8 Properties of Composite Systems 8.1 Introduction The mechanical properties of simple unidirectional continuous fiber composites depend on the volume fraction and properties of the fibers (including flaw and strength distribution),the fiber/matrix bond strength,and the mechanical properties of the matrix.The alignment (waviness)of the fibers also has a significant effect on some properties-notably,compression strength. Elevated temperature and moist environments also significantly affect properties dependent on matrix properties or interfacial strength. Because of these and several other factors,the efficiency of translation of fiber properties into those of the composite is not always as high as may be expected. Stiffness can be predicted more reliably than strength,although static tensile strength is easier to predict than other strength properties.Chapter 2 provides some elementary equations for estimating the mechanical properties of unidirectional composites,which are reasonably accurate in estimating elastic properties,providing fiber alignment is good.The equations can also provide ball-park figures for the matrix-dominated shear and transverse elastic properties and the fiber-dominated tensile strength properties.However,estimation of matrix-dominated or fiber/matrix bond strength-dominated strength properties, including shear and compression,is complex.Prediction problems also arise when the fibers are sensitive to compression loading,as is the case for aramid fibers,as discussed later. Chapter 7 describes the experimental procedures for measuring the mechanical properties,including those for assessing tolerance to damage and fatigue.These tests are used to develop a database for design of aerospace components and as part of the information required for airworthiness certification,as described in Chapter 12. Table 8.1 lists relevant mechanical and physical properties of the composites discussed in this chapter.Details of aerospace structural alloys aluminum 2024 T3 and titanium 6Al4V are also provided for comparison.The nomenclature used for the properties is similar to that used in Chapter 2.The data provided for the composites can be used as an estimate of ply properties for making an approximate prediction of laminate properties. The first four sections of this chapter provide an overview of the mechanical properties of composite systems based on glass,boron,aramid,or carbon fibers. 239
8 Properties of Composite Systems 8.1 Introduction The mechanical properties of simple unidirectional continuous fiber composites depend on the volume fraction and properties of the fibers (including flaw and strength distribution), the fiber/matrix bond strength, and the mechanical properties of the matrix. The alignment (waviness) of the fibers also has a significant effect on some properties--notably, compression strength. Elevated temperature and moist environments also significantly affect properties dependent on matrix properties or interfacial strength. Because of these and several other factors, the efficiency of translation of fiber properties into those of the composite is not always as high as may be expected. Stiffness can be predicted more reliably than strength, although static tensile strength is easier to predict than other strength properties. Chapter 2 provides some elementary equations for estimating the mechanical properties of unidirectional composites, which are reasonably accurate in estimating elastic properties, providing fiber alignment is good. The equations can also provide ball-park figures for the matrix-dominated shear and transverse elastic properties and the fiber-dominated tensile strength properties. However, estimation of matrix-dominated or fiber/matrix bond strength-dominated strength properties, including shear and compression, is complex. Prediction problems also arise when the fibers are sensitive to compression loading, as is the case for aramid fibers, as discussed later. Chapter 7 describes the experimental procedures for measuring the mechanical properties, including those for assessing tolerance to damage and fatigue. These tests are used to develop a database for design of aerospace components and as part of the information required for airworthiness certification, as described in Chapter 12. Table 8.1 lists relevant mechanical and physical properties of the composites discussed in this chapter. Details of aerospace structural alloys aluminum 2024 T3 and titanium 6A14V are also provided for comparison. The nomenclature used for the properties is similar to that used in Chapter 2. The data provided for the composites can be used as an estimate of ply properties for making an approximate prediction of laminate properties. The first four sections of this chapter provide an overview of the mechanical properties of composite systems based on glass, boron, aramid, or carbon fibers. 239
