《纺织复合材料》课程参考文献（Composite Materials for Aircraft Structures）03 Fibers for Polymer-Matrix Composites

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3 Fibers for Polymer-Matrix Composites 3.1 Overview As a result of their strong directional interatomic bonds,elements of low atomic number,including C,B,Al,and Si,can be formed into stiff,low-density materials.These materials may be made entirely from the elements themselves (e.g,C or B),or from their compounds (e.g.,SiC),or with oxygen or nitrogen, (e.g.,Al203,SiO2 or SiN4). The strong bonding'also inhibits plastic flow,at least at temperatures below around half the melting temperature.Because these materials are unable to relieve stress concentrations by plastic flow,they are markedly weakened by sub- microscopic faws,particularly those open to the surface.Thus,it is generally only when made in the form of fibers that the inherent very high strength of these materials can be realized.2.3 There are several reasons for this,including the following: The probability of a flaw being present (per unit length)in a sample is an inverse function of volume of the material,as described by Weibull statistics. Hence a fiber having a very low volume(per unit length)is much stronger on average than the bulk material.However,the bulk material,having a much higher content of weakening flaws,exhibits a much lower variability in strength,as shown in Figure 3.1.It follows similarly that the smaller the fiber diameter and the shorter the length,the higher the average and maximum strength,but the greater the variability. Flaws can be minimized by appropriate fiber manufacturing and coating procedures to minimize surface damage.Also,the precursor materials used in fiber making must be of a high purity,including freedom from inclusions.The effect of flaws on strength can be estimated from thermodynamic (energy balance)and elasticity considerations. Fiber manufacturing processes that involve drawing or spinning can impose very high strains in the direction of the fiber axis,thus producing a more favorable orientation of the crystal or atomic structure. Some fiber manufacturing processes involve a very high cooling rate or rapid molecular deposition to produce metastable,often ultra-fine grained structures, having properties not achievable in the bulk material. 55

3 Fibers for Polymer-Matrix Composites 3.1 Overview As a result of their strong directional interatomic bonds, elements of low atomic number, including C, B, A1, and Si, can be formed into stiff, low-density materials. These materials may be made entirely from the elements themselves (e.g., C or B), or from their compounds (e.g., SIC), or with oxygen or nitrogen, (e.g., A1203, SiO 2 or Si3N4). The strong bonding I also inhibits plastic flow, at least at temperatures below around half the melting temperature. Because these materials are unable to relieve stress concentrations by plastic flow, they are markedly weakened by submicroscopic flaws, particularly those open to the surface. Thus, it is generally only when made in the form of fibers that the inherent very high strength of these materials can be realized. 2'3 There are several reasons for this, including the following: • The probability of a flaw being present (per unit length) in a sample is an inverse function of volume of the material, as described by Weibull statistics.4 Hence a fiber having a very low volume (per unit length) is much stronger on average than the bulk material. However, the bulk material, having a much higher content of weakening flaws, exhibits a much lower variability in strength, as shown in Figure 3.1. It follows similarly that the smaller the fiber diameter and the shorter the length, the higher the average and maximum strength, but the greater the variability. • Flaws can be minimized by appropriate fiber manufacturing and coating procedures to minimize surface damage. Also, the precursor materials used in fiber making must be of a high purity, including freedom from inclusions. The effect of flaws on strength can be estimated from thermodynamic (energy balance) and elasticity considerations. • Fiber manufacturing processes that involve drawing or spinning can impose very high strains in the direction of the fiber axis, thus producing a more favorable orientation of the crystal or atomic structure. • Some fiber manufacturing processes involve a very high cooling rate or rapid molecular deposition to produce metastable, often ultra-fine grained structures, having properties not achievable in the bulk material. 55

