Design parameters for high temperature creep and the minimum-commitment method

Abstract

Materials used in energy conversion systems are often subjected to stresses at elevated temperature where resistance to creep and creep/fatigue damage is important. Consequently, more sophisticated and accurate techniques are needed to characterize elevated-temperature material behaviour. A new approach in the analysis of elevated-temperature creep data has recently been developed, called the Minimum-Commitment-Method (MCM), which allows the development of design equations from analysis of creep-rupture data using a station to station or data point to data point approach. The power and general applicability of the minimum-commitment, station-function approach to the analysis of creep-rupture data is demonstrated. It is shown that equivalence in the best value for the variable constant A in the generalized time-temperature-stress equation, logt +AP(T) logt +P(T) =G (log σ), is obtained, whether derived from convergence of isostress creep-rupture data or by the focal-point convergence of isothermal creep-rupture data. Data conforming exactly to restrictions imposed by several common time-temperature parameters were analysed, and the corresponding values for A were derived by computer analysis of the isostress and the isothermal data. Multiple heat analyses on real data for Type 316 stainless steel was undertaken as well. In all cases, the analyses yielded equivalence in the A-value whether it was obtained from isothermal or isostress creep data. Finally, it is demonstrated that application of the Minimum-Commitment-Method using the derived Avalues yields a minimum in error of log-time for interpolation and extrapolation of the data.