器 Table 8.1 Unidirectional Properties(Mostly Approximate)of Various Composites Considered in This Chapter and,for Comparison, Airframe Titanium and Aluminum Alloys.Based Largely on Ref.2 Glass fiber Carbon fiber composites composites COMPOSITE Units E s Boron Aramid K 49 HT HM UHM Al Ti SG 2.1 2.0 2.0 1.38 1.58 1.64 1.7 2.76 4.4 ai Lm/C 7.1 6.3 4.5 -1 -0.16 MATERIALS 02 um/C 20 70 24 二 二 23 9 23 Gitu MPa 1020 1620 1520 1240 1240 760 620 454 1102 FOR Glcu MPa 620 690 2930 275 1100 690 620 280* 1030 02u MPa 40 40 70 30 41 28 21 441 1102 Tu MPa 70 80 90 60 80 70 60 275 640 ILS MPa 70 80 90 60 80 70 60 AIRCRAFT E GPa 45 55 210 76 145 220 290 110 GPa 12 16 19 5.5 10 6.9 6.2 7 110 G12 GPa 5.5 7.6 4.8 2.1 4.8 4.8 4.8 4 V12 0.28 0.28 0.25 0.34 0.25 0.25 0.25 0.33 .31 Etu 0.022 0.029 0.006 0.016 0.01 0.03 0.02 0.12 0.06 E2u 0.4 0.4 0.4 0.5 0.4 0.4 0.3 STRUCTURES Notes:Ti Ti 6Al4V;Al 2024 T3 *yield value V60%in the composites;SG,specific gravity:ILS,interlaminar shear.See Chapters 2 and 12 for definition of other terms
240 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES ~3 "a f~ [.. J j r,j E .| ! r~ ,,,., o m- t'q ,.-~ O r., o ..= Cq t'4 .8 q= ~;,.., q= III ~
PROPERTIES OF COMPOSITE SYSTEMS 241 Further information on the properties of carbon-fiber composites is also provided throughout this book.In the last three sections,more generic discussion is provided on important impact,fatigue,and environmental properties,while a focus on carbon-fiber systems is maintained. 8.2 Glass-Fiber Composite Systems As described in Chapter 3,several types of glass reinforcements are suitable for the manufacture of aircraft and helicopter composite components.E-glass composites are used extensively in gliders and in non-structural components that do not require high stiffness,such as radomes.S-glass composites have better mechanical properties and therefore are used in more demanding applications.A third type of reinforcement known as D-glass has good dielectric properties and is occasionally used in aircraft to minimize the impact of lighting strikes.E-and S-glass are used in the form of epoxy-based pre-preg or as fabrics containing unidirectional,woven,or chopped strand filaments. A major advantage of E-glass fibers over the other types of fibers used in aircraft is their low cost.Figure 8.1 compares typical material costs for E-glass composites against costs for carbon,aramid (trade name,Kevlar),and boron/ epoxy composites;the relative cost of boron pre-preg shown is divided by a factor of 10 to make the chart readable.Costs are given for composites made of pre-preg or fabric(woven roving,chopped strand mat).The costs are approximate and do not include the expense of fabricating the composite into an aircraft component, which is usually much higher than the raw material cost.E-glass composites are 6 3 2 0 E-Glass Aramid HS Carbon IM Carbon Boron Fig 8.1 Relative costs of some fiber composite systems used in aerospace applications.Boron is shown at about 1/10 of its actual relative cost
PROPERTIES OF COMPOSITE SYSTEMS 241 Further information on the properties of carbon-fiber composites is also provided throughout this book. In the last three sections, more generic discussion is provided on important impact, fatigue, and environmental properties, while a focus on carbon-fiber systems is maintained. 8.2 Glass-Fiber Composite Systems As described in Chapter 3, several types of glass reinforcements are suitable for the manufacture of aircraft and helicopter composite components. E-glass composites are used extensively in gliders and in non-structural components that do not require high stiffness, such as radomes. S-glass composites have better mechanical properties and therefore are used in more demanding applications. A third type of reinforcement known as D-glass has good dielectric properties and is occasionally used in aircraft to minimize the impact of lighting strikes. E- and S-glass are used in the form of epoxy-based pre-preg or as fabrics containing unidirectional, woven, or chopped strand filaments. A major advantage of E-glass fibers over the other types of fibers used in aircraft is their low cost. Figure 8.1 compares typical material costs for E-glass composites against costs for carbon, aramid (trade name, Kevlar), and boron/ epoxy composites; the relative cost of boron pre-preg shown is divided by a factor of 10 to make the chart readable. Costs are given for composites made of pre-preg or fabric (woven roving, chopped strand mat). The costs are approximate and do not include the expense of fabricating the composite into an aircraft component, which is usually much higher than the raw material cost. E-glass composites are 6 0} .= ~5 .= a. 4 o o3 .m ~2 1 E-Glass Aramid HS Carbon IM Carbon Fig 8.1 Relative costs of some fiber composite systems used applications. Boron is shown at about 1/10 of its actual relative cost. Boron in aerospace
242 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES by far the cheapest,particularly when chopped strand mat or woven fabric is used.S-glass composites are much more expensive than the E-glass composites and only marginally less expensive than carbon/epoxy. Figures 8.2 and 8.3 provide comparisons'of the strength and stiffness of some of the available forms of E-glass fiber materials.The forms are chopped-stand mat,woven rovings,and unidirectional pre-preg material.The comparisons in these figures are based on relativities that will also be relevant to the other fiber types if made from similar geometrical forms. Table 8.1 provides relevant physical,thermal,and mechanical property data for unidirectional E-and S-glass/epoxy composites.Glass fibers have a specific gravity of about 2.5 g cm,which is slightly lower than the density of boron fibers (2.6g cm)but is appreciably higher than carbon (~1.8 