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Chapter 1

Introduction to Composite Materials

A composite material can be defined sheets of continuous fibers in different orienta- as a combination of two or more materials that tions to obtain the desired strength and stiffness results in better properties than those of the indi- properties with fiber volumes as high as 60 to vidual components used alone. In contrast to 70 percent. Fibers produce high-strength com- metallic alloys, each material retains its separate posites because of their small diameter; they con- chemical, physical, and mechanical properties. tain far fewer defects (normally surface defects) The two constituents are a reinforcement and a compared to the material produced in bulk. As a matrix. The main advantages of composite ma- general rule, the smaller the diameter of the fiber, terials are their high strength and stiffness, com- the higher its strength, but often the cost increases bined with low density, when compared with as the diameter becomes smaller. In addition, bulk materials, allowing for a weight reduction smaller-diameter high-strength fibers have greater in the finished part. flexibility and are more amenable to fabrication The reinforcing phase provides the strength processes such as weaving or forming over radii. and stiffness. In most cases, the reinforcement is Typical fibers include glass, aramid, and carbon, harder, stronger, and stiffer than the matrix. The which may be continuous or discontinuous. reinforcement is usually a fiber or a particulate. The continuous phase is the matrix, which is a Particulate composites have dimensions that are polymer, metal, or ceramic. Polymers have low approximately equal in all directions. They may strength and stiffness, metals have intermediate be spherical, platelets, or any other regular or ir- strength and stiffness but high ductility, and ce- regular geometry. Particulate composites tend to ramics have high strength and stiffness but are be much weaker and less stiff than continuous- brittle. The matrix (continuous phase) performs fiber composites, but they are usually much less several critical functions, including maintaining expensive. Particulate reinforced composites usu- the fibers in the proper orientation and spacing ally contain less reinforcement (up to 40 to 50 and protecting them from abrasion and the envi- volume percent) due to processing difficulties ronment. In polymer and metal matrix compos- and brittleness. ites that form a strong bond between the fiber A fiber has a length that is much greater than and the matrix, the matrix transmits loads from its diameter. The length-to-diameter (l/d) ratio is the matrix to the fibers through shear loading at known as the aspect ratio and can vary greatly. the interface. In ceramic matrix composites, the Continuous fibers have long aspect ratios, while objective is often to increase the toughness rather discontinuous fibers have short aspect ratios. than the strength and stiffness; therefore, a low Continuous-fiber composites normally have a interfacial strength bond is desirable. preferred orientation, while discontinuous fibers The type and quantity of the reinforcement generally have a random orientation. Examples determine the final properties. Figure 1.2 shows of continuous reinforcements include unidirec- that the highest strength and modulus are ob- tional, woven cloth, and helical winding (Fig. tained with continuous-fiber composites. There is 1.1a), while examples of discontinuous rein- a practical limit of about 70 volume percent reinforcements are chopped fibers and random mat forcement that can be added to form a composite. (Fig. 1.1b). Continuous-fiber composites are At higher percentages, there is too little matrix to often made into laminates by stacking single support the fibers effectively. The theoretical 2 / Structural Composite Materials

Fig. 1.1 Typical reinforcement types strength of discontinuous-fiber composites can a low-viscosity resin that reacts and cures during approach that of continuous-fiber composites processing, forming an intractable solid. A ther- if their aspect ratios are great enough and they moplastic is a high-viscosity resin that is pro- are aligned, but it is difficult in practice to main- cessed by heating it above its melting tempera- tain good alignment with discontinuous fibers. ture. Because a thermoset resin sets up and cures Discontinuous-fiber composites are normally during processing, it cannot be reprocessed by somewhat random in alignment, which dramati- reheating. By comparison, a thermoplastic can cally reduces their strength and modulus. How- be reheated above its melting temperature for ad- ever, discontinuous-fiber composites are gen ditional processing. There are processes for both erally much less costly than continuous-fiber classes of resins that are more amenable to dis- composites. Therefore, continuous-fiber com- continuous fibers and others that are more ame- posites are used where higher strength and stiff- nable to continuous fibers. In general, because ness are required (but at a higher cost), and metal and ceramic matrix composites require discontinuous-fiber composites are used where very high temperatures and sometimes high pres- cost is the main driver and strength and stiffness sures for processing, they are normally much are less important. more expensive than polymer matrix composites. Both the reinforcement type and the matrix af- However, they have much better thermal stabil- fect processing. The major processing routes for ity, a requirement in applications where the com- polymer matrix composites are shown in Fig. 1.3. posite is exposed to high temperatures. Two types of polymer matrices are shown: ther- This book will deal with both continuous and mosets and thermoplastics. A thermoset starts as discontinuous polymer, metal, and ceramic matrix Chapter 1: Introduction to Composite Materials / 3

