COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 1.2 Approximate Actual(US$/kg)Values of Saving One Unit of Weight: Costing Based on Some Late 1980s Estimates ●small civil$80 advanced fighter $500 .civil helicopter $80-$200 ·VTOL$800 military helicopter $400 ·SST$1500 ·large transport$300 Space Shuttle $45,000 large commercial $500 Chapter 2 describes the basic principles(micromechanics)of fiber composite materials.As an example,to a good first approximation,the stiffness under loading in the fiber direction(unidirectional fibers)may be determined by the simple law of mixtures.This is simply a sum of the volume (or area)fraction of the fibers and the matrix multiplied by the elastic modulus.The strength estimation is similar(for a reasonably high fiber-volume fraction)but with each elastic modulus multiplied by the breaking strain of the first-failing component. In the case of carbon fiber/epoxy composites,this is generally the fiber-breaking strain.If,however,the lowest failure strain is that of the matrix,the first failure event may be the development of extensive matrix cracking,rather than total fracture.This damage may or may not be defined as failure of the composite. However,toughness is usually much more than the sum of the toughness of each of the components because it depends also on the properties of the fiber/ matrix interface.Therefore,brittle materials such as glass fibers and polyester resin,when combined,produce a tough,strong composite,most familiarly known as fiberglass,used in a wide range of structural applications. Control of the strength of the fiber/matrix interface is of paramount importance for toughness,particularly when both the fiber and the matrix are brittle.If the interface is too strong,a crack in the matrix can propagate directly through fibers in its path.Thus it is important that the interface is able to disbond Table 1.3 Summary of the Approach for Development of a High-Performance Fiber Composite ·Fibers ·Polymer Matrix 。Composite stiff/strong/brittle/low -low stiffness and strength toughness through density ductile or brittle synergistic action -high temperature -can be polymer,metal, (woodlike) capability or ceramic -high strength and able to carry major load -transmits load to and stiffness in fiber as reinforcement from fiber direction,weak at -usually continuous -forms shape and protects angles to fiber axis -oriented for principal fiber tailor fiber directions to stresses optimize properties
COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Table 1.2 Approximate Actual (US$/kg) Values of Saving One Unit of Weight: Costing Based on Some Late 1980s Estimates • small civil $80 • civil helicopter $80-$200 • military helicopter $400 • large transport $300 • large commercial $500 • advanced fighter $500 • VTOL $800 * SST $1500 • Space Shuttle $45,000 Chapter 2 describes the basic principles (micromechanics) of fiber composite materials. As an example, to a good first approximation, the stiffness under loading in the fiber direction (unidirectional fibers) may be determined by the simple law of mixtures. This is simply a sum of the volume (or area) fraction of the fibers and the matrix multiplied by the elastic modulus. The strength estimation is similar (for a reasonably high fiber-volume fraction) but with each elastic modulus multiplied by the breaking strain of the first-failing component. In the case of carbon fiber/epoxy composites, this is generally the fiber-breaking strain. If, however, the lowest failure strain is that of the matrix, the first failure event may be the development of extensive matrix cracking, rather than total fracture. This damage may or may not be defined as failure of the composite. However, toughness is usually much more than the sum of the toughness of each of the components because it depends also on the properties of the fiber/ matrix interface. Therefore, brittle materials such as glass fibers and polyester resin, when combined, produce a tough, strong composite, most familiarly known as fiberglass, used in a wide range of structural applications. Control of the strength of the fiber/matrix interface is of paramount importance for toughness, particularly when both the fiber and the matrix are brittle. If the interface is too strong, a crack in the matrix can propagate directly through fibers in its path. Thus it is important that the interface is able to disbond Table 1.3 Summary of the Approach for Development of a High-Performance Fiber Composite • Fibers t Polymer Matrix • Composite - stiff/strong/brittle/low - low stiffness and strength - toughness through density ductile or brittle synergistic action - high temperature - can be polymer, metal, (woodlike) capability or ceramic - high strength and - able to carry major load - transmits load to and stiffness in fiber as reinforcement from fiber direction, weak at - usually continuous - forms shape and protects angles to fiber axis - oriented for principal fiber - tailor fiber directions to stresses optimize properties
