7 COMPOSITE MATERIALS AND AEROSPACE CONSTRUCTION Aeronautical constructors have been looking for light weight and robustness from composites since the earlier times.As a brief history: In 1938,the Morane 406 plane (FRA)utilized sandwich panels with wood core covered with light alloy skins. In 1943,composites made of hemp fiber and phenolic resin were used on the Spitfire (U.K.)airplane. Glass/resin has been used since 1950,with honeycombs.This allows the construction of the fairings with complex forms. Boron/epoxy was introduced around 1960,with moderate development since that time. Carbon/epoxy has been used since 1970. Kevlar/epoxy has been used since 1972. Experiences have proved that the use of composites allows one to obtain weight reduction varying from 10%to 50%,with equal performance,together with a cost reduction of 10%to 20%,compared with making the same piece with conventional metallic materials. 7.1 AIRCRAFT 7.1.1 Composite Components in Aircraft Currently a large variety of composite components are used in aircrafts.Following the more or less important role that composites play to assure the integrity of the aircraft,one can cite the following: Primary structure components (integrity of which is vital for the aircraft): 2003 by CRC Press LLC
7 COMPOSITE MATERIALS AND AEROSPACE CONSTRUCTION Aeronautical constructors have been looking for light weight and robustness from composites since the earlier times. As a brief history: In 1938, the Morane 406 plane (FRA) utilized sandwich panels with wood core covered with light alloy skins. In 1943, composites made of hemp fiber and phenolic resin were used on the Spitfire (U.K.) airplane. Glass/resin has been used since 1950, with honeycombs. This allows the construction of the fairings with complex forms. Boron/epoxy was introduced around 1960, with moderate development since that time. Carbon/epoxy has been used since 1970. Kevlar/epoxy has been used since 1972. Experiences have proved that the use of composites allows one to obtain weight reduction varying from 10% to 50%, with equal performance, together with a cost reduction of 10% to 20%, compared with making the same piece with conventional metallic materials. 7.1 AIRCRAFT 7.1.1 Composite Components in Aircraft Currently a large variety of composite components are used in aircrafts. Following the more or less important role that composites play to assure the integrity of the aircraft, one can cite the following: Primary structure components (integrity of which is vital for the aircraft): TX846_Frame_C07 Page 135 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
Wing box Empennage box Fuselage The control components: Ailerons Control components for direction and elevation High lift devices Spoilers Exterior components: Fairings “Karmans'” Storage room doors Landing gear trap doors Radomes,front cauls ■ Interior components: Floors Partitions,bulkheads Doors,etc. Example:The vertical stabilizer of the Tristar transporter (Lockheed Company, USA) With classical construction,it consists of 175 elements assembled by 40,000 rivets. With composite construction,it consists only with 18 elements assembled by 5,000 rivets. 7.1.2 Characteristics of Composites One can indicate the qualities and weak points of the principal composites used. These serve to justify their use in the corresponding components. 7.1.2.1 Glass/Epoxy,Kevlar/Epoxy These are used in fairings,storage room doors,landing gear trap doors,karmans, radomes,front cauls,leading edges,floors,and passenger compartments. ■Pluses: High rupture strength' Very good fatigue resistance ■Minuses: High elastic elongation Maximum operating temperature around 80C Nonconducting material TSee Section 3.3.3. 2003 by CRC Press LLC
Wing box Empennage box Fuselage The control components: Ailerons Control components for direction and elevation High lift devices Spoilers Exterior components: Fairings “Karmans” Storage room doors Landing gear trap doors Radomes, front cauls Interior components: Floors Partitions, bulkheads Doors, etc. Example: The vertical stabilizer of the Tristar transporter (Lockheed Company, USA) With classical construction, it consists of 175 elements assembled by 40,000 rivets. With composite construction, it consists only with 18 elements assembled by 5,000 rivets. 7.1.2 Characteristics of Composites One can indicate the qualities and weak points of the principal composites used. These serve to justify their use in the corresponding components. 7.1.2.1 Glass/Epoxy, Kevlar/Epoxy These are used in fairings, storage room doors, landing gear trap doors, karmans, radomes, front cauls, leading edges, floors, and passenger compartments. Pluses: High rupture strength1 Very good fatigue resistance Minuses: High elastic elongation Maximum operating temperature around 80∞C Nonconducting material 1 See Section 3.3.3. TX846_Frame_C07 Page 136 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