60 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES dielectric properties,glass-fiber PMCs are also widely used in applications in which transparency to electromagnetic radiation is required,including radomes and aerial covers. Glass is an amorphous solid produced by cooling a viscous liquid at a sufficiently high rate to prevent the formation of ordered or crystalline regions. Compounds that make up the glass in glass fibers can include (in addition to silica)oxides of aluminum,boron,calcium,magnesium,sodium,and potassium. Additives are used to lower the melting point of silica so that the required viscosity is obtained at a lower temperature.In addition,they facilitate the removal of gas bubbles and have a significant effect on the mechanical and chemical properties of the final product. Glass fibers are manufactured by a viscous drawing process depicted in Figure 3.3 in which glass,melted in a furnace at temperatures of about 1400C, flows into an electrically heated platinum-rhodium alloy bushing or spinneret, containing a large number(400-8000)of holes in its base.The emerging glass drops are drawn into fibers by pulling at speeds of up to 50 m s.They are then cooled by a fine water spray and coated with a size by contact with a rotating applicator.Finally,the fibers are combined into a strand as they are wound onto a take-up spool. The fiber diameter,typically around 5-20 um,is a function of the size of the holes in the bushing,the viscosity of the melt (which is dependent on the composition of the glass and the temperature),the head of glass in the furnace, and the rate of winding.Depending on the number of holes in the bushing,the strand typically consists of 52,102,or 204 fibers. The cooling rate experienced by the fibers is very high,>10,000C s-1 A parameter called the fictive temperature is the apparent temperature at which Drawing of glass filaments(A) glass melt feed spinning hole bushing appr0x.1250°C molten glass spinning holes 8ppr0x.1250°C rapid cooling filaments cooling drawing at high speed size assembler The formation of a single strand fllament according to the traversing and process shown in A. winding Fig.3.3 Schematic illustration of the process used to manufacture glass fibers

60 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES dielectric properties, glass-fiber PMCs are also widely used in applications in which transparency to electromagnetic radiation is required, including radomes and aerial covers. Glass is an amorphous solid produced by cooling a viscous liquid at a sufficiently high rate to prevent the formation of ordered or crystalline regions. Compounds that make up the glass in glass fibers can include (in addition to silica) oxides of aluminum, boron, calcium, magnesium, sodium, and potassium. Additives are used to lower the melting point of silica so that the required viscosity is obtained at a lower temperature. In addition, they facilitate the removal of gas bubbles and have a significant effect on the mechanical and chemical properties of the final product. Glass fibers are manufactured by a viscous drawing process depicted in Figure 3.3 in which glass, melted in a furnace at temperatures of about 1400 °C, flows into an electrically heated platinum-rhodium alloy bushing or spinneret, containing a large number (400-8000) of holes in its base. The emerging glass drops are drawn into fibers by pulling at speeds of up to 50 m s.- 1 They are then cooled by a fine water spray and coated with a size by contact with a rotating applicator. Finally, the fibers are combined into a strand as they are wound onto a take-up spool. The fiber diameter, typically around 5-20 Ixm, is a function of the size of the holes in the bushing, the viscosity of the melt (which is dependent on the composition of the glass and the temperature), the head of glass in the furnace, and the rate of winding. Depending on the number of holes in the bushing, the strand typically consists of 52, 102, or 204 fibers. The cooling rate experienced by the fibers is very high, > 10,000 °C s -1. A parameter 5 called thefictive temperature is the apparent temperature at which Drawing of glass filaments (A) | gloss melt feed | bushing approx. 1250°C ,J spinning holes filaments cooling size II -- spinning hole molten glass approx, 1250*C rapid cooling drawing at high speed Q Fig. 3.3 assembler The formation of a single strand filamenl according to the traversing and process shown in A. winding Schematic illustration of the process used to manufacture glass fibers