INTRODUCTION AND OVERVIEW 5 at a modest stress level,deflecting the crack and thereby avoiding fiber failure. However,if the interface is too weak,the composite will have unacceptably low transverse properties.As discussed in more detail in Chapter 2,several other mechanisms contribute to energy absorbed in fracture and thus to toughness, including fiber disbonding and pullout,matrix deformation,and bridging of the cracked region by unbroken fibers. The composite structure is arranged (tailored)during manufacture of the component with the fibers orientated in various directions in sufficient concentrations to provide the required strength and stiffness(Chapter 12).For in-plane loading,this is usually achieved using a laminated or plywood type of construction consisting of layers or plies of unidirectional or bi-directional orientated fibers.This concept is illustrated in Figure 1.3 for an aircraft wing. Alternatively,the fibers may be arranged by a variety of advanced textile techniques,such as weaving,braiding,or filament winding. Thus to obtain the desired mechanical properties,the fiber layers or plies in a laminate are arranged at angles from 0 to 90 relative to the 0 primary loading direction.However,certain sequence and symmetry rules must be obeyed to avoid distortion of the component after cure or under service loading (as described in Chapters 6 and 12).For simplicity the plies are most often based on combinations of0°，±45°，and90°orientations.The laminate is stiffest and strongest(in-plane)in the direction with the highest concentratio of 0 fibers, Ref.Axis (spanwise) Torque Spanwise bending moment Shear Fig.1.3 Tailoring of fiber directions for the applied loads in a composite wing skin Taken from Ref.1
INTRODUCTION AND OVERVIEW 5 at a modest stress level, deflecting the crack and thereby avoiding fiber failure. However, if the interface is too weak, the composite will have unacceptably low transverse properties. As discussed in more detail in Chapter 2, several other mechanisms contribute to energy absorbed in fracture and thus to toughness, including fiber disbonding and pullout, matrix deformation, and bridging of the cracked region by unbroken fibers. The composite structure is arranged (tailored) during manufacture of the component with the fibers orientated in various directions in sufficient concentrations to provide the required strength and stiffness (Chapter 12). For in-plane loading, this is usually achieved using a laminated or plywood type of construction consisting of layers or plies of unidirectional or bi-directional orientated fibers. This concept is illustrated in Figure 1.3 for an aircraft wing. Alternatively, the fibers may be arranged by a variety of advanced textile techniques, such as weaving, braiding, or filament winding. Thus to obtain the desired mechanical properties, the fiber layers or plies in a laminate are arranged at angles from 0 ° to 90 ° relative to the 0 ° primary loading direction. However, certain sequence and symmetry rules must be obeyed to avoid distortion of the component after cure or under service loading (as described in Chapters 6 and 12). For simplicity the plies are most often based on combinations of 0 °, + 45 °, and 90 ° orientations. The laminate is ~tiffest and strongest (in-plane) in the direction with the highest concentratio'~ of 0 ° fibers, Ref. Axis (spanwtse) ~-~ Torque Vertk Sheer Fig. 1.3 Tailoring of fiber directions for the applied loads in a composite wing skin. Taken from Ref. 1
6 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES but it will have much reduced strength and stiffness in other directions-the laminate is then said to be orthotropic. When the ply configuration is made of equal numbers of plies at0°，±45°，and 90the in-plane mechanical properties do not vary with loading direction and the composite is then said to be quasi-isotropic.A similar situation arises with a 060 ply configuration.The quasi-isotropic ply configuration is used when in-plane loading is bi-directional.Because the quasi-isotropic configuration has a stress concentration factor(similar to that of an isotropic material),it is also used where local stresses are high,such as in a mechanical joint.However,for most cases,the quasi-isotropic configuration is an inefficient use of the composite material. 