Ceramics are well known for their elevatedtemperature

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Unformatted text preview: Boron (unidirectional) Carbon Chopped glass Epoxy Epoxy Epoxy Polyester Tensile strength (MPa) Elastic modulus (GPa) Volume fraction Matrix Density (g/cm3) Long. Trans. 0.50 0.35 0.60 0.60 2.65 3.86 3.45 2.0 210 300 262 40 150 150 152 10 1500 1750 690 780 140 410 180 28 0.35 0.60 1.7 2.1 16.5 215 16.5 9.3 280 1400 280 63 0.60 0.70 1.9 1.8 145 55–138 9.4 — 1860 103–206 65 — Long. Trans. The traditional application of PMCs was in secondary load-bearing aerospace structures. Now they are used in I-beams in civil structures, various automotive parts, steel-belted tires, and in sports goods. The cost of PMCs has been steadily decreasing, which makes them attractive enough for use in consumer goods. Another burgeoning application of PMCs is in architectural structures such as the Georgia Dome: the roof is composed of seven acres of woven glass fiber cloth in a PTFE matrix. 14.4.3 Ceramic-Matrix Composites Ceramic-matrix composites (CMCs) have been used in the form of reinforced concrete for a long time. Because of their low tensile strength, their use has been limited to structures subject to compressive loading. Ceramics are well known for their elevatedtemperature oxidation and creep resistance. Therefore, if their brittle behavior could be controlled, these materials would be excellent candidates for use in automotive and aircraft gas turbine hot-section components such as blades, disks, pistons, and rotors. The fracture toughness of conventional ceramics is on the order of 1 to 5 MPa- m. The toughness of CMCs has been boosted to levels of 15 to 20 MPa- m by embedding ceramic fibers in ceramic-matrix materials. Examples include SiC-reinforced SiC, SiCreinforced silicon nitride (Si3 N4), and carbon fiber-reinforced glass. Table 14.4–2 lists properties of typical CMCs. TABLE 14.4–2 Properties of some representative ceramics and CMC composites. Flexure strength (MPa)* Matrix Fiber Al2 O3 MgO SiC SiO2 glass Al2 O3 SiO2 glass Al2 O3 — — — — SiC whiskers SiC fibers BN particulates Fracture toughness ~MPa- m! 350–700 200–500 500–800 70–150 800 1000 350 2–5 1–3 3–6 1 10 20 7 *Flexural strength of brittle materials is determined by using a four-point bend test instead of tensile specimens used for ductile materials to prevent failure from occurring in the grips. It represents an estimation of the tensile strength of the material. | v v 594 iq | e-Text Main Menu | Textbook Table of Contents pg595 [R] G1 7-27060 / IRWIN / Schaffer iq Chapter 14 (a) (b) (c) Composite Materials (d) FIGURE 14.4–1 A schematic illustration of how the presence of fibers can be used for toughening of brittle ceramic matrices: (a) matrix crack approaching a fiber, (b) crack developing on the other side of the fiber while the fiber remains intact, (c) fiber break occurring away from the matrix crack plane, and (d) fiber pullout accompanied by energy absorption. Figure 14.4–1 shows the principle behind fiber toughening in CMCs. The interface in these...
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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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