34 typical matrix materials and their properties are

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Unformatted text preview: on fibers in a class of composites known as carbon-carbon composites. The primary purposes of the matrix materials are to provide lateral support to the fibers and transfer loads. They also are a source of toughness in the composite, since the majority of fiber materials are brittle. Cracks that have propagated through a brittle fiber are stopped when their tips encounter relatively tougher matrix materials. An exception to ductile matrix materials is ceramic matrix materials, which are inherently brittle. Composites using ceramic matrices, such as reinforced concrete, are used in compressive load applications, or the brittle behavior is countered by carefully tailoring the interface properties. This approach is discussed in Section 14.3.4. Typical matrix materials and their properties are given in Table 14.3–2. TABLE 14.3–2 Typical properties of matrix materials. Density (g/cm3) Material Epoxy Polyester Copper Ti–6Al–4V Stainless steel High-strength aluminum alloys Magnesia (MgO) Lithium-alumino-silicate (glass ceramic) Silicon carbide Note: CTE | v v 588 iq Elastic modulus (GPa) Tensile strength (MPa) 1.05–1.35 1.12–1.46 8.9 4.5 8.0 2.7 2.8–4.5 2–4.4 120 110 198 70 55–130 30–70 400 1000 700–1000 250–480 30–45 40–60 16.5 10 10 10 6 18 23.6 10 10 6 3.6 2.0 210–310 100 97–130 100–150 13.8 1.5 10 10 6 3.2 400–440 310 4.8 10 6 c oefficient of thermal expansion. | e-Text Main Menu | Textbook Table of Contents CTE ( C 1) 6 6 6 6 pg589 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 Composite Materials 14.3.4 Role of Interfaces | v v Interfaces play an important role in determining the properties of composites. Consider 1 cm3 of a unidirectional composite made from 25- m -diameter continuous fibers embedded in a matrix. If the fibers are arranged in a square array and are on average 50 m apart, the volume fraction of fibers is approximately 20%. The total fiber-matrix interface area is approximately 314 cm2, compared with the 6 cm2 external surface area. The typical fiber volume fraction in composites is 2 to 3 times the above amount, and the interfacial area increases proportionally. The large interfacial area can significantly affects the properties of composites, in particular the crucial properties of toughness and ductility. If the matrix and the fibers have different coefficients of thermal expansion, then cooling from a high fabrication temperature causes differential thermal contraction between the fiber and the matrix and results in thermal stresses at the interfaces (see Section 13.5). This problem can be minimized by matching the expansion coefficients of the fibers and matrix. However, some differential expansion will always be present, and the fiber-matrix interface must have sufficient strength to survive temperature changes. Interfaces also provide a preferential path for oxygen diffusion or moisture uptake that may facilitate composite property degradation. For example, if moisture diffuses...
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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 Tech.

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