Lecture23%20with%20slides

Lecture23%20with%20slides - Advanced Materials for...

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1 1 Lecture 23 Creep 1 2 Advanced Materials for Efficient Power Generation 39 ft Increase T to increase efficiency 3 Creep deformation Time-dependent deformation (flow) in response to a shear stress ˜ σ H = 1 2 σ 1 2 () 2 + 1 3 2 + 2 3 2 [] Loading parameter: Effective strain: ˜ ε H = 2 9 ε 1 2 2 + 1 3 2 + 2 3 2 Constitutive Eqn.: d ˜ H dt = f ˜ H 4 Linear creep H H H d B dt == % & % % • In solids linear creep occurs by (grain boundary or lattice) diffusion of atoms • Newtonian flow where, B = B o exp( Q / RT ) 3 H H H d dt η % % & % Or, , with = o exp( + Q / RT ) 5 Non-linear creep / n n HH o H o A εσ & %& %% • In metals and ceramics non-linear creep occurs by dislocation motion (thermal energy allows them to climb over obstacles at stresses lower than yield) • Non- Newtonian flow where, A = A o exp( Q / RT ) • In polymers non-linear creep and time-dependent yield are essentially the same process • Power-law creep: 6 X strain time creep rupture tertiary creep secondary creep primary creep initial strain (elastic or plastic) increase stress X Creep curve for constant applied uniaxial stress Secondary creep is the steady-state creep for which the previous laws were stated
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2 7 W. Blumenthal Creep rupture of an alumina bar 8 Creep crack growth in alumina W. Blumenthal 9 W. Blumenthal Creep cavities in alumina 10 Design criterion for creep ˜ ε < ˜ f during design life of component Failure strain may be established by: (i) Strain for creep rupture (ii) Maximum deflection allowed by design Creep strain less than failure strain ˜ r Integrate creep equations (found from experiment) to find life-time 11 Example AISI 304 stainless steel
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This note was uploaded on 09/15/2010 for the course MATSCIE 250 taught by Professor Yalisove during the Fall '08 term at University of Michigan.

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Lecture23%20with%20slides - Advanced Materials for...

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