7.1.2.2 Carbon/Epoxy This is used in wing box,horizontal stabilizers,fuselage,ailerons,wings,spoilers (air brakes)vertical stabilizers,traps,and struts. ■Pluses: High rupture resistance Very good fatigue strength Very good heat and electricity conductor High operating temperature (limited by the resin) No dilatation until 600C Smaller specific mass than that of glass/epoxy ■Minuses: More delicate fabrication Impact resistance two or three times less than that of glass/epoxy Material susceptible to lightning 7.1.2.3 Boron/Epoxy This is used for vertical stabilizer boxes and horizontal stabilizer boxes. ■Pluses: High rupture resistance High rigidity Very good compatibility with epoxy resins Good fatigue resistance ■Minuses: Higher density than previous composites? Delicate fabrication and forming High cost 7.1.2.4 Honeycombs Honeycombs are used for forming the core of components made of sandwich structures. ■Pluses: Low specific mass Very high specific modulus and specific strength Very good fatigue resistance ■Minuses: Susceptible to corrosion Difficult to detect defects See Section 3.3.3. 2003 by CRC Press LLC
7.1.2.2 Carbon/Epoxy This is used in wing box, horizontal stabilizers, fuselage, ailerons, wings, spoilers (air brakes) vertical stabilizers, traps, and struts. Pluses: High rupture resistance Very good fatigue strength Very good heat and electricity conductor High operating temperature (limited by the resin) No dilatation until 600∞C Smaller specific mass than that of glass/epoxy Minuses: More delicate fabrication Impact resistance two or three times less than that of glass/epoxy Material susceptible to lightning 7.1.2.3 Boron/Epoxy This is used for vertical stabilizer boxes and horizontal stabilizer boxes. Pluses: High rupture resistance High rigidity Very good compatibility with epoxy resins Good fatigue resistance Minuses: Higher density than previous composites2 Delicate fabrication and forming High cost 7.1.2.4 Honeycombs Honeycombs are used for forming the core of components made of sandwich structures. Pluses: Low specific mass Very high specific modulus and specific strength Very good fatigue resistance Minuses: Susceptible to corrosion Difficult to detect defects 2 See Section 3.3.3. TX846_Frame_C07 Page 137 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
7.1.3 A Few Remarks The construction using only glass fibers is less and less favored in comparison with a combination of Kevlar fibers and carbon fibers for weight saving reasons: If one would like to have maximum strength,use Kevlar. If one would like to have maximum rigidity,use carbon. Kevlar fibers possess excellent vibration damping resistance. Due to bird impacts,freezing rain,impact from other particles (sand,dirt), one usually avoids the use of composites in the leading edges without metallic protection.3 Carbon/epoxy composite is a good electrical conductor and susceptible to lightning,with the following consequences: Damages at the point of impact:delamination,burning of resin Risk of lightning in attachments (bolts) The necessity to conduct to the mass for the electrical circuits situated under the composite element Remedies consist of the following: Glass fabric in conjunction with a very thin sheet of aluminum (20 um) The use of a protective aluminum film (aluminum flam spray) Temperature is an important parameter that limits the usage of epoxy resins. A few experimental components have been made of bismaleimide resins (ther- mosets that soften'at temperatures higher than 350C rather than 210C for epoxies).One other remedy would be to use a thermoplastic resin with high temperature resistance such as poly-ether-ether-ketone "peek"s that softens at 380C.Laminates made of carbon/peek are more expensive than products made of carbon/epoxy.However,they present good performance at higher operating temperatures (continuously at 130C and periodically at 160C)and have the following additional advantages: ■ Superior impact resistance ■ Negligible moisture absorption Very low smoke generation in case of fire 3The impacts can create internal damages that are invisible from the outside.This can also happen on the wing panels (for example,drop of tools on the panels during fabrication or during maintenance work). 4The mechanical properties of the thermoset resins diminish when the temperature reaches the "glass transition temperature." 5 See section 1.6 for the physical properties. 2003 by CRC Press LLC
