ch14 - pg576[V G6 7-27060 IRWIN Schaffer H A P T E 14 R...

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14 C H A P T E R COMPOSITE MATERIALS 14.1 Introduction 14.2 History and Classification of Composites 14.3 General Concepts 14.4 Practical Composite Systems 14.5 Prediction of Composite Properties 14.6 Other Applications of Composites
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MATERIALS IN ACTION Composites Composites are a class of materials that incorporate two or more different materials. Composites are designed to take advantage of the most desirable characteristics of each material while eliminating or at least reducing the negative properties of the constituents. A highly publicized use of composite materials occurs in sporting equipment such as fishing rods, tennis rackets, golf clubs, and skis, to name but a few. These items were traditionally made from wood, which in itself is a natural composite. While acceptable equipment can be made using wood, high performance man-made fibrous composites offer superior performance. With the space program as a technology driver, many new materials and processes were developed. For example, carbon fibers, composed of graphite-like crystals oriented along the fiber axis, can be very strong and lightweight. Epoxy glue, on the other hand, is not very strong but can serve as a glue to bind carbon fibers. By incorporating carbon fibers in epoxy glue it is possible to obtain a material that is light, tough, and strong! By changing the size, volume fraction, and geometric arrangement of the fibers, elastic and strength properties can be tailored to meet specific demands. Suddenly new possibilities are available for creative design that were previously unavailable. Nowhere have these opportunities been more vigorously pursued than in the sporting equipment field. New equipment appeared that made extensive use of composites to achieve new possibilities. Suddenly club players had the power and control of champions of the past. The technological revolution in sports was thrust to new heights—bobsleds, skis, vaulting poles, running shoes, tennis rackets, golf clubs, and so on. Companies who failed to grasp this new technology faded from the scene, while new ones with a strong technology and marketing bases appeared and became household words—Nike, Calloway, and so on. In this chapter we will develop the fundamentals of composite materials. 577
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578 Part III Properties 14.1 INTRODUCTION In the last few chapters we learned that some properties of materials are directly related to one another. Examples include: (1) the link among elastic modulus, bond strength, and thermal expansion coefficient (all characteristics of the bond-energy curve); (2) the inverse relationship between strength and ductility (or toughness) within a class of mate- rials; and (3) the proportionality of electrical and thermal conductivity in metals (the Wiedemann-Franz law). Often these correlated properties cause problems for design engineers. In particular, the inverse relationship between strength and toughness is unfor- tunate, since in many applications one would like to maximize both properties. When no
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