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
56 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES ing ueaw Bulk material Fibrous material Load Fig.3.1 Effect of sample cross-section on distribution of strength.4 Polymeric materials,based on a suitable carbon backbone structure,can also form strong,stiff fibers.Some of these materials rely on a very high drawing ratio to orientate the polymer chains,as well as high purity to develop their stiffness and strength. Finally,some polymeric fiber materials can be used as precursors for producing inorganic fibers,through a process of controlled pyrolysis. Thus,commercially available continuous fibers used in structural polymer- matrix composites (PMCs)for aerospace applications can be loosely classed as ceramic or as polymeric.Ceramic fibers,for the purposes of this discussion, include silica,carbon,and boron,although strictly these last two are not classed as ceramics.True ceramic fibers include silicon carbide and alumina,whereas polymeric fibers include aramid and high-density polyethylene. Ceramic fibers,including glass,are typically flaw-sensitive and fail in an elastic brittle fashion from surface or internal flaws and inclusions. Polymer fibers exhibit a complex fibrous type of fracture,as they essentially are made of a bundle of relatively weakly bonded sub-filaments or fibrils.As a result these fibers,compared with the ceramic fibers,are relatively insensitive to flaws.However,under compression loading they can defibrillate,resulting in poor compression properties. Figure 3.2 summarizes the specific properties of several fiber types and includes,for comparison,structural metals.As a result of fiber volume fraction
56 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES O 5 Z Fig. 3.1 === co • - Bulk material ~ Fibrous Load Effect of sample cross-section on distribution of strength. 4 Polymeric materials, based on a suitable carbon backbone structure, can also form strong, stiff fibers. Some of these materials rely on a very high drawing ratio to orientate the polymer chains, as well as high purity to develop their stiffness and strength. Finally, some polymeric fiber materials can be used as precursors for producing inorganic fibers, through a process of controlled pyrolysis. Thus, commercially available continuous fibers used in structural polymermatrix composites (PMCs) for aerospace applications can be loosely classed as ceramic or as polymeric. Ceramic fibers, for the purposes of this discussion, include silica, carbon, and boron, although strictly these last two are not classed as ceramics. True ceramic fibers include silicon carbide and alumina, whereas polymeric fibers include aramid and high-density polyethylene. Ceramic fibers, including glass, are typically flaw-sensitive and fail in an elastic brittle fashion from surface or internal flaws and inclusions. Polymer fibers exhibit a complex fibrous type of fracture, as they essentially are made of a bundle of relatively weakly bonded sub-filaments or fibrils. As a result these fibers, compared with the ceramic fibers, are relatively insensitive to flaws. However, under compression loading they can defibrillate, resulting in poor compression properties. Figure 3.2 summarizes the specific properties of several fiber types and includes, for comparison, structural metals. As a result of fiber volume fraction
FIBERS FOR POLYMER-MATRIX COMPOSITES 57 and other limitations,maximum properties for a PMC with unidirectional fibers are around 60%of the values shown.It is apparent from this plot that significant improvements in specific stiffness compared with the metals are achieved only by using some of the more advanced fibers,including carbon and boron.More details on fiber properties are provided in Table 3.1. 3.2.Glass Fibers 3.2.1 Manufacture Glass fibers,s based on silica(SiO2)melted with oxides,are the mainstay of PMCs because of their high strength and low cost.High-strength glass fibers have been used in demanding structural applications such as pressure vessels and rocket casings since the early 1960s.Structural applications in airframes are limited because glass fibers have a relatively low specific stiffness,as shown in Table 3.1 Nevertheless,they are widely exploited for airframes of gliders and other aircraft,where their low specific stiffness is not a design limitation,and in secondary structures such as fairings,with which relatively low cost (compared with the high-performance fibers)is attractive.Because of the suitability of their 500 450 ●P140 400 P100 350 300 M60 sn 250 ○M50 PE 200 Be ●BORON 150 ●Sic CV ● K149 T800 T1000 100 ●NICALON T300 METALS 50 E-GLASS(●●S-GLASS 0.5 1 1.52 2.53 3.54 4.55 Specific strength[o GPa)/SG] Fig.3.2 Plot of specific fibers versus specific strength;the zone in which structural metals fall is shown for comparison.SG,specific gravity