7.1.3 A Few Remarks The construction using only glass fibers is less and less favored in comparison with a combination of Kevlar fibers and carbon fibers for weight saving reasons: If one would like to have maximum strength, use Kevlar. If one would like to have maximum rigidity, use carbon. Kevlar fibers possess excellent vibration damping resistance. Due to bird impacts, freezing rain, impact from other particles (sand, dirt), one usually avoids the use of composites in the leading edges without metallic protection.3 Carbon/epoxy composite is a good electrical conductor and susceptible to lightning, with the following consequences: Damages at the point of impact: delamination, burning of resin Risk of lightning in attachments (bolts) The necessity to conduct to the mass for the electrical circuits situated under the composite element Remedies consist of the following: Glass fabric in conjunction with a very thin sheet of aluminum (20 mm) The use of a protective aluminum film (aluminum flam spray) Temperature is an important parameter that limits the usage of epoxy resins. A few experimental components have been made of bismaleimide resins (thermosets that soften4 at temperatures higher than 350∞C rather than 210∞C for epoxies). One other remedy would be to use a thermoplastic resin with high temperature resistance such as poly-ether-ether-ketone “peek”5 that softens at 380∞C. Laminates made of carbon/peek are more expensive than products made of carbon/epoxy. However, they present good performance at higher operating temperatures (continuously at 130∞C and periodically at 160∞C) and have the following additional advantages: Superior impact resistance Negligible moisture absorption Very low smoke generation in case of fire 3 The impacts can create internal damages that are invisible from the outside. This can also happen on the wing panels (for example, drop of tools on the panels during fabrication or during maintenance work). 4 The mechanical properties of the thermoset resins diminish when the temperature reaches the “glass transition temperature.” 5 See Section 1.6 for the physical properties. TX846_Frame_C07 Page 138 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
t(c) percent of ground maximum load flight 30% 20 2 1 time time -10% ground -30 flight Figure 7.1 Temperature Cycle and Load Cycle for Components of an Aircraft 7.1.4 Specific Aspects of Structural Resistance One must apply to composite components the technique called fail safe in aerodynamics,which consists of foreseeing the mode of rupture (delam- ination,for example)and acting in such a manner that this does not lead to the destruction of the component during the period between inspections. Composite components are repairable.Methods of reparation are analogous to those for laminates made of unidirectionals or fabrics. Considering the very important reduction of the number of rivets used as compared with the conventional construction,one obtains a smother surface,which can lead to better aerodynamic performance. One also considers that the attack of the environment and the cycles of fatigue over the years do not lead to significant deterioration of the composite pieces(shown in Figure 7.1 are two types of fatigue cycles for the components of aircraft structure). The failure aspect subject to a moderate impact is more problematic with the structures made of composite materials,because the energy absorbed by plastic deformation does not exist. For the cabins,one uses phenolic resins.These have good fire resistance, with low smoke emission.For the same reason,one prefers replacing Kevlar fibers with a combination of glass/carbon (lighter than glass alone and less expensive than carbon alone). It is possible to benefit from anisotropy of the laminates for the control of dynamic and aeroelastic behavior of the wing structures.' 7.1.5 Large Carriers The following examples give an idea on the evolution of use of composites in aircrafts over two decades: Example:Aerospatiale (FRA);Airbus Industry (EU)(Figure 7.2) Example:Boeing (USA)(Figure 7.3) 6 See Section 4.4.4. See Section 7.1.8. 2003 by CRC Press LLC
