Calculate creep elongation and stress required

daskywalker

1. The problem statement, all variables and given/known data

A 10 inch long 316stainless steel structure is in service at 1200°F. The maximum creep elongation in service permitted is 0.05 inches in 500 hours. The minimum creep rate vs. applied stress is known at 1100 and 1300°F and given by the following equations σ = 31.91[itex]\dot{\epsilon}[/itex]^(0.1292) (1300°F) and σ = 68.23[itex]\dot{\epsilon}[/itex]^(0.1142) (1100°F). Assuming the exponent and log of the pre-exponent for 1200°F are interpolated averages of the values at 1100 and 1300° determine the maximum stress that can be used to obtain this creep lifetime. Explain the likelihood of the accuracy of this calculation in light of the data given.

2. Relevant equations

Given in problem

3. The attempt at a solution

I wrote down the 2 equations and took the log of both sides, found the equation for 1200F to be σ=46.66[itex]\dot{\epsilon}[/itex]^(146.79)
But then plugging in the strain rate (0.05/1800000s) gives me zero for stress