FIBERS FOR POLYMER-MATRIX COMPOSITES 57 and other limitations, maximum properties for a PMC with unidirectional fibers are around 60% of the values shown. It is apparent from this plot that significant improvements in specific stiffness compared with the metals are achieved only by using some of the more advanced fibers, including carbon and boron. More details on fiber properties are provided in Table 3.1. 3.2. Glass Fibers 3.2.1 Manufacture Glass fibers, 5 based on silica (SiO2) melted with oxides, are the mainstay of PMCs because of their high strength and low cost. High-strength glass fibers have been used in demanding structural applications such as pressure vessels and rocket casings since the early 1960s. Structural applications in airframes are limited because glass fibers have a relatively low specific stiffness, as shown in Table 3.1 Nevertheless, they are widely exploited for airframes of gliders and other aircraft, where their low specific stiffness is not a design limitation, and in secondary structures such as fairings, with which relatively low cost (compared with the high-performance fibers) is attractive. Because of the suitability of their ~P uJ (n "o o E 500 450- 400- 350- 300" 250- 200- 150" 100" 50" 0 0 • O PIO0 0 M60 (~ i50 PE • • .o.o. O SiC CV • O K149 T800 T300 ~ NIC~LON METALS E-G sseO s.G ss ® TIO00 | | | ! ! ! ! | ! 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 Specific strength[GuGPa)lSG ] Fig. 3.2 Plot of specific fibers versus specific strength; the zone in which structural metals fall is shown for comparison. SG, specific gravity
Table 3.1 Details of the Mechanical Properties of Various Fiber Types(the Temperature Column is the Nominal 8粉 Maximum Operating Temperature in an Inert Environment) Coefficient of thermal Maximum Fiber Ultimate expansion use diameter Specific Stiffness Specific strain Strength Specific (×10-6 temperature Commercial Fiber (m) gravity (GPa) stiffness (%) (GPa) strength m/m/C) (C) name Glass E-Electrical 5-20 2.6 73 1.1 3.5 3.5 11.2 5.0 350 S-High strength 8-14 2.5 87 1.3 4.5 4.6 15.3 5.6 Carbon PAN Toray TERIALS based High strength" 8 1.76 230 4.9 1.5 3.5 16.6 -0.4 T300 1.80 294 6.1 2.4 5.9 32.9 -1.0 >2000 T800 FOR Intermediate modulus High modulus 86 1.90 490 9.7 0.5 2.5 11.0 -1.0 M-50 High modulus 1.94 588 14.3 0.7 3.9 16.8 -1.2 M-60 y Carbon pitch based Amoco High modulus 10 2.03 520 9.6 0.4 2.1 8.6 -1.4 P.75 High modulus 10 2.15 725 12.7 0.3 2.2 8.5 -1.4 >2000 P-100 High modulus 10 2.18 830 14.3 0.3 2.2 8.4 -1.4 P-120 Boron CVD 140 2.50 400 0.7 2.8 9.3 4.9 1500 Textron Silicon carbide Textron RUCTURES Monofilament 140 2.50 430 6.5 0.8 3.4 11.3 1400 SCS8 Nippon Carbon Multifilament 15 2.60 200 2.9 1.5 2.8 9.0 3.1 1200 Nicalon
58 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES e~ °~ ! 0 r~ ~ ~× r~3 r~ ",~ O O o § A A ~ " " II II III I ~,.- o ~ ~ ~.~.~=== ~
Alumina Du Pont Monofilament 20 3.90 380 3.7 0.5 1.8 3.8 5.7 1000 FP Sumitomo Multifilament 17 3.30 210 2.4 0.7 2.1 5.3 4.0 1100 Alumina Aramid Du Pont Ballistic 1.43 80 2.1 3.6 2.9 9.7 16.9 250 Kevlar 29 Structural 12 1.45 120 3.1 2.8 2.9 9.7 17.1 Kevlar 49 High modulus 12 1.47 185 4.7 1.5 2.3 7.7 17.3 Kevlar 149 FIBERS Polyethylene DSC 10-12 0.97 87 3.4 3.5 2.7 9.0 23.2 120 Dyneema FOR Allied Signal 38 0.97 117 4.5 3.5 2.6 8.7 22.3 100 Spectra 900 28 0.97 172 6.7 2.7 3.0 10.0 25.8 Spectra 1000 N.B.The specific stiffness and strength is normalized to aluminum alloy 2024 T3;strength is based on stress at nominal yield. POLYMER-MATRIX COMPOSITES 名
FIBERS FOR POLYMER-MATRIX COMPOSITES I E .9 -5 o o [.. o ..= r~ © © z =.. [-., 59