Fiber Reinforced Composites

Fiber Reinforced Composites

Fiber-reinforced composite materials consist of fibers of high strength and modulus embedded in or bonded to a matrix with distinct interfaces (boundaries) between them. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that cannot be achieved with either of the constituents acting alone. In general, fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired location and orientation, acts as a load transfer medium between them, and protects them from environmental damages due to elevated temperatures and humidity, for example. Thus, even though the fibers provide reinforcement for the matrix, the latter also serves a number of useful functions in a fiberreinforced composite material. The principal fibers in commercial use are various types of glass and carbon as well as Kevlar 49. Other fibers, such as boron, silicon carbide, and aluminum oxide, are used in limited quantities. All these fibers can be incorporated into a matrix either in continuous lengths or in discontinuous (short) lengths. The matrix material may be a polymer, a metal, or a ceramic. Various chemical compositions and microstructural arrangements are possible in each matrix category. The most common form in which fiber-reinforced composites are used in structural applications is called a laminate, which is made by stacking a number of thin layers of fibers and matrix and consolidating them into the desired thickness. Fiber orientation in each layer as well as the stacking sequence of various layers in a composite laminate can be controlled to generate a wide range of physical and mechanical properties for the composite laminate. In this book, we focus our attention on the mechanics, performance, manufacturing, and design of fiber-reinforced polymers. Most of the data presented in this book are related to continuous fiber-reinforced epoxy laminates, although other polymeric matrices, including thermoplastic matrices, are also considered. Metal and ceramic matrix composites are comparatively new, but significant developments of these composites have also occurred. They are included in a separate chapter in this book. Injection-molded or reaction injection-molded (RIM) discontinuous fiber-reinforced polymers are not discussed; however, some of the mechanics and design principles included in this book are applicable to these composites as well. Another material of great
2007 by Taylor & Francis Group, LLC. commercial interest is classified as particulate composites. The major constituents in these composites are particles of mica, silica, glass spheres, calcium carbonate, and others. In general, these particles do not contribute to the loadcarrying capacity of the material and act more like a filler than a reinforcement for the matrix. Particulate composites, by themselves, deserve a special attention and are not addressed in this book. Another type of composites that have the potential of becoming an important material in the future is the nanocomposites. Even though nanocomposites are in the early stages of development, they are now receiving a high degree of attention from academia as well as a large number of industries, including aerospace, automotive, and biomedical industries. The reinforcement in nanocomposites is either nanoparticles, nanofibers, or carbon nanotubes. The effective diameter of these reinforcementsis of the order of 10
9 m, whereasthe effective diameter of the reinforcements used in traditional fiber-reinforced composites is of the order of 10
6 m.

GENERAL CHARACTERISTICS

Many fiber-reinforced polymers offer a combination of strength and modulus that are either comparable to or better than many traditional metallic materials. Because of their low density, the strength–weight ratios and modulus–weight ratios of these composite materials are markedly superior to those of metallic materials (Table 1.1). In addition, fatigue strength as well as fatigue damage tolerance of many composite laminates are excellent. For these reasons, fiberreinforced polymers have emerged as a major class of structural materials and are either used or being considered for use as substitution for metals in many weight-critical components in aerospace, automotive, and other industries. Traditional structural metals, such as steel and aluminum alloys, are considered isotropic, since they exhibit equal or nearly equal properties irrespective of the direction of measurement. In general, the properties of a fiber-reinforced composite depend strongly on the direction of measurement, and therefore, they are not isotropic materials. For example, the tensile strength and modulus of a unidirectionally oriented fiber-reinforced polymer are maximum when these properties are measured in the longitudinal direction of fibers. At any other angle of measurement, these properties are lower. The minimum value is observed when they are measured in the transverse direction of fibers, that is, at 908 to the longitudinal direction. Similar angular dependence is observed for other mechanical and thermal properties, such as impact strength, coefficient of thermal expansion (CTE), and thermal conductivity. Bi- or multidirectional reinforcement yields a more balanced set of properties. Although these properties are lower than the longitudinal properties of a unidirectional composite, they still represent a considerable advantage over common structural metals on a unit weight basis. The design of a fiber-reinforced composite structure is considerably more difficult than that of a metal structure, principally due to the difference in its.

