Chapter 1:Introduction to Composite Materials /15 Carbon /Epoxy (AS/3501-6) Fatigue 2 Static Titanium leingonns (Ti-6A-4V) Aluminum (7075-T6) Static 1 Static NNWWVWWWWwWw Xw Fatigue 0 Fig.1.19 Relative structural efficiency of aerospace materials resistance of composites compared to high- bly cost is another major cost driver,accounting strength metals is shown in Fig.1.20.As long for about 50 percent of the total part cost.As pre- as reasonable strain levels are used during de- viously stated,one of the potential advantages of sign,fatigue of carbon fiber composites should composites is the ability to cure or bond a number not be a problem. of detail parts together to reduce assembly costs Assembly costs can account for as much as and the number of required fasteners. 50 percent of the cost of an airframe.Compos- Temperature has an effect on composite me- ites offer the opportunity to significantly reduce chanical properties.Typically,matrix-dominated the amount of assembly labor and the number of mechanical properties decrease with increas- required fasteners.Detail parts can be combined ing temperature.Fiber-dominated properties are into a single cured assembly either during initial somewhat affected by cold temperatures,but the cure or by secondary adhesive bonding. effects are not as severe as those of elevated Disadvantages of composites include high raw temperature on the matrix-dominated properties. material costs and usually high fabrication and Design parameters for carbon/epoxy are cold- assembly costs;adverse effects of both tempera- dry tension and hot-wet compression(Fig.1.22). ture and moisture;poor strength in the out-of- An important design factor in the selection of a plane direction where the matrix carries the pri- matrix resin for elevated-temperature applica- mary load (they should not be used where load tions is the cured glass transition temperature paths are complex,such as with lugs and fittings); The cured glass transition temperature(T,)of a susceptibility to impact damage and delamina- polymeric material is the temperature at which it tions or ply separations;and greater difficulty in changes from a rigid,glassy solid into a softer, repairing them compared to metallic structures. semiflexible material.At this point,the polymer The major cost driver in fabrication for a com- structure is still intact but the crosslinks are no posite part using conventional hand lay-up is the longer locked in position.Therefore,the T de- cost of laying up or collating the plies.This cost is termines the upper use temperature for a com- generally 40 to 60 percent of the fabrication cost, posite or an adhesive and is the temperature depending on part complexity(Fig.1.21).Assem- above which the material will exhibit significantly
Chapter 1: Introduction to Composite Materials / 15 Fig. 1.19 Relative structural efficiency of aerospace materials resistance of composites compared to highstrength metals is shown in Fig. 1.20. As long as reasonable strain levels are used during design, fatigue of carbon fiber composites should not be a problem. Assembly costs can account for as much as 50 percent of the cost of an airframe. Composites offer the opportunity to significantly reduce the amount of assembly labor and the number of required fasteners. Detail parts can be combined into a single cured assembly either during initial cure or by secondary adhesive bonding. Disadvantages of composites include high raw material costs and usually high fabrication and assembly costs; adverse effects of both temperature and moisture; poor strength in the out-ofplane direction where the matrix carries the primary load (they should not be used where load paths are complex, such as with lugs and fittings); susceptibility to impact damage and delaminations or ply separations; and greater difficulty in repairing them compared to metallic structures. The major cost driver in fabrication for a composite part using conventional hand lay-up is the cost of laying up or collating the plies. This cost is generally 40 to 60 percent of the fabrication cost, depending on part complexity (Fig. 1.21). Assembly cost is another major cost driver, accounting for about 50 percent of the total part cost. As previously stated, one of the potential advantages of composites is the ability to cure or bond a number of detail parts together to reduce assembly costs and the number of required fasteners. Temperature has an effect on composite mechanical properties. Typically, matrix-dominated mechanical properties decrease with increas ing temperature. Fiber-dominated properties are somewhat affected by cold temperatures, but the effects are not as severe as those of elevated temperature on the matrix-dominated properties. Design parameters for carbon/epoxy are colddry tension and hot-wet compression (Fig. 1.22). An important design factor in the selection of a matrix resin for elevated-temperature applications is the cured glass transition temperature. The cured glass transition temperature (Tg) of a polymeric material is the temperature at which it changes from a rigid, glassy solid into a softer, semiflexible material. At this point, the polymer structure is still intact but the crosslinks are no longer locked in position. Therefore, the Tg determines the upper use temperature for a composite or an adhesive and is the temperature above which the material will exhibit significantly
