PROPERTIES OF COMPOSITE SYSTEMS 249 method has not changed since the early 1960s.This is the main reason that boron fibers are more costly than carbon fibers (an equivalent quantity of boron/epoxy pre-preg is roughly 12 times the price of carbon/epoxy pre-preg).The high cost of boron fiber was,initially,not critically important in defense applications and, because of its excellent specific mechanical properties,was selected for some of the empennage skins in the F-14 and F-15,and is also used in the B-1 bomber,in several components.However,in the 1970s,as the quantity of carbon fiber production rapidly increased,the cost of carbon fibers fell considerably,so that for most common aircraft applications,it became a more cost-effective fiber than boron in other than specialized applications. One application for which boron/epoxy is well suited is as a repair material for defective metallic structures.3 When repairs to aircraft components are considered,for example,the amount of boron/epoxy required is usually not great,and so the comparatively high cost of the material is not a critical factor. The high specific tensile and compressive properties of b/ep are ideally suited to repair applications.Carbon/epoxy can also be used for these applications; however,this material has several disadvantages.Because repairs are adhesively bonded to the structure with high-temperature curing adhesives,the lower coefficient of expansion of carbon/epoxy results in higher residual stresses in the repaired structure.These residual stresses can increase the local stresses at the defect.In addition,carbon fibers are electrically conducting,which inhibits the use of eddy-current non-destructive inspection methods through the repair material to confirm that there has been no growth of the damage.Boron fibers do not produce a galvanic couple with aluminum,so there is no danger of a boron repair causing corrosion of an aluminum aircraft structure. 8.4 Aramid Fiber Composite Systems When Kevlar 49/epoxy composites were introduced by DuPont in the mid 1960s,they had a higher specific tensile strength than similar composites,based on the then available carbon fibers.However,the subsequent development of carbon fibers with greatly improved strength properties displaced aramid composites from this position.Now they fill a property gap in specific strength and stiffness between glass and carbon fibers.4 In contrast to their high tensile properties,compression strength of aramid composites is low.Under compression loading,aramid fibers undergo non- linear deformation at strain levels around 0.5%by the formation of kink bands. Essentially,this mode of deformation occurs because the extended chain structure of the aramid fibers is unstable under compression loading.Figure 8.8 illustrates the extreme asymmetry in stress/strain behavior tension and compression loading for a typical aramid/epoxy composite
PROPERTIES OF COMPOSITE SYSTEMS 249 method has not changed since the early 1960s. This is the main reason that boron fibers are more costly than carbon fibers (an equivalent quantity of boron/epoxy pre-preg is roughly 12 times the price of carbon/epoxy pre-preg). The high cost of boron fiber was, initially, not critically important in defense applications and, because of its excellent specific mechanical properties, was selected for some of the empennage skins in the F-14 and F-15, and is also used in the B-1 bomber, in several components. However, in the 1970s, as the quantity of carbon fiber production rapidly increased, the cost of carbon fibers fell considerably, so that for most common aircraft applications, it became a more cost-effective fiber than boron in other than specialized applications. One application for which boron/epoxy is well suited is as a repair material for defective metallic structures. 3 When repairs to aircraft components are considered, for example, the amount of boron/epoxy required is usually not great, and so the comparatively high cost of the material is not a critical factor. The high specific tensile and compressive properties of b/ep are ideally suited to repair applications. Carbon/epoxy can also be used for these applications; however, this material has several disadvantages. Because repairs are adhesively bonded to the structure with high-temperature curing adhesives, the lower coefficient of expansion of carbon/epoxy results in higher residual stresses in the repaired structure. These residual stresses can increase the local stresses at the defect. In addition, carbon fibers are electrically conducting, which inhibits the use of eddy-current non-destructive inspection methods through the repair material to confirm that there has been no growth of the damage. Boron fibers do not produce a galvanic couple with aluminum, so there is no danger of a boron repair causing corrosion of an aluminum aircraft structure. 