APPLICATIONS

Commercial and industrial applications of fiber-reinforced polymer composites are so varied that it is impossible to list them all. In this section, we highlight only the major structural application areas, which include aircraft, space, automotive, sporting goods, marine, and infrastructure. Fiber-reinforced polymer composites are also used in electronics (e.g., printed circuit boards), building construction (e.g., floor beams), furniture (e.g., chair springs), power industry (e.g., transformer housing), oil industry (e.g., offshore oil platforms and oil suckerrods used in lifting underground oil), medical industry (e.g., bone plates for fracture fixation, implants, and prosthetics), and in many industrial products, such as step ladders, oxygen tanks, and power transmission shafts. Potential use of fiber-reinforced composites exists in many engineering fields. Putting them to actual use requires careful design practice and appropriate process development based on the understanding of their unique mechanical, physical, and thermal characteristics.

AIRCRAFT AND MILITARY APPLICATIONS The major structural applications for fiber-reinforced composites are in the field of military and commercial aircrafts, for which weight reduction is critical for higher speeds and increased payloads. Ever since the production application of boron fiber-reinforced epoxy skins for F-14 horizontal stabilizers in 1969, the use of fiber-reinforced polymers has experienced a steady growth in the aircraft industry. With the introduction of carbon fibers in the 1970s, carbon fiber-reinforced epoxy has become the primary material in many wing, fuselage, and empennage components (Table 1.3). The structural integrity and durability of these early components have built up confidence in their performance and prompted developments of other structural aircraft components, resulting in an increasing amount of composites being used in military aircrafts. For example, the airframe of AV-8B, a vertical and short take-off and landing (VSTOL) aircraft introduced in 1982, contains nearly 25% by weight of carbon fiberreinforced epoxy. The F-22 fighter aircraft also contains ~25% by weight of carbon fiber-reinforced polymers; the other major materials are titanium (39%) and aluminum (16%). The outer skin of B-2 (Figure 1.1) and other stealth aircrafts is almost all made of carbon fiber-reinforced polymers. The stealth characteristics of these aircrafts are due to the use of carbon fibers, special coatings, and other design features that reduce radar reflection and heat radiation.

The composite applications on commercial aircrafts began with a few selective secondary structural components, all of which were made of a highstrength carbon fiber-reinforced epoxy (Table 1.4). They were designed and produced under the NASA Aircraft Energy Efficiency (ACEE) program and were installed in various airplanes during 1972–1986 [1]. By 1987, 350 composite components were placed in service in various commercial aircrafts, and over the next few years, they accumulated millions of flight hours. Periodic inspection and evaluation of these components showed some damages caused by ground handling accidents, foreign object impacts, and lightning strikes.

FIGURE 1.1 Stealth aircraft (note that the carbon fibers in the construction of the aircraft contributes to its stealth characteristics).

Apart from these damages, there was no degradation of residual strengths due to either fatigue or environmental exposure. A good correlation was found between the on-ground environmental test program and the performance of the composite components after flight exposure. Airbus was the first commercial aircraft manufacturer to make extensive use of composites in their A310 aircraft, which was introduced in 1987. The composite components weighed about 10% of the aircraft’s weight and included such components as the lower access panels and top panels of the wing leading edge, outer deflector doors, nose wheel doors, main wheel leg fairing doors, engine cowling panels, elevators and fin box, leading and trailing edges of fins, flap track fairings, flap access doors, rear and forward wing–body fairings, pylon fairings, nose radome, cooling air inlet fairings, tail leading edges, upper surface skin panels above the main wheel bay, glide slope antenna cover, and rudder. The composite vertical stabilizer, which is 8.3 m high by 7.8 m wide at the base, is about 400 kg lighter than the aluminum vertical stabilizer previously used [2]. The Airbus A320, introduced in 1988, was the first commercial aircraft to use an all-composite tail, which includes the tail cone, vertical stabilizer, and horizontal stabilizer. Figure 1.2 schematically shows the composite usage in Airbus A380 introduced in 2006. About 25% of its weight is made of composites. Among the major composite components in A380 are the central torsion box (which links the left and right wings under the fuselage), rear-pressure bulkhead (a dome-shaped partition that separates the passenger cabin from the rear part of the plane that is not pressurized), the tail, and the flight control surfaces, such as the flaps, spoilers, and ailerons.

