Lecture - Chapter 8

Lecture - Chapter 8 - Chapter 8 Failure Why study Failure?...

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    Chapter 8  Chapter 8  Failure Failure
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    Why study Failure? Design of a structure should be done with a damage tolerance design concept –that is a design engineer assumes that there is a certain amount of damage tolerable or flaws present in the structure and they also assume that it is going to fail at some future point by one of the various failure modes. Thus by learning all the failure modes (i.e fracture, fatigue, creep, etc.) a design engineer can design more effectively.
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    Why study Failure? For example, if the fatigue life of an airplane is 20,000 cycles then it should be recommended to run the airplane for no more than 20,000 cycles.
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    Learning Objectives Define stress concentration factor, stress intensity factor and fracture toughness. Impact testing and ductile to brittle transition. Define fatigue failure, fatigue life time, fatigue limit and fatigue strength. Creep, steady state creep rate and rupture life time.
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    Fracture When a single body separates into two or more pieces in response to an imposed stress, which may be tensile, compressive, torsional, or shear, it can be regarded as fracture.
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    Fundamentals of Fracture Fracture of a structure could be ductile or brittle. When the structure undergoes a large amount of plastic deformation before fracture that is a ductile fracture. On the other hand fracture without any plastic deformation is a brittle fracture (Figures 8.1, 8.2, and 8.3).
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    Ductile Fracture Ductile fracture can be determined in a number of ways. For one, it is always characterized by a large amount of deformation prior to failure, i.e. a large percent elongation. Secondly, a cup and cone type fracture is typical for ductile fractures (reference Fig. 8.3).
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    Ductile Fracture cont. Finally, the fracture surface should appear to be dimpled or porous, resulting from the formation of microvoids (reference Fig. 8.2 & 8.4).
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    Fig 8.1 (a) Highly ductile fracture   (b) Moderate ductile  fracture   (c) Brittle fracture  
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    Fig 8.2 (a) Initial necking (b) Small cavity formation (c) Coalescence  of cavities to form a crack (d) Crack propagation (e) Final shear  fracture.
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    Fig 8.3 (a) Cup and cone fracture in aluminium     (b) Brittle fracture in mild steel
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    Fig 8.4 (a) Scanning electron fractograph showing  spherical  dimples characteristic of ductile fracture resulting from uniaxial  tensile loads.  (b) Scanning electron fractograph showing  parabolic-shaped dimple characteristic of ductile fracture  resulting from shear loading.
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  There are two kinds of brittle fractures– cleavage (or transgranular fracture) and intergranular fracture. Brittle Fracture
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This note was uploaded on 05/03/2010 for the course ME 250-750 taught by Professor Signer during the Summer '10 term at Wichita State.

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Lecture - Chapter 8 - Chapter 8 Failure Why study Failure?...

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