Fig. 1.2 Influence of reinforcement type and quantity on composite performance

Fig. 1.3 Major polymer matrix composite fabrication processes 4 / Structural Composite Materials

composites, with an emphasis on continuous- material is anisotropic (for example, the compos- fiber, high-performance polymer composites. ite ply shown in Fig. 1.5), it has properties that vary with direction within the material. In this example, the moduli are different in each direction (E0° ≠ E45° ≠ E90°). While the modulus of 1.1 Isotropic, Anisotropic, and elasticity is used in the example, the same depen- Orthotropic Materials dence on direction can occur for other material properties, such as ultimate strength, Poisson’s Materials can be classified as either isotropic ratio, and thermal expansion coefficient. or anisotropic. Isotropic materials have the same Bulk materials, such as metals and polymers, material properties in all directions, and normal are normally treated as isotropic materials, while loads create only normal strains. By compari- composites are treated as anisotropic. However, son, anisotropic materials have different mate- even bulk materials such as metals can become rial properties in all directions at a point in the anisotropic––for example, if they are highly cold body. There are no material planes of symmetry, worked to produce grain alignment in a certain and normal loads create both normal strains and direction. shear strains. A material is isotropic if the prop- Consider the unidirectional fiber-reinforced erties are independent of direction within the composite ply (also known as a lamina) shown material. in Fig. 1.6. The coordinate system used to de- For example, consider the element of an iso- scribe the ply is labeled the 1-2-3 axes. In this tropic material shown in Fig. 1.4. If the material case, the 1-axis is defined to be parallel to the is loaded along its 0°, 45°, and 90° directions, fibers (0°), the 2-axis is defined to lie within the the modulus of elasticity (E) is the same in each plane of the plate and is perpendicular to the fi- direction (E0° = E45° = E90°). However, if the bers (90°), and the 3-axis is defined to be normal

Fig. 1.4 Element of isotropic material under stress Chapter 1: Introduction to Composite Materials / 5

Fig. 1.5 Element of composite ply material under stress

Fig. 1.6 Ply angle definition 6 / Structural Composite Materials

to the plane of the plate. The 1-2-3 coordinate Consider the unidirectional composite shown system is referred to as the principal material in the upper portion of Fig. 1.7, where the unidi- coordinate system. If the plate is loaded parallel rectional fibers are oriented at an angle of 45 de- to the fibers (one- or zero-degree direction), the grees with respect to the x-axis. In the small, modulus of elasticity E11 approaches that of the isolated square element from the gage region, be- fibers. If the plate is loaded perpendicular to cause the element is initially square (in this ex- the fibers in the two- or 90-degree direction, the ample), the fibers are parallel to diagonal AD of modulus E22 is much lower, approaching that of the element. In contrast, fibers are perpendicular the relatively less stiff matrix. Since E11 >> E22 to diagonal BC. This implies that the element is and the modulus varies with direction within the stiffer along diagonal AD than along diagonal material, the material is anisotropic. BC. When a tensile stress is applied, the square Composites are a subclass of anisotropic mate- element deforms. Because the stiffness is higher rials that are classified as orthotropic. Ortho- along diagonal AD than along diagonal BC, the tropic materials have properties that are different length of diagonal AD is not increased as much in three mutually perpendicular directions. They as that of diagonal BC. Therefore, the initially have three mutually perpendicular axes of sym- square element deforms into the shape of a par- metry, and a load applied parallel to these axes allelogram. Because the element has been dis- produces only normal strains. However, loads torted into a parallelogram, a shear strain gxy is that are not applied parallel to these axes produce induced as a result of coupling between the axial both normal and shear strains. Therefore, ortho- strains exx and eyy. tropic mechanical properties are a function of If the fibers are aligned parallel to the direc- orientation. tion of applied stress, as in the lower portion of

