and modulus,a lack of moisture absorp- tion,a higher thermal conductivity,a lower electrical resistivity,better radiation resistance,and absence of outgassing. On the other hand,a metal-matrix composite has the following disadvantages compared to the metal itself and the corresponding polymer-matrix composite: higher fabrication cost and limited service experience. Fibers used for load-bearing metal-matrix composites are mostly in the form of continuous fibers,but short fibers are also used.The matrices used include aluminum,magnesium,copper,nickel,tin alloys,silver-copper,and lead alloys. Aluminum is by far the most widely used matrix metal because of its low density, low melting temperature(which makes composite fabrication and joining relatively convenient),low cost,and good machinability.Magnesium is comparably low in melting temperature,but its density is even lower than aluminum.Applications include structures(aluminum,magnesium),electronic heat sinks and substrates (aluminum,copper),soldering and bearings(tin alloys),brazing (silver-copper), and high-temperature applications(nickel). Although cement is a ceramic material,ceramic-matrix composites usually refer to those with silicon carbide,silicon nitride,alumina,mullite,glasses and other ceramic matrices that are not cement. Ceramic-matrix fiber composites are gaining increasing attention because the good oxidation resistance of the ceramic matrix(compared to a carbon matrix) makes the composites attractive for high-temperature applications (e.g.,aerospace and engine components).The fibers serve mainly to increase the toughness and strength(tensile and flexural)of the composite due to their tendency to be partially pulled out during the deformation.This pullout absorbs energy,thereby tough- ening the composite.Although the fiber pullout is advantageous,the bonding between the fibers and the matrix must still be sufficiently strong for the fibers to strengthen the composite effectively.Therefore,control over the bonding between the fibers and the matrix is important for the development of these composites. When the reinforcement is provided by carbon fibers,the reinforcement has a second function,which is to increase the thermal conductivity of the composite, as the ceramic is mostly thermally insulating whereas carbon fibers are thermally conductive.In electronic,aerospace,and engine components,the enhanced ther- mal conductivity is attractive for heat dissipation
1.1 Introduction 3 thermal conductivity makes them very attractive for electronic packaging (e.g., heat sinks). Besides good thermal properties, their low density makes them particularly desirable for aerospace electronics and orbiting space structures; orbiters are thermally cycled by moving through the Earth’s shadow. Compared to the metal itself, a carbon fiber metal-matrix composite is characterized by a higher strength-to-density ratio (i.e., specific strength), a higher modulus-to-density ratio (i.e., specific modulus), better fatigue resistance, better high-temperature mechanical properties (a higher strength and a lower creep rate), a lower CTE, and better wear resistance. Compared to carbon fiber polymer-matrix composites, a carbon fiber metalmatrix composite is characterized by higher temperature resistance, higher fire resistance, higher transverse strength and modulus, a lack of moisture absorption, a higher thermal conductivity, a lower electrical resistivity, better radiation resistance, and absence of outgassing. On the other hand, a metal-matrix composite has the following disadvantages compared to the metal itself and the corresponding polymer-matrix composite: higher fabrication cost and limited service experience. Fibers used for load-bearing metal-matrix composites are mostly in the form of continuous fibers, but short fibers are also used. The matrices used include aluminum, magnesium, copper, nickel, tin alloys, silver-copper, and lead alloys. Aluminum is by far the most widely used matrix metal because of its low density, lowmeltingtemperature(whichmakescompositefabricationandjoiningrelatively convenient), low cost, and good machinability. Magnesium is comparably low in melting temperature, but its density is even lower than aluminum. Applications include structures (aluminum, magnesium), electronic heat sinks and substrates (aluminum, copper), soldering and bearings (tin alloys), brazing (silver-copper), and high-temperature applications (nickel). Although cement is a ceramic material, ceramic-matrix composites usually refer to those with silicon carbide, silicon nitride, alumina, mullite, glasses and other ceramic matrices that are not cement. Ceramic-matrix fiber composites are gaining increasing attention because the good oxidation resistance of the ceramic matrix (compared to a carbon matrix) makes the composites attractive for high-temperature applications (e.g., aerospace and engine components). The fibers serve mainly to increase the toughness and strength (tensile and flexural) of the composite due to their tendency to be partially pulled out during the deformation. This pullout absorbs energy, thereby toughening the composite. Although the fiber pullout is advantageous, the bonding between the fibers and the matrix must still be sufficiently strong for the fibers to strengthen the composite effectively. Therefore, control over the bonding between the fibers and the matrix is important for the development of these composites. When the reinforcement is provided by carbon fibers, the reinforcement has a second function, which is to increase the thermal conductivity of the composite, as the ceramic is mostly thermally insulating whereas carbon fibers are thermally conductive. In electronic, aerospace, and engine components, the enhanced thermal conductivity is attractive for heat dissipation