FIBERS FOR POLYMER-MATRIX COMPOSITES 61 the glass is frozen,generally found to be 200-300C above the liquidus. As a result,the fiber structure is somewhat different from that of bulk glass, resulting in a higher tensile strength but lower elastic modulus and chemical resistance. 3.2.2 Effect of Flaws Glass fibers,being essentially monolithic,linearly elastic brittle materials, depend for their high strength on the absence of flaws and defects.These take the form of sub-microscopic inclusions and cracks The inclusions can often be seen with a scanning electron microscope,but "cracks"sufficient to reduce strength significantly can be very difficult to find because they are of nanometre dimensions.The origin of flaws is,however,generally obvious when examining the fracture surface because growth starts from the region of the flaw as a flat (mirror)surface and transforms to hackles radiating from this region as growth accelerates. Commercial glass fibers are particularly prone to the formation of flaws by abrasion against other fibers,resulting in a reduction in strength of the order of 20%compared with pristine fibers made under laboratory conditions. The tensile strength is probably significantly dependent on the composition, structure,and internal stresses in the surface layer,all of which differ signi- ficantly from those in the internal structure due in part to the high cooling rate. Although this layer is only of the order of a nanometer thick,it is of the order of the size of the flaws that control the strength of high strength fibers>2 GPa. Generally,surface flaws have a similar strength-reducing effect compared with internal flaws of twice the length. Humid environments reduce the strength of glass fibers under sustained loading,as the moisture adsorbed onto the surface of the flaw reduces the surface energy,thus facilitating slow growth to critical size.This phenomenon in glass is called static fatigue. The strength of the glass fibers is reduced by about a further 50%when they are formed into a polymer-matrix composite.However,because of the bundle effect described in Chapter 2,this reduction is not noticeable.Essentially,the gauge length for a bundle of fibers is the length of the bundle,whereas,due to load transfer from the matrix,for a composite it is only of the order of 1 mm, depending on fiber diameter and fiber/matrix bond strength.Further reductions in strength can occur if the composite is exposed to wet conditions because components leached out of the polymer can cause acidic or basic conditions to develop at the fiber surface. 3.2.3 Types of Glass Fiber The compositions of glass made into fibers for PMCs are listed in Table 3.2 There are two types of glass fiber used for structural applications:"E,"a calcium

FIBERS FOR POLYMER-MATRIX COMPOSITES 61 the glass is frozen, generally found to be 200-300 °C above the liquidus. As a result, the fiber structure is somewhat different from that of bulk glass, resulting in a higher tensile strength but lower elastic modulus and chemical resistance. 3.2.2 Effect of Flaws Glass fibers, being essentially monolithic, linearly elastic brittle materials, depend for their high strength on the absence of flaws and defects. These take the form of sub-microscopic inclusions and cracks The inclusions can often be seen with a scanning electron microscope, but "cracks" sufficient to reduce strength significantly can be very difficult to find because they are of nanometre dimensions. The origin of flaws is, however, generally obvious when examining the fracture surface because growth starts from the region of the flaw as a flat (mirror) surface and transforms to hackles radiating from this region as growth accelerates. Commercial glass fibers are particularly prone to the formation of flaws by abrasion against other fibers, resulting in a reduction in strength of the order of 20% compared with pristine fibers made under laboratory conditions. The tensile strength is probably significantly dependent on the composition, structure, and internal stresses in the surface layer, all of which differ significantly from those in the internal structure due in part to the high cooling rate. Although this layer is only of the order of a nanometer thick, it is of the order of the size of the flaws that control the strength of high strength fibers > 2 GPa. Generally, surface flaws have a similar strength-reducing effect compared with internal flaws of twice the length. Humid environments reduce the strength of glass fibers under sustained loading, as the moisture adsorbed onto the surface of the flaw reduces the surface energy, thus facilitating slow growth to critical size. This phenomenon in glass is called static fatigue. The strength of the glass fibers is reduced by about a further 50% when they are formed into a polymer-matrix composite. However, because of the bundle effect described in Chapter 2, this reduction is not noticeable. Essentially, the gauge length for a bundle of fibers is the length of the bundle, whereas, due to load transfer from the matrix, for a composite it is only of the order of 1 mm, depending on fiber diameter and fiber/matrix bond strength. Further reductions in strength can occur if the composite is exposed to wet conditions because components leached out of the polymer can cause acidic or basic conditions to develop at the fiber surface. 3.2.3 Types of Glass Fiber The compositions of glass made into fibers for PMCs are listed in Table 3.2 There are two types of glass fiber used for structural applications: "E," a calcium

