In the present study, particular attempts have been made to propose a new interpretation methodology and a physically-based model of the transient creep behaviors using the effective stress concept, and to investigate the relationship between the transient creep and the steady state creep. In order to accomplish these objectives, more sophisticated creep experiments such as cyclic stress dip tests as well as constant tensile stress creep tests were carried out at 573K (~ 0.6$T_m$) under the stress range of 4 to 15 MPa on 99.95% pure aluminum specimens. Part of these special efforts was dedicated to improving the accuracy of the creep data by using PC-based digital technologies to assure the reliabilities of the model and experiments.
The proposed model for the transient creep behavior was originated from two important findings in this study: Firstly, the creep curves, including both primary stage and steady state, could be normalized by the steady state creep rates to give a unique master curve in different applied stresses. Secondly, the creep equation, including the primary stage as well as the steady state, could be rationalized with two stress terms; the applied stress and the effective stress. Therefore, the current creep rate normalized with the steady state creep rate could be described by two terms: the ratio of the internal stress to the applied stress and the creep time compensated by the steady state creep rate. Assuming the internal stress can represent the microstructural state which has been developed during the prior deformation history, the predicted transient creep behaviors after stress change showed a good correlation with the experimental results in various loading conditions.
Constant structure creep behaviors after a sudden stress reduction were also investigated to elaborate the existing theories of time-dependent plastic flow. The kinetic law for the thermally activated dislocation flow was found to show an exponential dependence on the reduced stresses, regardless of the applied stress and the strain at the stress reduction. The measured activation area is found to be inversely proportional to the the applied stress and it is not strongly dependent on the strain, which represents that the main dislocation mechanism in the soft cell interior may not be changed with creep strain in a given condition. Therefore, the main reason for the decrease in the creep rate during primary stage seems to be the decrease of the effective stress acting on the mobile dislocations in the soft cell interior.
Consequently, the creep equation expressed by the effective stress could describe the primary transient as well as the steady state creep. What is more, the transient creep behaviors after stress change could also be very well predicted by the effective stress concept. Those results can make a conclusion that the effective stress may be the strongest factor to influence the creep rate.
