# Mechanical Properties of Metasandstone under Uniaxial Graded Cyclic Loading and Unloading
## 3 Test Results and Analysis
### Acoustic Emission Characteristics Analysis  
Considering the large amount of data, this paper uses B02 rock sample as an example to study and analyze the acoustic emission characteristics of metamorphic sandstone in uniaxial graded cyclic loading and unloading tests. The stress-strain curve and acoustic emission (AE) event characteristics of each cycle of the rock sample are shown in Figure 16.  
In this study, we found that the acoustic emission signals of metasandstone rock in uniaxial graded cyclic loading and unloading tests were mainly generated in the loading phase, and almost no acoustic emission activity was generated in the unloading phase. The Kaiser effect and Felicity effect were also found in the uniaxial graded cyclic loading and unloading tests.  
In Figure 16, during the first unloading cycle of the B02 rock sample, some acoustic emission signals were generated due to the friction and extrusion of crystal particles and microcracks in the rock sample, but the overall intensity of the signals was weak. The acoustic emission activity was further weakened during the second and third levels of cyclic addition and unloading of the rock sample compared with the first level, indicating that the internal damage development in the initial stage of linear deformation of the rock was low. The rock sample underwent nonlinear deformation during the fourth level of loading and unloading, and the acoustic emission activity increased compared with the previous levels. It is noteworthy that the rock samples exhibited a significant Kaiser effect during the loading and unloading tests in the second, third, and fourth levels [23; 24].  
Figure 15: Relationship between final cycle elastic modulus and initial elastic modulus of rock sample.  
The Kaiser effect means that the material will not produce obvious acoustic emissions when the loading stress does not exceed the upper limit of the upper-level stress. The cause of the Kaiser effect is the irreversible damage inside the material. However, during the fifth level of loading, the rock sample B02 produced significant acoustic emission before loading is observed. The reason is that the material is not a  
Figure 16: Relationship between stress–strain curve and AE events of each cycle of rock sample. (**a**) Level 1 complete cycle; (**b**) Level 2 complete cycle; (**c**) Level 3 complete cycle; (**d**) Level 4 complete cycle; (**e**) Level 5 uniaxial compression.  
reaching the upper stress limit of the previous level. This is the Felicity effect, also known as the inverse Kaiser effect. The appearance of the Felicity effect shows that under the action of cyclic load, the internal damage of the rock continues to intensify, which reduces the upper limit of stress, leading to the continuous development of internal cracks in rock.  
The B02 rock sample was finally damaged during the loading process of the fifth stage. It was found that, when the rock sample is macroscopically damaged, the acoustic emission number of the rock reaches the peak. However, between the last two obvious AE peaks before rock sample failure, the AE activity of rock samples tends to be relatively quiet, which is called the \"quiet period\" of AE activity.  
The existence of the \"quiet period\" can be understood as the existing fissures in the rock needing a certain time to accumulate energy during further development and expansion. When the energy accumulation is sufficient, the internal cracks in the rock will immediately intersect and penetrate, and the rock samples will be destroyed, sending out strong acoustic emission signals. It can be assumed that the acoustic emission activity of the metasandstone will have a sudden rise and then a sudden fall immediately prior to damage, and the rock will be damaged soon after the sudden fall. This acoustic emission feature can be used to monitor and predict the destabilization of the rock in practical engineering.