62 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES alumino-borosilicate glass,and "S,"a magnesium alumino-silicate glass.E stands for electrical grade,because compared with other standard forms of glass, its electrical resistivity is high and its dielectric constant low.These are by far the most widely exploited in structural applications,particularly in the non-aerospace area,because of their relatively low cost and high strength.A modified (low boron and fluorine)version of E glass fiber,ECR(E glass chemically resistant),is used where improved chemical properties are required.S stands for high-strength grade,although stiffness is also somewhat increased.These fibers can also withstand significantly higher temperatures than E glass fibers.Thus S glass fibers are used in more demanding structural applications.However,this marginal increase in stiffness is obtained at a relatively high cost.Where high specific strength and stiffness are required (with good dielectric properties) aramid fibers,described later,may be more attractive.More recently,a boron- free E glass has been developed that has markedly improved resistance to corrosive environments,but with no loss in mechanical properties. 3.2.4 Glass Fiber Coatings As mentioned earlier,glass fibers are highly sensitive to surface damage. Because the coefficient of friction between glass fibers is around unity, mechanical damage sufficient to cause a significant loss in strength can result from fiber-to-fiber abrasion during the forming process.To prevent contact damage,within milliseconds of solidifying,the fibers are coated with a protective size that also serves to minimize losses in strength due to atmospheric moisture absorption.For example,the tensile strength of as-drawn fibers can be reduced by over 20%after contact with air during drawing under normal ambient conditions. It seems likely that the atmospheric moisture is absorbed into microscopic flaws,reducing fracture energy because time would be too limited chemical attack.In any case,the tensile strength of the glass fibers drops significantly during the manufacturing process,from as high as 5 GPa immediately after drawing to typically around 2-3 GPa postproduction. The size consists of several components.The simplest is a lubricant,such as a light mineral oil for protection and to aid further processing such as weaving, filament winding,and pultrusion.Binders such as starch and polyvinyl alcohol Table 3.2 Chemical Composition of the Two Main Glass Fiber Types Glass type Si Al203 Cao B203 Mgo Na2O K20 E-Electrical 53 14 18 10 5 <1 S-High strength 65 25 10

62 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES alumino-borosilicate glass, and "S," a magnesium alumino-silicate glass. E stands for electrical grade, because compared with other standard forms of glass, its electrical resistivity is high and its dielectric constant low. These are by far the most widely exploited in structural applications, particularly in the non-aerospace area, because of their relatively low cost and high strength. A modified (low boron and fluorine) version of E glass fiber, ECR (E glass chemically resistant), is used where improved chemical properties are required. S stands for high-strength grade, although stiffness is also somewhat increased. These fibers can also withstand significantly higher temperatures than E glass fibers. Thus S glass fibers are used in more demanding structural applications. However, this marginal increase in stiffness is obtained at a relatively high cost. Where high specific strength and stiffness are required (with good dielectric properties) aramid fibers, described later, may be more attractive. More recently, a boronfree E glass has been developed that has markedly improved resistance to corrosive environments, but with no loss in mechanical properties. 3.2.4 Glass Fiber Coatings As mentioned earlier, glass fibers are highly sensitive to surface damage. Because the coefficient of friction between glass fibers is around unity, mechanical damage sufficient to cause a significant loss in strength can result from fiber-to-fiber abrasion during the forming process. To prevent contact damage, within milliseconds of solidifying, the fibers are coated with a protective size that also serves to minimize losses in strength due to atmospheric moisture absorption. For example, the tensile strength of as-drawn fibers can be reduced by over 20% after contact with air during drawing under normal ambient conditions. It seems likely that the atmospheric moisture is absorbed into microscopic flaws, reducing fracture energy because time would be too limited chemical attack. In any case, the tensile strength of the glass fibers drops significantly during the manufacturing process, from as high as 5 GPa immediately after drawing to typically around 2-3 GPa postproduction. The size consists of several components. The simplest is a lubricant, such as a light mineral oil for protection and to aid further processing such as weaving, filament winding, and pultrusion. Binders such as starch and polyvinyl alcohol Table 3.2 Chemical Composition of the Two Main Glass Fiber Types Glass type Si A1203 CaO B203 MgO Na20 K20 E-Electrical 5 3 14 18 10 5 < 1 S-High strength 65 25 -- -- 10 --

