17~chapter 17

17~chapter 17 - Materials: engineering, science, processing...

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Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon
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Durability Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Durability is a key material attribute, but is one of the more difficult attributes to characterize, quantify, and use for selection It is a function not just of the material but of the environment in which it operates There are many mechanisms, some general, some peculiar to particular materials and environments Material combinations and configurations play a role
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Factors Affecting Durability Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 17.1
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Oxidation Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 17.2 Figure 17.3 The most stable form of most elements is an oxide If the oxide adheres to the material, it gains weight in either a linear or parabolic way – if the oxide is volatile, the sample loses weight in a linear way
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Flammability Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Table 17.1 Limiting oxygen index LOI The oxygen concentration in % required to maintain steady burning Underwriters Laboratory (UL) rating Response to a 1.6 mm polymer strip that is ignited while being held horizontally or vertically – codes such as HB (horizontal burn) describes the response Durability of polymers is dependant on their flammability
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Photo-Degradation Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Polymers and elastomers age when exposed to light and oxygen, causing loss of strength, stiffness, and toughness as well as discoloration Additives such ass anti-oxidants, light stabilizers, and fluorescent whitening agents are used to counteract this
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Mechanisms of Oxidation Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon When most metals are exposed to air, an ultra-thin surface film of oxide forms The oxide film separates the metal from the oxygen – to react farther either oxygen atoms must diffuse inward through the film to reach the metal or metal atoms must diffuse outward through the film to reach the oxygen
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Materials: engineering, science, processing and design, 2nd edition Copyright (c)2010 Michael Ashby, Hugh Shercliff, David Cebon Figure 17.4 The oxidation reaction M + O = MO occurs in two steps: 1)The metal forms an ion and releases electrons M = M 2+ + 2e 2)Electrons are absorbed by oxygen to give an oxygen ion O + 2e = O 2 - The first of these reactions occurs at the metal side of the oxide film while
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17~chapter 17 - Materials: engineering, science, processing...

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