The solution is spun into a ber with extremely high

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Unformatted text preview: generally used in soft matrices, such as reinforcement of rubbers in tires, belts, and hoses. About two decades ago polymer fibers with much higher molecular orientation and without chain folds were developed. These materials represented a significant increase in strength and modulus over conventional nylons and polyesters, revolutionizing the development of stiff, strong, lightweight (all polymer) composites. Examples of these polymers are Kevlar and Spectra. The high molecular orientation of Kevlar and other similar aramids (short for aromatic polyamides) is achieved by dissolving the inherently stiff polymer in a solvent so that a liquid crystalline solution is obtained. The solution is spun into a fiber with extremely high axial molecular alignment. Spectra is formed by dissolving ultrahigh– molecular weight flexible polyethylene chains in a solvent to form a gel. The gel is spun into a fiber and collapsed by multiple drawings at high temperatures. In both of these | e-Text Main Menu | Textbook Table of Contents 12.01.98 plm QC2 rps MP 587 pg588 [V] G2 7-27060 / IRWIN / Schaffer Part III 12.01.98 plm QC2 rps MP Properties processes, which are described in more detail in Chapter 16, chain folding is almost completely absent and molecular orientation is high. While these fibers have excellent tensile properties at room temperature, their use temperature is limited to a few hundred degrees C or less. In addition, their axial and lateral compressive properties are extremely poor. The most common ceramic reinforcing materials include fibers of Al2 O3 and SiC as well as SiC whiskers and particles. Other ceramic fibers with good properties include silicon nitride, boron carbide, and boron nitride. SiC fibers are made using a chemical vapor deposition process or by controlled pyrolysis of organosilane precursors, while whiskers are obtained by vapor phase growth. These processes are discussed in Chapter 16. Much like carbon fibers, the properties of ceramic fibers depend on the processing method used to fabricate them. Because ceramic fibers are brittle, it is important to produce them in small diameters to achieve flexibility and good strength. A variety of metals can be used to draw high-strength wires, which can serve as metal fibers. The most prominent metal fibers include beryllium alloys—which have high strength, high modulus, and low density—steel, and tungsten. The strengths of metal fibers are consistent and reproducible. The properties of typical carbon, polymer, ceramic, and metal fibers are given in Table 14.3–1. In summary, the ideal fiber material for strengthing and stiffening a matrix requires the following attributes: low density, high tensile strength, high modulus of elasticity, high flexibility, and the ability to form a strong interface with the matrix. 14.3.3 Characteristics of Matrix Materials Like fibers, matrix materials can be polymers, ceramics, or metals. Carbon is also used as a matrix material with carb...
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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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