During a stress relief anneal, creep occurs, which converts elastic strains (and therefore stresses) into plastic strains. The decrease in stress should be t>σ = -Eε, or dσ/dt = -E dε/dt = -E ε?. Assume that the stress relief anneal is done in a temperature range where the basic creep rate is given by dε/dt = ε and that the residual stress before annealing was σ o. = Cσm
A. If m = 5, as is typical of creep at the low temperatures used for stress relieving, σ o = 5000 psi, E = 10 × 106 psi, and C = 6 × 10-22(psi)-5/h at the annealing temperature, how long would it take for the stress to drop to 2500 psi?
B. For the same temperature, how long would it take the stress to drop to 1000 psi?