## 5 Conclusions  
In this study, a specially-developed high-speed Instron loading system is adopted to perform high-speed compression tests on natural lake ice, combining a two-load-cell system for dynamic load measurement, and a digital speckle correlation method for deformation measurement. According to results of present experiments, the following general conclusions can be drawn:  
1. When loading rate is not greater than 2.0 m\(\cdot\)s\({}^{-1}\), the forces obtained at the two ends of a five-centimeter-high ice specimen are balanced or approximately balanced. Therefore, the difference in forces between the upper-end and the lower-end, resulting from the inertial effect, could be ignored.
2. In ice dynamic compression experiments, there may exist a significant difference between true deformation and nominal deformation, so it is not recommended to regard nominal strain-rate and nominal ultimate strain directly as true strain-rate and true ultimate strain.
3. By constructing an artificial speckle on ice surface, it is feasible to measure the deformation of ice specimen under dynamic compression tests through the digital speckle correlation method.
4. In the employed strain-rate range (0.4-10 s\({}^{-1}\)), the relationships between uniaxial compressive strength of natural lake ice and its influencing factors, such as strain-rate, temperature, loading direction, as well as air porosity, are almost similar to that under quasi-static conditions. The compressive strength increases with increasing strain-rate, and they obey a power-law relationship with the power exponent of 0.14-0.17. Temperature has a significant effect on the compressive strength, no matter whether the air porosity in ice is low or high and whether the loading direction is horizontal or vertical. The compressive strength shows an increasing trend with decreasing temperature. The strength at \(-10\)\({}^{\circ}\)C is about 1.4-1.8 times that at \(-5\)\({}^{\circ}\)C. Moreover, the vertical compressive strength is slightly greater than the horizontal compressive strength.
5. In the employed strain-rate range, the fracture mode of ice is not sensitive to the loading rate and loading direction. Failure of ice specimen exhibits features of longitudinal splitting failure, accompanied with feature of forming a large number of small fragments, just like that in crushing failure mode. The fracture mode is a combination of splitting failure and crushing failure.  
**Acknowledgments:** The authors acknowledge financial support from the National Key Research and Development Program of China (2016 YFC0401905) and the Tianjin Research Program of the Application Foundation and Advanced Technology (13 JCZDJC35100) in conducting this research. Additionally, the authors thank [PERSON], [PERSON] and [PERSON] for their helpful suggestions and advice. Finally, the authors would like to express their sincere gratitude to all of those who have offered selfless help during the course of this research.  
**Author Contributions:** All authors ([PERSON], [PERSON], [PERSON], [PERSON] and [PERSON]) contributed to the research work, and designed the new method and planned the experiments. [PERSON], [PERSON] and [PERSON] led and reviewed the research work. [PERSON] and [PERSON] performed the experiments and wrote the paper.  
**Conflicts of Interest:** The authors declare no conflict of interest.