## 3 Results and Discussion
### DIC Analysis of the Dynamic Impact of Layered Sandstone After High-Temperature Treatment  
Figure 14 depicts the development process of primary cracks along with the strain field evolution nephograms of layered sandstone during impact loading following heat treatment at different temperatures. As shown in this figure, no distinct strain concentration areas exist prior to the failure of the layered sandstone. These represent the locations where failure is initiated. At low temperatures (T \(\leq\) 400 \({}^{\circ}\)C), high-strain areas are mainly concentrated on each side of the involved specimen, gradually expanding toward the center. As the temperature rises, the ductility of layered sandstone increases, and the contact area between the two sides of the sample and the loading bars increases, reducing the stress concentration at the ends. In addition, as the heat-treatment temperature rises, cracks in the sandstone become more developed. In particular, cracks around the bedding bands are likely to propagate and interconnect with one another, causing high-strain areas to form easily within the internal region of the sandstone at high temperatures. Temperature has a significant impact on the propagation and interconnection speeds of cracks in sandstone. As the temperature rises, the number of cracks that form during the failure of layered sandstone increases greatly, and the range of the pre-failure strain concentration areas expands significantly. This is because at lower temperatures, as clay minerals gradually dehydrate, the cementation between sandstone grains becomes weaker. Hot cracks form on grain boundaries and propagate along these boundaries where the strain is more concentrated. When the temperature increases above 400 \({}^{\circ}\)C, hot cracks start to form within grains due to the anisotropic thermal expansion of mineral grains. This increases the number of internal pores and cracks within sandstone, as well as their lengths and widths, by a significant degree. These cracks along grain boundaries interconnect with intragranular cracks, thus expanding the range of the strain concentration area and significantly reducing the specimen failure time. Figure 12: Variation of the dynamic compressive strength (**a**) and dynamic elastic modulus of layered sandstone (**b**). Figure 13: Impact fracture characteristics of bedding sandstone after high-temperature treatment. Cracks formed during the impact failure in layered sandstone mainly propagate along the weak bedding planes. Increasing the heat-treatment temperature only negligibly influences the propagation direction of primary cracks due to the presence of a large amount of magnetite grains embedded between quartz and feldspar mineral grains along the bedding bands. Pores in these areas are more developed than those in clay cementation matrix areas, making it easier for cracks to interconnect with one another. Thus, cracks in horizontally layered sandstone are mainly controlled by weak bedding planes. During high-temperature treatment, the differences in thermal expansion between various minerals become pronounced. For example, the thermal expansion coefficients of quartz and feldspar  
Figure 14: Images of the sandstone failure process and strain field evolution nephograms after layered sandstone is heat-treated at different temperatures. (**a**) Rock sample at room temperature; (**b**) Rock sample heated at 200 \({}^{\circ}\)C; (**c**) Rock sample heated at 400 \({}^{\circ}\)C; (**d**) Rock sample heated at 600 \({}^{\circ}\)C; (**e**) Rock sample heated at 800 \({}^{\circ}\)C.  
minerals exhibit an order of magnitude of \(10^{-6}\)/K, while the coefficient of magnetite minerals exhibits a substantially higher order of magnitude of \(10^{-5}\)/K. Therefore, in bedding bands, differences in thermal expansion between different mineral grains are more pronounced, resulting in higher expansion stress and denser hot cracks within these bands.