The creep behaviors of pure Aluminum have been compared under constant peak stress and constant peak load in the temperature range of 0.32-0.34 Tm. Creep tests under constant peak stress were carried out using Andrade-Charlmers' machine (A-C machine) and those under constant peak load were carried out using Instron-type machine (I-machine).
In both cases, cyclic creep acceleration was found to occur. Cyclic creep rate under constant peak load was much greater than that under constant peak stress. This phenomena was more significant with decreasing temperature. The measured apparent activation energy for cyclic creep under constant peak load was lower than that under constant peak stress by about 20KJ/mole. The difference in apparent activation may be interpreted in terms of difference in internal stress (σi). The apparent activation energy required to overcome the barrier to deformation will vary with the effective stress (σe, applied stress minus internal stress) as follows; $Q_a=Q_o-V.\sigma_e$ where $Q_o$=energy barrier to be overcome, V=activation volume, $Q_a$=apparent activation energy at stress σ.
The measured internal stress for cyclic creep under constant peak load was found to be lower than that under constant peak stress. It has been reported that lower internal stress is representative of softer substructure. Therefore the softer substructure probably accounts for the accelerated cyclic creep rate and the lower apparent activation energy for cyclic creep under constant peak load. It is thought this cyclic creep behavior is due to increasing stress amplitude (Δσ) and increasing peak stress (σp). In this work the increasing of stress amplitude has larger effect on cyclic creep rate than the increase of peak stress.
Since, like the above mentioned phenomena, creep behavior under constant peak load is different from creep behavior under constant peak stress, creep test using I-machine which keeps constant peak load can't be substituted for creep test using A-C machine which keeps constant peak stress to analyze physical meanings about creep behavior under constant peak stress.
