# Anisotropy and Directivity Effects on Uniaxial Compression of Carbonaceous Slate Form Jinman Mine
## 3 Results and Discussion
### Failure Characteristics
#### 3.4.2 Micro-Cracks and Fracture Characteristics
Compared with slate samples under vertical loading, slate samples under parallel loading primarily show a tensile fracture, and the fractured sections have a lower roughness. The scattered fractures and sections are smooth without protrusions or transversdalline cracks. This is mainly due to the tensile fracture of weak surface structures due to the principal stress disturbance, and the fracture surface formed after the cement cracking dominates the slate fracture trend [32; 33], and only local mineral crystals have a tensile fracture. Structural stress adjustment in the gradual intersection of internal cracks leads to mechanical effects such as friction and tearing on the macroscopic fracture surface, which in turn, causes tensile and tearing fractures, clustered tensile fractures, concave tensile fractures, friction scratches, or intergranular cracks. Figure 14 shows that the principal stress causes microscopic penetration cracks with no smooth fractures. The tearing and tensioning effects on the weak surface of slate are caused by transverse tension which results in layered fractures (Figure 14b) and produces smooth sections without microcracks (Figure 14c,d). This is significantly different from the micro-cracks and fracture characteristics under vertical loading. An analysis of the mechanism of action indicates that when the principal stress is parallel to the weak structure surface, the rock pillar causes the stress interaction between mineral crystals due to the compression rod effect [34; 35], following which the damage degree of slate is significantly improved. The differences in the load-bearing strength of mineral crystals cause stronger mineral crystals to wedge into the lower-strength structural layer, resulting in cracks in surrounding rock formations, which increases structural damage. The wedging action of crystals leads to general convergence through the cracks and decomposition of the mineral structure. The degree of damage is closely related to the cohesive force in mineral crystals and the cementation strength between the mineral crystal and cement. Therefore, microstructural changes are the cause of the deterioration of the mechanical properties of slate. Figure 14: Micro-cracks and fracture characteristics under parallel loading: (**a**) Gray sample (2000\(\times\)); (**b**) Gray sample (5000\(\times\)); (**c**) Carbonaceous sample (2000\(\times\)); (**d**) Carbonaceous sample (5000\(\times\)). The fracture morphology and micro-fracture characteristics of slate samples indicate that slate microstructure has three types of microstructure fracture modes; intergranular fractures (cracks appear on the edge of the crystal and cement), transgranular fractures (stress concentration causes cracks to pass through the crystal), and intergranular-transgranular composite fractures (cracks gradually expand through the cement and crystal structure), which is consistent with shale micromechanical features [36; 37; 38; 39; 40]. There are differences in fracture smoothness, fracture area, and the presence of micro-cracks between vertical and parallel loading modes.