g cm)and Kevlar(1.45 g cm)fibers.The specific gravity of thermoset resins is around 1.3 gcm,and as a result,glass/epoxy composites have a specific gravity that is higher than for other types of aerospace composites(except boron/epoxy)with the same fiber volume content.However,depending on the fiber volume fraction, it is still somewhat lower than that of aircraft-grade aluminum alloys (2.8 g cm).The Young's moduli and strengths of both E-and S-glass composites are lower than those of other aerospace structural composites and metals.The combined effects of low stiffness and high specific gravity makes glass/epoxy or 60 UD unidirectional 50 WR woven rovings CSM=chopped strand mat UD/epoxy 40 sninpow 30 20 WR/polyester 10 CSM/polyester 0 0 20 40 60 80 100 Glass Content by weight Fig 8.2 Typical Young's modulus for various types of glass-fiber composites. Adapted from Ref.1
242 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES by far the cheapest, particularly when chopped strand mat or woven fabric is used. S-glass composites are much more expensive than the E-glass composites and only marginally less expensive than carbon/epoxy. Figures 8.2 and 8.3 provide comparisons 1 of the strength and stiffness of some of the available forms of E-glass fiber materials. The forms are chopped-stand mat, woven rovings, and unidirectional pre-preg material. The comparisons in these figures are based on relativities that will also be relevant to the other fiber types if made from similar geometrical forms. Table 8.1 provides relevant physical, thermal, and mechanical property data for unidirectional E- and S-glass/epoxy composites. Glass fibers have a specific gravity of about 2.5 g cm -3, which is slightly lower than the density of boron fibers (2.6 g cm -3) but is appreciably higher than carbon (~ 1.8 g cm -3) and Kevlar (1.45 g cm -3) fibers. The specific gravity of thermoset resins is around 1.3 g cm -3, and as a result, glass/epoxy composites have a specific gravity that is higher than for other types of aerospace composites (except boron/epoxy) with the same fiber volume content. However, depending on the fiber volume fraction, it is still somewhat lower than that of aircraft-grade aluminum alloys (2.8 g cm-3). The Young's moduli and strengths of both E- and S-glass composites are lower than those of other aerospace structural composites and metals. The combined effects of low stiffness and high specific gravity makes glass/epoxy or 60 50 e~ t~ 40 "g 3o -g ~ 2o 0 10 UD = unidirectional WR = woven rovings CSM = chopped strand mat 0 20 40 60 80 100 Glass Content % by weight Fig 8.2 Typical Young's modulus for various types of glass-fiber composites. Adapted from Ref. I
PROPERTIES OF COMPOSITE SYSTEMS 243 900 800 700 UD=unidirectional WR woven rovings 600 CSM chopped strand mat 500 400 300 WR/polyester 200 100 CSM/polyester 0 0 20 40 60 80 100 Glass Content by Weight Fig 8.3 Typical strengths of various types of glass-fiber composites.Adapted from Ref.1. other glass fiber composites unattractive for use in weight-critical load-bearing primary structures on larger aircraft. 8.2.1 Fatigue Performance of Glass-Fiber Systems Another drawback of using glass/epoxy composites in aircraft structures is their relatively poor fatigue performance compared with the other composites discussed in this chapter.Glass/epoxy composites are more prone to fatigue- induced damage (e.g.,microscopic cracks,delaminations)and failure than other aerospace composite materials.Figure 8.4 shows a typical fatigue-life curve for a unidirectional glass/epoxy composite that was tested under cyclic tension- tension loading.Fatigue-life curves for unidirectional carbon/epoxy and Kevlar/ epoxy laminates that were also tested under tension-tension loading are shown for comparison.In the figure,the normalized fatigue strain (er/e)is the maximum applied cyclic tensile strain (er)divided by the static tensile failure strain of the composite().Of the three materials,the fatigue-life curve for the glass/epoxy
PROPERTIES OF COMPOSITE SYSTEMS 243 900 800 700 600 t~ ~" =E 500 400 ~ 300 " 200 o 1- 100 0 UD = unidirectional WR = woven rovings CSM = chopped strand mat II O 0 20 40 60 80 100 Glass Content % by Weight Fig 8.3 Typical strengths of various types of glass-fiber composites. Adapted from Ref. 1. other glass fiber composites unattractive for use in weight-critical load-bearing primary structures on larger aircraft. 8.2.1 Fatigue Performance of Glass-Fiber Systems Another drawback of using glass/epoxy composites in aircraft structures is their relatively poor fatigue performance compared with the other composites discussed in this chapter. Glass/epoxy composites are more prone to fatigueinduced damage (e.g., microscopic cracks, delaminations) and failure than other aerospace composite materials. Figure 8.4 shows a typical fatigue-life curve for a unidirectional glass/epoxy composite that was tested under cyclic tensiontension loading. Fatigue-life curves for unidirectional carbon/epoxy and Kevlar/ epoxy laminates that were also tested under tension-tension loading are shown for comparison. In the figure, the normalized fatigue strain (ef/eo) is the maximum applied cyclic tensile strain (ef) divided by the static tensile failure strain of the composite (eo). Of the three materials, the fatigue-life curve for the glass/epoxy