The Rock-Breaking Mechanism of Thermal Spalling-Assisted Rock Cutting by PDC Cutter

The challenges pertaining to low rock-breaking efficiency and high drilling cost in deep hard formation and hot dry rock (HDR) are highly pronounced. A novel drilling technology, the combined thermomechanical drilling technology, has demonstrated its potential to enhance the rate of penetration. However, the rock-breaking mechanism for thermal spalling-assisted rock breaking by the drilling bit remains inadequately understood. This paper establishes a model of thermal spalling-assisted rock cutting (TSARC) by polycrystalline diamond compact (PDC) cutter to investigate the influence of different heating and cutting parameters on the rock-breaking efficiency. The outcomes indicate that the brittleness of heterogeneous rock decreases while its plasticity increases after undergoing high-temperature heating. Consequently, this reduces the cutting force and mechanical specific energy (MSE) of the PDC cutter. In the TSARC process, an increase in heating power corresponds to an increase in rock-breaking efficiency. However, once a certain temperature threshold is reached, the rock surface suffers complete destruction due to the high temperature, while the MSE remains unchanged. When considering the same heating parameters, the MSE of TSARC and traditional rock cutting (TRC) is directly proportional to the cutting velocity. The larger the cutting depth and rake angle of TRC, the higher is the rock-breaking efficiency. In contrast, the smaller the cutting depth and rake angle of TSARC, the higher is the rock-breaking efficiency, and the cutting force can be reduced by up to 94%. This study also includes an experimental program to examine the effect of thermal source height and velocity on the rock-breaking behavior of TSARC. It was observed that in comparison to TRC, when the thermal source movement speed of TSARC is 7.5 cm/s or the thermal source height is 5 cm, a significant reduction of 47.37% and 54.1% in the average cutting force, respectively, can be achieved. The results are of positive significance for increasing the rock-breaking efficiency and drilling speed in deep hard and HDR formations.