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

Although primary stage is often neglected in creep design, in several alloys for high temperature applications, most of the allowable design strain can occurs before reaching the minimum creep rate. The kinetic of the primary creep stage is important since it determines the microstuctural conditions for the subsequent creep regime. Under rapid stress change creep transient occurs and, according to the developed dislocation substructure, a stress dependence of the creep rate is observed. In order to account for these features in the material response, an appropriate modelling is required. Assuming that the current creep rate in the transient regime can be expressed as a function of the steady state creep rate, a model based on the evolution of the internal stress is derived. The proposed model does not rely on any particular formulation for the steady state creep and therefore is of general applicability. In the present paper, a mechanism based secondary creep stage model, with an explicit dependence of the creep exponent n on stress, as proposed by the authors, has been used1). An application to polycrystalline copper is presented