4 1 Composite Material Structure and Processing A third function of the reinforcement is to decrease the drying shrinkage in the case of ceramic matrices prepared using slurries or slips.In general,the drying shrinkage decreases with increasing solid content in the slurry.Fibers are more ef- fective than particles at decreasing the drying shrinkage.This function is attractive for the dimensional control of parts made from the composites. Fiber-reinforced glasses are useful for space structural applications,such as mirror back structures and supports,booms and antenna structures.In low Earth orbit,these structures experience a temperature range from -100 to 80C,so they need an improved thermal conductivity and a reduced coefficient of thermal expansion.In addition,increased toughness,strength and modulus are desirable. Due to the environmental degradation resistance of carbon fiber reinforced glasses, they are also potentially useful for gas turbine engine components.Additional attractions are low friction,high wear resistance,and low density. The glass matrices used for fiber-reinforced glasses include borosilicate glasses (e.g.,Pyrex),aluminosilicate glasses,soda lime glasses and fused quartz.Moreover, a lithia aluminosilicate glass-ceramic and a CaO-MgO-Al2O3-SiO2 glass-ceramic have been used. 1.2 Composite Material Structure The structure of a composite is commonly such that one of the components is the matrix while the other components are fillers bound by the matrix,which is often called the binder.For example,in carbon fiber reinforced polymer,which is important for lightweight structures,the polymer is the matrix,while the carbon fiber is the filler.In case of a structural composite,the filler usually serves as a reinforcement.For example,carbon fiber is a reinforcement in the polymer- matrix composite. Composites can be classified according to the matrix material,which can be a polymer,a metal,a carbon,a ceramic or a cement (e.g.,Portland cement).They can also be classified according to the shape of the filler.A composite that has particles as the filler is said to be a particulate composite.For example,concrete is a particulate composite in which cement is the matrix and sand and stones are the two types of particles that are present together.A composite with fibers used as the filler is said to be a fibrous composite. The components in a composite can also take the form of layers.An example is laminate flooring that consists of layers of polymer,paper and fiberboard that are joined together during fabrication. A composite material can be in bulk or film form.The film form can be such that the composite is a standalone film or a film that is attached to a substrate.Less commonly,a composite material takes the form of particles or fibers;i.e.,a single particle or fiber consisting of more than one component
4 1 Composite Material Structure and Processing A third function of the reinforcement is to decrease the drying shrinkage in the case of ceramic matrices prepared using slurries or slips. In general, the drying shrinkage decreases with increasing solid content in the slurry. Fibers are more effective than particles at decreasing the drying shrinkage. This function is attractive for the dimensional control of parts made from the composites. Fiber-reinforced glasses are useful for space structural applications, such as mirror back structures and supports, booms and antenna structures. In low Earth orbit, these structures experience a temperature range from –100 to 80°C, so they need an improved thermal conductivity and a reduced coefficient of thermal expansion. In addition, increased toughness, strength and modulus are desirable. Due to the environmental degradation resistance of carbon fiber reinforced glasses, they are also potentially useful for gas turbine engine components. Additional attractions are low friction, high wear resistance, and low density. The glass matrices used for fiber-reinforced glasses include borosilicate glasses (e.g., Pyrex), aluminosilicate glasses, soda lime glasses and fused quartz. Moreover, a lithia aluminosilicate glass-ceramic and a CaO-MgO-Al2O3-SiO2 glass-ceramic have been used. 1.2 Composite Material Structure The structure of a composite is commonly such that one of the components is the matrix while the other components are fillers bound by the matrix, which is often called the binder. For example, in carbon fiber reinforced polymer, which is important for lightweight structures, the polymer is the matrix, while the carbon fiber is the filler. In case of a structural composite, the filler usually serves as a reinforcement. For example, carbon fiber is a reinforcement in the polymermatrix composite. Composites can be classified according to the matrix material, which can be a polymer, a metal, a carbon, a ceramic or a cement (e.g., Portland cement). They can also be classified according to the shape of the filler. A composite that has particles as the filler is said to be a particulate composite. For example, concrete is a particulate composite in which cement is the matrix and sand and stones are the two types of particles that are present together. A composite with fibers used as the filler is said to be a fibrous composite. The components in a composite can also take the form of layers. An example is laminate flooring that consists of layers of polymer, paper and fiberboard that are joined together during fabrication. A composite material can be in bulk or film form. The film form can be such that the composite is a standalone film or a film that is attached to a substrate. Less commonly, a composite material takes the form of particles or fibers; i.e., a single particle or fiber consisting of more than one component
