# Dynamic Characteristics of Sandstone under Coupled Static-Dynamic Loads after Freeze-Thaw Cycles
## 5 Conclusions  
This study performed dynamic experiments on sandstone to investigate the effects of freeze-thaw cycles and pre-compression stress by using the SHPB system. The conclusions are as follows:  
1. The stress-strain curves of the sandstone can be divided into three stages under axial pre-compression stresses and do not exhibit a densification stage. With increasing axial pre-compression stress, the slope of the elastic stage and the peak strength first increase and then decrease. The dynamic peak strain, on the other hand, first decreases and then increases for the same number of freeze-thaw cycles. When the axial pre-compression stress is 15 MPa, with increasing number of freeze-thaw cycles, the peak stress decreases and the peak strain increases almost linearly. When the axial pre-compression stress is 30 MPa, the peak stress decreases and then increases while the peak strain decreases. The peak stress and strain both increase under a pre-compression stress of 55 MPa.
2. The rock failure characteristics under different freeze-thaw processes and pre-compression stresses indicate that the number of fragments increases with both the increasing axial pre- compression stress and the increasing number of freeze-thaw cycles. The fragments become more uniform when more freeze-thaw cycles are applied due to the increasing internal damage within the rock. The failure modes change from splitting at 0 MPa axial pre-compression stress to shear failure at higher pre-compression stresses. The energy absorption rate of the sandstone specimen increases first and then decreases with increasing axial pre-compression stresses. With increasing number of freeze-thaw cycles, the energy absorption rates of the sandstone increase.  
However, these test results and the proposed model were obtained for one particular set of conditions. The specimens were frozen and thawed in a temperature-controlled container with the temperature varying from +20 to -20 \({}^{\circ}\)C, and the stress state of the sample was coupled with the static-dynamic compression. In the future, other test conditions should be investigated such as the stress state in triaxial compression and how the freeze-thaw and dynamic loads affect spalling in rock slopes and rock tunnels.  
Figure 13: The variation of energy absorption rates at different freeze-thaw cycles.  
**Author Contributions:** Methodology: B.K.; formal analysis: B.K. and H.D.; resources: H.D.; data curation: B.K., J.Z. and X.Y.; writing--original draft preparation: B.K.; writing--review and editing: B.K. and J.Z.; supervision: H.D. All authors have read and agreed to the published version of the manuscript.  
**Funding:** The research was funded by the National Natural Science Foundation of China (Grant No. 51474252 and No. 51874352), the China Postdoctoral Science Foundation (Grant No. 2018M632936) and the Fundamental Research Funds for the Central Universities (WUT: 2019 IVA092).  
**Acknowledgments:** The first author would like to thank the Chinese Scholarship Council for financial support for the joint Ph.D. at the University of Adelaide. [PERSON] is also acknowledged for his support in conducting the experiments.  
**Conflicts of Interest:** The authors declare no conflict of interest.