# Triaxial Loading and Unloading Tests on Dry and Saturated Sandstone Specimens
## 3 Results
### Energy Evolution of Rock Failure  
Based on the relevant strain energy calculation formula and the experimental data, the strain energy evolution curves can be obtained with loading time [41; 42]. The typical time history curves of strain energy for dry and saturated sandstone specimens under different initial confining pressures are shown in Figure 9. The initial confining pressures of 10 MPa and 40 MPa represent low and high confining pressures respectively. The strain energy curves for other specimens under low or high confining pressures are similar to these curves in each testing group. For conventional triaxial compression tests, as shown in Figure 9a-d, several typical stages can be divided corresponding to the points in the stress-time curves. When the rock specimen is under low initial confining pressure (\(a_{3}^{0}=10\) MPa), as shown in Figure 9a,c, the curves of total strain energy (\(U\)) and elastic strain energy (\(U_{e}\)) almost overlap, and the dissipative strain energy (\(U_{d}\)) is relatively low in the micro-cracks compaction stage (OA) and the elastic deformation stage (AB). It indicates that the energy absorbed from the test machine is basically converted into \(U_{e}\). However, while the rock specimen is under high initial confining pressure (\(a_{3}^{0}=40\) MPa), particularly for the saturated sandstone as shown in Figure 9d, the curves of \(U\) and \(U_{e}\) begin to separate gradually and there are  
Figure 8: Typical SEM images of sandstone fractures with the initial confining pressure 10 and 40 MPa in unloading confining pressure tests: (**a**,**b**) TC-D-10; (**c**,**d**) TC-S-10; (**e**,**f**) TC-D-40; (**g**,**h**) TC-S-40 (Opposite arrows and inverse arrows represent slip trace and tensile trace respectively; and the letters F, S and T represent fragments, shear crack and tensile crack, respectively). few increases of \(U_{d}\) in the elastic deformation stage (AB). In the crack initiation and expansion stage (BC), when the rock is under low initial confining pressure, the curves of \(U\) and \(U_{e}\) separate gradually at point B. Meanwhile, \(U_{d}\) starts to increase from this point, owing to the development of plastic deformation and propagation of micro-cracks of the specimens. When the rock is under high initial confining pressure, \(U\), \(U_{e}\), and \(U_{d}\) increase steadily where the increase rate of \(U_{d}\) is the minimum among them. During the unstable crack development up to the failure stage (CD), the increase rate of \(U_{e}\) slows down, and \(U_{e}\) reaches the elastic energy storage limit at the peak strength point D. \(U_{d}\) increases rapidly in this stage, illustrating accelerated growth of micro-cracks, and the failure approaches. In the post-failure stage (DE), \(U_{e}\) releases quickly to a small value, and \(U_{d}\) increases rapidly to a large value. The absorbed strain energy is basically transformed into \(U_{d}\), which contributes to the development of internal cracks and a large shear deformation along the fracture surface. However, \(U\) still increases rapidly as the large axial deformation at this stage. Finally, the specimen shows a brittle failure. Figure 9: _Cont._  
For the unloading confining pressure tests, the strain energy time curves of sandstone specimens are shown in Figure 9e-h. The characteristics of strain energy curves in unloading tests are quite different from conventional triaxial compression tests, especially from the unloading point. Before the unloading point, in the micro-cracks compaction stage and the elastic stage (OB'), the curves of \(U\) and \(U_{e}\) almost overlap with a low initial confining pressure (10 MPa), while the curves of U and \(U_{e}\) begin to separate under a high initial confining pressure (40 MPa). The variation trend is similar with the loading test. However, after unloading point (B'C'), \(U\) and \(U_{d}\) increase steadily while \(U_{e}\) shows a slow rate of increase.