1.2 Composite Material Structure 5 1.2.1 Continuous Fiber Composites A fibrous composite involving continuous fibers is particularly attractive as a struc- tural material due to the high strength and modulus of the fibers,which bear most of the load.A continuous carbon fiber polymer-matrix composite is an example. The use of steel reinforcing bars(called "rebars")to reinforce concrete gives steel reinforced concrete,which is another example(even though steel rebars are not re- ferred to as fibers).A fibrous composite is also attractive in that it can be tailored by choosing the orientation of the fibers.A common configuration involves the fibers being in the form of plies.A ply,also known as a lamina,is a sheet that has fibers with the same orientation in the plane of the sheet.Each lamina has thousands of fibers along its thickness because each fiber tow consists of thousands of fibers. The composite is made up of a number of laminae such that the fiber orientations can be different among the laminae.For example,the fibers in the consecutive laminae can be oriented at 0,90,+45 and-45,resulting in a two-dimensionally "quasi-isotropic"configuration.A conventional system of notation for describing the lay-up configuration of the laminae is illustrated below. [O]s means an eight-lamina composite with all laminae having fibers in the same direction (0).[0/90]2s (where the subscript "s"means "symmetric")means an eight-lamina composite with the stacking order 0,90,0,90,90,0,90,0,where the first four laminae and the remaining four laminae are mirror images and the mirror plane is the center plane of the composite.[0/45/90/-45]s means an eight- lamina composite with the stacking order 0,45,90,-45,-45,90,45,0,where the first four laminae and the remaining four laminae are mirror images.[0/45/90/- 45]2s means a 16-lamina composite with the stacking order 0,45,90,-45,0,45,90, -45,-45,90,45,0,-45,90,45,0,where the first eight laminae and the remaining eight laminae are mirror images.[0/45/90/-45]2s means a 16-lamina composite with the stacking order0,45,90,-45,0,45,90,-45,-45,90,45,0,-45,90,45,0°， where the first eight laminae and the remaining eight laminae are mirror images. [0/45/90/-45]3s means a 24-lamina composite with the stacking order 0,45,90, -45,0,45,90,-45,0,45,90,-45,-45,90,45,0,-45,90,45,0,-45,90,45,0°，where the first 12 laminae and the remaining 12 laminae are mirror images. An optical micrograph of the interlaminar interface between two laminae that are at 90 to one another (i.e.,a crossply configuration).The interlaminar interface is the region between the two parallel lines that are separated by 8.7 um.The fibers above the interface are in the plane of the paper,whereas those below the interface are perpendicular to the paper The direction perpendicular to the laminae is known as the through-thickness direction.The interface between two adjacentlaminae is known as the interlaminar interface,which is the mechanically weak link in the laminate.Figure 1.1 is an optical micrograph of the interlaminar interface between two laminae that are 90 relative to one another (i.e.,a crossply configuration).This means that the through-thickness direction is relatively weak mechanically,and delamination (local separation of the laminae from one another)is a common form of damage in these composites.When the fibers are carbon fibers,which are much more conductive electrically than the polymer matrix,the through-thickness direction
1.2 Composite Material Structure 5 1.2.1 Continuous Fiber Composites A fibrous composite involving continuous fibers is particularly attractive as a structural material due to the high strength and modulus of the fibers, which bear most of the load. A continuous carbon fiber polymer-matrix composite is an example. The use of steel reinforcing bars (called “rebars”) to reinforce concrete gives steel reinforced concrete, which is another example (even though steel rebars are not referred to as fibers). A fibrous composite is also attractive in that it can be tailored by choosing the orientation of the fibers. A common configuration involves the fibers being in the form of plies. A ply, also known as a lamina, is a sheet that has fibers with the same orientation in the plane of the sheet. Each lamina has thousands of fibers along its thickness because each fiber tow consists of thousands of fibers. The composite is made up of a number of laminae such that the fiber orientations can be different among the laminae. For example, the fibers in the consecutive laminae can be oriented at 0, 90, +45 and –45°, resulting in a two-dimensionally “quasi-isotropic” configuration. A conventional system of notation for describing the lay-up configuration of the laminae is illustrated below. [0]8 means an eight-lamina composite with all laminae having fibers in the same direction (0°). [0/90]2s (where the subscript “s” means “symmetric”) means an eight-lamina composite with the stacking order 0, 90, 0, 90, 90, 0, 90, 0°, where the first four laminae and the remaining four laminae are mirror images and the mirror plane is the center plane of the composite. [0/45/90/–45]s means an eightlamina composite with the stacking order 0, 45, 90, –45, –45, 90, 45, 0°, where the first four laminae and the remaining four laminae are mirror images. [0/45/90/– 45]2s means a 16-lamina composite with the stacking order 0, 45, 90, –45, 0, 45, 90, –45, –45, 90, 45, 0, –45, 90, 45, 0°, where the first eight laminae and the remaining eight laminae are mirror images. [0/45/90/–45]2s means a 16-lamina composite with the stacking order 0, 45, 90, –45, 0, 45, 90, –45, –45, 90, 45, 0, –45, 90, 45, 0°, where the first eight laminae and the remaining eight laminae are mirror images. [0/45/90/–45]3s means a 24-lamina composite with the stacking order 0, 45, 90, –45, 0, 45, 90, –45, 0, 45, 90, –45, –45, 90, 45, 0, –45, 90, 45, 0, –45, 