FL&O_6_failure[1] - Chapter 6 Fracture Fatigue and...

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Chapter 6. Fracture, Fatigue and Creep. After studying this chapter you will be able to: Distinguish between ductile and brittle fracture. Describe a stress concentrator (also called stress raiser) and the danger it represents. Distinguish between the resistance to deformation and to fracture, and name their units. Explain why hard materials are more brittle than soft metals. Explain why ceramics are unreliable in tensile stresses. Describe fatigue, its mechanism, its consequences and what can be done to the material in order to improve fatigue resistance. Measure and describe creep and name the important engineering situations where it plays a major role. Under certain circumstances, materials break. Fracture is a failure that can cost human lives, lead to the destruction of costly structures or machines or interfere with manufacture. If we want to avoid fracture, we must know how and under what conditions materials break and what can be done to prevent it. In this chapter we will discuss the ductile and brittle fracture of metals and the fracture of ceramics and glasses, which is always brittle. We will examine the concept of stress concentration and learn why strong and hard materials are brittle while softer materials are capable of absorbing much more energy before they break. Finally, we will examine the phenomena of fatigue and creep which can lead to failure at stresses well below the yield stress, where one would expect the piece to be safe. Figure 6.1. Schematic Stress-strain curve of a metal. 6.1. Ductile Fracture of Metals. Let us look again at the stress-strain curve of a metal (Figure 6.1). After a certain amount of plastic deformation, a neck forms in the tensile test specimen, the force for
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When fracture occurs after extensive plastic deformation, the test piece fails by ductile fracture. Ductile metals neck and display the cup-cone fracture morphology shown in Fig.6.2. A. The process of ductile fracture may be broadly viewed in terms of the sequential microvoid nucleation and growth processes, schematically indicated in Fig.6.3. Te earliest stage of fracture spawns isolated microscopic cavities. These nucleate at inclusions, second phase particles and probably at grain boundary junctions. Microvoids coalesce then form an elliptical crack that spreads outward toward the periphery of the neck. Finally, an overloaded outer ring of material is all that is left to connect the specimen halves, and it fails by shear. Further examination reveals equiaxed or spherical dimples on the flat crater bottom loaded in tension, and elongated ellipsoidal dimples on the shear lips oriented at 45 o (Fig.6.4). In high purity FCC and BCC metals free of inclusions, necking to ~ 100 percent area reduction (i.e., to a point) is possible. A
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FL&O_6_failure[1] - Chapter 6 Fracture Fatigue and...

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