Non-linear modeling of stress relaxation curves for Grade 91 steel

We focused on the modeling of the high-temperature Stress relaxation (SR) curves for Gr. 91 steel using two nonlinear models of a spring-dashpot and Peleg model to get a better fit to experimental relaxation data. A series of SR tests were performed under a constant strain of 0.6 % at 500, 550, 600 and 650 °C. The SR behavior of Gr. 91 steel was clarified based on the experimental data. In the modeling of the SR curves, each parameter value for the two models was optimized to give the best fit to the experiment SR data. The springdashpot model did not match the experimental data at 500 and 550 °C, although the model showed good agreement with the experimental data at 600 and 650 °C. However, the Peleg model was superior to the spring-dashpot model for all temperature conditions, and its parameter values followed temperature dependence well. Using the Peleg model, the SR curves for Gr. 91 steel could be suitably modeled at different temperature ranges. The confidence of the Peleg model was validated from elastic-creep finite element analysis. It is suggested that the Peleg model can be a useful tool to characterize the relaxation behavior of Gr. 91 steel.