## 3 Experimental Results and Analysis
### Sample V-3  
Figure 5e displays the principal strain states of the observation surface of the sample V-3 in the process of uniaxial loading. Similarly, several types of cracks appeared sequentially at the periphery of the two openings during sample loading. In other words, firstly, two elementary tensile cracks 1\({}^{\text{a}}\) and 1\({}^{\text{b}}\) occurred on the bottom of the lower and upper openings, respectively. Secondly, a posterior tensile crack 2\({}^{\text{a}}\) initiated from the lower-left corner of the upper opening and propagated straight to the opening below with the increasing stress. At the peak stress, it was observed that the other two  
posterior tensile cracks \(2^{\rm c}\) and \(2^{\rm d}\) had emerged at the upper-right corners of the two openings. During the propagation of these posterior tensile cracks, the slabbing cracks were found to appear on the opening sides. At the post-peak failure stage, a shear crack appeared in the upper-left corner of the specimen. Once it gets connected with the crack \(2^{\rm a}\) and the new formed posterior tensile crack \(2^{\rm d}\) at the lower-right corner of the lower opening, the failure would take place. Based on the DIC experimental technique, we could also visualize the displacement contours of these samples at different loading times. Figure 6 presents the horizontal displacement states of the above samples at the same stresses. The displacement symbol is defined as: if rock particles move to the right, the displacement is positive. Otherwise, the displacement is negative. In Figure 6a, the displacement of the left blue part of the sample C-3 was negative at the early loading stages, suggesting that this part of rock moved towards the left. As the stress grows, a yellow curved area with positive displacement was gradually formed near the right border. It was symmetrically distributed with the blue curved area near the left end. Due to the end friction, the displacements of regions near the loading  
Figure 5: Variation of principal strain with the stress during uniaxial loading: (**a**) Sample C-3; (**b**) Sample S-2; (**c**) Sample H-3; (**d**) Sample D-1 and (**e**) Sample V-3. ends were basically zero. As a result, the shear band 4 would be formed on the diagonal and the right yellow part would be split when the two cracks 1\({}^{\text{a}}\) and 1\({}^{\text{b}}\) get connected. For the sample S-2, first, a triangular cyan area with negative displacement on the left side of the opening gradually appeared as the stress rose. At 40 MPa, it could be seen that there was a short dividing line between the green and cyan areas on the floor of the opening. This formed boundary was the crack 1\({}^{\text{a}}\). At 65 MPa, the cyan area evolved into a trapezoidal blue area. Obviously, the right boundary of this area represents the cracks 2\({}^{\text{a}}\) and 2\({}^{\text{b}}\). Meanwhile, a triangular yellow area with positive displacement appears on the right side of the opening. At the maximum stress, it was found that the boundary of the cyan area at the upper-left corner of the opening disappeared, while that at the lower-left corner prolongs. This proved that the crack 2\({}^{\text{b}}\) lengthened. Besides, at the upper-right corner, a vertical line representing the crack 2\({}^{\text{c}}\) emerged as well. At 78 MPa after the peak, a yellow shear band appeared in the upper-right corner of the sample. Similarly, according to the displacement distributions of the rest samples in Figure 6b-e, it can be summarized that the displacement variation during the loading was agreeable with the fracture development. That is to say, the dividing lines between two different color areas on the sample surface would occur at the places where the cracks appear. Figure 6: Variation of horizontal displacement with the stress during uniaxial loading: (**a**) Sample C-3; (**b**) Sample S-2; (**c**) Sample H-3; (**d**) Sample D-1 and (**e**) Sample V-3.