# Energy Evolution Analysis of Coal Fracture Damage Process Based on Digital Image Processing
## 5 Conclusions  
In this study, uniaxial compression tests were conducted on coal rocks filled with calcite veins using RFPA2D-DIP software, and their mechanical properties and fracture damage energy evolution characteristics were analyzed, which can be summarized by the following rules:  
1. The calcite vein-bearing coal rocks show strong anisotropic characteristics. The different azimuth angles of calcite veins change the internal stress distribution of coal, resulting in higher compressive strength of coal at low dip angle and lower compressive strength of coal at high dip angle.
2. The fracture mode of coal is significantly affected by calcite. At low dip angles (0\({}^{\circ}\), 15\({}^{\circ}\) and 30\({}^{\circ}\)), the coal fracture modes are complex, which are inclined Z-type (0\({}^{\circ}\)), V-type (15\({}^{\circ}\)) and inverted V-type (30\({}^{\circ}\)) failure modes, respectively, and mainly shear failure. At high dip angles (45\({}^{\circ}\), 60\({}^{\circ}\), 75\({}^{\circ}\) and 90\({}^{\circ}\)), the fracture mode of coal and rock is single, which is type I failure mode, mainly tensile failure.
3. Calcite vein has a significant effect on energy evolution during coal fracture. At low dip angle, the input energy, accumulated elastic energy and impact energy index of coal are large. At high dip angle, the input energy, accumulated elastic energy and impact energy index of coal are low.
4. Calcite vein has a considerable impact on the fracture damage process of coal. Under low dip angle, the internal stress distribution of coal is relatively uniform, the weak cementation between coal matrix and calcite vein is not easy to be destroyed, and the stress and energy required for coal destruction are large. Under high dip angle, the internal distribution of coal is uneven, and the weak cementitious material between coal matrix and calcite vein is easy to be damaged, resulting in coal fracture and damage.  
Conceptualization, Z.W.; methodology, L.L.; software, Y.L.; writing--original draft preparation, W.W. All authors have read and agreed to the published version of the manuscript.  
This study was supported by the Project of Special Fund for Science and Technology of The Guizhou Province Graduate Research Fund, the National Natural Science Foundation of China (Project Nos. 52104080, 51964007, 51774101), the High-Level Innovative Talents Training Project in Guizhou Province (Project No. 2016-4011), Project: Scientific Research Project of Guiyang Rail Transit Line 2 Phase I Project No. D2(I)--FW--YJ--2019--001--WT), Teaching Reform Project of Guizhou University (Project No.JG201990), the Science and Technology Support Plan of Guizhou Province (Project No. [2021]401, [2020]4Y046, [2020]227), the Science and Technology Support Plan of Guizhou Province (Science Support of Guizhou Province [2019] 2861) and the Science and Technology Project for Outing and Young Talents of Guizhou (Talents of Science Platform in Guizhou [2019] 5674).  
Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.  
The authors declare no conflict of interest.