Study on dynamic mechanical properties and microscopic fracture characteristics of coal measures mudstone at real-time temperature

In China's open-pit coal mines, there are generally goaf areas with old kilns. The spontaneous combustion of coal seams in the area will have a significant impact on the macroscopic mechanical properties and microscopic structure of the surrounding rock mass. The impact load of blasting is a key factor inducing the instability of the surrounding rock in the coal fire combustion area, which is prone to causing safety accidents such as collapse of working benches and fires, posing a serious threat to the production safety of open-pit coal mines. Therefore, conducting research on the dynamic mechanical characteristics and micro fracture mechanism of coal bearing mudstone under real-time temperature is of great significance for the working steps and safe mining of open-pit coal mines. With the help of the self-developed high-temperature separated Hopkinson pressure bar (H-SHPB) testing device, dynamic compression tests of coal bearing mudstone samples were carried out at room temperature (25 ℃) and real-time temperatures of 100, 200, 300, 400, and 500 ℃. Combined with scanning electron microscopy, energy spectrum analysis, and component testing experiments, the dynamic mechanical properties and macroscopic and microscopic fracture characteristics of coal bearing mudstone were systematically studied. The research results indicate that the dynamic peak strength, elastic modulus, and peak strain of coal bearing mudstone increase with the increase of strain rate, exhibiting a significant strain rate strengthening effect; Under a fixed strain rate, the dynamic peak strength and elastic modulus show a trend of first increasing and then decreasing with the increase of temperature, fluctuating upwards in the temperature range of 25−200 ℃, and monotonically decreasing in the temperature range of 200 ℃ to 500 ℃; The degree of damage to coal bearing mudstone is positively correlated with strain rate, but it exhibits a significant interval effect with increasing temperature; as the strain rate increases, the fracture morphology of coal bearing mudstone after failure gradually transforms from shear fracture morphology to tensile fracture morphology. Moreover, at high strain rates, the failure mode of sodium feldspar and orthoclase grain skeletons gradually changes from intergranular failure to transgranular failure mode, which is the main reason for the increase in the degree of damage of coal bearing mudstone; The effect of temperature on coal bearing mudstone can be divided into two stages. At 25−200 ℃, the clay minerals inside the coal bearing mudstone expand due to heat, filling the gaps between skeleton particles and clay minerals, making the coal bearing mudstone more compact as a whole. The degree of damage gradually decreases, showing a strengthening effect; Under the influence of high temperature thermal stress at temperatures between 200 and 500 ℃, coal bearing mudstone experiences an increase in internal cracks and a decrease in grain size, resulting in a decrease in overall strength and a gradual increase in degree of damage, exhibiting a deterioration effect.