Chapter 8.6

Chapter 8.6 - in long-life applications. Another parameter,...

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26 MSE 2090: Introduction to Materials Science Chapter 8, Failure Creep Furnace Creep is a time-dependent and permanent deformation of materials when subjected to a constant load at a high temperature (> 0.4 T m ). Examples: turbine blades, steam generators. Creep testing:
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27 MSE 2090: Introduction to Materials Science Chapter 8, Failure Stages of creep 1. Instantaneous deformation , mainly elastic. 2. Primary/transient creep . Slope of strain vs. time decreases with time: work-hardening 3. Secondary/steady-state creep . Rate of straining is constant: balance of work-hardening and recovery . 4. Tertiary. Rapidly accelerating strain rate up to failure: formation of internal cracks, voids, grain boundary separation, necking, etc.
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28 MSE 2090: Introduction to Materials Science Chapter 8, Failure Parameters of creep behavior The stage of secondary/steady-state creep is of longest duration and the steady-state creep rate . is the most important parameter of the creep behavior
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Unformatted text preview: in long-life applications. Another parameter, especially important in short-life creep situations, is time to rupture, or the rupture lifetime, t r . t / s = & t r / t 29 MSE 2090: Introduction to Materials Science Chapter 8, Failure Creep: stress and temperature effects With increasing stress or temperature: The instantaneous strain increases The steady-state creep rate increases The time to rupture decreases 30 MSE 2090: Introduction to Materials Science Chapter 8, Failure Creep: stress and temperature effects The stress/temperature dependence of the steady-state creep rate can be described by = RT Q exp K c n 2 s & where Q c is the activation energy for creep, K 2 and n are material constants. (Remember the Arrhenius dependence on temperature for thermally activated processes that we discussed for diffusion)...
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Chapter 8.6 - in long-life applications. Another parameter,...

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