## 2 Material and Experiment
### Experiment Results and Analysis  
In order to avoid the results being disturbed by external factors and random error, twenty specimens with ISCSC configuration were employed and impacted with different speeds, from 6.0 m/s  
Figure 3: Improved Single Cleavage Semi-Circle Specim (ISCSC) specimen sample geometry. Figure 2: Schematic of the Split Hopkins Pressure Bar (SHPB) test system and crack propagation gauge (CPG) test system. to 8.5 m/s. According to the laws of processing data, the data were filtrated and the signals collected from the experiment were analyzed. Based on the voltage signals recorded by the strain gauges, the strain signals could be converted with Equation (4):  
\[\varepsilon=\frac{4U_{0}}{nE_{g}K} \tag{4}\]  
where \(\varepsilon\) is the strain, \(U_{0}\) is the voltage recorded by strain gauges, \(E_{g}\) is the bridge voltage, \(K\) is the sensitivity coefficient of the strain gauges, and \(n\) is the gain coefficient of an amplifier. Then, the loads acting on the top and bottom of the specimen could be calculated with Equation (5), and their curves of loads versus time are shown in Figure 4a. \[ \begin{split} P_{i}(t)&=A_{i}E[\varepsilon_{i}(t)+ \varepsilon_{r}(t)]\\ P_{t}(t)&=A_{t}E\varepsilon_{t}(t)\end{split}  \tag{5}\]  
where \(\varepsilon_{i}(t)\) and \(\varepsilon_{r}(t)\) are the strains of incident bar caused by incident wave and reflected wave, \(\varepsilon_{t}(t)\) is the strain of transmitted bar caused by the transmitted wave, \(P_{i}(t)\) is the loads on the top of the specimen and \(P_{t}(t)\) is the loads on the bottom of the specimen. According to the curves of the loads detected in different tests, the maximum loading rate of each experiment could be obtained technically by utilizing Origin software, as Figure 4b shows. Thus, three typical crack propagation patterns in the experiment were selected, each with a different maximum loading rate--1000 GPa\(\cdot\)s\({}^{-1}\), 1500 GPa\(\cdot\)s\({}^{-1}\), and 2000 GPa\(\cdot\)s\({}^{-1}\), respectively. The incident and reflected waves which were used to calculate the loading rate and the transmitted wave in Figure 4b are the waves obtained in the incident and transmitted bar showing stress equilibrium condition. However, the specimens in this experiment didn't satisfy the dynamic force equilibrium condition. We agree that it is a critical step to determine whether or not the test meets the dynamic force equilibrium condition in the SHPB experiment. Assuming that the stress/strain of the specimen is uniformly distributed along its length, its purpose is to decouple the stress wave effect and strain rate effect. It is the basic assumption of the quasi-static method. But, in this paper, the ISCSC specimen was mainly employed to investigate crack propagation behaviour and loading rate effect, whereas the numerical method was introduced to continue the thorough research. The whole research method is a hybrid experimental-numerical approach [35], and it is already beyond the scope of quasi-static study. Therefore, it is not necessary to satisfy the assumption of the stress balance. Figure 5 presents three typical crack propagation patterns of specimens in the experiment. The crack propagation paths on the specimen HT-1, HT-2, and HT-3 in Figure 5 were in a straight  
Figure 4: Curves of loads versus time (**a**) The dynamic stress curves detected from the incident bar and transmitted bar. (**b**) Determination method of the loading rate. line before the running crack reached the two holes' zone, where two empty holes were deployed, which accorded with the feature of mode I of fracture. As the running cracks went across the two holes area, the surfaces of two empty holes were damaged at different levels, with secondary cracks appearing around the two empty holes. The microstructure of the rock material played a crucial role in dominating the crack initiation and propagation, helping the running crack behave as shown above.