Fig. 1.7 Shear coupling in a 45° ply. Source: Ref 1 Chapter 1: Introduction to Composite Materials / 7

Fig. 1.7, the coupling between exx and eyy does ites are normally laminated materials (Fig. 1.8) not occur. In this case, the application of a ten- in which the individual layers, plies, or laminae sile stress produces elongation in the x-direction are oriented in directions that will enhance the and contraction in the y-direction, and the dis- strength in the primary load direction. Unidirec- torted element remains rectangular. Therefore, tional (0°) laminae are extremely strong and stiff the coupling effects exhibited by composites occur in the 0° direction. However, they are very weak only if stress and strain are referenced to a non– in the 90° direction because the load must be car- principal material coordinate system. Thus, when ried by the much weaker polymeric matrix. the fibers are aligned parallel (0°) or perpendic- While a high-strength fiber can have a tensile ular (90°) to the direction of applied stress, the strength of 500 ksi (3500 MPa) or more, a typical lamina is known as a specially orthotropic lam- polymeric matrix normally has a tensile strength ina (θ = 0° or 90°). A lamina that is not aligned of only 5 to 10 ksi (35 to 70 MPa) (Fig. 1.9). The parallel or perpendicular to the direction of ap- longitudinal tension and compression loads are plied stress is called a general orthotropic lam- carried by the fibers, while the matrix distributes ina (θ ≠ 0° or 90°). the loads between the fibers in tension and stabi- lizes the fibers and prevents them from buckling in compression. The matrix is also the primary 1.2 Laminates load carrier for interlaminar shear (i.e., shear between the layers) and transverse (90°) tension. When there is a single ply or a lay-up in which The relative roles of the fiber and the matrix in all of the layers or plies are stacked in the same detemining mechanical properties are summa- orientation, the lay-up is called a lamina. When rized in Table 1.1. the plies are stacked at various angles, the lay-up Because the fiber orientation directly impacts is called a laminate. Continuous-fiber compos- mechanical properties, it seems logical to orient

Fig. 1.8 Lamina and laminate lay-ups 8 / Structural Composite Materials

Fig. 1.9 Comparison of tensile properties of fiber, matrix, and composite

Table 1.1 effect of fiber and matrix on 1.3 Fundamental Property Relationships mechanical properties Dominating composite constituent When a unidirectional continuous-fiber lam- Mechanical property Fiber Matrix ina or laminate (Fig. 1.11) is loaded in a di Unidirectional rection parallel to its fibers (0° or 11-direction), 0º tension √ … 0º compression √ √ the longitudinal modulus E11 can be estimated Shear … √ from its constituent properties by using what is 90º tension … √ known as the rule of mixtures: Laminate … Tension √ E E V E V (Eq 1.1) Compression √ √ 11 = f f + m m In-plane shear √ √ … Interlaminar shear √ where Ef is the fiber modulus, Vf is the fiber volume percentage, Em is the matrix modulus, and Vm is the matrix volume percentage. The longitudinal tensile strength s11 also can as many of the layers as possible in the main be estimated by the rule of mixtures: load-carrying direction. While this approach may work for some structures, it is usually nec- s11 = sVf + smVm (Eq 1.2) essary to balance the load-carrying capability in a number of different directions, such as the where sf and sm are the ultimate fiber and ma- 0°, +45°, -45°, and 90° directions. Figure 1.10 trix strengths, respectively. Because the proper- shows a photomicrograph of a cross-plied con- ties of the fiber dominate for all practical vol- tinuous carbon fiber/epoxy laminate. A balanced ume percentages, the values of the matrix can laminate having equal numbers of plies in the often be ignored; therefore: 0°, +45°, –45°, and 90° degrees directions is E E V (Eq 1.3) called a quasi-isotropic laminate, because it car- 11 ≈ f f ries equal loads in all four directions. s11 ≈ sVf (Eq 1.4) Chapter 1: Introduction to Composite Materials / 9

Fig. 1.10 Cross section of a cross-plied carbon/epoxy laminate

Fig. 1.11 Unidirectional continuous-fiber lamina or laminate 10 / Structural Composite Materials