1.4 Fiber Reinforcements As described in Chapter 3,continuous strong,stiff fibers can be made from the light elements;carbon and boron,and the compounds silicone oxide(silica and silica-based glasses),silicon carbide,and silicon nitride.Fibers can also be made from organic materials based on long-chain molecules of carbon,hydrogen,and nitrogen.Such fibers include aramid (Kevlar)fibers.Fibers may be available in the form of single large-diameter filaments or as tows (or rovings)consisting of many thousands of filaments.For example,boron fibers formed by chemical vapor deposition (CVD)are produced as single filaments with a diameter of over 100 um.Carbon fibers,formed by pyrolysis of a polymer precursor (polyacrylonitrile;PAN),are produced as a filament diameter of about 8 um and supplied in a tow (bundle of filaments)with up to 2.5 x 10 filaments. Chemical Vapor deposition and other techniques can make short ultra-strong and stiff fibers called whiskers.These are filamentary single crystals having diameters in the range 1-10 um and length-to-diameter ratios up to 10,000.With the correct deposition techniques,whiskers can have strengths approaching the theoretical maximum of one tenth of the Young's modulus.This high level of strength results from the perfection of the crystal structure and freedom from cracklike flaws.Whiskers can be made from various materials,including SiC, Al2O3,C,and B4C. In the early 1990s,a new form of carbon called carbon nanotubes was discovered.These are essentially sheets of hexagonal graphite basal plane rolled up into a tube,with a morphology determined by the way in which the sheet is rolled up.The tube walls may be made of single or double layers;typically, length is in the range 0.6-8 nm.They can be produced by a variety of processes, including arc-discharge and CVD.As may be expected,carbon in this form has exceptionally high strength and stiffness.Elastic moduli of over 1000 GPa (1 TPa)and strengths over 100 GPa are quoted,although the minute dimensions and wall geometry of the tubes makes measurement extremely difficult
6 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES but it will have much reduced strength and stiffness in other directions--the laminate is then said to be orthotropic. When the ply configuration is made of equal numbers of plies at 0 °, ___ 45 °, and 90 ° the in-plane mechanical properties do not vary with loading direction and the composite is then said to be quasi-isotropic. A similar situation arises with a 0 ° +__ 60 ° ply configuration. The quasi-isotropic ply configuration is used when in-plane loading is bi-directional. Because the quasi-isotropic configuration has a stress concentration factor (similar to that of an isotropic material), it is also used where local stresses are high, such as in a mechanical joint. However, for most cases, the quasi-isotropic configuration is an inefficient use of the composite material. 1.4 Fiber Reinforcements As described in Chapter 3, continuous strong, stiff fibers can be made from the light elements; carbon and boron, and the compounds silicone oxide (silica and silica-based glasses), silicon carbide, and silicon nitride. Fibers can also be made from organic materials based on long-chain molecules of carbon, hydrogen, and nitrogen. Such fibers include aramid (Kevlar) fibers. Fibers may be available in the form of single large-diameter filaments or as tows (or rovings) consisting of many thousands of filaments. For example, boron fibers formed by chemical vapor deposition (CVD) are produced as single filaments with a diameter of over 100 p~m. Carbon fibers, formed by pyrolysis of a polymer precursor (polyacrylonitrile; PAN), are produced as a filament diameter of about 8 Ixm and supplied in a tow (bundle of filaments) with up to 2.5 x 10 4 filaments. Chemical Vapor deposition and other techniques can make short ultra-strong and stiff fibers called whiskers. These are filamentary single crystals having diameters in the range 1 - 10 Izm and length-to-diameter ratios up to 10,000. With the correct deposition techniques, whiskers can have strengths approaching the theoretical maximum of one tenth of the Young's modulus. This high level of strength results from the perfection of the crystal structure and freedom from cracklike flaws. Whiskers can be made from various materials, including SiC, A1203, C, and B4C. In the early 1990s, a new form of carbon called carbon nanotubes was discovered. 3 These are essentially sheets of hexagonal graphite basal plane rolled up into a tube, with a morphology determined by the way in which the sheet is rolled up. The tube walls may be made of single or double layers; typically, length is in the range 0.6-8 nm. They can be produced by a variety of processes, including arc-discharge and CVD. As may be expected, carbon in this form has exceptionally high strength and stiffness. Elastic moduli of over 1000 GPa (1 TPa) and strengths over 100 GPa are quoted, although the minute dimensions and wall geometry of the tubes makes measurement extremely difficult
