## 5 Conclusions and Recommendations  
1. A device testing micro-crack evolution characteristics of shale has been designed to realize simultaneous monitoring of ultrasonic, mechanical and video information in crack evolution processes of shale under load so as to analyze acoustic and mechanical characteristic responses at key points of crack propagation.
2. With increases in load, shale undergoes four stages of destruction, crack initiation, propagation, penetration and overall failure. The dominant amplitude curve of acoustic wave matches well with the stress-strain curve in fluctuating points, which indicates that during the process of shale shear failure, acoustic characteristics and mechanical characteristics are in good agreement, proving the validity of predicting rock mechanical parameters with acoustic data.
3. Combined with the video monitoring, it was found that at several points when shale crack occurred, the mechanical curve did not change, while acoustic characteristics had big changes. This indicates that during the load process of shale, the dominant amplitude of acoustic waves is more sensitive to rock cracking than mechanical parameters.
4. Due to the lighting, the video only monitored one surface of the shale sample. For further studies, test equipment needs to be modified. Adding CT scanning technology is recommended, which will help improve the completeness of the description of crack state changes during loading process of the rock sample.
5. This research can provide theoretical foundations for the evaluation of deformation mechanism and stability of host rocks in underground engineering.  
Conceptualization, Q.C.; Data curation, F.X., P.S., Y.Z., Y.L. and D.J.; Project administration, H.Z.; Writing-review & editing, X.G., J.C. and D.L. All authors have read and agreed to the published version of the manuscript.  
The study was funded by Key Laboratory of Mine Geological Hazard Mechanism and Prevention and Control (no. 2018-08), Xigaze City Science and Technology Plan Project (RKZ2020 KJ01), Key Laboratory of Rock Mechanics and Geohazards of Zhejiang Province (no. ZJRMC-2019-01), Chongqing Natural Science Foundation project (no. cstc2019)cyj-msxm0749, cstc2021 icyj-msxmX0187), Ecological environment survey and ecological restoration technology demonstration project in the water-level-fluctuating of the Three Gorges Reservoir (no. 5000002021 BF40001), The Tibet Autonomous Region Natural Resources Department's prevention and control capacity system construction project (no. 2020-0890-2).  
Not applicable.  
The data used to support the findings of this study are available from the corresponding author upon request.  
The study was funded by Key Laboratory of Mine Geological Hazard Mechanism and Prevention and Control (no. 2018-08); Xigaze City Science and Technology Plan Project (RKZ2020 KJ01); Key Laboratory of Rock Mechanics and Geohazards of Zhejiang Province (no.  
msxmX0187); Ecological environment survey and ecological restoration technology demonstration project in the water-level-fluctuating of the Three Gorges Reservoir (no. 5000002021 BF40001); and the Tibet Autonomous Region Natural Resources Department's prevention and control capacity system construction project (no. 2020-0890-2). Their support is gratefully acknowledged.  
The authors declare no conflict of interest.