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