90, 45, 0°, where the first 12 laminae and the remaining 12 laminae are mirror images. An optical micrograph of the interlaminar interface between two laminae that are at 90° to one another (i.e., a crossply configuration). The interlaminar interface is the region between the two parallel lines that are separated by 8.7μm. The fibers above the interface are in the plane of the paper, whereas those below the interface are perpendicular to the paper The direction perpendicular to the laminae is known as the through-thickness direction. The interface between two adjacent laminae is known as the interlaminar interface, which is the mechanically weak link in the laminate. Figure 1.1 is an optical micrograph of the interlaminar interface between two laminae that are 90° relative to one another (i.e., a crossply configuration). This means that the through-thickness direction is relatively weak mechanically, and delamination (local separation of the laminae from one another) is a common form of damage in these composites. When the fibers are carbon fibers, which are much more conductive electrically than the polymer matrix, the through-thickness direction
6 1 Composite Material Structure and Processing 8.676μm 40μm Figure 1.1.An optical micrograph of the interlaminar interface between two laminae that are at 90to one another(i.e, a crossply configuration).The interlaminar interface is the region between the two parallel lines that are separated by 8.7 um.The fibers above the interface are in the plane of the paper,whereas those below the interface are perpendicular to the paper also has a relatively high electrical resistivity (i.e.,low electrical conductivity). In other words,the composites are strongly anisotropic (i.e.,the properties are different in different directions)both mechanically and electrically. 1.2.2 Carbon-Carbon Composites The carbon fibers used for carbon-carbon composites are usually continuous and woven.Both two-dimensional and higher-dimensional weaves are used,though the latter has the advantage of enhanced interlaminar shear strength. The weave pattern of the carbon fabric affects the densification of the carbon- carbon composite during composite fabrication.An 8H satin weave is preferred over a plain weave because of the inhomogeneous matrix distribution around the crossed bundles in the plain weave.Microcracks tend to develop beneath the bundle crossover points. For two-dimensional carbon-carbon composites containing plain weave fabric reinforcements under tension,the mode of failure of the fiber bundles depends on their curvature.Fiber bundles with small curvatures fail due to tensile stress or due to a combination of tensile and bending stresses.Fiber bundles with large curvatures fail due to shear stresses at the point where the local fiber direction is most inclined to the applied load. Circular fibers are preferred to irregularly shaped fibers,as the latter leads to stress concentration points in the matrix around the fiber corners.Microcrack initiation occurs at these points,thus resulting in low strength in the carbon- carbon composite
6 1 Composite Material Structure and Processing Figure 1.1. An optical micrograph of the interlaminar interface between two laminae that are at 90° to one another (i.e., a crossply configuration). The interlaminar interface is the region between the two parallel lines that are separated by 8.7μm. The fibers above the interface are in the plane of the paper, whereas those below the interface are perpendicular to the paper also has a relatively high electrical resistivity (i.e., low electrical conductivity). In other words, the composites are strongly anisotropic (i.e., the properties are different in different directions) both mechanically and electrically. 1.2.2 Carbon–Carbon Composites The carbon fibers used for carbon–carbon composites are usually continuous and woven. Both two-dimensional and higher-dimensional weaves are used, though the latter has the advantage of enhanced interlaminar shear strength. The weave pattern of the carbon fabric affects the densification of the carbon– carbon composite during composite fabrication. An 8H satin weave is preferred over a plain weave because of the inhomogeneous matrix distribution around the crossed bundles in the plain weave. Microcracks tend to develop beneath the bundle crossover points. For two-dimensional carbon–carbon composites containing plain weave fabric reinforcements under tension, the mode of failure of the fiber bundles depends on their curvature. Fiber bundles with small curvatures fail due to tensile stress or due to a combination of tensile and bending stresses. Fiber bundles with large curvatures fail due to shear stresses at the point where the local fiber direction is most inclined to the applied load. Circular fibers are preferred to irregularly shaped fibers, as the latter leads to stress concentration points in the matrix around the fiber corners. Microcrack initiation occurs at these points, thus resulting in low strength in the carbon– carbon composite