16 Structural Composite Materials 800 700 Carbon/Epoxy (46%0°150%±4514%90) 600 (=0) R=0.1 Unloaded Hole 500 Ti-6Al-4V 400 4340 Steel(210 ksi) 300 7075-T6A1 200 100 103 104 105 105 107 108 Cycles to Failure Fig.1.20 Fatigue properties of aerospace materials reduced mechanical properties.Since most ther- rate of moisture absorption.Absorbed mois- moset polymers will absorb moisture that se- ture reduces the matrix-dominated mechani- verely depresses the T,the actual use tempera- cal properties and causes the matrix to swell, ture should be about 50F(30C)lower than the which relieves locked-in thermal strains from wet or saturated T elevated-temperature curing.These strains can be large,and large panels fixed at their edges Upper Use Temperature=WetT-50F (Eq 1.9) can buckle due to strains caused by swelling. During freeze-thaw cycles,absorbed moisture In general,thermoset resins absorb more mois- expands during freezing,which can crack the ture than comparable thermoplastic resins. matrix,and it can turn into steam during thermal The cured glass transition temperature (T) spikes.When the internal steam pressure ex- can be determined by several methods that are ceeds the flatwise tensile(through-the-thickness) outlined in Chapter 3,"Matrix Resin Systems." strength of the composite,the laminate will The amount of absorbed moisture (Fig.1.23) delaminate. depends on the matrix material and the relative Composites are susceptible to delaminations(ply humidity.Elevated temperatures increase the separations)during fabrication,during assembly
16 / Structural Composite Materials reduced mechanical properties. Since most thermoset polymers will absorb moisture that severely depresses the Tg, the actual use temperature should be about 50 ºF (30 ºC) lower than the wet or saturated Tg. Upper Use Temperature = Wet Tg – 50 °F (Eq 1.9) In general, thermoset resins absorb more moisture than comparable thermoplastic resins. The cured glass transition temperature (Tg) can be determined by several methods that are outlined in Chapter 3, “Matrix Resin Systems.” The amount of absorbed moisture (Fig. 1.23) depends on the matrix material and the relative humidity. Elevated temperatures increase the rate of moisture absorption. Absorbed moisture reduces the matrix-dominated mechanical properties and causes the matrix to swell, which relieves locked-in thermal strains from elevated-temperature curing. These strains can be large, and large panels fixed at their edges can buckle due to strains caused by swelling. During freeze-thaw cycles, absorbed moisture expands during freezing, which can crack the matrix, and it can turn into steam during thermal spikes. When the internal steam pressure exceeds the flatwise tensile (through-the-thickness) strength of the composite, the laminate will delaminate. Composites are susceptible to delaminations (ply separations) during fabrication, during assembly, Fig. 1.20 Fatigue properties of aerospace materials
Chapter 1:Introduction to Composite Materials /17 Tool Prep (12%) Collate Plies Bag Cure (46%) (13%) Trim (6%) NDI (15%) Cut Plies (8%) Fabrication Cost Materials Fasteners and and Fabrication Assembly 50% 50% Total Cost Fig1.21 Cost drivers for composite hand lay-up.NDI,nondestructive inspection and in service.During fabrication,foreign mate- through the laminates,forming a complex net- rials such as prepreg backing paper can be inad- work of delaminations and matrix cracks,as vertently left in the lay-up.During assembly. shown in Fig.1.24.Depending on the size of the improper part handling or incorrectly installed delamination,it can reduce the static and fatigue fasteners can cause delaminations.In service, strength and the compression buckling strength. low-velocity impact damage from dropped tools If it is large enough,it can grow under fatigue or forklifts running into aircraft can cause dam- loading. age.The damage may appear as only a small Typically,damage tolerance is a resin-domi- indentation on the surface but it can propagate nated property.The selection of a toughened
Chapter 1: Introduction to Composite Materials / 17 and in service. During fabrication, foreign materials such as prepreg backing paper can be inadvertently left in the lay-up. During assembly, improper part handling or incorrectly installed fasteners can cause delaminations. In service, low-velocity impact damage from dropped tools or forklifts running into aircraft can cause damage. The damage may appear as only a small indentation on the surface but it can propagate through the laminates, forming a complex network of delaminations and matrix cracks, as shown in Fig. 1.24. Depending on the size of the delamination, it can reduce the static and fatigue strength and the compression buckling strength. If it is large enough, it can grow under fatigue loading. Typically, damage tolerance is a resin-dominated property. The selection of a toughened Fig. 1.21 Cost drivers for composite hand lay-up. NDI, nondestructive inspection
