# Dynamic Mechanical Properties of Heat-Treated Shale under Different Temperatures
## 4 Shale SHPB Experiment  
At present, a split Hopkinson pressure bar is commonly used to test the dynamic characteristics of brittle materials, such as concrete and ceramics [32; 33; 34]. The impact experiment of the shale was carried out using a SHPB device with a diameter of 50 mm. The test was carried out on the split Hopkinson experimental system in the structural dynamics laboratory of Southwest University of Science and Technology. It is mainly composed of a bullet, an incident bar, a specimen, a transmission bar, an absorption bar, a measuring device, and a data processing system. The schematic diagram of the experimental device is shown in Figure 6, where the lengths of the bullet, incident bar, and transmission bar are 600 mm, 3000 mm, and 3000 mm, respectively. Each bar has a diameter of 50 mm and is made of a steel alloy with a modulus of elasticity (E) of 72 gPa, a yield strength of 300 MPa, and an ultimate strength of 460 MPa. The shale specimen was placed between the incident bar and the transfer bar, where the contact surfaces are parallel. Strain gauges were installed on the incident bar and the transmission bar to collect waveform data. The actual experiment is shown in Figure 7. The position of the strain gauge was two times the length of the bullet from the incident end face. The strain gauge of the incident bar and the strain gauge of the transmission bar were symmetrical with respect to the specimen. \begin{table}
\begin{tabular}{c c c c} \hline
**Specimen Number** & **Failure Load Value (kN)** & **Splitting Strength (MPa)** & **Average Value (MPa)** \\ \hline H10 & 39.99 & 18.76 & \\ H11 & 39.91 & 18.72 & 18.36 \\ H12 & 37.56 & 17.62 & \\ \hline \end{tabular}
\end{table} 
Table 2: Splitting tensile strength. Figure 5: The ultimate failure mode of specimens with a loading angle of 0\({}^{\circ}\). Figure 6: SHPB experimental device schematic diagram. The pulse-shaping technique was used in SHPB testing to promote dynamic stress equilibrium in the sample to reduce the effects of the heterogeneity of the shale material and the effects of high-frequency impacts, which can improve the stress homogeneity of the sample. In this paper, by comparing the effects of different materials as forming sheets, an appropriate material was selected so that the rise time of the incident wave was 300 \(\upmu\)s or more, i.e., the stress of the specimen was much less than the peak stress when the stress was uniform. It can be assumed that the experimental data obtained achieve a constant strain rate loading process of shale materials and satisfy the assumption of stress uniformity. In the process of the waveform shaping experiment, according to the experience of predecessors [35], butter, copper, brass, rubber, and other materials were added to the front end of the incident bar to shape the curves. The circular sheets with a diameter of 15 mm and a thickness of 2 mm were prepared, and the SHPB empty bar experiment was completed, and the incident waveform diagrams of the different materials are shown in Figure 8.  
According to the experiment, the loading time reached 400 ms when rubber was added as the shaping piece, and the waveform was similar to a sine wave. After adding the rubber shaping sheet, high-frequency impacts were effectively avoided. After analyzing  
Figure 8: The incident waveform of different shapers. Figure 7: Physical picture of the SHPB experimental device. the incident pulses of shaping pieces of different materials, rubber was selected as the shaping piece in this experiment, and its average loading time reached more than 300 ms. By calculating the stress balance time required by the shale, it can be seen that the stress balance time of the test pieces is far less than the time required to reach the peak stress. Therefore, the selected rubber shaping sheet can sufficiently meet the requirements of stress balance in the later analysis. At present, there are few SHPB compression experiments on shale with temperature. High-temperature dynamic mechanical experiments have been proposed much earlier.