4. Discussion
4.1 Stress-strain curves
The typical stress-strain curve under cyclic creep is shown in Fig. 7,
where \(\sigma^{\max}\) and \(\sigma^{\min}\) are of the peak and valley
applied stresses, \(\ \sigma_{e}^{\max}\) and \(\sigma_{e}^{\min}\) are
the upper and lower limits of the elastic domain respectively. The
strain within the two adjacent cycles can be divided into four parts, as
illustrated in Fig. 7 (a), including (I)\(\ \varepsilon^{r}\), the
ratcheting deformation which contains elastic and plastic strains,
loading from the lower-elastic limit to the peak stress;
(II)\(\ \varepsilon^{c}\), which contains the creep strain generated
during peak stress hold and a minor additional viscoplastic strain that
is produced during the unloading process from peak stress to
upper-elastic limit, In order to simplify the classification, the part
of viscoplastic strain is considered as a continuation of creep.
Previous works40,41 have concluded that the
viscoplasticity of materials has a contribution to creep during the
loading and unloading stage; (III)\(\ \varepsilon^{\text{re}}\ \), the
total recovery strain when stress unloads from the upper-elastic limit
to the valley stress and then reload to the lower-elastic limit; and
(IV)\(\ \varepsilon^{\text{act}}\),
the
actual increased deformation compared to the last adjacent cycle.