INTRODUCTION AND OVERVIEW 7 Whiskers(with some exceptions)are expensive and difficult to incorporate into composites with high degrees of orientation and alignment.So,despite their early discovery,they have not been exploited in any practical composites. Although nanotubes are also expensive and similarly difficult to process into composites,they have such attractive mechanical properties and potential for relatively cheap manufacture that many R&D programs are focused on their exploitation.However,significant technological developments will be required to make composites based on these materials practically and economically feasible. Textile technology has been developed to produce special reinforcing fabrics from continuous fibers,mainly glass,carbon,or aramid.Small-diameter fiber tows may be woven to produce a wide range of fabrics;simple examples are plain weave or satin weave cloths.Fabrics can also be woven from two or more types of fiber,for example,with carbon fibers in the 0 or warp direction (the roll direction)and glass or aramid in the 90 weft direction. To avoid fiber crimping (waviness)associated with weaving,a textile approach can be used in which the fibers are held in place by a knitting yarn.The resulting materials are called non-crimp fabrics,and these can contain fibers orientated at0°，90°，and±45°in any specified proportions..Because of the elimination of fiber waviness,composites based on non-crimp fabric show a significant improvement in compression strength compared with those based on woven materials.Stiffness in both tension and compression is also improved by around 10%. Fiber preforms ready for matrix impregnation to form the component can be produced by several techniques including weaving,braiding,and knitting. Advanced weaving and braiding techniques are used to produce preforms with 3-D reinforcement,as described in Chapter 14.Three-dimensional weaving is extensively employed for the manufacture of carbon/carbon composites, described later. 1.5 Matrices The matrix,which may be a polymer,metal,or ceramic,forms the shape of the component and serves the following additional functions:1)transfers load into and out of the fibers,2)separates the fibers to prevent failure of adjacent fibers when one fails,and 3)protects the fiber from the environment.The strength of the fiber/matrix interfacial bond is crucial in determining toughness of the composite.The interface,known as the interphase,is regarded as the third phase in the composite because the matrix structure is modified close to the fiber surface.The interface is even more complex in some fibers,notably glass fibers, which are pre-coated with a sizing agent to improve bond strength,to improve environmental durability,or simply to reduce handling damage
INTRODUCTION AND OVERVIEW 7 Whiskers (with some exceptions) are expensive and difficult to incorporate into composites with high degrees of orientation and alignment. So, despite their early discovery, they have not been exploited in any practical composites. Although nanotubes are also expensive and similarly difficult to process into composites, they have such attractive mechanical properties and potential for relatively cheap manufacture that many R&D programs are focused on their exploitation. However, significant technological developments will be required to make composites based on these materials practically and economically feasible. Textile technology has been developed to produce special reinforcing fabrics from continuous fibers, mainly glass, carbon, or aramid. Small-diameter fiber tows may be woven to produce a wide range of fabrics; simple examples are plain weave or satin weave cloths. Fabrics can also be woven from two or more types of fiber, for example, with carbon fibers in the 0 ° or warp direction (the roll direction) and glass or aramid in the 90 ° weft direction. To avoid fiber crimping (waviness) associated with weaving, a textile approach can be used in which the fibers are held in place by a knitting yam. The resulting materials are called non-crimp fabrics, and these can contain fibers orientated at 0 °, 90 °, and _ 45 ° in any specified proportions. Because of the elimination of fiber waviness, composites based on non-crimp fabric show a significant improvement in compression strength compared with those based on woven materials. Stiffness in both tension and compression is also improved by around 10%. Fiber preforms ready for matrix impregnation to form the component can be produced by several techniques including weaving, braiding, and knitting. Advanced weaving and braiding techniques are used to produce preforms with 3-D reinforcement, as described in Chapter 14. Three-dimensional weaving is extensively employed for the manufacture of carbon/carbon composites, described later. 