Experimental research on failure mechanism and evolution characteristics of thermal-treated granites under action of impact load with PDC cutter

High rock temperatures and complex engineering environments are the most significant problems in enhancing the rate of penetration (ROP) and drilling tool efficiency for hot dry rocks (HDR). Studying the failure characteristics of different thermal-treated granites by PDC cutters during percussive drilling has profound significance in enhancing the compatibility between drilling teeth and rock during HDR mining and optimizing the drilling toolʼs efficiency in breaking rocks. Through drillability tests and SHPB tests, the present study explored the relationship between drillability and the dynamic behavior of granites. In addition, granites with different heating temperatures and cooling methods were subjected to dynamic rock-breaking tests by the SHPB system installed with the PDC cutter. Furthermore, the impact crater was quantified by 3D contour scanning technology, and the fractal theory was utilized to analyze the fragments. Then, the equations for the dynamic magnification factor of intrusion, fractal dimension, and specific rock-breaking energy of granite upon impact loading are discussed. According to the results, the impact crater size is closely associated with the heating temperature of the rock. Heating and rapid cooling treatments will affect the thermal shock damage and strength of rock, and then change the penetration depth, damage volume, and area of impact craters generated from the PDC cutter. The granite at 600 ℃ exhibits about 45% smaller fragment size than that at room temperature, while the damage degree of the liquid nitrogen-cooled specimen will be greater. According to the variation of dynamic magnification factor, fractal dimension, and specific rock-breaking energy of rock with thermal shock damage, it can be found that with the temperature increasing, the problems drilling tools need to solve gradually shift from increasing ROP to improving cleaning efficiency. The experimental results are expectedly to lay the ground for high compatibility between drilling tools and formation in HDR.