8.4 Aramid Fiber Composite Systems When Kevlar 49/epoxy composites were introduced by DuPont in the mid 1960s, they had a higher specific tensile strength than similar composites, based on the then available carbon fibers. However, the subsequent development of carbon fibers with greatly improved strength properties displaced aramid composites from this position. Now they fill a property gap in specific strength and stiffness between glass and carbon fibers. 4 In contrast to their high tensile properties, compression strength of aramid composites is low. Under compression loading, aramid fibers 5 undergo nonlinear deformation at strain levels around 0.5% by the formation of kink bands. Essentially, this mode of deformation occurs because the extended chain structure of the aramid fibers is unstable under compression loading. Figure 8.8 illustrates the extreme asymmetry in stress/strain behavior tension and compression loading for a typical aramid/epoxy composite
250 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES 1400 1200 (edW) 1000 800 600 400 200 2 -1.5 .1 0.5 0 0.5 1 1.5 Strain (% Fig 8.8 Typical tensile and compression stress-strain curves for aramid composites at ambient temperature.Adapted from Ref.4. The low compression resistance of aramid composites is a major disadvantage in applications requiring high strength or stiffness under compression or flexural loading.However,the non-linear behavior in compression,combined with a high strain capacity under tension,is a significant advantage in applications in which resistance to severe mechanical contact or penetration is required.Thus,in aerospace applications,aramid composites were favored for use in secondary structures such as fairings subject to impact damage.Thin-skin honeycomb panels based on aramid fibers were used extensively in some civil applications; however,the skins suffered from severe moisture penetration.This problem was mainly attributed to microcracking of the skins,possibly caused in part by moisture absorption and swelling of the fibers,coupled with the relatively weak fiber-to-resin bond strength. The properties of high tensile strength and resistance to penetration damage continue to favor aramid composites for use in filament-wound vessels and for containment rings in engines.Ballistic protection is another important use of aramid composites,for example,in structural or non-structural components on helicopters for protection against small arms fire. Finally,aramid fibers are used as the reinforcement in aircraft radomes,as they have favorable dielectric properties. For components that require both good compressive properties and impact resistance,aramid fibers may be used in combination with carbon or glass fibers. They can be used to enhance the toughness properties of carbon-fiber composites or to improve strength in the presence of stress raisers.Hybrid aramid/carbon composites have been used in helicopter fuselage panels and in civil aircraft for fairings
1400 1200 ft. ~1ooo ~ 8oo 600 40O -2 -1.5 -1 -0.5 0 0.5 1 1.5 2 250 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Strain (%) Fig 8.8 Typical tensile and compression stress-strain curves for aramid composites at ambient temperature. Adapted from Ref. 4. The low compression resistance of aramid composites is a major disadvantage in applications requiring high strength or stiffness under compression or flexural loading. However, the non-linear behavior in compression, combined with a high strain capacity under tension, is a significant advantage in applications in which resistance to severe mechanical contact or penetration is required. Thus, in aerospace applications, aramid composites were favored for use in secondary structures such as fairings subject to impact damage. Thin-skin honeycomb panels based on aramid fibers were used extensively in some civil applications; however, the skins suffered from severe moisture penetration. This problem was mainly attributed to microcracking of the skins, possibly caused in part by moisture absorption and swelling of the fibers, coupled with the relatively weak fiber-to-resin bond strength. The properties of high tensile strength and resistance to penetration damage continue to favor aramid composites for use in filament-wound vessels and for containment rings in engines. Ballistic protection is another important use of aramid composites, for example, in structural or non-structural components on helicopters for protection against small arms fire. Finally, aramid fibers are used as the reinforcement in aircraft radomes, as they have favorable dielectric properties. For components that require both good compressive properties and impact resistance, aramid fibers may be used in combination with carbon or glass fibers. They can be used to enhance the toughness properties of carbon-fiber composites or to improve strength in the presence of stress raisers. Hybrid aramid/carbon composites have been used in helicopter fuselage panels and in civil aircraft for fairings