FIBERS FOR POLYMER-MATRIX COMPOSITES 63 (PVA)are included in the size to bond or hold the filaments together into strands and tows.Finishes,also called primers,are used in the size to improve the adhesive bonding between the fiber and the polymer matrix.Primers may be added to the size or applied later after removal of the size components by heat treatment. The finish is often based on a coupling agent that for most polymer-matrix resins is an organo-silane compound.Organo-silanes effectively have dual functionality,with their organo portion interacting with the organic resins or adhesives and the silane portion interacting with the inorganic fibers.Thus,these compounds are used to improve the interfacial(resin/fiber)properties of PMCs. Briefly,the silane molecule on hydration in water can be represented by the following simplified formula: R...Si(OH) The Si(OH)3 bonds with the oxide film at the surface of the inorganic fiber-glass in this case,while the organic functional group R is incorporated into the organic matrix during its cure.R must therefore be a group that is chemically compatible with the matrix resin.For example,for an epoxy resin,an epoxy silane may be used.The following lists some of the coupling agents used as finishes for various resins: Vinyl silane (methacrylate silane),suitable for polyester resins .Volan(methacrylate chromic chloride),suitable for polyester and epoxy resins .Amino silane,suitable for epoxy,phenolic,or melamine resins Epoxy silane,suitable for epoxy and phenolic resins 3.3 Carbon Fibers 3.3.1 Manufacture Carbon fibers are widely used for airframes and engines and other aerospace applications.High modulus (HM,Type I),high strength (HS,Type II)and intermediate modulus(IM,Type III)form the three broad categories of carbon fibers available commercially,shown in Table 3.3 The name graphite for these fibers is sometimes used interchangeably with carbon,but this is actually incorrect.Graphite is a form of carbon in which strong covalently bonded hexagonal basal planes are aligned in a three-dimentional lattice.The weak dispersive atomic Van der Waals'bonding allows easy slip between the basal planes,the basis for the lubricating properties of graphite.As discussed later,the atomic structure of carbon fibers differs in that the basal planes have only a two-dimensional order,which inhibits slip

FIBERS FOR POLYMER-MATRIX COMPOSITES 63 (PVA) are included in the size to bond or hold the filaments together into strands and tows. Finishes, also called primers, are used in the size to improve the adhesive bonding between the fiber and the polymer matrix. Primers may be added to the size or applied later after removal of the size components by heat treatment. The finish is often based on a coupling agent that for most polymer-matrix resins is an organo-silane compound. Organo-silanes effectively have dual functionality, with their organo portion interacting with the organic resins or adhesives and the silane portion interacting with the inorganic fibers. Thus, these compounds are used to improve the interfacial (resin/fiber) properties of PMCs. Briefly, the silane molecule on hydration in water can be represented by the following simplified formula: R... Si(OH)3 The Si(OH)3 bonds with the oxide film at the surface of the inorganic fiber-glass in this case, while the organic functional group R is incorporated into the organic matrix during its cure. R must therefore be a group that is chemically compatible with the matrix resin. For example, for an epoxy resin, an epoxy silane may be used. The following lists some of the coupling agents used as finishes for various resins: • Vinyl silane (methacrylate silane), suitable for polyester resins • Volan (methacrylate chromic chloride), suitable for polyester and epoxy resins • Amino silane, suitable for epoxy, phenolic, or melamine resins • Epoxy silane, suitable for epoxy and phenolic resins 3,3 Carbon Fibers 3.3.1 Manufacture Carbon fibers are widely used for airframes and engines and other aerospace applications. High modulus (HM, Type I), high strength (HS, Type II) and intermediate modulus (IM, Type III) form the three broad categories of carbon fibers available commercially, shown in Table 3.3 The name graphite for these fibers is sometimes used interchangeably with carbon, but this is actually incorrect. Graphite is a form of carbon in which strong covalently bonded hexagonal basal planes are aligned in a three-dimentional lattice. The weak dispersive atomic Van der Waals' bonding allows easy slip between the basal planes, the basis for the lubricating properties of graphite. As discussed later, the atomic structure of carbon fibers differs in that the basal planes have only a two-dimensional order, which inhibits slip

