## 2 Material and Methods
### Experimental Facilities and Parameter Settings  
Figure 3 presents the equipment used for laboratory experiments. It was made up of a set of servo-controlled loading device, a set of AE monitoring device and a set of DIC\({}^{\text{2D}}\) (digital image correlation) monitoring device. In order to ensure the time correspondence of different monitoring parameters, these devices need to be turned on simultaneously.  
In this research, a high-capacity universal material testing system (Instron model 1346) at Central South University was applied to carry out the uniaxial compression tests. This apparatus can also perform other types of tests, such as the biaxial compression test, inclined plane shear test, tension  
Figure 3: Equipment for laboratory experiments.  
Figure 2: Dimensions of sandstone samples and fabricated openings (unit: mm): (**a**) Group C; (**b**) Group S; (**c**) Group H; (**d**) Group D and (**e**) Group V.  
test and flexure test. Its load precision and force capacity reach \(\pm 0.5\%\) and 450,000 lbf, respectively. By using displacement-controlled loading method, loads were continuously exerted on the samples until failure occurred. The loading speed was 0.01 mm/s. Additionally, a LVDT deflection sensor was placed between the two compression platens to monitor the real-time displacements of the samples during the tests.  
Due to the mighty merits like non-contact, non-intervention, high-precision, simple apparatus and easy operation, DIC approach was widespread devoted to monitoring the full-field deformation of rock samples under different loads. The calculation principle of this method is that, by comparing the photos of the sample before and after deformation, the new locations of all the points on the surface can be searched using correlation criteria, and then the displacement and strain can be calculated and visualized via digital image processing. To effectively distinguish the points, a speckle field was required to cover the monitoring surface of the sample before testing. The speckle distribution was random, which can be generated by subtly spraying the black and white paint on the observation surface. For more information regarding the speckle production, please refer to references [47; 50]. In general, strain concentration occurs before the crack appears, i.e., strain localized areas indicate impending cracks. Consequently, we adopted a set of DIC device to monitor the crack development in this study. The main components of this device were charge coupled device (CCD) camera, photo acquisition control terminal and fill light. The model of the camera used was [PERSON] piA2400-17 gm, which could deliver 17 frames per second at 5 MP resolution; it was placed about one meter directly in front of the sample. With the aid of an operation software installed on the photo acquisition control terminal, the sizes of photos and sampling rate were set to 1100 (width) pixels \(\times\) 1500 (length) pixels and 15 FPS, respectively. Besides, a LED lamp served as fill light was arranged next to the sample to provide illumination for picking high-resolution photos. Based on the consecutively collected photos, both the strain and displacement of the sample under any load can be visually presented through GOM Correlate software.  
Currently, it is universally accepted that the damage of rock samples during loading can be evaluated based on AE signals [56; 57; 58]. This is because elastic wave will be generated when fracture occurs in samples. Thus, an AE device composed of an PCI-2 AE apparatus (including AE-win operating software), two MISTRAS 2/4/6 preamplifiers with a gain of 40 decibels and two pico-type resonant sensors with 250 KHz frequency was employed to detect the AE activities. An adhesive tape coupled with Vaseline was used to fix the sensors at the back surface of the specimen. Some basic AE acquisition parameters set for the tests are shown in Table 2.