PROPERTIES OF COMPOSITE SYSTEMS 251 8.4.1 Manufacturing Issues with Aramid Composites A significant issue in manufacturing aramid-fiber composites is the difficulty in achieving adhesion between the fibers and polymer matrix.Thus,the fibers must be surface-treated to enhance adhesion.However,in some applications, notably those requiring good ballistic properties,a fairly low-level of adhesive strength between fiber and matrix is desirable to obtain optimum energy absorption properties.In the case of aramid filament-wound pressure vessels, burst strength is highest at some intermediate level of bond strength Various treatments have been used to improve fiber/matrix adhesion, including gas plasma treatment in Ar,N2,or CO2,which typically results in a 20%improvement in interfacial bond strength to epoxy. Aramid fibers absorb moisture,up to around 6%by weight,if exposed to a humid environment.This can affect fiber/matrix adhesion and other prop- erties,so the fibers are either stored in low humidity conditions or dried before usage. 8.4.1.1 Matrix Systems for Aramid Composites.Some thermoset resin systems such as anhydride-cured bisphenol A epoxies are inherently more compatible with aramid fibers than other matrix resins and provide relatively high interlaminar strengths.Vinyl esters are more compatible with aramid fiber than polyesters and are used for marine-type applications.To obtain optimum tensile properties,it is important that the resin has high elongation.About 6%appears to provide the best balance of properties.Thermoplastic such as PEEK and polysulphones can also be successfully used.However,as processing temp- eratures can exceed 260C in the case of polysulphones,and as high as 400C in case of PEEK,there is some degradation of the fiber strength. 8.4.1.2 Cutting,Drilling and Machining Aramid Composites.The high toughness of aramid fibers,including their tendency to defibrillate (separate into microfilaments)under high compressive and shear stresses,makes aramid composites very difficult to cut or machine.Indeed,dry aramid cloths themselves are difficult to cut and require the use of special shears,although heavy-duty upholstery scissors can be used.Special carbide-tipped tools are required for drilling and machining.Water jet is an excellent method for cutting aramid composites and also minimizes the creation of airborne fibers. 8.4.2 Mechanical Properties of Aramid Composites As mentioned previously,under tensile loading,the strength of aramid/epoxy pre-preg laminates can match or exceed those of similar carbon/epoxy or glass/ epoxy composites.Their elastic modulus is below that of carbon/epoxy but exceeds that of glass/epoxy.Typical values,including those for similar carbon and E-glass/epoxy composites,are listed in Table 8.1 The Table shows that
PROPERTIES OF COMPOSITE SYSTEMS 251 8.4.1 Manufacturing Issues with Aramid Composites A significant issue in manufacturing aramid-fiber composites is the difficulty in achieving adhesion between the fibers and polymer matrix. Thus, the fibers must be surface-treated to enhance adhesion. However, in some applications, notably those requiring good ballistic properties, a fairly low-level of adhesive strength between fiber and matrix is desirable to obtain optimum energy absorption properties. In the case of aramid filament-wound pressure vessels, burst strength is highest at some intermediate level of bond strength. Various treatments have been used to improve fiber/matrix adhesion, 6 including gas plasma treatment in Ar, N2, or CO2, which typically results in a 20% improvement in interfacial bond strength to epoxy. Aramid fibers absorb moisture, up to around 6% by weight, if exposed to a humid environment. This can affect fiber/matrix adhesion and other properties, so the fibers are either stored in low humidity conditions or dried before usage. 