64 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 3.3 Typical Properties for the Major Types of Commercial Carbon Fibers Property HM Type I HS TypeⅡ M TypeⅢ Specific gravity 1.9 1.8 1.8 Tensile modulus (GPa) 276-380 228-241 296 Tensile strength (MPa) 2415-2555 3105-4555 4800 Ultimate strain (% 0.6-0.7 1.3-1.8 2.0 Coefficient of thermal expansion -0.7 -0.5 N/A (×10-6mm-1K) Thermal conductivity (Wm-K-) 64-70 8.1-9.3 N/A Electrical resistivity (un m) 9-10 15-18 N/A Carbon fibers are made from organic precursor materials by a process of carbonization.The bulk of carbon fibers used in aerospace and other structural applications,are made from polyacrylonitrile (PAN)fibers.Carbon fibers are also made from various forms of pitch.'Early carbon fibers were manufactured from rayon,however,these fibers have been gradually phased out due to their low carbon yield (20-25%)and their generally poorer mechanical properties compared to PAN and pitch-based carbon fibers. 3.3.2 PAN-Based fibers PAN is an acrylic textile fiber produced by wet or dry spinning of the basic polymer or copolymer.Dry spinning produces round smooth fibers whereas wet spinning (extrusion into a coagulating bath)produces a variety of cross-sections, including dog-bone,elliptical,and kidney-shaped.There are some advantages in the non-circular cross-sections;for example,the larger relative surface area improves effective bonding.The fibers are stretched during the spinning process. The greater the stretch,the smaller the fiber diameter and the higher the preferred orientation of the molecular chain along the fiber axis,resulting in a stiffer carbon fiber when processed.PAN fiber tows typically contain around 104 fibers, although much larger or smaller tows are also produced.The finished carbon fibers are between 5-10 um in diameter. Figure 3.4 schematically illustrates the process of conversion of the PAN fibers into carbon fibers.The PAN is first stabilized in air at around 250C by oxidation to form a thermally stable ladder polymer,having a high glass transition temperature (Ts),which is resistant to melting at the higher temperatures.The cyclic groups in the ladder polymer are rather similar in molecular structure to the carbon basal plane,except that they also contain nitrogen and hydrogen atoms.The fibers are maintained under tension to prevent them from contracting during oxidation and,through the resulting deformation, to align further the ladder structure with the fiber axis

64 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 3.3 Typical Properties for the Major Types of Commercial Carbon Fibers Property HM Type I HS Type II IM Type III Specific gravity 1.9 1.8 1.8 Tensile modulus (GPa) 276-380 228-241 296 Tensile strength (MPa) 2415-2555 3105-4555 4800 Ultimate strain (%) 0.6-0.7 1.3-1.8 2.0 Coefficient of thermal expansion - 0.7 - 0.5 N/A (× 10 -6 mrn -1 K -1) Thermal conductivity (Wm- 1 K- 1) 64-70 8.1-9.3 N/A Electrical resistivity (V~ m) 9-10 15-18 N/A Carbon fibers are made from organic precursor materials by a process of carbonization. The bulk of carbon fibers used in aerospace and other structural applications, are made from polyacrylonitrile (PAN) fibers. 6 Carbon fibers are also made from various forms of pitch. 7 Early carbon fibers were manufactured from rayon, however, these fibers have been gradually phased out due to their low carbon yield (20-25%) and their generally poorer mechanical properties compared to PAN and pitch-based carbon fibers. 3.3.2 PAN-Based fibers PAN is an acrylic textile fiber produced by wet or dry spinning of the basic polymer or copolymer. Dry spinning produces round smooth fibers whereas wet spinning (extrusion into a coagulating bath) produces a variety of cross-sections, including dog-bone, elliptical, and kidney-shaped. There are some advantages in the non-circular cross-sections; for example, the larger relative surface area improves effective bonding. The fibers are stretched during the spinning process. The greater the stretch, the smaller the fiber diameter and the higher the preferred orientation of the molecular chain along the fiber axis, resulting in a stiffer carbon fiber when processed. PAN fiber tows typically contain around 10 4 fibers, although much larger or smaller tows are also produced. The finished carbon fibers are between 5-10 txm in diameter. Figure 3.4 schematically illustrates the process of conversion of the PAN fibers into carbon fibers. The PAN is first stabilized in air at around 250 °C by oxidation to form a thermally stable ladder polymer, having a high glass transition temperature (Tg), which is resistant to melting at the higher temperatures. The cyclic groups in the ladder polymer are rather similar in molecular structure to the carbon basal plane, except that they also contain nitrogen and hydrogen atoms. The fibers are maintained under tension to prevent them from contracting during oxidation and, through the resulting deformation, to align further the ladder structure with the fiber axis

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