Experimental and simulation study on tensile mechanical characteristics and crack development law of sandstone after thermal treatment

The thermal damage caused by high temperature will accelerate the deterioration of rock mechanical properties, affecting the stability of surrounding rock in underground engineering. For further explore the tensile mechanical properties and crack development law of rock after thermal treatment, Brazilian splitting test and ThreeDimensional Particle Flow Code (PFC3D) numerical simulation of sandstone after thermal treatment at 25 degrees C-900 degrees C were carried out. Based on scanning electron microscopy (SEM), X-ray diffraction (XRD) tests, it is determined that the thermal cracks in the rock show the characteristics of "disorderly distribution, centralized development, a large number of microcracks develop to form penetration trend, and local penetration to form complex crack network". By using image threshold segmentation to quantitatively analyze the high temperature thermal damage, it is determined that the high temperature thermal damage presents the characteristics of "slow increase, rapid increase". With the increase of thermal treatment temperature, the tensile strength gradually decreases, and the peak displacement gradually increases. The thermal decomposition of kaolinite at 450 degrees C is the turning point of accelerated deterioration of sandstone mechanical properties. High temperature will increase the internal strain concentration factor and range to accelerate the crack growth. PFC3D numerical results show that high temperature will promote the generation of shear cracks, resulting in the formation of more complex three-dimensional crack penetration network. The type and distribution characteristics of mineral composition and the thermal expansion characteristics of mineral crystals are the main controlling factors for the difference of initial thermal damage, and the penetration of thermal cracks and loading cracks under external force is the direct cause of rock strength instability, which can provide help for the stability of deep underground engineering.