1.2 Composite Material Structure 7 1.2.3 Cement-Matrix Composites Cement-matrix composites include concrete,which is a cement-matrix composite with a fine aggregate (sand),a coarse aggregate (gravel)and optionally other additives(called admixtures).Concrete is the most widely used civil structural material.When the coarse aggregate is absent,the composite is known as a mortar, which is used in masonry(for joining bricks)and for filling cracks.When both coarse and fine aggregates are absent,the material is known as cement paste. Cement paste is rigid after curing(the hydration reaction involving cement- a silicate-and water to form a rigid gel). The admixtures can be a fine particulate such as silica(SiOz)fume to decrease the porosity in the composite.It can be a polymer(used in either a liquid solution form or a solid dispersion form)such as latex,again to decrease the porosity.It can be short fibers(such as carbon fibers,glass fibers,polymer fibers and steel fibers) to increase the toughness and decrease the drying shrinkage (shrinkage during curing-undesirable,as it can cause cracks to form).Continuous fibers are seldom used because of their high cost and the impossibility of incorporating continuous fibers into a cement mix.Due to the bidding system used for many construction projects,low cost is essential. Fibrous cement-matrix composites are structural materials that are gaining in importance quite rapidly due to the increasing demand for superior structural and functional properties.Discontinuous fibers used in concrete include steel, glass,polymer and carbon fibers.Among these fibers,carbon and glass fibers are micrometer scale (e.g.,10 um)in diameter,whereas steel and polymer fibers are usually much larger in diameter (e.g.,100um).For the microfibers,the fiber length is typically around 5mm,as fiber dispersion becomes more difficult as the fiber length increases.Due to the weak bond between fiber and the cement matrix,continuous fibers are much more effective than short fibers at reinforcing concrete.However,continuous fibers cannot be incorporated into a concrete mix, and it is difficult for the concrete mix to penetrate into the space between adjacent fibers,even in the absence of aggregates.The alignment of the continuous fibers in concrete also adds to the implementation difficulty.Therefore,short fibers are typically used. The effect of short fiber addition on the properties of cement increases with increasing fiber volume fraction unless the fiber volume fraction is so high that the air void content becomes excessively high.(The air void content increases with fiber content,and air voids tend to have a negative effect on many properties,such as the compressive strength.)In addition,the workability of the mix decreases with increasing fiber content.Moreover,the cost increases with increasing fiber content. Therefore,a rather low volume fraction of fibers is desirable.A fiber content as low as 0.2 vol%is effective,although fiber contents exceeding 1 vol%are common. The required fiber volume fraction increases with increasing fiber diameter and increases with increasing particle size of the aggregate. The improvement in the structural properties due to the addition of discon- tinuous fibers to cement includes increases in the tensile ductility and flexural toughness and a decrease in the drying shrinkage.A low drying shrinkage is par-
1.2 Composite Material Structure 7 1.2.3 Cement-Matrix Composites Cement-matrix composites include concrete, which is a cement-matrix composite with a fine aggregate (sand), a coarse aggregate (gravel) and optionally other additives (called admixtures). Concrete is the most widely used civil structural material. When the coarse aggregate is absent, the composite is known as a mortar, which is used in masonry (for joining bricks) and for filling cracks. When both coarse and fine aggregates are absent, the material is known as cement paste. Cement paste is rigid after curing (the hydration reaction involving cement – a silicate – and water to form a rigid gel). The admixtures can be a fine particulate such as silica (SiO2) fume to decrease the porosity in the composite. It can be a polymer (used in either a liquid solution form or a solid dispersion form) such as latex, again to decrease the porosity. It can be short fibers (such as carbon fibers, glass fibers, polymer fibers and steel fibers) to increase the toughness and decrease the drying shrinkage (shrinkage during curing – undesirable, as it can cause cracks to form). Continuous fibers are seldom used because of their high cost and the impossibility of incorporating continuous fibers into a cement mix. Due to the bidding system used for many construction projects, low cost is essential. Fibrous cement-matrix composites are structural materials that are gaining in importance quite rapidly due to the increasing demand for superior structural and functional properties. Discontinuous fibers used in concrete include steel, glass, polymer and carbon fibers. Among these fibers, carbon and glass fibers are micrometer scale (e.g., 10μm) in diameter, whereas steel and polymer fibers are usually much larger in diameter (e.g., 100μm). For the microfibers, the fiber length is typically around 5mm, as fiber dispersion becomes more difficult as the fiber length increases. Due to the weak bond between fiber and the cement matrix, continuous fibers are much more effective than short fibers at reinforcing concrete. However, continuous fibers cannot be incorporated into a concrete mix, and it is difficult for the concrete mix to penetrate into the space between adjacent fibers, even in the absence of aggregates. The alignment of the continuous fibers in concrete also adds to the implementation difficulty. Therefore, short fibers are typically used. The effect of short fiber addition on the properties of cement increases with increasing fiber volume fraction unless the fiber volume fraction is so high that the air void content becomes excessively high. (The air void content increases with fiber content, and air voids tend to have a negative effect on many properties, such as the compressive strength.) In addition, the workability of the mix decreases with increasing fiber content. Moreover, the cost increases with increasing fiber content. Therefore, a rather low volume fraction of fibers is desirable. A fiber content as low as 0.2vol% is effective, although fiber contents exceeding 1vol% are common. The required fiber volume fraction increases with increasing fiber diameter and increases with increasing particle size of the aggregate. The improvement in the structural properties due to the addition of discontinuous fibers to cement includes increases in the tensile ductility and flexural toughness and a decrease in the drying shrinkage. A low drying shrinkage is par-
