## 3 Results and Discussion
### Dynamic Mechanical Properties of Layered Sandstone After High-Temperature Treatment  
#### 3.7.1 Evolutionary Patterns of the Dynamic Mechanical Properties  
The dynamic stress-strain curves of layered sandstone after heat treatment at different temperatures, along with the calculated average strain rates, are depicted in Figure 11. Under identical impact pressure conditions, the average strain rate of the experiment  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**No.**} & \multirow{2}{*}{**T/\({}^{\circ}\)C**} & **Height** & **Diameter** & **Density** & **Peak Stress** & **Elastic Modulus** \\  & & **/mm** & **/mm** & **/g\(\cdot\)cm\({}^{-3}\)** & **/MPa** & **/Gpa** \\ \hline C-0-1-1 & 25 & 100.24 & 50.02 & 2.10 & 102.0 & 15.2 \\ C-0-1-2 & 200 & 100.04 & 50.12 & 2.10 & 100.0 & 10.8 \\ C-0-1-3 & 400 & 100.50 & 50.08 & 2.09 & 86.6 & 10.2 \\ C-0-1-4 & 600 & 100.26 & 50.07 & 2.06 & 75.3 & 9.03 \\ C-0-1-5 & 800 & 100.04 & 50.22 & 2.06 & 54.8 & 5.39 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The physical and static mechanical parameters of rock samples.  
Figure 10: Static stress–strain curves of layered sandstone.  
generally increases as the heating temperature rises. However, an exception is observed at 600 \({}^{\circ}\)C, where the average strain rate is notably lower compared to those at 400 \({}^{\circ}\)C and 800 \({}^{\circ}\)C. We hypothesize that this discrepancy can primarily be attributed to variations in sample heterogeneity. In some of the rock samples heated to 600 \({}^{\circ}\)C, the density of bedding planes formed by magnetite particle aggregation is notably low. The decrease in the number of these bedding planes may be a crucial factor contributing to the reduction in the average strain rate observed at this temperature. As shown in this figure, the dynamic stress-strain curves of layered sandstone heat-treated at different temperatures follow a similar pattern, which is divided into four stages, namely compaction, linear elasticity, plasticity, and failure. Stage 1 is the initial compaction stage. Stage 1 is not noticeable when the temperature is between 25 \({}^{\circ}\)C and 600 \({}^{\circ}\)C but is visibly extensive when the temperature reaches 800 \({}^{\circ}\)C, indicating that the volumes of pores and cracks in sandstone grow significantly at this temperature. Stage 2 is the elastic deformation stage, where the stress-strain curves remain generally stable, while strain energy continues to accumulate. The slope in the linear elasticity stage decreases with rising temperature, indicating a gradual reduction in the ability of layered sandstone to resist impact loads and accumulate elastic strain energy. Stage 3 is the plasticity stage. As the temperature rises, the stress in the specimen increases at a slower rate, while the strain increases sharply. The slope of each dynamic stress-strain curve decreases until peak stress is reached, where the slope is zero. Stage 4 is the failure stage, where the residual elastic strain energy is released, resulting in a negative slope in the stress-strain curve. The variation in the peak stress and elastic modulus of layered sandstone following heat treatment at different temperatures is depicted in Figure 12. Evidently, as the temperature rises, the peak stress in the specimen drops at an increasing rate. When the temperature is above the threshold of 400 \({}^{\circ}\)C, the peak stress drops at a significantly faster rate.