7.1.4 Specific Aspects of Structural Resistance One must apply to composite components the technique called fail safe in aerodynamics, which consists of foreseeing the mode of rupture (delamination, for example) and acting in such a manner that this does not lead to the destruction of the component during the period between inspections. Composite components are repairable. Methods of reparation are analogous to those for laminates made of unidirectionals or fabrics.6 Considering the very important reduction of the number of rivets used as compared with the conventional construction, one obtains a smother surface, which can lead to better aerodynamic performance. One also considers that the attack of the environment and the cycles of fatigue over the years do not lead to significant deterioration of the composite pieces (shown in Figure 7.1 are two types of fatigue cycles for the components of aircraft structure). The failure aspect subject to a moderate impact is more problematic with the structures made of composite materials, because the energy absorbed by plastic deformation does not exist. For the cabins, one uses phenolic resins. These have good fire resistance, with low smoke emission. For the same reason, one prefers replacing Kevlar fibers with a combination of glass/carbon (lighter than glass alone and less expensive than carbon alone). It is possible to benefit from anisotropy of the laminates for the control of dynamic and aeroelastic behavior of the wing structures.7 7.1.5 Large Carriers The following examples give an idea on the evolution of use of composites in aircrafts over two decades: Example: Aerospatiale (FRA); Airbus Industry (EU) (Figure 7.2) Example: Boeing (USA) (Figure 7.3) Figure 7.1 Temperature Cycle and Load Cycle for Components of an Aircraft 6 See Section 4.4.4. 7 See Section 7.1.8. TX846_Frame_C07 Page 139 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
composite mass in 20% of structural mass A320 A340 A310/300 10% A300/600 A A concorde300310 vautour 6% (3.3 tons) 1970 1980 1990 Figure 7.2 Evolution of Mass of Composites in Aircraft composite in of aerodynamic 767 surface 30% 747 757 0 20% 737 10% 727 0 707 0 1960 1970 1980 1990 Figure 7.3 Use of Composite in Boeing Aircraft How to evaluate the gains: In theory:For example,a study was made by Lockheed Company (USA)for the design of a large carrier having the following principal characteristics:payload 68 tons,range 8300 km.This study gives the following significant results: for an aircraft made using conventional metallic construction: total mass at take-off:363 tons mass of the structure:175 tons ■for an aircraft made using“maximum”composite construction: total mass at take-off:245 tons mass of the structure:96 tons. Such a difference can be explained by the cascading consequences that can be illustrated as in Figure 7.4. In practice:In reality,introduction of composites in the aircrafts is limited to certain parts of the structures.It is done case by case and in a progressive manner during the life of the aircraft (re-evaluation operation).One is then led to consider the different notions: 2003 by CRC Press LLC
How to evaluate the gains: In theory: For example, a study was made by Lockheed Company (USA) for the design of a large carrier having the following principal characteristics: payload 68 tons, range 8300 km. This study gives the following significant results: for an aircraft made using conventional metallic construction: total mass at take-off: 363 tons mass of the structure: 175 tons for an aircraft made using “maximum” composite construction: total mass at take-off: 245 tons mass of the structure: 96 tons. Such a difference can be explained by the cascading consequences that can be illustrated as in Figure 7.4. In practice: In reality, introduction of composites in the aircrafts is limited to certain parts of the structures. It is done case by case and in a progressive manner during the life of the aircraft (re-evaluation operation). One is then led to consider the different notions: Figure 7.2 Evolution of Mass of Composites in Aircraft Figure 7.3 Use of Composite in Boeing Aircraft TX846_Frame_C07 Page 140 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