8 1 Composite Material Structure and Processing ticularly valuable for large structures,as cracks can form due to the shrinkage and the cracks are wide for the same fractional shrinkage if the structure is large. In the case of the fiber being carbon fiber,improvements in the tensile strength and the flexural strength also occur.Carbon fibers(made from isotropic pitch)are advantageous in their superior ability to increase the tensile strength of cement, even though the tensile strength,modulus and ductility of the isotropic pitch based carbon fibers are low compared to most other fibers.Carbon fibers are also advantageous because of their relative chemical inertness. In relation to most functional properties,carbon fibers are exceptional compared to the other fiber types.Carbon fibers are electrically conducting,in contrast to glass and polymer fibers,which are not conducting.Steel fibers are conductive,but their typical diameter(>60 um)is much larger than the diameter ofa typical carbon fiber(10 um).The combination ofelectrical conductivity and small diameter makes carbon fibers attractive for use in composite functional property tailoring. 1.3 Processing of Composite Materials The technology and cost of composite materials depend largely on the process- ability;i.e.,how the components are combined to form a composite material.The processability depends largely on the ability of the components to join,thereby forming a cohesive material.The processing often involves elevated temperatures and/or pressures.The required temperature and pressure,as well as the processing time,are typically dictated by the matrix material.The bonding of the filler with the matrix at an elevated temperature has a disadvantage in that bond weakening or even debonding may occur during the subsequent cooling,due to the difference in the thermal contraction(related to the coefficient of thermal expansion,or CTE) between filler and matrix.The bond weakening will result in the filler being less effective as a reinforcement,thus causing the mechanical properties of the com- posite to diminish.This problem tends to be particularly serious in metal-matrix composites,due to the relatively high processing temperatures involved. Fiber composites are most commonly fabricated by the impregnation(or infil- tration)of the matrix or matrix precursor in the liquid state into the fiber preform, which can take the form of a woven fabric.In the case of composites in the shape of tubes,the fibers may be impregnated in the form of a continuous bundle(called a tow)from a spool and subsequently the bundles can by wound on a mandrel. Instead of impregnation,the fibers and matrix material may be intermixed in the solid state by commingling reinforcing fibers and matrix fibers,by coating the reinforcing fibers with the matrix material,by sandwiching reinforcing fibers with foils of the matrix material,and in other ways.After impregnation or intermixing, consolidation is carried out,often under heat and pressure. 1.3.1 Polymer-Matrix Composites Polymer-matrix composites (abbreviated PMC)can be classified according to whether the matrix is a thermoset or a thermoplastic polymer.Thermoset-
8 1 Composite Material Structure and Processing ticularly valuable for large structures, as cracks can form due to the shrinkage and the cracks are wide for the same fractional shrinkage if the structure is large. In the case of the fiber being carbon fiber, improvements in the tensile strength and the flexural strength also occur. Carbon fibers (made from isotropic pitch) are advantageous in their superior ability to increase the tensile strength of cement, even though the tensile strength, modulus and ductility of the isotropic pitch based carbon fibers are low compared to most other fibers. Carbon fibers are also advantageous because of their relative chemical inertness. Inrelationtomostfunctionalproperties,carbonfibersareexceptionalcompared to the other fiber types. Carbon fibers are electrically conducting, in contrast to glass and polymer fibers, which are not conducting. Steel fibers are conductive, but theirtypicaldiameter(≥60μm)ismuchlargerthanthediameterofatypicalcarbon fiber(10μm).Thecombinationofelectricalconductivityandsmalldiametermakes carbon fibers attractive for use in composite functional property tailoring. 1.3 Processing of Composite Materials The technology and cost of composite materials depend largely on the processability; i.e., how the components are combined to form a composite material. The processability depends largely on the ability of the components to join, thereby forming a cohesive material. The processing often involves elevated temperatures and/or pressures. The required temperature and pressure, as well as the processing time, are typically dictated by the matrix material. The bonding of the filler with the matrix at an elevated temperature has a disadvantage in that bond weakening or even debonding may occur during the subsequent cooling, due to the difference in the thermal contraction (related to the coefficient of thermal expansion, or CTE) between filler and matrix. The bond weakening will result in the filler being less effective as a reinforcement, thus causing the mechanical properties of the composite to diminish. This problem tends to be particularly serious in metal-matrix composites, due to the relatively high processing temperatures involved. Fiber composites are most commonly fabricated by the impregnation (or infiltration) of the matrix or matrix precursor in the liquid state into the fiber preform, which can take the form of a woven fabric. In the case of composites in the shape of tubes, the fibers may be impregnated in the form of a continuous bundle (called a tow) from a spool and subsequently the bundles can by wound on a mandrel. Instead of impregnation, the fibers and matrix material may be intermixed in the solid state by commingling reinforcing fibers and matrix fibers, by coating the reinforcing fibers with the matrix material, by sandwiching reinforcing fibers with foils of the matrix material, and in other ways. After impregnation or intermixing, consolidation is carried out, often under heat and pressure. 1.3.1 Polymer-Matrix Composites Polymer-matrix composites (abbreviated PMC) can be classified according to whether the matrix is a thermoset or a thermoplastic polymer. Thermoset-
