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26 Creep II

26 Creep II - 530.352 Materials Selection Lecture#26 High...

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Unformatted text preview: 530.352 Materials Selection Lecture #26 High Temperature Creep - II Tuesday November 15 th , 2005 Sherby-Dorn Equation ε ss = C σ n exp (- Q diffusion / RT) . Temperature dependence Stress dependence Constants Use this equation to calculate creep rate at any given or new stress or temperature !! Example of creep based design: Ni-base superalloys that are used for jet turbine applications exhibit Q creep = 320 kJ/mol and n=5. Creep rate; T=925 C and σ =350 MPa . ε ss = 1.7 x 10-7 350 5 exp( -320,000 / 8.314 x 119 = 1.7 x 10-7 x 5.25 x 10 12 x 11.1 x 10-15 = 10-8 sec-1 . Is 10 sec fast ? year per or year year days day hours hour ss ss % 31 31 . 25 . 365 24 sec 600 , 3 sec 10 1 8-- = = ε ε Is short for a service life but long for a graduate student-- must extrapolate from short tests to long times !! Increasing by 25 MPa : . . ε 1 / ε 2 = ( σ 1 / σ 2 29 n = (350/375) 5 = 0.708 ε 2 = 1.4 x 10-8 sec ....
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26 Creep II - 530.352 Materials Selection Lecture#26 High...

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