decrease of empty mass decrease in motor-mass decrease in consumed fuel (minus 33%for the same mission)】 decrease in total mass Figure 7.4 Cascading Effect in Mass Reduction Notion of the exchange rate is the cost for a kilogram saved when one substitutes a classical metallic piece with a piece made mainly with com- posite.For the substitution light alloy-carbon/epoxy-this cost is on the order of $160 (1984)per kilogram when the piece is calculated in terms of rigidity (similar deformation for the same load).It is amortized over a period of at least one year for the gain in "paying passenger." Notion of gain in paying passenger is the gain in terms of the number of passengers,of freight,or in fuel cost;for example,for a large carrier: An aircraft of 150 tons,with 250 passengers consists of 60 tons of structure.A progressive introduction of 1600 kg of high performance composite materials leads to a gain of 16 more passengers along with their luggage. A reduction of 1 kg mass leads to the reduction of fuel consumption of around 120 liters per year. Why are the reductions of mass (average about 20%)not more spectacular? Consider the example of a vertical stabilizer.The distribution of mass of a composite vertical stabilizer can be presented as follows: Facings in carbon/epoxy:30%of total mass Honeycombs,adhesives:35%of total mass Attachments:25%of total mass Connections between carbon/epoxy components and attachments:over- layers of carbon/epoxy Allowance for the aging of the carbon/epoxy:overdimensions of the facings (the stresses are magnified about 10%more for a subsonic aircraft and 13%for a supersonic aircraft) In consequence,the global gain of mass in comparison with a classical metallic construction for the vertical stabilizer is not more than an order of about 15%. Example:European aircraft Airbus A-310-300 (Figure 7.5). 2003 by CRC Press LLC
Notion of the exchange rate is the cost for a kilogram saved when one substitutes a classical metallic piece with a piece made mainly with composite. For the substitution light alloy—carbon/epoxy—this cost is on the order of $160 (1984) per kilogram when the piece is calculated in terms of rigidity (similar deformation for the same load). It is amortized over a period of at least one year for the gain in “paying passenger.” Notion of gain in paying passenger is the gain in terms of the number of passengers, of freight, or in fuel cost; for example, for a large carrier: An aircraft of 150 tons, with 250 passengers consists of 60 tons of structure. A progressive introduction of 1600 kg of high performance composite materials leads to a gain of 16 more passengers along with their luggage. A reduction of 1 kg mass leads to the reduction of fuel consumption of around 120 liters per year. Why are the reductions of mass (average about 20%) not more spectacular? Consider the example of a vertical stabilizer. The distribution of mass of a composite vertical stabilizer can be presented as follows: Facings in carbon/epoxy: 30% of total mass Honeycombs, adhesives: 35% of total mass Attachments: 25% of total mass Connections between carbon/epoxy components and attachments: overlayers of carbon/epoxy Allowance for the aging of the carbon/epoxy: overdimensions of the facings (the stresses are magnified about 10% more for a subsonic aircraft and 13% for a supersonic aircraft) In consequence, the global gain of mass in comparison with a classical metallic construction for the vertical stabilizer is not more than an order of about 15%. Example: European aircraft Airbus A-310–300 (Figure 7.5). Figure 7.4 Cascading Effect in Mass Reduction TX846_Frame_C07 Page 141 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
vertical stabilizer:Kevlar/carbon/glass number of metal components divided by 20 rudder: Kevlar/carbon flap fairing horizontal stabilizer Kevlar motor mast reinforcement:Kevlar (19 kg instead of 26 kg of metal) air conditioning components:Kevlar. flaps,ailerons,spoilers 29 kg instead of 43 kg in metal main landing gear hatch and fairing disk brakes front landing gear hatch carbon/carbon karman:Kevlar 1.8 kg/m2 radome:glass top of vertical stabilizer(Kevlar/glass) top of the rudder (no carbon to avoid lightning strike) (Kevlar) floors leading edge of vertical stabilizer (Kevlar/carbon /glass) vertical stabilizer edge trusses: (Kevlar/carbon/glass) rudder light alloy:2 kg carbon:800 g (Kevlar/carbon/glass) Karman vertical stabilizer (Kevlar) trailing edge:Kevlar Airbus A-310 vertical stabilizer:the number of components and rivets is divided by 20 in comparison with the classical solution Figure 7.5 Composite Components in an Airbus A-310 ■Total mass:180tons Mass of structure:44.7 tons Mass of composites:6.2 tons Mass of high performance composites:1.1 tons Reduction of mass of structure:1.4 tons ■ Percentage of composites:13.8%of mass of structure.A reduction of mass of the structure of 1 kg augments the range of the aircraft by 1 nautical mile. 2003 by CRC Press LLC
