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