1.3 Processing of Composite Materials 9 matrix composites are traditionally far more common,but thermoplastic-matrix composites are currently the focus of rapid development.The advantages of thermoplastic-matrix composites compared to thermoset-matrix composites in- clude the following: Lower manufacturing costs: ·No cure Unlimited shelf-life Reprocessing possible(for repair and recycling) Fewer health risks due to chemicals during processing ·Low moisture content .Thermal shaping possible .Weldability(fusion bonding possible). Better performance: High toughness(damage tolerance) Good hot/wet properties High environmental tolerance. The disadvantages of thermoplastic-matrix composites include the following: Limitations in relation to processing methods High processing temperatures ·High viscosities Prepreg (collection of continuous fibers aligned to form a sheet that has been impregnated with the polymer or polymer precursor)is stiff and dry when a solvent is not used (ie.,not drapeable or tacky) Fiber surface treatments less developed. Fibrous polymer-matrix composites can be classified according to whether the fibers are short or continuous.Continuous fibers have much more effect than short fibers on the composite's mechanical properties,electrical resistivity,thermal conductivity,and on other properties too.However,they give rise to composites that are more anisotropic.Continuous fibers can be utilized in unidirectionally aligned tape or woven fabric form. Polymer-matrix composites are much easier to fabricate than metal-matrix, carbon-matrix,and ceramic-matrix composites,whether the polymer is a ther- moset or a thermoplastic.This is because of the relatively low processing temper- atures required to fabricate polymer-matrix composites.For thermosets,such as epoxy,phenolic,and furfuryl resin,the processing temperature typically ranges from room temperature to about 200C;for thermoplastic polymers,such as poly- imide(PI),polyethersulfone(PES),polyetheretherketone(PEEK),polyetherimide
1.3 Processing of Composite Materials 9 matrix composites are traditionally far more common, but thermoplastic-matrix composites are currently the focus of rapid development. The advantages of thermoplastic-matrix composites compared to thermoset-matrix composites include the following: Lower manufacturing costs: No cure Unlimited shelf-life Reprocessing possible (for repair and recycling) Fewer health risks due to chemicals during processing Low moisture content Thermal shaping possible Weldability (fusion bonding possible). Better performance: High toughness (damage tolerance) Good hot/wet properties High environmental tolerance. The disadvantages of thermoplastic-matrix composites include the following: Limitations in relation to processing methods High processing temperatures High viscosities Prepreg (collection of continuous fibers aligned to form a sheet that has been impregnated with the polymer or polymer precursor) is stiff and dry when a solvent is not used (i.e., not drapeable or tacky) Fiber surface treatments less developed. Fibrous polymer-matrix composites can be classified according to whether the fibers are short or continuous. Continuous fibers have much more effect than short fibersonthecomposite’smechanicalproperties,electricalresistivity,thermal conductivity, and on other properties too. However, they give rise to composites that are more anisotropic. Continuous fibers can be utilized in unidirectionally aligned tape or woven fabric form. Polymer-matrix composites are much easier to fabricate than metal-matrix, carbon-matrix, and ceramic-matrix composites, whether the polymer is a thermoset or a thermoplastic. This is because of the relatively low processing temperatures required to fabricate polymer-matrix composites. For thermosets, such as epoxy, phenolic, and furfuryl resin, the processing temperature typically ranges from room temperature to about 200°C; for thermoplastic polymers, such as polyimide (PI), polyethersulfone (PES), polyetheretherketone (PEEK), polyetherimide
10 1 Composite Material Structure and Processing (PEI),and polyphenyl sulfide(PPS),the processing temperature typically ranges from 300 to 400C. Thermosets(especially epoxy)have long been used as polymer matrices for car- bon fiber composites.During curing,usually performed in the presence ofheat and pressure,a thermoset resin hardens gradually due to the completion of polymer- ization and the associated crosslinking of the polymer molecules.Thermoplastic polymers have recently become important because of their greater ductility and processing speed compared to thermosets,and the recent availability of thermo- plastics that can withstand high temperatures.The higher processing speed of thermoplastics arises from the fact that amorphous thermoplastics soften im- mediately upon heating above the glass transition temperature(Ta),and so the softened material can be shaped easily.Subsequent cooling completes the process- ing.In contrast,the curing of a thermoset resin is