Figure 1.12 shows the dominant role of the fi- n12 = nfVf + nmVm (Eq 1.6) bers in determining strength and stiffness. When 1/G12 = Vf /Gf + Vm/Gm (Eq 1.7) loads are parallel to the fibers (0°), the ply is much stronger and stiffer than when loads are These expressions are somewhat less useful transverse (90°) to the fiber direction. There is a than the previous ones, because the values for dramatic decrease in strength and stiffness re- Poisson’s ratio (nf) and the shear modulus (Gf) sulting from only a few degrees of misalignment of the fibers are usually not readily available. off of 0°. Physical properties, such as density (r), can When the lamina shown in Fig. 1.11 is loaded also be expressed using rule of mixture relations: in the transverse (90° or 22-direction), the fibers and the matrix function in series, with both car- r = r V + r V (Eq 1.8) rying the same load. The transverse modulus of 12 f f m m elasticity E22 is given as: While these micromechanics equations are useful for a first estimation of lamina properties 1/E22 = Vf /Ef + Vm/Em (Eq 1.5) when no data are available, they generally do not yield sufficiently accurate values for design pur- Figure 1.13 shows the variation of modulus as poses. For design purposes, basic lamina and a function of fiber volume percentage. When the laminate properties should be determined using fiber percentage is zero, the modulus is essen- actual mechanical property testing. tially the modulus of the polymer, which increases up to 100 percent (where it is the modulus of the fiber). At all other fiber volumes, the 1.4 Composites versus Metallics E22 or 90° modulus is lower than the E11 or zero degrees modulus, because it is dependent on the As previously discussed, the physical character- much weaker matrix. istics of composites and metals are significantly Other rule of mixture expressions for lamina different. Table 1.2 compares some properties of properties include those for the Poisson’s ratio composites and metals. Because composites are n12 and for the shear modulus G12: highly anisotropic, their in-plane strength and

Fig. 1.12 Influence of ply angle on strength and modulus Chapter 1: Introduction to Composite Materials / 11

Fig. 1.13 Variation of composite modulus of a unidirectional 0° lamina as a function of fiber volume fraction

Table 1.2 Composites versus metals Metals typically have reasonable ductility, con- comparison tinuing to elongate or compress considerably Condition Comparative behavior relative to metals when they reach a certain load (through yielding) Load-strain relationship More linear strain to failure without picking up more load and without fail- Notch sensitivity ure. Two important benefits of this ductile yield- Static Greater sensitivity Fatigue Less sensitivity ing are that (1) it provides for local load relief by Transverse properties Weaker distributing excess load to an adjacent material Mechanical property Higher variability or structure; therefore, ductile metals have a great Fatigue strength Higher capacity to provide relief from stress concentra- Sensitivity to hydrothermal Greater tions when statically loaded; and (2) it provides environment Sensitivity to corrosion Much less great energy-absorbing capability (indicated by Damage growth mechanism In-plane delamination instead of the area under a stress-strain curve). As a result, through thickness cracks when impacted, a metal structure typically de- Source: Ref 2 forms but does not actually fracture. In contrast, composites are relatively brittle. Figure 1.15 shows a comparison of typical tensile stress-strain curves for two materials. The brittleness of the stiffness are usually high and directionally vari- composite is reflected in its poor ability to toler- able, depending on the orientation of the rein- ate stress concentrations, as shown in Fig. 1.16. forcing fibers. Properties that do not benefit from The characteristically brittle composite material this reinforcement (at least for polymer matrix has poor ability to resist impact damage without composites) are comparatively low in strength extensive internal matrix fracturing. and stiffness—for example, the through-the- The response of damaged composites to cyclic thickness tensile strength where the relatively loading is also significantly different from that of weak matrix is loaded rather than the high- metals. The ability of composites to withstand strength fibers. Figure 1.14 shows the low cyclic loading is far superior to that of metals, in through-the-thickness strength of a typical com- contrast to the poor composite static strength posite laminate compared with aluminum. when it has damage or defects. Figure 1.17 12 / Structural Composite Materials

Fig. 1.14 Comparison of through-the-thickness tensile strength of a composite laminate with aluminum alloy sheet. Source: Ref 3

Fig. 1.15 Comparison of typical stress-strain curves for a composite laminate and aluminum alloy sheet. Source: Ref 3 Chapter 1: Introduction to Composite Materials / 13

Fig. 1.16 Compared with aluminum alloy sheet, a composite laminate has poor tolerance of stress concentration because of its brittle nature. Source: Ref 3

