The cyclic creep behaviors have been studied with various stress frequencies, amplitudes and unloading times in the temperature range of 0.4-0.5 $T_m$. The cyclic creep conditions such as peak stress, amplitude and frequency were kept constant during creep deformation.
Creep tests have been carried out at stress frequency ; f=0.2-3 cpm, ratio of amplitude to peak stress ; Δσ =0-0.9 and ratio of unloading time to a period ; $t_u/t_p=0-0.7$.
The creep rate and activation energy for creep deformation in the steady state have been measured from each creep test. For high peak stress, frequency and large amplitude, creep rate ($&εgrave;$_c$) was noticeably higher and activation energy for cyclic creep($Q_c$) decreased in the range of cyclic creep acceleration, while those changed slightly in the range of retardation.
The effect of varying the unloading time to a period ($t_u/t_p$) on the ratio of creep rate ($&εgrave;$_c/$&εgrave;$_s$) and activation energy were examined at the condition of constant frequency and amplitude in the vicinity of 0.4 $T_m$. The activation energy and the ratio of creep rate were minimum and maximum at $t_u/t_p$ 0.5, respectively. These indicate that the recovery process during the unloaded period (anealstic recovery) operates more effectively than that during the loaded period (dynamic recovery).
After specimens were allowed to attain the steady state, unloading tests were carried out and anelastic strain was measured with unloading time. The activation energy for anelastic deformation ($Q_a$) was about the same as that for cyclic creep ($Q_c$). The difference in activation energy for static and cyclic creep ($ΔQ=Q_s-Q_c$) were formulated in terms of unloading time (half a period) and amplitude.
$Δ{Q}=0.9V(Δσ- σ_o) exp(-Kt)$
ΔQ is an additionally mechanical work term which increased with amplitude and frequency. The calculated values of ΔQ were nearly consistent with experimental values. From the test results of stress mode change in differential creep tests, it seems clear that the accelerated creep rate is associated with the development of softer substructure which allows more creep deformation by anelastic recovery in the cyclic creep.
Based on a consideration of mechanistic model mentioned above, it may be concluded that cyclic creep deformation is controlled by anelastic recovery occurred under repeated stress.