Starting with Boeing 777, which was first introduced in 1995, Boeing has started making use of composites in the empennage (which include horizontal stabilizer, vertical stabilizer, elevator, and rudder), most of the control surfaces, engine cowlings, and fuselage floor beams (Figure 1.3). About 10% of Boeing 777’s structural weight is made of carbon fiber-reinforced epoxy and about 50% is made of aluminum alloys. About 50% of the structural weight of Boeing’s

next line of airplanes, called the Boeing 787 Dreamliner, will be made of carbon fiber-reinforced polymers. The other major materials in Boeing 787 will be aluminum alloys (20%), titanium alloys (15%), and steel (10%). Two of the major composite components in 787 will be the fuselage and the forward section, both of which will use carbon fiber-reinforced epoxy as the major material of construction. There are several pioneering examples of using larger quantities of composite materials in smaller aircrafts. One of these examples is the Lear Fan 2100, a business aircraft built in 1983, in which carbon fiber–epoxy and Kevlar 49 fiber–epoxy accounted for ~70% of the aircraft’s airframe weight. The composite components in this aircraft included wing skins, main spar, fuselage, empennage, and various control surfaces [3]. Another example is the Rutan Voyager (Figure 1.4), which was an all-composite airplane and made the firstever nonstop flight around the world in 1986. To travel 25,000 miles without refueling, the Voyager airplane had to be extremely light and contain as much fuel as needed. Fiber-reinforced polymers are used in many military and commercial helicopters for making baggage doors, fairings, vertical fins, tail rotor spars, and so on. One key helicopter application of composite materials is the rotor blades. Carbon or glass fiber-reinforced epoxy is used in this application. In addition to significant weight reduction over aluminum, they provide a better control over the vibration characteristics of the blades. With aluminum, the critical flopping

FIGURE 1.4 Rutan Voyager all-composite plane

and twisting frequencies are controlled principally by the classical method of mass distribution [4]. With fiber-reinforced polymers, they can also be controlled by varying the type, concentration, distribution, as well as orientation of fibers along the blade’s chord length. Another advantage of using fiberreinforced polymers in blade applications is the manufacturing flexibility of these materials. The composite blades can be filament-wound or molded into complex airfoil shapes with little or no additional manufacturing costs, but conventional metal blades are limited to shapes that can only be extruded, machined, or rolled. The principal reason for using fiber-reinforced polymers in aircraft and helicopter applications is weight saving, which can lead to significant fuel saving and increase in payload. There are several other advantages of using them over aluminum and titanium alloys. 1. Reduction in the number of components and fasteners, which results in a reduction of fabrication and assembly costs. For example, the vertical fin assembly of the Lockheed L-1011 has 72% fewer components and 83% fewer fasteners when it is made of carbon fiber-reinforced epoxy than when it is made of aluminum. The total weight saving is 25.2%. 2. Higher fatigue resistance and corrosion resistance, which result in a reduction of maintenance and repair costs. For example, metal fins used in helicopters flying near ocean coasts use an 18 month repair cycle for patching corrosion pits. After a few years in service, the patches can add enough weight to the fins to cause a shift in the center of gravity of the helicopter, and therefore the fin must then be rebuilt or replaced. The carbon fiber-reinforced epoxy fins do not require any repair for corrosion, and therefore, the rebuilding or replacement cost is eliminated. 3. The laminated construction used with fiber-reinforced polymers allows the possibility of aeroelastically tailoring the stiffness of the airframe structure. For example, the airfoil shape of an aircraft wing can be controlled by appropriately adjusting the fiber orientation angle in each lamina and the stacking sequence to resist the varying lift and drag loads along its span. This produces a more favorable airfoil shape and enhances the aerodynamic characteristics critical to the aircraft’s maneuverability. The key limiting factors in using carbon fiber-reinforced epoxy in aircraft structures are their high cost, relatively low impact damage tolerance (from bird strikes, tool drop, etc.), and susceptibility to lightning damage. When they are used in contact with aluminum or titanium, they can induce galvanic corrosion in the metal components. The protection of the metal components from corrosion can be achieved by coating the contacting surfaces with a corrosion-inhibiting paint, but it is an additional cost.

Reference: Fiber-Reinforced Composites: Materials, Manufacturing, and Design, Third EditionP.K. Mallick