Total mass: 180 tons Mass of structure: 44.7 tons Mass of composites: 6.2 tons Mass of high performance composites: 1.1 tons Reduction of mass of structure: 1.4 tons Percentage of composites: 13.8% of mass of structure. A reduction of mass of the structure of 1 kg augments the range of the aircraft by 1 nautical mile. Figure 7.5 Composite Components in an Airbus A-310 TX846_Frame_C07 Page 142 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
leading and trailing edge extemal ailerons of vertical stabilizer external spoilers horizontal rudder stabilizer flap rails and covers fairing intemal spoilers floors cabin compartment motor case karman brake disks radome carbon/carbon hatch of landing gear hatch and cover of main landing gear Figure 7.6 Composite Components in an Airbus A-320 Example:European aircraft Airbus A-320 (Figure 7.6). ■Total mass:72tons ■Empty mass:40tons Mass of structure:21 tons Mass of composite materials:4.5 tons,corresponding to a reduction of mass of the structure of 1.1 tons.The percent of composite mass is 21.5% of the mass of the structure. A few other characteristics:Length:37.6 m;breadth:34 m;150 to 176 passengers transported from 3,500 to 5,500 km;maximum cruising speed: 868 km/h Example:European aircraft Airbus A-340 Total mass:253.5 tons Mass of structure:76 tons Mass of composites:11 tons,corresponding to a reduction of structure mass of 3 tons Percentage of composites:14.5%of the mass of the structure Example:Future supersonic aircraft ATSF (Figure 7.7),Aerospatiale (FRA)and Britisb Aerospace (UK).Principal characteristics defined at the stage before the project Transport of 200 passengers over a distance of 12,000 km Cruising speed between Mach 2(2,200 km/hr)and Mach 2.4(2,600 km/hr) 2003 by CRC Press LLC
Example: European aircraft Airbus A-320 (Figure 7.6). Total mass: 72 tons Empty mass: 40 tons Mass of structure: 21 tons Mass of composite materials: 4.5 tons, corresponding to a reduction of mass of the structure of 1.1 tons. The percent of composite mass is 21.5% of the mass of the structure. A few other characteristics: Length: 37.6 m; breadth: 34 m; 150 to 176 passengers transported from 3,500 to 5,500 km; maximum cruising speed: 868 km/h Example: European aircraft Airbus A-340 Total mass: 253.5 tons Mass of structure: 76 tons Mass of composites: 11 tons, corresponding to a reduction of structure mass of 3 tons Percentage of composites: 14.5% of the mass of the structure Example: Future supersonic aircraft ATSF (Figure 7.7), Aerospatiale (FRA) and British Aerospace (UK). Principal characteristics defined at the stage before the project Transport of 200 passengers over a distance of 12,000 km Cruising speed between Mach 2 (2,200 km/hr) and Mach 2.4 (2,600 km/hr) Figure 7.6 Composite Components in an Airbus A-320 TX846_Frame_C07 Page 143 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC
radome temperature:130°C (mach2,05) front cone auxiliary gear hatch main landing gear hatch leading flaps temperature 130C(mach2.05) flaps extemal flap rudder Figure 7.7 Composite Components in a Future Supersonic Aircraft Economically viable for a single type of aircraft on the market (enlarged international cooperation) 7.1.6 Regional Jets Example:Regional transport aircraft ATR 72,ATR (FRA-ITA)(Figure 7.8): ■Total mass:20tons Percentage of composite materials more than 25%of the mass of the structure ■ Transports 66 passengers over a distance of 2,600 kilometers Interior equipment:Facings of panels for portholes and ceiling,baggage compartment,bulkheads,toilets,storing armors in glass-carbon/phenolic resins/NOMEX honeycomb;decoration by a film of "TEDLAR" 2003 by CRC Press LLC
Economically viable for a single type of aircraft on the market (enlarged international cooperation) 7.1.6 Regional Jets Example: Regional transport aircraft ATR 72, ATR (FRA–ITA) (Figure 7.8): Total mass: 20 tons Percentage of composite materials more than 25% of the mass of the structure Transports 66 passengers over a distance of 2,600 kilometers Interior equipment: Facings of panels for portholes and ceiling, baggage compartment, bulkheads, toilets, storing armors in glass-carbon/phenolic resins/NOMEX honeycomb; decoration by a film of “TEDLAR” Figure 7.7 Composite Components in a Future Supersonic Aircraft TX846_Frame_C07 Page 144 Monday, November 18, 2002 12:17 PM © 2003 by CRC Press LLC