# A Study on the Effects of Hob Temperature on the Rock-Breaking Characteristics of Sandstone Strata
## 1 Introduction
In the study of thermal effects on rock breaking, it was observed that when the thermal expansion coefficient of embedded particle galena exceeds that of matrix particle calcite, cracks extend in a direction perpendicular to the intersection of embedded and matrix particles. This phenomenon occurs irrespective of the size or shape of the embedded particles. [PERSON] et al. [11] utilized PFC to explore the effects of different cooling methods on the Brazilian cleavage of granite post high-temperature treatment. They constructed numerical models for particles of varying diameters, based on the distribution characteristics of fine components on the surfaces of standard disc specimens used in Brazilian splitting tests. These simulations revealed that for specimens without heat treatment, Brazilian splitting strength diminished with increasing particle size. However, below 400 \({}^{\circ}\)C, the compression strength remained relatively unaffected. For the same particle size, heat-treated granite exhibited decreased Brazilian splitting strength, with higher temperatures correlating to lower strength. Interestingly, the impact of two cooling methods--water and natural cooling--on the Brazilian splitting strength of granite was found to be negligible. Additionally, as particle sizes increased, the length of thermally induced cracks grew, while their number decreased. [PERSON] and [PERSON] [12] established that the rock-breaking load exerted by a hob is not only influenced by the projected area of the hob blade on the rock surface but also by the rock's compressive strength. This was concluded by studying the stress intensity caused by hob action when the rock's compressive strength was exceeded. [PERSON] et al. [13]developed a mechanical model of rock breaking via hobbing, based on rock extrusion and tensioning mechanisms. They deduced that the spacing of hob-cutting grooves relates to the length of tension cracks and derived an equation for the normal thrust and hobbing force of disc-shaped hobbing based on zero moment of force. [PERSON] et al. [14] conducted uniaxial compression experiments on laminated gneiss, mud slate, and schist with varying dip angles, analyzing that the uniaxial compression damage patterns in laminated rocks of different dip angles are essentially similar. [PERSON] et al. [15] employed discrete element software to simulate the rock-breaking processes of single and double knives. [PERSON] et al. [16] utilized the discrete element method to analyze the dynamic process of rock cutting via hobbing. [PERSON] [17] developed a discrete element model to correlate the geometric distribution of rock joints with deformation characteristics. [PERSON] et al. [18] used discrete element software for simulating rock breaking via hobbing and analyzed the crushing pattern of the rock exposed to hobbing. [PERSON] et al. [19] investigated the influence of confining pressure conditions and rock-breaking surfaces on the rock-breaking process during hobbing, using a two-dimensional discrete element numerical simulation model. When rocks suffer from multidirectional damage, their fundamental mechanical properties undergo significant changes. [PERSON] et al. [20] investigated the constitutive relationship of sand by introducing the coupling of damage and consolidation stress. The coupling of stress and heat can also be considered in waste storage and in relation to the thermal effects of tunnel boring machine (TBM) cutterheads during cutting. From the comprehensive review of theoretical, experimental, and numerical simulation studies, it becomes evident that the current understanding of rock breaking via hobbing does not adequately consider the impact of hob temperature on rock-breaking characteristics. During excavation, the hob generates substantial heat, which alters the internal micro-contact dynamics of sandstone under these high-temperature conditions [6; 7; 8; 9; 10]. These changes significantly influence the force exerted by the hob and the resulting damage patterns in the surrounding rock.