1.5 Matrices The matrix, which may be a polymer, metal, or ceramic, forms the shape of the component and serves the following additional functions: 1) transfers load into and out of the fibers, 2) separates the fibers to prevent failure of adjacent fibers when one fails, and 3) protects the fiber from the environment. The strength of the fiber/matrix interfacial bond is crucial in determining toughness of the composite. The interface, known as the interphase, is regarded as the third phase in the composite because the matrix structure is modified close to the fiber surface. The interface is even more complex in some fibers, notably glass fibers, which are pre-coated with a sizing agent to improve bond strength, to improve environmental durability, or simply to reduce handling damage
8 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Properties of the composite that are significantly affected by the properties of the matrix (matrix-dominated properties)include:1)temperature and environmental resistance,2)longitudinal compression strength,3)transverse tensile strength,and 4)shear strength. The matrix may be brittle or tough.Figure 1.4 shows the inherent toughness of some candidate materials. Economic production requires that the techniques used for matrix introduction allow simple low-cost formation of the composite without damaging or misaligning the fibers.The simplest method is to infiltrate an aligned fiber bed with a low-viscosity liquid that is then converted by chemical reaction or by cooling to form a continuous solid matrix with the desired properties. Alternatively,single fibers,tows of fibers,or sheets of aligned fibers may be coated or intermingled with solid matrix or matrix precursor and the continuous matrix formed by flowing the coatings together (and curing if required)under heat and pressure. 1.5.1 Polymers Chapter 4 discusses the thermosetting or thermoplastic polymers that are used for the matrix of polymer composites.Thermosetting polymers are long-chain molecules that cure by cross-linking to form a fully three-dimensional network and cannot be melted and reformed.They have the great advantage that they allow fabrication of composites at relatively low temperatures and pressures since they pass through a low-viscosity stage before polymerization and cross- 3 Aluminium Thermoplastics Toughened Polymethyl Unmodified Glass Alloys Epoxies methacrylate Epoxies Fig.1.4 Toughness of some materials used as matrices in advanced fiber composites
8 COMPOSITE MATERIALS FOR AIRCRAFT STRUCTURES Properties of the composite that are significantly affected by the properties of the matrix (matrix-dominated properties) include: 1) temperature and environmental resistance, 2) longitudinal compression strength, 3) transverse tensile strength, and 4) shear strength. The matrix may be brittle or tough. Figure 1.4 shows the inherent toughness of some candidate materials. Economic production requires that the techniques used for matrix introduction allow simple low-cost formation of the composite without damaging or misaligning the fibers. The simplest method is to infiltrate an aligned fiber bed with a low-viscosity liquid that is then converted by chemical reaction or by cooling to form a continuous solid matrix with the desired properties. Alternatively, single fibers, tows of fibers, or sheets of aligned fibers may be coated or intermingled with solid matrix or matrix precursor and the continuous matrix formed by flowing the coatings together (and curing if required) under heat and pressure. 1.5.1 Polymers Chapter 4 discusses the thermosetting or thermoplastic polymers that are used for the matrix of polymer composites. Thermosetting polymers are long-chain molecules that cure by cross-linking to form a fully three-dimensional network and cannot be melted and reformed. They have the great advantage that they allow fabrication of composites at relatively low temperatures and pressures since they pass through a low-viscosity stage before polymerization and crossLu Alumlnlum Thermoplastics Alloys Fig. 1.4 Toughness of some composites. Toughened Polymethyl Unmodified Glass Epoxies methacrylate Epoxies materials used as matrices in advanced fiber