Abstract
Creep deformation in turbomachinery applications is a highly non-linear phenomenon. Very small alterations in stress or temperature will substantially change the progression of creep. Common rules-of-thumb would suggest that a change in temperature of +/−10 to 15°C may halve or double the average rupture life respectively. Even in well-controlled laboratory conditions the stochastic nature of creep means that identical tests may differ by a factor of two. Even if an analyst has incorporated a very accurate creep model into an implicit finite element user creep subroutine, they may lack an appreciation of the uncertainty of their solution, or the design changes required to account for uncertainty in either the boundary conditions or the solution itself. Common practice would be to design with maximum strain limit, which may be inadequate given the sudden acceleration of tertiary creep rates. Alternatively, analysts might apply a time factor to their model, necessitating extending the simulation time by factors of two or more. This paper will present a methodology where two estimations of the progression of creep damage are made in parallel to the creep simulation at actual operating temperatures. These two estimations consider the predicted change in stress relaxation at higher or lower temperatures to provide the analyst with an estimate of what change in temperature would result in the model being at the onset of creep at any given node in the model, at any given time in the analysis. This margin calculation is then further expanded to consider the margin required to account for the uncertainty in analytical creep prediction. A time-based scatter factor is then incorporated into the temperature margin calculation to provide a minimum temperature margin including model uncertainty. The analyst is then free to also consider the uncertainty of the temperature prediction and boundary conditions to produce a robust creep prediction in a real-world simulation. The methodology is validated through FEA of example cases and has been applied to the analysis of a creep limited 2 stage industrial gas turbine blade.
