In order to optimally design a hot section component for creep, the designer and turbine durability specialist must have confidence in their predictive tools and be able to gain design insight from these analytical tools. The modified theta projection (MTP) creep model was previously presented as an accurate means of describing creep behavior as a function of stress, temperature and time. The MTP was then implemented in an analytical model using a life fraction hardening (LFH) rule to calculate creep in the presence of time-varying stresses, and the results presented in a second paper. This paper presents improvements to the technique through the use of state variables in addition to the previously shown strain life fraction (ELF) and temperature margin (TMar). The need for performing multiple creep analyses is avoided by adding state variables to that track estimates of the effect of temperature changes on stress relaxation and life fraction, as well as an allowance for material variability and an inexact fit of material behavior.
The results of creep tests, on a nickel blade alloy, with incrementally increasing or decreasing loads are presented to provide validation of the accuracy of the life fraction hardening rule. The use of MTP and LFH has now been expanded to steels. Incremental testing results are examined for a NiCrMoV rotor steel to further validate the technique. The effect of true stress on model accuracy is also presented. Now that an accurate creep model and hardening rule have been implemented, expansion of the techniques to provides more useable design information and allows us to improve the structural integrity of turbine blades, vanes and rotors.