# Mechanical Properties of Metasandstone under Uniaxial Graded Cyclic Loading and Unloading
## 3 Test Results and Analysis
### Strength Characteristic Analysis  
#### 3.3.1 Variation Characteristics of Rock Compressive Strength  
The stress-strain curves of metasandstone under uniaxial compression and uniaxial graded cyclic loading and unloading are shown in Figures 12 and 13.  
From the uniaxial compression stress-strain curve of the rock sample in Figure 12, it can be seen that the stress-strain curve does not show a yielding phase that obviously produces plastic deformation when the rock sample is about to reach the peak strength, which indicates that this metasandstone is a typical plastic-elastic rock. The deformation characteristics after the peak of the curve indicate that the metasandstone rock sample experienced obvious brittle damage.  
As can be seen in Figure 13, the area of the plastic hysteresis loop in the cyclic stress-strain curve of the rock sample becomes larger as the cyclic level rises, which indicates that the irreversible damage within the rock is increasing.  
Tables 2 and 3 show the data of peak strength (UCS) and modulus of elasticity (E) of the rock samples under uniaxial compression and uniaxial graded cyclic unloading conditions.  
Figure 11: External damage of rock sample during uniaxial graded cyclic loading and unloading. (a) B02; (b) B04; (c) B05.  
Figure 12: Uniaxial compressive stress–strain curves.  
Figure 13: Uniaxial graded cyclic loading and unloading stress–strain curves of rock. (**a**) Stress–strain curve of B01. (**b**) Stress–strain curve of B02. (**c**) Stress–strain curve of B03. (**d**) Stress–strain curve of B04. (**e**) Stress–strain curve of B05. (**f**) –strain curve of B06.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Rock Samples** & **UCS/MPa** & **E/GPa** & **R\({}_{\textbf{0}}\)/MPa** & **E\({}_{\textbf{0}}\)/GPa** \\ \hline A01 & 124.6 & 29.9 & & \\ A02 & 137.9 & 28.8 & 126.9 & 29.8 \\ A03 & 118.2 & 30.8 & & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Strength data of rock sample in the uniaxial compression test.  
From Table 3, it can be seen that none of the six rock samples experienced a complete 5 th stage unloading cycle. Rock samples B03 and B05 were destroyed after only two complete unloading cycles, and the compressive strength was significantly lower than that of the other four samples. It is assumed that the reason for this phenomenon is that there are relatively developed joints and fissures or structural surfaces inside the B03 and B05 samples, and most of these joints and fissures or structural surfaces are not in the monitoring area of the radial wave velocity measurement point and are approximately parallel to the axis of the sample, so the internal defects are not well reflected in the initial wave velocity.  
The average cyclic compressive strength of the B01-B06 rock samples at final damage R\({}_{\rm b}\) was 87.4 MPa, which was only 68.9% of the average uniaxial compressive strength R\({}_{0}\). Even if the results of B03 and B05 are not taken into account, the average compressive strength of the remaining four rock samples was 100.3 MPa, which was significantly less than the uniaxial compressive strength of the metasandstone of 126.9 MPa, and the former was 79.0% of the latter. This shows that uniaxial graded cyclic loading and unloading can cause significant damage to the metasandstone rock samples, which in turn reduces the compressive strength of the rock.