Fig. 1.17 Comparative notched fatigue strength of composite laminate and aluminum alloy sheet. Source: Ref 3 14 / Structural Composite Materials

shows a comparison of the normalized notched The specific strength (strength/density) and specimen fatigue response of a common 7075- specific modulus (modulus/density) of high- T6 aluminum aircraft metal and a carbon/epoxy strength fibers (especially carbon) are higher laminate. The fatigue strength of the composite than those of other comparable aerospace metal- is much higher relative to its static or residual lic alloys (Fig. 1.18). This translates into greater strength. The static or residual strength require- weight savings resulting in improved perfor- ment for structures is typically much higher than mance, greater payloads, longer range, and fuel the fatigue requirement. Therefore, because the savings. Figure 1.19 compares the overall struc- fatigue threshold of composites is a high percent- tural efficiency of carbon/epoxy, Ti-6Al-4V, and age of their static or damaged residual strength, 7075-T6 aluminum. they are usually not fatigue critical. In metal The chief engineer of aircraft structures for structures, fatigue is typically a critical design the U.S. Navy once told the author that he liked consideration. composites because “they don’t rot [corrode] and they don’t get tired [fatigue].” Corrosion of aluminum alloys is a major cost and a con- 1.5 Advantages and Disadvantages of stant maintenance problem for both commer- Composite Materials cial and military aircraft. The corrosion resistance of composites can result in major savings T he advantages of composites are many, in- in supportability costs. Carbon fiber composites cluding lighter weight, the ability to tailor the lay- cause galvanic corrosion of aluminum if the fi- up for optimum strength and stiffness, improved bers are placed in direct contact with the metal fatigue life, corrosion resistance, and, with good surface, but bonding a glass fabric electrical design practice, reduced assembly costs due to insulation layer on all interfaces that contact fewer detail parts and fasteners. aluminum eliminates this problem. The fatigue

Fig. 1.18 Comparison of specific strength and modulus of high-strength composites and some aerospace alloys Chapter 1: Introduction to Composite Materials / 15

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 (Tg) 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 Tg 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 16 / Structural Composite Materials

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 Tg, the actual use tempera- cal properties and causes the matrix to swell, ture should be about 50 ºF (30 ºC) lower than the which relieves locked-in thermal strains from wet or saturated Tg. elevated-temperature curing. These strains can be large, and large panels fixed at their edges

Upper Use Temperature = Wet Tg – 50 °F (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 (Tg) 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, Chapter 1: Introduction to Composite Materials / 17

Fig. 1.21 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 18 / Structural Composite Materials

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 Chapter 1: Introduction to Composite Materials / 19

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. 20 / Structural Composite Materials

Fig. 1.24 Delaminations and matrix cracking in polymer matrix composite due to impact damage

T he major automakers (Fig. 1.28) are in- For high-performance Formula 1 racing cars, creasingly turning to composites to help them where cost is not an impediment, most of the meet performance and weight requirements, chassis, including the monocoque, suspension, thus improving fuel efficiency. Cost is a major wings, and engine cover, is made from carbon driver for commercial transportation, and com- fiber composites. posites offer lower weight and lower mainte- Corrosion is a major headache and expense for nance costs. Typical materials are fiberglass/ the marine industry. Composites help minimize polyurethane made by liquid or compression these problems, primarily because they do not molding and fiberglass/ polyester made by corrode like metals or rot like wood. Hulls of compression molding. Recreational vehicles boats ranging from small fishing boats to large have long used glass fibers, mostly for their du- racing yachts (Fig. 1.29) are routinely made of rability and weight savings over metal. The glass fibers and polyester or vinyl ester resins. product form is typically fiberglass sheet mold- Masts are frequently fabricated from carbon fiber ing compound made by compression molding. composites. Fiberglass filament-wound SCUBA Chapter 1: Introduction to Composite Materials / 21

