# Research on the Development Mechanism of Three Kinds of Cracks in Rock Cutting
## 1 Introduction  
Rock cutting is one of the primary methods of mechanical rock fragmentation. The efficiency of rock cutting may be increased via research, which can also shorten construction cycles and lower construction costs. Early research on rock-cutting mainly focused on the rock-cutting properties of big drill rigs, which examined the rock-breaking efficiency under various depths and tooth configurations [1]. Numerous academics have expanded on the subsequent study, and various theoretical conclusions have been reached. Substantial experiments and observational data demonstrated that the basic damage of rocks is in the form of ductile collapse and brittle fracture [2; 3; 4]. Brittle fracture is the result of crack development, whereas plastic collapse is the outcome of plastic deformation and flow, which seldom results in additional micro-cracks. For rock cutting, when the depth of it is shallow, the damage belongs to the ductile collapse. However, as the depth of the cut increases, the damage converts to the brittle one. [PERSON] and [PERSON] [5] investigated that when the cutting depth is less than 1\(\upmu\)m, tiny fragments occur and are not accompanied by micro-cracks. It is called ductile failure, which occurs in the surface failure experiments of rock materials. It is commonly accepted that the size of the rock fragmentation in brittle fracture generated by rock cutting is closely proportional to the depth of rock cutting, which was proven in the investigation by [PERSON] and [PERSON] [6; 7]. Namely, [PERSON]'s Size Effect Law forquasi-brittle material was proved to be applicable to rock cutting. There was little research on the form of crack propagation and the fine mechanism of chip flaking in the related research of rock cutting, which was mainly focused on the essence of ductile collapse and brittle fracture and the mechanism of the ductile-brittle transition. In many experimental studies of rock cutting, the cutting depth is usually used as an experimental variable to study the rock breaking mechanism and rock breaking effect at different depths. [PERSON]'s [8] team conducted a variety of deep downcutting experiments and proved that the formation mechanism of small debris also exists between plastic failure and brittle failure. Microcracks that form as a result of drilling teeth pushing outward cause small fragmentations. These small fragmentations appear as thin flakes, and the lithology and cutting depth have an impact on the chip size. [PERSON] et al. [9] analyzed three stages of cutting process from weight on bit, torque on bit, depth of cutting and speed, and proposed mathematical models of parameters such as weight on bit and torque on bit in different stages. There is intermittently generated little fragmentation in addition to the discontinuous development of huge fragmentation in the brittle damage. The study conducted by [PERSON] et al. [10] proposed a novel approach to determine the transformation of rock failure mode based on the analysis of cutting forces at various cutting depths. The effectiveness of this method is validated through a series of single PDC cutter tests. Many other scholars have also carried out cutting experiments at various depths, such as Refs. [11; 12; 13; 14; 15]. The study of damage cracks and debris formation has an important impact on the subsequent fracture generation and even determines the basic form of fracture generation in the damaged area. PDC teeth linear cutting trials were seen using high-speed cameras and post-experimental sections by [PERSON] et al. [16]. It was discovered that the primary fracture in rock cutting creates curved rock fragmentation that is either directed toward the rock matrix or the free surface. In certain numerical calculations and tests, such as those by [PERSON] et al. [17] and [PERSON] et al. [18], damage fragmentation beneath cutting pits have been examined. Potential fragmentation can only be analyzed in sections after the trials since they are challenging to see in rock-cutting experiments.