18 Structural Composite Materials Carbon/Epoxy 0.60%Moisture 42%0°150%±4518%90°Plies 125 115 Wet 105 6 95 85 75 -65 75 220 250 Temperature(F) Carbon/Epoxy 0.6%Moisture 42%0°/50%±45°/8%90°Plies 130 110 Dry 90 Wet 70 50 -65 75 220 250 Temperature(F) Fig.1.22 Effects of temperature and moisture on strength of carbon/epoxy.R.T.,room temperature resin can significantly improve the resistance 1.6 Applications to impact damage.In addition,S-2 glass and aramid fibers are extremely tough and damage Applications include aerospace,transpor- tolerant.During the design phase,it is impor- tation,construction,marine goods,sporting tant to recognize the potential for delamina- goods,and more recently infrastructure,with tions and use sufficiently conservative design construction and transportation being the largest. strains so that a damaged structure can be In general.high-performance but more costly repaired. continuous-carbon-fiber composites are used
18 / Structural Composite Materials resin can significantly improve the resistance to impact damage. In addition, S-2 glass and aramid fibers are extremely tough and damage tolerant. During the design phase, it is important to recognize the potential for delaminations and use sufficiently conservative design strains so that a damaged structure can be repaired. 1.6 Applications Applications include aerospace, transpor tation, construction, marine goods, sporting goods, and more recently infrastructure, with construction and transportation being the largest. In general, high-performance but more costly continuous-carbon-fiber composites are used Fig. 1.22 Effects of temperature and moisture on strength of carbon/epoxy. R.T., room temperature
Chapter 1:Introduction to Composite Materials /19 Surface Thickness Moisture (%) Time=0 Time>0 Time= Saturation 1.6 Carbon/Bismaleimide Carbon/Epoxy 1.2 8 0.17in. 0.8 Carbon/Amorphous Thermoplastic 0.4 160F,95%RH Carbon/Semicrystalline Thermoplastic 0 25 5075 100 125150175200 Time(Days) Fig.1.23 Absorption of moisture for polymer matrix composites.RH,relative humidity where high strength and stiffness along with drove the development of much of the technology light weight are required,and much lower-cost now being used by other industries.Both small fiberglass composites are used in less demand- and large commercial aircraft rely on composites ing applications where weight is not as critical. to decrease weight and increase fuel performance, In military aircraft,low weight is "king"for the most striking example being the 50 percent performance and payload reasons,and compos- composite airframe for the new Boeing 787 ites often approach 20 to 40 percent of the air- (Fig.1.26).All future Airbus and Boeing aircraft frame weight(Fig.1.25).For decades,helicop- will use large amounts of high-performance ters have incorporated glass fiber-reinforced composites.Composites are also used exten- rotor blades for improved fatigue resistance,and sively in both weight-critical reusable and ex- in recent years helicopter airframes have been pendable launch vehicles and satellite structures built largely of carbon-fiber composites.Mili- (Fig.1.27).Weight savings due to the use of tary aircraft applications,the first to use high- composite materials in aerospace applications performance continuous-carbon-fiber composites, generally range from 15 to 25 percent
Chapter 1: Introduction to Composite Materials / 19 where high strength and stiffness along with light weight are required, and much lower-cost fiberglass composites are used in less demanding applications where weight is not as critical. In military aircraft, low weight is “king” for performance and payload reasons, and composites often approach 20 to 40 percent of the airframe weight (Fig. 1.25). For decades, helicopters have incorporated glass fiber–reinforced rotor blades for improved fatigue resistance, and in recent years helicopter airframes have been built largely of carbon-fiber composites. Military aircraft applications, the first to use highperformance continuous-carbon-fiber composites, drove the development of much of the technology now being used by other industries. Both small and large commercial aircraft rely on composites to decrease weight and increase fuel performance, the most striking example being the 50 percent composite airframe for the new Boeing 787 (Fig. 1.26). All future Airbus and Boeing aircraft will use large amounts of high-performance composites. Composites are also used extensively in both weight-critical reusable and expendable launch vehicles and satellite structures (Fig. 1.27). Weight savings due to the use of composite materials in aerospace applications generally range from 15 to 25 percent. Fig. 1.23 Absorption of moisture for polymer matrix composites. RH, relative humidity