8.4.1.1 Matrix Systems for Aramid Composites. Some thermoset resin systems such as anhydride-cured bisphenol A epoxies are inherently more compatible with aramid fibers than other matrix resins and provide relatively high interlaminar strengths. Vinyl esters are more compatible with aramid fiber than polyesters and are used for marine-type applications. To obtain optimum tensile properties, it is important that the resin has high elongation. About 6% appears to provide the best balance of properties. Thermoplastic such as PEEK and polysulphones can also be successfully used. However, as processing temperatures can exceed 260°C in the case of polysulphones, and as high as 400°C in case of PEEK, there is some degradation of the fiber strength. 8.4.1.2 Cutting, Drilling and Machining Aramid Composites. The high toughness of aramid fibers, including their tendency to defibrillate (separate into microfilaments) under high compressive and shear stresses, makes aramid composites very difficult to cut or machine. Indeed, dry aramid cloths themselves are difficult to cut and require the use of special shears, although heavy-duty upholstery scissors can be used. Special carbide-tipped tools are required for drilling and machining. Water jet is an excellent method for cutting aramid composites and also minimizes the creation of airborne fibers. 8.4.2 Mechanical Properties of Aramid Composites As mentioned previously, under tensile loading, the strength of aramid/epoxy pre-preg laminates can match or exceed those of similar carbon/epoxy or glass/ epoxy composites. Their elastic modulus is below that of carbon/epoxy but exceeds that of glass/epoxy. Typical values, including those for similar carbon and E-glass/epoxy composites, are listed in Table 8.1 The Table shows that
252 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES although the elastic modulus is similar under tension and compression loading, strength is much reduced.Apparent interlaminar shear strength(ILSS)is also relatively low compared with the other composites.One reason for this is the low fiber/matrix bond strength.Another reason is the poor compression properties of these composites because failure in the standard short-beam ILSS test is rarely pure shear and often includes a significant component of compression failure. 8.4.2.1 Fatigue Resistance.Under tensile-dominated cyclic loading,as illustrated schematically in Figure 8.9,unidirectional aramid composites are superior to aluminum alloys and to S-glass/epoxy composites but inferior to carbon/epoxy(not shown).For unidirectional composites,the fatigue damage occurs mainly as matrix microcracking.As may be expected,the rate of damage accumulation depends on the strain level experienced by the matrix,which is directly dependent on the fiber elastic modulus and volume fraction-hence,the relative ranking.The relative advantage of the composites over aluminum alloys is reduced in cross-plied laminates,normally used in aircraft structures. Nevertheless,a marked advantage over aluminum alloys is maintained for the aramid-and carbon-fiber composites. As is to be expected from the poor compression strength of the fibers,aramid composites are inferior to both glass and carbon composites under compression- dominated fatigue. 8.4.2.2 Creep and Stress Rupture.Aramid fibers and composites have a similar low creep rate to glass fibers but,as illustrated in Figure 8.10,they are less 1400 1200 Kevlar 49/Epoxy 1000 (edW) 800 600 S-Glass/Epoxy 400 2024-T3 Aluminum 200 0 10 10 10 10 Cycles Fig 8.9 Plot of tension-tension fatigue results for unidirectional composites and for an aluminum alloy.Adapted from Ref.4
252 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES although the elastic modulus is similar under tension and compression loading, strength is much reduced. Apparent interlaminar shear strength (ILSS) is also relatively low compared with the other composites. One reason for this is the low fiber/matrix bond strength. Another reason is the poor compression properties of these composites because failure in the standard short-beam ILSS test is rarely pure shear and often includes a significant component of compression failure. 8.4.2.1 Fatigue Resistance. Under tensile-dominated cyclic loading, as illustrated schematically in Figure 8.9, unidirectional aramid composites are superior to aluminum alloys and to S-glass/epoxy composites but inferior to carbon/epoxy (not shown). For unidirectional composites, the fatigue damage occurs mainly as matrix microcracking. As may be expected, the rate of damage accumulation depends on the strain level experienced by the matrix, which is directly dependent on the fiber elastic modulus and volume fraction--hence, the relative ranking. The relative advantage of the composites over aluminum alloys is reduced in cross-plied laminates, normally used in aircraft structures. Nevertheless, a marked advantage over aluminum alloys is maintained for the aramid- and carbon-fiber composites. As is to be expected from the poor compression strength of the fibers, aramid composites are inferior to both glass and carbon composites under compressiondominated fatigue. 