Continuous Aligned Fiber-Reinforced Composites

Schematic stress-strain behavior of aligned composites. It is assumed that the matrix is ductile and the fibers fail by brittle fracture. The yield strength of the composite is o-yc and its tensile strength is erre- 

As additional stress is applied, the matrix will continue to transfer load to the fibers as it yields plastically until the strain at fracture Cp of the fiber is reached. The tensile strength of the composite is given by 

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For a continuous and aligned fiber-reinforced composite, modulus of elasticity in the longitudinal direction... 

The superalloys, as well as alloys of aluminum, magnesium, titanium, and copper, are used as matrix materials. The reinforcement may be in the form of particulates, both continuous and discontinuous fibers, and whiskers concentrations normally range between 10 and 60 vol%. Continuous-fiber materials include carbon, silicon carbide, boron, aluminum oxide, and the refractory metals. However, discontinuous reinforcements consist primarily of silicon carbide whiskers, chopped fibers of aluminum oxide and carbon, or particulates of silicon carbide and aluminum oxide. In a sense, the cermets (Section 16.2) fall within this MMC scheme. Table 16.9 presents the properties of several common metal-matrix, continuous and aligned fiber-reinforced composites. 

Stress and strain relationships for laminates have been developed that are analogous to Equations 16.10 and 16.16 for continuous and aligned fiber-reinforced composites. However, these expressions use tensor algebra, which is beyond the scope of this discussion. 

We will see in Section 5.4.2 that the elastic modulus of a unidirectional, continuous-fiber-reinforced composite depends on whether the composite is tested along the direction of fiber orientation (parallel) or normal to the fiber direction (transverse). In fact, the elastic modulus parallel to the fibers, Ei, is given by Eq. (1.62), whereas the transverse modulus, 2, is given by Eq. (1.63). Consider a composite material that consists of 40% (by volume) continuous, uniaxially aligned, glass fibers (Ef =16 GPa) in a polyester matrix (Em = 3 GPa). 

The main functions of the matrix in a fiber-reinforced composite are to bind the fibers and to transfer loads to and between them only a small amount of the applied load is supported by the matrix. Let us consider a bunch of unidirectionally aligned continuous fibers subjected to a tensile stress. If a fiber breaks down, it becomes useless but if the fibers are embedded in a polymer matrix (see Fig. 15.1a), the load distributes around the break point and the fiber remains useful. Furthermore, the matrix protects the fibers from self-abrasion and scratches on handling, keeps the reinforcement in... 

As we have seen, the presence of fibers in the matrix has the effect of stiffening and strengthening it. The tensile deformation behavior of fiber-reinforced composites depends largely on the direction of the applied stress in relation to the orientation of the fibers, as illustrated in Figure 3.48. The maximum strength and modulus are achieved with unidirectional fiber reinforcement when the stress is aligned with the fibers (0°), but there is no enhancement of matrix properties when the stress is applied perpendicular to the fibers. With random orientation of fibers the properties of the composite are approximately the same in all directions, but the strength and modulus are somewhat less than for the continuous-fiber reinforcement. 

There are three basic types of engineered composites (1) laminates, (2) particle-reinforced composites, and (3) fiber-reinforced composites. In particle-reinforced composites, one can make the distinction between small (submicron) particle composites, where the particles are incorporated in the microstructure, vs. large particle composites, where the particles themselves actually do the work or carry the load. The reinforcing fibers can be discontinuous or continuous. The fibers in discontinuous fiber-reinforced composites can be randomly oriented to provide isotropic properties or aligned to enhance a specific property in a specific direction. Continuous fiber composites are generally designed for their unidirectional properties but can be crisscrossed to obtain multidirectional property enhancement such as in a filament-woimd pressure container. All possible permutations of metal, ceramic, and pol)uner are foimd in the laminated as well as in the reinforced composites. 

Both nature and man have made extensive use of composite materials in which two or more different materials are joined in such a manner that they maintain their identity but work together to add their strengths and decrease their weaknesses. Composites can be classified into three categories (1) Laminates, in which sheets of different materials are laminated together (2) particle-reinforced composites, in which particles of one material are imbedded in a matrix of a second material and (3) fiber-reinforced composites, in which fibers of one material are encapsulated in a matrix of a second material. Particle-reinforced composites can be subdivided into small particle composites, where the particles are incorporated into the microstructure, such as dispersion-hardened alloys, and large particle composites, where the matrix simply supports the particles. Fiber-reinforced composites may have continuous versus discontinuous fibers and aligned versus randomly oriented fibers, which can provide anisotropic versus isotropic properties. Composites combine all combinations of metals, ceramics, and polymers into MMCs, where a metal... 

Calculate longitudinal modulus and longitudinal strength for an aligned and continuous fiber-reinforced composite. 

Figure 16.8 Schematic representations of (a) continuous and aligned, (b) discontinuous and aligned, and (c) discontinuous and randomly oriented fiber-reinforced composites.

A continuous and aligned glass fiber-reinforced composite eonsists of 40 vol% glass fibers having a modulus of elasticity of 69 GPa (10 X 10 psi) and 60 vol% polyester resin that, when hardened, displays a modulus of 3.4 GPa (0.5 X 10 psi). 

Figure 5.97 Plot of composite tensile strength versus fiber volume fraction for an aligned, short fiber-reinforced polymer matrix composite. The dotted lines show the corresponding values for continuous fibers for comparison purposes. Reprinted, by permission, from N. G. McCrum, C. P. Buckley, and C. B. Bucknall, Principles of Polymer Engineering, 2nd ed., p. 281. Copyright 1997 by Oxford University Press.

Polymer matrix composites (PMCs), or fiber-reinforced plastics (FRPs). provide a wide range of properties and behavior. Materials with discontinuous fibers are slightly stiffer than conventional unreinforced plastics, whereas the fully aligned continuous fiber systems can record exceptionally high specific properties (property divided by density), exeeeding those of competing materials such as steel and aluminum. There are a virtually infinite number of materials, and material formats that can be combined to form a composite material, as shown in Table 1. 

Aveston, Mercer and Sillwood [71,72] derived crack theory for cement reinforced with continuous and short fibers and found that the tensile strength of the composite for the aligned fibers was ... 

Even though reinforcement efficiency is lower for discontinuous than for continuous fibers, discontinuous and aligned-fiber composites (Figure 16.8ii) are becoming increasingly more important in the commercial market. Chopped-glass fibers are used most extensively however, carbon and aramid discontinuous fibers are also used. These short-fiber composites can be produced with moduli of elasticity and tensile strengths that approach 90% and 50%, respectively, of their continuous-fiber counterparts. 

Consideration of orientation and fiber length for a particular composite depends on the level and nature of the applied stress as well as on the fabrication cost. Production rates for short-fiber composites (both aligned and randomly oriented) are rapid, and intricate shapes can be formed that are not possible with continuous fiber reinforcement. Furthermore, fabrication costs are considerably lower than for continuous and aligned fibers fabrication techniques applied to short-fiber composite materials include compression, injection, and extrusion molding, which are described for unreinforced polymers in Section 15.22. 

The properties of continuous and aligned glass, carbon, and aramid fiber-reinforced epoxy composites are given in Table 16.5. A comparison of the mechanical characteristics of these three materials may be made in both longitudinal and transverse directions. 

Table 16.9 Properties of Several Metal-Matrbc Composites Reinforced with Continuous and Aligned Fibers...

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