FL&O_6_mechanical[1] - Section 3 Structural...

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1 Section 3. Structural Materials. Structural materials are the ones we use in the construction of buildings and machines where we rely on their response to applied forces. In buildings and machines, we rely on the ability of the material to resist deformation under loads; in springs we utilize the elastic deformation of the materials; we utilize the ability of metals to undergo extensive plastic deformation in order to give them shape, a method that is not possible with ceramics and polymers. In what follows, we will first examine the strength of metals and their elastic and plastic deformation under stress. In a following chapter, we will examine the resistance of metals and ceramics to fracture, fatigue and creep. The mechanical properties of polymers depend very much on their molecular structure; they are best discussed in conjunction with the properties and processing of these materials. Chapter 5. The Strength of Metals After having studied this chapter you will be able to: Distinguish between forces applied to a solid and the resulting stresses. Define tensile, compressive and shear stresses. Draw a stress-strain curve and identify elastic and plastic deformation, yield stress, ultimate tensile stress, ductility, work-hardening. Describe the effect of residual stresses on the strength of structural materials. Perform a hardness test; describe the different hardness tests and their significance Explain why the defects make a material strong. Name and explain the main means of strengthening of a metal: strain hardening (i.e. work hardening), solution strengthening, precipitation-strengthening, fine grain structure. Describe annealing, how it changes mechanical properties and why. 5.1. Structural Materials Let us take a wire and bend it around a mandrel into a helical spring. We have performed a plastic deformation of the material in order to give it a permanent shape. We can now hang different weights on the ends of the spring and obtain a reversible, elastic deformation that is proportional to the weight we applied. If the weight is too large, the spring elongates permanently, by plastic deformation. We can also use the wire to support a weight. If the latter is too heavy, the wire will not hold its shape, it will yield, become longer, and finally break. We can perform these experiments with different metals, a hard or a soft steel wire or a copper wire, and we will observe that the strength of the wires or possible amount of plastic deformation is different for each metal. We know from everyday experience that we can not bend a glass or a porcelain plate: when the stress is too large, it breaks before any deformation. We also know that we can bend a piece of polymer but it does not keep its new shape as a metal does.
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