## 2 Experimental Material and Method
### Experimental Apparatus and Principle  
Since the deformation rate of rock-like materials under explosive loading belongs to a medium-high level, we selected the modified SHPB testing equipment of CSU to conduct the dynamic compressive tests on pre-holed specimens. In this device, the impact component is designed from the original cylindrical shape to a spindle shape, which could produce the half-sine loading waves, solving the issues of waveform distortion and P-C oscillation compared with the previous rectangular loading wave. The experimental equipment is composed of a launch system, a bar system and a monitoring system. The launch system includes a nitrogen storage device, a striker excitation device and a special-shaped striker. The bar system is made up of an incident bar, a transmitted bar and an absorption bar. The monitoring system consists of a pedestal, a dynamic strain apparatus along with an oscilloscope, two strain gauges together with two bridge boxes, a floodlight and an industrial camera, a speedometer and a rock debris collection box, as shown in Figure 4.  
A valve installed on the nitrogen tank can control the pressure of nitrogen charged into the excitation device, and there is a group of combined switches on the excitation device, which can realize the charging and discharging of nitrogen. The incident, transmitted and buffer bars made up of 40 Cr alloy have lengths of 2.0 m, 1.5 m and 0.5 m, respectively. The density, elastic modulus and wave velocity are 7821 kg/m\({}^{3}\), 233 GPa and 5461 m/s. All the bars have a diameter of 50 mm. The dynamic strain-meter (DL-850E) and oscilloscope (CS-1D) are applied to monitor the dynamic strain and display the waveform, while the industrial camera (Fastcam Sa1.1) with a maximum shooting speed of 675,000 fps is used to capture the photos of rock specimens during loading. In this work, the shooting speed was set as 75,000 fps, that is, one image was collected every 13.33 us. To clearly capture the sample image, a high-power supplementary light source was set up next to the camera during the tests. Also, a self-made rock debris collection box was arranged on the pedestal to collect the rock fragments during the tests. Note that the several monitoring devices need to be turned on simultaneously to ensure that the monitored parameters are synchronized. The principle of the dynamic impact tests employing the SHPB system is as follows [5]: Firstly, by opening the excitation device, the high-pressure gas delivered from the nitrogen tank drives the spindle-shaped striker to strike the tail of the incident bar, resulting in the occurrence of elastic stress wave. The strike speed of the striker could be captured using the laser velocimeter (model TT-02). Then the elastic stress wave extends steadily at a speed of \(C_{e}=(E_{e}/\rho_{e})^{\mathrm{\char 32}}\) along the incident bar, and it will reach the interface I through a time of \((L_{e}/C_{e})\). At this time, the stress wave could transmit and reflect because of the different wave impedance in samples and bars. Afterward, the stress wave keeps on propagating in the sample. When the propagation time is \((L_{s}/C_{s})\), it approaches the interface II and the stress wave generates transmission and reflection again. Further on, the voltage signals of the stress wave on the incident and transmitted bars can be captured using the dynamic  
Figure 3: Experimental results of mechanical properties: (**a**) stress–strain curve of specimen subjected to axial loading, and (**b**) Mohr–Coulomb fitting curve of sandstone on the basis of shear results. strain-meter through the bridge box and displayed on the oscilloscope. Finally, the dynamic mechanical parameters, including the dynamic stress, dynamic strain and strain rate of the specimen could be derived based on the stress wave theory.