## 2 Materials and Methods
### Testing Equipment  
Figure 3a shows the \(\varphi\)50 mm SHPB experiment system independently developed by the coal or rock dynamic load damage parameter testing laboratory, which lies in the school of emergency management and safety engineering, China University of Mining and Technology (Beijing, China). The system consisted of a power supply subsystem, a rod subsystem, an information acquisition subsystem, and a data processing subsystem. As shown in Figure 3a, the power supply subsystem, i.e., the stress-wave generator, used a combination of an air compressor, high-pressure air storage chamber, air cavity, and SHPB electronic control system; this controlled the impact velocity of the bullet by setting the magnitude of the air pressure. The rod subsystem consisted of a cylindrical impact rod (bullet), incidence rod, transmission rod, and energy-absorbing rod. Each rod was cylindrical with a diameter of 50 mm and lengths of 400 mm, 3000 mm, 2500 mm, and 1000 mm, respectively. The rod was made of mild steel with a density of 7740 kg/m\({}^{3}\) and a modulus of elasticity of 206 GPa. The data acquisition subsystem consisted of an infrared velocimetry device and a strain signal acquisition system. To ensure the accuracy of the test, double strain gauges were affixed to the incident and transmission rods, dual channels were set up for data acquisition, and the incident, reflected, and transmission wave signals were recorded with an ultra-dynamic strain gauge and researched in a targeted way using the data processing subsystem.  
Figure 2: An illustration of coal and rock specimens used in the experiment. (**a**) Coal specimens; (**b**) rock specimens.  
Notably, to be able to accurately reflect the destruction process of the coal and rock specimens during impact dynamic loading, the data acquisition subsystem also included a high-speed photography device for recording the rupture process of the sample. As shown in Figure 3b, the device consisted of a FASTCAM SA5 high-speed camera developed by PHOTRON, China Shenzhen Shenniu Photographic Equipment Co., Ltd. (Shenzhen, China), a produced VL300 fill light and synchronous trigger device, and other components. The camera frame speed was set to 100,000 frames per second with a pixel resolution of 320 \(\times\) 192, and a shield made of high-transmittance acrylic panels was installed around the coal and rock samples to prevent the camera and the filler lamp from being damaged by the splashing of the samples during the destruction of the coal and rock, as well as to facilitate the collection and sieving of crushed coal and rock fragments [26].  
Figure 4 shows the instruments used in the pre-treatment of coal and rock. Figure 4a shows the electric blast dryer, which adopts a hot air circulation system to ensure uniform heating of coal and rock samples. The temperature control range is room temperature (RT) +50-250 \({}^{\circ}\)C, and the constant temperature fluctuation is \(\pm\)1 \({}^{\circ}\)C. Figure 4b is a natural saturated container. Figure 4c is the constant-temperature test chamber, which adopts an intelligent and high-precision proportional integral derivative temperature control system with strong temperature stability and a temperature control range from \(-\)40 \({}^{\circ}\)C to  
Figure 3: Experimental system diagram. (**a**) Schematic diagram; (**b**) physical diagram.  
100 \({}^{\circ}\)C. Figure 4d shows the ZBL-U5100 nonmetallic ultrasonic detector produced by Beijing Zhibo Lian Technology Co. (Beijing, China). The acoustic time measurement range is \(\pm\)1,677,700 \(\upmu\)s, the gain accuracy is 0.5 db, and the acceptance sensitivity is less than 10 \(\upmu\)v.