8.4.2.2 Creep and Stress Rupture. Aramid fibers and composites have a similar low creep rate to glass fibers but, as illustrated in Figure 8.10, they are less 1400 1200 ,-, 1000 800 600 400 2O0 0 3 4 5 6 7 10 10 10 10 10 Cycles Fig 8.9 Plot of tension-tension fatigue results for unidirectional composites and for an aluminum alloy. Adapted from Ref. 4
PROPERTIES OF COMPOSITE SYSTEMS 253 100 5605530751066 Kevlar 49/Epoxy S-Glass/Epoxy ajewpin 5 89 0.1 1 10 100 1000 10000 100000 Time to Failure (hours) Fig 8.10 Stress rupture properties of unidirectional aramid and glass fibers in epoxy resin.Adapted from Ref.4. prone to stress rupture.Glass fibers are particularly sensitive to humid environments,where they have much lower stress rupture properties.Generally, carbon fibers are significantly more resistant to creep and stress rupture than glass or aramid fibers.Although,in unidirectional composites,the creep behavior is dominated by the fiber properties,the relaxation of the matrix makes a small contribution to the relatively short-term creep behavior.The creep rate increases and the stress rupture decreases as a function of both temperature and humidity. 8.4.2.3 Environmental Effects.Aramid fibers absorb moisture;at 60% relative humidity,the equilibrium moisture content is about 4%,which rises to around 6%when the RH is 100%.The result is a decrease of tensile strength and stiffness at room temperature of around 5%(probably significantly greater at elevated temperature),which would be reflected in the properties of the composite.However,the effect of moisture on the fibers appears to be reversible. Tensile strength of the dry fiber is reduced by up to 20%at 180C.Room temperature strength is also reduced by about 20%after prolonged (80 h) exposure at200°C. The effects of temperature and moisture on tensile and compression properties are illustrated in Figure 8.11.Tensile properties are unaffected up to a relatively high temperature (177C)when the loss is around 30%hot/wet.The loss in compression strength at this temperature is quite dramatic and is around 70%. However,similar carbon/epoxy composites would also experience a significant loss of compression strength under wet conditions close to the cure temperature
100 95 90 85 _~ 80 '~ 75 70 65 E 60 55 50 PROPERTIES OF COMPOSITE SYSTEMS ~S.Glass/Epoxy 253 0.1 1 10 100 1000 10000 100000 Time to Failure (hours) Fig 8.10 Stress rupture properties of unidirectional aramid and glass fibers in epoxy resin. Adapted from Ref. 4. prone to stress rupture. Glass fibers are particularly sensitive to humid environments, where they have much lower stress rupture properties. Generally, carbon fibers are significantly more resistant to creep and stress rupture than glass or aramid fibers. Although, in unidirectional composites, the creep behavior is dominated by the fiber properties, the relaxation of the matrix makes a small contribution to the relatively short-term creep behavior. The creep rate increases and the stress rupture decreases as a function of both temperature and humidity. 8.4.2.3 Environmental Effects. Ararnid fibers absorb moisture; at 60% relative humidity, the equilibrium moisture content is about 4%, which rises to around 6% when the RH is 100%. The result is a decrease of tensile strength and stiffness at room temperature of around 5% (probably significantly greater at elevated temperature), which would be reflected in the properties of the composite. However, the effect of moisture on the fibers appears to be reversible. Tensile strength of the dry fiber is reduced by up to 20% at 180°C. Room temperature strength is also reduced by about 20% after prolonged (80 h) exposure at 200°C. The effects of temperature and moisture on tensile and compression properties are illustrated in Figure 8.11. Tensile properties are unaffected up to a relatively high temperature (177°C) when the loss is around 30% hot/wet. The loss in compression strength at this temperature is quite dramatic and is around 70%. However, similar carbon/epoxy composites would also experience a significant loss of compression strength under wet conditions close to the cure temperature