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Unformatted text preview: fibers. Boron fibers are used for stiffening aluminum matrices. Since boron is inherently brittle, it is chemically deposited on a tungsten (W) wire or a carbon-coated glass fiber. The fibers containing the W filament are expensive, but have superior properties compared with glass-filament fibers. Since the chemical vapor deposition process (discussed in Chapter 16) often results in surface defects, the surface of boron fibers is polished to remove defects and is also coated with a thin layer of SiC to produce compressive surface stresses and pacify the surface, as shown in Figure 14.3–4. The surface treatments considerably enhance the fracture strength of B fibers; however, they also add to their cost. Properties of boron fibers are given in Table 14.3–1. Carbon fibers have a highly distorted and defective graphitic-like structure. The atoms within the basal planes are covalently bonded. Parallel basal planes are joined together by | v v 586 iq | e-Text Main Menu | Textbook Table of Contents pg587 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 Composite Materials σθ (+) 0.7 ... 0.8 kN-mm–2 r (–) 0.3 ... 0.5 kN-mm–2 SiC jacket (–) 1.0 ... 1.4 kN-mm–2 Tungsten core σθ Tungsten-boride intermetallic layer Boron fiber Core (a) σr Mantle (b) FIGURE 14.3–4 Schematic cross sections of a boron fiber. (a) The fiber is a composite consisting of a series of concentric layers. (b) The residual stress pattern across a section of the boron fiber. In this view the intermetallic layer and SiC jacket are not shown. (Source: K. K. Chawla, Composites Material Science and Engineering, 1987, SpringerVerlag, New York, Reprinted with permission of Springer-Verlag, New York Publishers.) | v v weak van der Waals bonds. The bonds perpendicular to the basal planes are secondary bonds. Carbon fibers are produced so that the basal plane lies along the fiber axis. Therefore, the elastic modulus along the fiber axis can be as high as 1000 GPa, as shown in Figure 14.3–3b. In the transverse direction (i.e., the direction perpendicular to the fiber axis), the modulus can be as low as 35 GPa. The mechanical, chemical, electrical, and other physical properties of carbon fibers vary widely depending on the raw material and the processing treatment. The raw material for carbon fiber is called a precursor and consists of a fiber itself. While practically any fiber can be transformed with heat into carbon fibers, only a few precursors are commercially viable. Polyacrylonitrile may be used to make high-modulus, high-strength fibers using pyrolysis, which is described in Chapter 16. In addition, carbon fibers are made on a commercial scale using pitch. Pitch is a product of oil refining (asphalt pitch) or of combining coal with appropriate fluids (coal tar pitch). The pitch is converted into a liquid crystalline material, melt-spun into fibers, and then pyrolyzed. The first synthetic polymer reinforcing fibers were nylon and polyester. These fibers have reasonable strength and modulus, giving good toughness. They are...
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This note was uploaded on 02/25/2013 for the course PHYS 2202 taught by Professor Sowell during the Spring '10 term at Georgia Institute of Technology.

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