a reaction that occurs gradually. Short-fiber or particulate composites are usually fabricated by mixing the fibers or particles with a liquid resin to form a slurry,and then molding to form a com- posite.The liquid resin is the unpolymerized or partially polymerized matrix material in the case of a thermoset;it is the molten polymer or the polymer dissolved in a solvent in the case of a thermoplastic.The molding methods are those conventionally used for polymers by themselves.For thermoplastics,the methods include injection molding(heating above the melting temperature of the thermoplastic and forcing the slurry into a closed die opening through the use of a screw mechanism),extrusion(forcing the slurry through a die opening via a screw mechanism),calendering (pouring the slurry into a set of rollers with a small opening between adjacent rollers to form a thin sheet),and thermoform- ing(heating above the softening temperature of the thermoplastic and forming over a die using matching dies,a vacuum or air pressure,or without a die using movable rollers).For thermosets,compression molding or matched die molding (applying a high pressure and temperature to the slurry in a die to harden the thermoset)is commonly used.The casting of the slurry into a mold is not usually suitable because the difference in density between the resin and the fibers causes the fibers to float or sink unless the viscosity of the resin is carefully adjusted.To form a composite coating,the fiber-resin or particle-resin slurry can be sprayed instead of molded. Instead of using a fiber-resin slurry,short fibers in the form of a mat or a contin- uous spun staple yarn can be impregnated with a resin and shaped using methods commonly used for continuous fiber composites.Yet another method involves us- ing continuous staple yarns in the form of an intimate blend of short carbon fibers and short thermoplastic fibers.The yarns may be woven,if desired.They do not need to be impregnated with a resin to form a composite,as the thermoplastic fibers melt during consolidation under heat and pressure. One method of forming unidirectional fiber composite parts with a constant cross-section (e.g.,round,rectangular,pipe,plate,I-shaped)is pultrusion,in which fibers are drawn from spools,passed through a polymer resin bath for impregnation,and gathered together to produce a particular shape before enter- ing a heated die
10 1 Composite Material Structure and Processing (PEI), and polyphenyl sulfide (PPS), the processing temperature typically ranges from 300 to 400°C. Thermosets (especially epoxy) have long been used as polymer matrices for carbon fiber composites. During curing, usually performed in the presence of heat and pressure, a thermoset resin hardens gradually due to the completion of polymerization and the associated crosslinking of the polymer molecules. Thermoplastic polymers have recently become important because of their greater ductility and processing speed compared to thermosets, and the recent availability of thermoplastics that can withstand high temperatures. The higher processing speed of thermoplastics arises from the fact that amorphous thermoplastics soften immediately upon heating above the glass transition temperature (Tg), and so the softened material can be shaped easily. Subsequent cooling completes the processing. In contrast, the curing of a thermoset resin is a reaction that occurs gradually. Short-fiber or particulate composites are usually fabricated by mixing the fibers or particles with a liquid resin to form a slurry, and then molding to form a composite. The liquid resin is the unpolymerized or partially polymerized matrix material in the case of a thermoset; it is the molten polymer or the polymer dissolved in a solvent in the case of a thermoplastic. The molding methods are those conventionally used for polymers by themselves. For thermoplastics, the methods include injection molding (heating above the melting temperature of the thermoplastic and forcing the slurry into a closed die opening through the use of a screw mechanism), extrusion (forcing the slurry through a die opening via a screw mechanism), calendering (pouring the slurry into a set of rollers with a small opening between adjacent rollers to form a thin sheet), and thermoforming (heating above the softening temperature of the thermoplastic and forming over a die using matching dies, a vacuum or air pressure, or without a die using movable rollers). For thermosets, compression molding or matched die molding (applying a high pressure and temperature to the slurry in a die to harden the thermoset) is commonly used. The casting of the slurry into a mold is not usually suitable because the difference in density between the resin and the fibers causes the fibers to float or sink unless the viscosity of the resin is carefully adjusted. To form a composite coating, the fiber-resin or particle-resin slurry can be sprayed instead of molded. Instead of using a fiber-resin slurry, short fibers in the form of a mat or a continuous spun staple yarn can be impregnated with a resin and shaped using methods commonly used for continuous fiber composites. Yet another method involves using continuous staple yarns in the form of an intimate blend of short carbon fibers and short thermoplastic fibers. The yarns may be woven, if desired. They do not need to be impregnated with a resin to form a composite, as the thermoplastic fibers melt during consolidation under heat and pressure. One method of forming unidirectional fiber composite parts with a constant cross-section (e.g., round, rectangular, pipe, plate, I-shaped) is pultrusion, in which fibers are drawn from spools, passed through a polymer resin bath for impregnation, and gathered together to produce a particular shape before entering a heated die