## 4 Analysis of Rock Fracture Patterns
### Microscopic Analysis of Blasting Fracture Modes  
Sandstone is composed of various minerals, with numerous fractures between mineral crystals. After blasting, the complexity of the stress wave propagation and the uncertainty of the internal material structure resulted in a complicated rock fracture morphology. The fracture mechanism of the sandstone under the coupling action of blasting stress and in situ stress can generally be classified into tensile fractures and shear fractures. Tensile fractures are caused by tensile stress and can be categorized into transgranular fractures, intergranular fractures, and their mixed forms based on the fracture path. Shear fractures are induced by shear stress, where the relative sliding causes friction marks on the crystal planes or cleavage surfaces, or large displacements along the grain boundaries. Depending on the deformation type, fractures can be classified into cleavage fractures, quasi-cleavage fractures, and ductile fractures, with rock-like mineral materials under blasting loading mainly exhibiting cleavage or quasi-cleavage fractures. Based on the macroscopic plastic deformation before the fracture, the types can be divided into brittle fractures, brittle-plastic fractures, and plastic fractures. The SEM sample fracture morphologies primarily featured tensile fractures, brittle cleavage fractures, and transgranular fractures, with shear fractures, brittle-plastic quasi-cleavage fractures, coupled fractures, and intergranular fractures being less common. Under 0 MPa in situ stress, the blasted fracture surface was influenced by both tensile and shear stresses, with the stemming segment showing a predominantly tensile fracture morphology (stepped patterns, river patterns, herringbone patterns, etc. ), while the charge and bottom segments exhibited some shear fracture features (slip patterns). A small amount of brittle-plastic quasi-cleavage fractures (dimple patterns) appeared in the charge and bottom segments, with brittle cleavage fractures predominating. A few coupled fractures appeared in the bottom segment, primarily transgranular fractures. In the stemming segment, the accumulation and rapid release of blasting energy, coupled with high-strain-rate tensile stress, caused the rock to fracture swiftly along the micro-voids, resulting in smooth tensile fracture surfaces that are characteristic of brittle cleavage fractures. The charge and bottom segments were subjected to both tensile and shear stresses, and, due to the weakening of the blasting energy, brittle cleavage and transgranular fractures were dominant, though quasi-cleavage and coupled fractures were also present. Under 0.5 MPa in situ stress, the blasted fracture surface was influenced by both tensile and shear stresses, with brittle cleavage fractures being predominant, accompanied by some brittle-plastic quasi-cleavage fractures, primarily transgranular. Coupled fractures appeared in the charge segment, and, in addition to the coupled fractures, intergranular fractures appeared in the bottom segment. The in situ stress made the rock more difficult to fracture, reducing the effective breaking energy in the bottom segment, with more fractures being driven by blasting gases, resulting in intergranular fractures. Under 1 MPa in situ stress, the overall fracture types of the blasted surface showed no significant difference in the mechanism and deformation forms compared to those under 0.5 MPa in situ stress. However, in the fracture path, in addition to transgranular fractures, a few coupled and intergranular fractures appeared. The charge segment was less smooth compared to the stemming segment, with numerous irregular particles, and transgranular fractures were increasingly replaced by intergranular and coupled fractures. With the increase in the in situ stress, the fracture surface under 2 MPa in situ stress showed more pronounced coupled and intergranular fractures compared to that under 1 MPa in situ stress.