This last step requires energy to overcome the

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Unformatted text preview: materials consists of mechanical bonding with some strength derived from interdiffusion between the fiber and the matrix. Cracking begins in the brittlest material, often the matrix. When a crack tip encounters a fiber, two things can happen. If the fiber-matrix bond is strong, the crack will continue to grow across the fiber. In this instance, having fibers offers no apparent advantage. However, if the bond is relatively weak, as in CMCs, delamination occurs at the interface, and the crack is temporarily stopped, with some of the energy going into the formation of new surfaces (Figure 14.4–1a). When the load is raised further, the matrix on the other side of the fiber begins to crack (Figure 14.4–1b), but the fiber is still capable of transferring load across the crack faces. This phenomenon is called fiber bridging. When the fiber breaks, usually at a location away from the matrix crack plane (Figure 14.4–1c) in a section containing a random flaw, the matrix crack advances further. However, some load is still carried by the fiber (Figure 14.4–1d) across the crack plane because of continued fiber bridging. During subsequent loading, the fiber is pulled out of the matrix near the short end. This last step requires energy to overcome the frictional forces between the fiber and the matrix and raises the toughness level of CMCs. CMCs represent a relatively new class of materials that have caused considerable excitement in the composite and ceramic industries. 14.4.4 Carbon-Carbon Composites As the name implies, in this class of fiber-reinforced composites both the matrix and the fibers are fabricated from carbon. These materials offer a unique combination of properties, including the ability to withstand extremely high service temperatures ( 3000 C), high specific strength, excellent resistance to wear (they can be self-lubricating), good resistance to thermal shock, and reasonable machinability. The maximum use temperature for these composites is limited by oxidation problems. Typical applications include brake components, heat shields, and rocket nozzles—not your household composites because of high costs. Carbon-carbon composites can be fabricated using the CVD methods described in Chapter 16, by impregnating graphite fibers with a carbon-based polymer that is then pyrolyzed to “burn off ” the noncarbon atoms in the polymer, or combinations of the two. 14.5 PREDICTION OF COMPOSITE PROPERTIES | v v Testing of materials is both expensive and time-consuming. It also causes delays in the time required for bringing a new material into the marketplace. Therefore, materials | e-Text Main Menu | Textbook Table of Contents 12.01.98 plm QC1 rps MP 595 . resistance R The extrinsic materials property that iq 13.01.98 plm QC2 rps describes the ability of a material to resist, or oppose, the transport of electrical charge in response to an external electric field. It is defined as R (L /A). v v pg596 [V] G2 The first solid that7-27060 / IRWIN / Schaffer appears upon the liquid phase. ite The first cementite that forms Part III Properties d steel is cooled596 through the resistivity The intrinsic materials property that ite two-phase field and held at a describe...
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