## 3 Analysis of Test Results
### Dynamic Mechanical Characteristics
#### 3.2.2 Dynamic Mechanical Characteristics of Coal and Rock during Impact  
To better analyse the effect of temperature on the coal and rock samples, the stress-strain diagrams and peak stress diagrams were plotted for analysis. Using the three-wave method, the stress-strain curve was obtained after the calculation of the stress time-course curve. As shown in Figure 10a,b, the stress-strain curves in the water-saturated state had velocities of 18 m/s for coal and rock. As shown in the figures, the trend process of the coal and rock stress-strain curves at different temperatures is very similar; however, some differences in the characteristics of each stage are present. First, the coal samples at 10 \({}^{\circ}\)C, 20 \({}^{\circ}\)C, and \(-20\)\({}^{\circ}\)C show a kind of \"saddle shape\", which was evidently a manifestation of the strain intensification, and the brittleness of the samples was also more significant in this stage; rocks did not have this shape. Second, at a freezing temperature of \(-10\)\({}^{\circ}\)C, the slope of the curve rises slowly in the initial phase, and the strain duration was shorter. The slow decrease in the unloading stage also showed that at \(-10\)\({}^{\circ}\)C, the sample had the worst load-carrying capacity and the most severe deformation. In the last part of the curve, a certain \"rebound\" phenomenon occurred at the end stage because after the peak value, the coal and rock samples still had the ability to resist external loads. Figure 9: Denoised stress-wave signals at different impact velocities. (**a**) Incident-reflected waves of coal at \(-10\)\({}^{\circ}\)C; (**b**) transmitted waves of coal at \(-10\)\({}^{\circ}\)C; (**c**) incident-reflected waves of rock at \(-10\)\({}^{\circ}\)C; and (**d**) transmitted waves of rock at \(-10\)\({}^{\circ}\)C.  
In the analysis of the dynamic load mechanics, a fixed trend for the change in stress at different temperatures among studies by different scholars has not been shown. In this study, the variation curve of the peak stress with the temperature gradient in water-saturated coal and rock was plotted based on the stress-strain curve in Figure 10a,b, as shown in Figure 11a,b. In Figure 11a, for the coal samples at temperatures from \(-20\)\({}^{\circ}\)C to \(-10\)\({}^{\circ}\)C, the peak strength rapidly decreased. From \(-10\)\({}^{\circ}\)C to \(10\)\({}^{\circ}\)C, the peak intensity slowly increased. A temperature of \(-10\)\({}^{\circ}\)C was an inflection point of change. In Figure 11b, the rock samples also maintained a similar pattern. The overall stress peaks all increased with increasing impact velocity. The inflection point decreased from \(-10\)\({}^{\circ}\)C to \(0\)\({}^{\circ}\)C. The same pattern was maintained with the peak stress curve of coal at 18 m/s. Overall, the peak strength of water-saturated rock maintained the trend of rising, falling, and then rising with the decrease in temperature, which is similar to an \"N\" shape. However, the coal and rock samples are also slightly different. The 10 m/s impact velocity for the rock shows a U-shaped pattern. Based on the above results, two aspects of the joint role are as follows: (1) With a decrease in the temperature, the cold coal or rock body lead to a gradual reduction in its own strength and volume shrinkage, resulting in the closure of the internal fissure. (2) In the freezing process, liquid water transforms into solid ice; it expands in volume and generates freezing expansion force. Under the action of frost heave force, micro-cracks sprout or expand the existing cracks at the cracks. The two factors are mutually restraining and mutually exist. At different temperatures, the change in its own strength dominates the  
Figure 10: Dynamic mechanical characteristic curve of coal and rock impact. (**a**) 18 m/s for coal; (**b**) 18 m/s or rock. strength of the material.