Fig. 1.25 Typical fighter aircraft applications. Source: The Boeing Company

tanks are another example of composites im- In the United States alone, it is estimated that proving the marine industry. Lighter tanks can more than 250,000 structures, such as bridges hold more air yet require less maintenance than and parking garages, need repair, retrofit, or re- their metallic counterparts. Jet skis and boat trail- placement. Composites offer much longer life ers often contain glass composites to help mini- with less maintenance due to their corrosion re- mize weight and reduce corrosion. More resistance. Typical processes/materials include wet cently, the topside structures of many naval ships lay-up repairs and corrosion-resistant fiberglass have been fabricated from composites. pultruded products. Using composites to improve the infrastruc- In construction (Fig. 1.31), pultruded fiber- ture (Fig. 1.30) of our roads and bridges is a rela- glass rebar is used to strengthen concrete, and tively new, exciting application. Many of the glass fibers are used in some shingling materials. world’s roads and bridges are badly corroded and With the number of mature tall trees dwindling, in need of continual maintenance or replacement. the use of composites for electrical towers and 22 / Structural Composite Materials

Fig. 1.26 Boeing 787 Dreamliner commercial airplane. Source: The Boeing Company

Fig. 1.27 Launch and spacecraft structures Chapter 1: Introduction to Composite Materials / 23

Fig. 1.28 Transportation applications

light poles is greatly increasing. Typically, these improve electrical energy generation efficiency. are pultruded or filament-wound glass. These blades can be as long as 120 ft (37 m) and Wind power is the world’s fastest-growing en- weigh up to 11,500 lb (5200 kg). In 2007, nearly ergy source. The blades for large wind turbines 50,000 blades for 17,000 turbines were deliv- (Fig. 1.32) are normally made of composites to ered, representing roughly 400 million pounds 24 / Structural Composite Materials

Fig. 1.29 Marine applications

(approximately 180 million kg) of composites. Tennis racquets (Fig. 1.33) have been made of The predominant material is continuous glass glass for years, and many golf club shafts are fibers manufactured by either lay-up or resin made of carbon. Processes include compression infusion. molding for tennis racquets and tape wrapping or Chapter 1: Introduction to Composite Materials / 25

Fig. 1.30 Infrastructure applications

filament winding for golf shafts. Lighter, stron- forming is a snowboard, which typically involves ger skis and surfboards also are possible using the use of a sandwich construction (composite composites. Another example of a composite ap- skins with a honeycomb core) for maximum spe- plication that takes a beating yet keeps on per- cific stiffness. 26 / Structural Composite Materials

With the number of mature tall trees dwindling, the use of composites for electrical towers and light poles is greatly increasing.

Fig. 1.31 Construction applications

Although metal and ceramic matrix com are used where high temperatures are involved. posites are normally very expensive, they have However, the much higher temperatures and found uses in specialized applications such pressures required for the fabrication of as those shown in Fig. 1.34. Frequently, they metal and ceramic matrix composites lead Chapter 1: Introduction to Composite Materials / 27

Fig. 1.32 Composite clean energy generation

to very high costs, which severely limits their tance. They are used in aerospace, automotive, application. marine, sporting goods, and, more recently, infra- Composites are not always the best solution. structure applications. The major disadvantage An example is the avionics rack for an ad- of composites is their high cost. However, the vanced fighter aircraft shown in Fig. 1.35. This proper selection of materials (fiber and matrix), part was machined from a single block of alu- product forms, and processes can have a major minum in about 8.5 hours and assembled into impact on the cost of the finished part. the final component in five hours. Such a part made of composites would probably not be cost competitive. References Advanced composites are a diversified and growing industry due to their distinct advantages 1. M.E. Tuttle, Structural Analysis of Poly- over competing metallics, including lighter meric Composite Materials, Marcel Dekker, weight, higher performance, and corrosion resis- Inc., 2004 28 / Structural Composite Materials

Fig. 1.33 Sporting goods applications

Fig. 1.34 Metal and ceramic matrix composite applications Chapter 1: Introduction to Composite Materials / 29

Fig. 1.35 Composites are not always the best choice. This avionics rack machined from an aluminum alloy block would not be cost-competitive if made of composites. Source: The Boeing Company

2. M.C.Y. Niu, Composite Airframe Struc- Selected References tures, 2nd ed., Hong Kong Conmilit Press • Limited, 2000 High-Performance Composites Sourcebook 3. r.E. Horton and J.E. McCarty, Damage 2009, Gardner Publications Inc Tolerance of Composites, Engineered Ma- • S.K. Mazumdar, Composites Manufacturing: terials Handbook, Vol 1, Composites, ASM Materials, Product, and Process Engineer- International, 1987 ing, CRC Press, 2002