Since exibility is the inverse of bending stiffness

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Unformatted text preview: nd flexibility all increase. ....................................................................................................................................... EXAMPLE 14.3–1 Flexible nylon fibers, with E 3 GPa and diameter 25 m, can be easily wound on spools. What must be the diameter of the following fibers (which have circular cross section) to show the same flexibility? a. b. c. d. SiC Pitch-based C fibers Kevlar-49 Boron Solution In Example 14.2–1, we used the result that the bending stiffness of a circular fiber is equal to EI d 4 64. Since flexibility is the inverse of bending stiffness, the values of 1 EI , or, with I equivalently, the EI product itself, for the various fibers must be equal. The elastic moduli for the fibers are listed in Table 14.3–1. Using these data we find: EI nylon 25 m 64 3 GPa 4 and EI SiC d SiC 3 430 d4 64 430 GPa Hence, 14 25 m 7.2 m Similarly, 3 140 14 25 m 9.5 m d Kevlar-49 3 131 14 25 m 9.7 m d boron 3 400 14 25 m 7.3 m d pitch-based C and | v v ....................................................................................................................................... | e-Text Main Menu | Textbook Table of Contents 12.01.98 plm QC2 rps MP 585 pg586 [V] G2 7-27060 / IRWIN / Schaffer Part III 12.01.98 plm QC2 rps MP MRP Properties TABLE 14.3–1 Properties of typical fibers used in composite materials. Density (g/cm3) Fibers E-glass S-glass PAN-based C-fiber Pitch-based C-fiber Single-crystal graphite Kevlar-49 Kevlar-149 Spectra (polyethylene) Boron FP (alumina) SiC particles SiC whiskers SiC fibers Stainless steel Tungsten Molybdenum Note: CTE Elastic modulus (GPa) Tensile strength (MPa) Axial CTE ( C 1) 2.6 2.5 1.7–1.9 72 87 230–370 1.7 2.5 1.8 10 3 10 3 10 3 5 5.6 0.5 10 10 10 6 1.6–1.8 41–140 1.4 10 3 0.9 10 6 2.25 1000 20.6 10 3 — 1.44 1.47 0.97 131 186 117 3.8 3.4 2.6 10 3 10 3 10 3 — — — 2.5 3.9 3.3 3.5 2.6–3.3 8.0 19.3 10.2 400 379 430 580 180–430 198 360 310 2.8 1.38 3.5 8 2–3.5 0.7–1.0 3.8 2.45 10 3 10 3 10 3 10 3 10 3 10 3 10 3 10 3 10 10 10 10 10 10 10 10 6 4.9 6.7 4.9 4.9 4.9 18 11.6 6.0 6 6 6 6 6 6 6 6 6 c oefficient of thermal expansion. Attractive fibers for composite reinforcement must have high strength and high elastic modulus. They must also be suitable for production in small diameters. Fibers have been successfully fabricated from metals, ceramics, and polymers. Properties of some common fiber materials are discussed in this section; the methods used for producing fibers are discussed in Chapter 16. The fiber properties tabulated include tensile strength, elastic modulus, density, and coefficient of thermal expansion. Polymers are frequently used to bind glass fibers together in composites. The fibers are made from oxide glasses containing various fractions of oxides of silicon, sodium, aluminum, calcium, magnesium, potassium, and boron. Glass fibers are categorized on the basis of their compositions and corresponding properties as E-glass, C-glass, or S-glass. E-glass fibers are very good electrical insulators, C-glass fibers have high chemical corrosion resistance, and S-glass fibers have high strength and can withstand high temperatures. Table 14.3–1 lists typical properties of glass...
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