Coal resources are rich in Ningxia. Long-term mining creates mine goaf, which causes coal to burn spontaneously for a very long time. Unavoidably, the rocks around the coal fire area are affected by high temperatures, which can alter the characteristics of rocks and lead to safety accidents. To explore the temperature influence of sandstone in coal fire areas under cyclic impact loading, the sandstone treated under different temperatures is tested by a split Hopkinson pressure bar (SHPB). The mechanical properties of rocks treated at different temperatures are obtained. The composition of rock is determined, and the energy dissipation is calculated. Meantime, the digital image correlation (DIC) method is applied to study the mechanical behaviors of sandstone. The results show that at the first impact, the peak stress of sandstone decreases as the temperature increases. However, there is no obvious trend in the peak strain. Under the SHPB cyclic impact, the sandstone specimen is completely destroyed after two to three times of impact at different temperatures. At 25~1000 °C, the dynamic peak stress of sandstone decreases with the increase in impact times, and brittle failure occurs. When the impact pressure is 0.6 MPa, the incident energy increases with the impact velocity; the dynamic peak stress increases with the transmitted energy. Using the DIC method, it is found that when the temperature is below 800 °C, the dynamic characteristics of rock specimen have a close correlation with the crack initiation point and extreme point. When the temperature exceeds 800 °C, the rock specimen is seriously damaged, the overall strain is small, and the stress transfer efficiency is low. These findings show that temperature significantly affects the mechanical properties and initial damage of the sandstone, and the performance and damage are abrupt at 800 °C. Meanwhile, the DIC technology can effectively characterize the strain evolution of rock materials and explain the formation and propagation process of cracks, which provides a valid means for studying the damage and crack evolution of materials.

The rocks in coal fire areas are often in high-temperature environments. The mechanical properties of rocks play an important role in mining and engineering disturbance, so we aim to study the static and dynamic properties of high-temperature sandstone in the coal fire zone under different cooling methods, and to evaluate the degree of thermal damage at different high temperatures. The mechanical properties and failure modes of two cooling methods (natural cooling and watering cooling) were analyzed. In addition, the influence of the longitudinal wave velocity (V_p) on the mechanical properties of the high-temperature sandstone was analyzed, and the composition of the high-temperature sandstone was tested by XRD method. Under two cooling conditions, the static and dynamic peak strength of sandstone increases continuously with the increase of longitudinal wave velocity (V_p), and reduces with the increase of heat-treated temperature, but the damage degree increases with the increase of the heat treatment temperature. In the Split Hopkinson Pressure Bar (SHPB) test, the dynamic compressive strength of naturally cooled sandstone is higher than that of water-cooled sandstone. The failure of natural cooling high-temperature sandstone mainly occurs “X-shape”, transverse splitting failure or breaks into pieces, but the watering cooling high-temperature sandstone presents transverse or longitudinal splitting failure.

Ningxia is located in the western part of China, which abounds with desert resources. The application of desert sand in concrete has not been fully studied. In order to study the failure mechanism of desert sand concrete under impact loading, dynamic impact experiments on desert sand concrete and ordinary concrete were carried out by using the 74 mm diameter split Hopkinson pressure bar (SHPB) experimental apparatus. The dynamic mechanical properties of desert sand concrete under different impact velocities were analyzed. The random distribution program of round coarse aggregate in desert sand concrete was designed to generate the random distribution of coarse aggregate by ANSYS/APDL language. The dynamic failure process of single-graded concrete and two-graded concrete was numerically simulated and compared. The results showed that desert sand concrete and ordinary concrete have obvious rate-dependent effect. Detailedly, the ordinary concrete is totally destroyed when impact velocity is 10 m/s, but the desert sand concrete still maintains the cone shape at impact velocity of 18 m/s, which indicates that the impact resistance of desert sand concrete is better than that of ordinary concrete. When impact velocity is 5 m/s, the single-graded concrete and two-graded concrete are both destroyed, but the desert sand concrete is not damaged. When impact velocity is 10 m/s, the dynamic failure mode of single-graded concrete, two-graded concrete, and desert sand concrete is basically the same. When impact velocity is 15 m/s, the single-grade concrete shows different dynamic failure mode from that of desert sand concrete and two-graded concrete.