## 3 Results and Discussion
### Damage Evolution
#### 3.2.1 Deformation Evolution Process
As the cycle number increases, \(E_{20}\)+ initially increases and then levels off, and finally decreases. In contrast, \(E_{20}\)- shows a tendency to decrease with the cycle number throughout the loading process. In each cycle, \(E_{20}\)+ is simultaneously strengthened by the compaction effect and weakened by the damage effect. In the initial stage, as the damage degree of the specimen is low, \(E_{20}\)+ is mainly strengthened by the compaction effect. Approaching the failure, the damage degree of the specimen becomes higher, and the damage effect plays the dominant role. On the other hand, \(E_{20}\)- is based on the end part of a cycle where the specimen has already experienced a high level of stress compaction, resulting in the specimen being mainly affected by the damage effect. Comparing \(E_{20}\)+ and \(E_{20}\)- within one cycle, it can be found that with the increasing cycle number the value of \(E_{20}\)+ is first smaller than, and then exceeds, the value of \(E_{20}\)-. In fact, there is one more compaction and damage process undergone by the specimen for calculating \(E_{20}\)- compared to \(E_{20}\)+ in each cycle. With a growing level of damage to the specimen in the later cycles, the damage effect plays a dominant role, and thus \(E_{20}\)- is weakened, to be less than \(E_{20}\)+. Figure 9 displays the evolution of \(E_{s}\) for specimens under cyclic loading at different loading-foliation angles. The results show that \(E_{s}\)+ is always larger than \(E_{s}\)- under the same cycle number, indicating that the deformation of the specimen during the loading  
\(E_{\rm s}\) can generally reflect the damage of a specimen caused by the previous (n \(-\) 1) loading-unloading cycles. With the exception of specimen C-0-20, the \(E_{\rm s}\)\({}^{+}\) of specimens decreases with the increase in cycles. To be specific, the stress-strain curve of specimen C-0-20 in the range of 20 MPa to 40 MPa is enlarged, as shown in Figure 10a. Interestingly, the strength of the specimen after unloading is obviously increased, which is similar to the finding of [PERSON] and [PERSON] [40], as shown in Figure 10b. This strengthening effect can be attributed to the increased internal friction resulting from the improved contact state of internal particles by the formed rock debris infilling the microdefects within the specimen. Moreover, \(E_{\rm s}\)\({}^{-}\) seems to better reflect the damage caused by the previous n loading-unloading cycles, because \(E_{\rm s}\)\({}^{-}\) is consistently decreased with the increase in cycles, including for the specimen C-0-20. Thus, it is suggested that the deformation modulus at the unloading stage is more capable of reflecting the damage inside the rock than the deformation modulus at the loading stage. Figure 10: (**a**) Stress\(-\)strain curve and partially enlarged inset of specimen C-0-20 and (**b**) similar finding of You and Su [40]. Figure 9: The evolution of \(E_{\rm s}\) with the cycle number for specimens at different loading\(-\)foliation angles: (**a**) \(\beta\) = 0\({}^{\circ}\); (**b**) \(\beta\) = 30\({}^{\circ}\); (**c**) \(\beta\) = 45\({}^{\circ}\); and (**d**) \(\beta\) = 90\({}^{\circ}\).