Criteria for Small Scale Creep Testing Condition and Estimation of C-t

Abstract

As new creep resistant materials are developed for the temperature range of 800-1100 degrees C, the improved creep properties sometimes require the modification of the conventional approaches in high temperature life assessment procedures which have been applied for low alloy steels and chromium steels usually used in the temperature range of 500-650 degrees C. In this study, finite element analyses were carried out by using the creep properties of a gas turbine material to establish the small scale creep testing requirements and to propose a corrected C-t estimation scheme. The maximum allowable creep zone size was estimated through the analogy between the small scale yielding conditions and the small scale creep conditions. The maximum time during which the small scale creep condition holds was also defined. The creep zone boundary with the definition of epsilon(c)=epsilon(e) was more suitable for C-t estimation than that with the definition of epsilon(c)=0.002, since it was easy to find a proper correction function by applying the former definition. The newly estimated value of beta(r) over tilde (c)=1/7=1/12 was derived for determining C, instead of the conventional value of beta(r) over tilde (c)=1/7.5. Excellent correlations were shown between the estimated C-t values obtained with the new beta(r) over tilde (c), and the finite element calculated C-t values in all creep condition ranges. This result strongly supports the validity of the suggested small scale creep criterion and the new correction value beta(r) over tilde (c)=1/12 proposed in this study.