Experimental study on the dynamic direct tensile fracture mechanism of thermally damaged sandstone

Abstract

Understanding the dynamic tensile fracture mechanism of thermally damaged coal-rock media is crucial for developing scientific prevention and early warning systems in geotechnical engineering, particularly for high-temperature dynamic environments like underground coal gasification. This study employs a high-temperature loading system and a split Hopkinson tension bar (SHTB) experiment system to conduct dynamic direct tensile failure experiments on high-temperature thermally damaged coal sandstone. Three-dimensional cross-sectional scanners, scanning electron microscopy (SEM), and computed tomography are used to reveal the macroscopic and microscopic mechanisms of dynamic direct tensile fracture in thermally damaged coal sandstone. Experimental results show that temperature has a more significant effect on the macroscopic fracture characteristics of coal-rock media than impact velocity. As the impact velocity increases, the number of macroscopic debris gradually increases. However, the rise in temperature causes a deviation between the fracture plane normal and the tensile load direction and reduces the size of macroscopic debris. The macroscopic cross-sectional structural parameters of tensile failure exhibit an exponential change with increasing temperature and impact velocity. However, the change in cross-sectional structural parameters with temperature is significantly greater than with impact velocity. Additionally, the brittleness of the samples initially increases and then rapidly decreases with rising temperature, with the influence of high temperature on the rocks’ brittle-ductile properties gradually intensifies. The evolutionary pattern of microcracks and microporous defects within the coal-rock media shows that the formation and expansion of microcracks and the decoupling of mineral interfaces due to temperature significantly influence the rock’s physical and mechanical properties. At lower temperatures, the coal-rock media exhibits relatively smooth brittle fracture characteristics. However, under high-temperature conditions, the rock damage effect intensifies, and the cross-sectional morphological characteristics transition from brittle to ductile, exhibiting more complex and rough fracture forms. The increase in impact velocity mainly affects the undulation and roughness characteristics of the cross-sectional structure. The impact velocity has a lesser effect on the morphological characteristics of the coal-rock media’s cross-section at a certain temperature.