Primary creep modeling based on the dependence of the activation energy on the internal stress

In the high temperature component design the accumulation of creep strain during the primary stage cannot be ignored, since most the allowable design strain occurs in this stage, for which appropriate modelling is needed. In this work a mechanism based model for primary creep has been derived assuming that the creep rate in the transient regime can be given as a fraction of the steady state creep rate and as a function of the internal stress. Taking into account that the apparent activation energy varies with the internal stress and that the internal stress kinetic can be given as a function of strain, an exponential form of the creep rate vs creep strain has been obtained. The proposed model has been applied to high chromium steel P91 and the evolution of the decay constant and scaled activation volume with the applied stress has been determined