# Energy Dissipation and Stress Equilibrium Behavior of Granite under Dynamic Impact
## 5 Discussion  
In the study, we illustrated the incident energy, reflected energy, transmitted energy, and dissipated energy curves of granite specimens with different length-diameter ratios in the SHPB test. With the increase of time, four types of energy increased. During the stress equilibrium time, the increment of dissipated energy is faster than the transmitted energy. The slope of the dissipated energy and transmitted energy curves is larger than the reflected energy curve. [PERSON] et al. [18] studied the energy dissipation test on the sandstone under the impact loading. They suggested that the incident stress is too high and the rock will be crushed excessively. However, the incident energy can only transfer energy to the transmission bar at the initial stress equilibrium state. When the rock is crushed, the transmitted energy transfer stops and tends to maintain a certain value. The same conclusion was reached by [PERSON] et al. [19] from the analysis of the energy transfer of magnetite.  
The best crushing effect is the stress uniform achieved before damage to the granite specimen occurs and it can improve the accuracy of the impact of SHPB on test results [20]. The stress equilibrium time is measured by the stress equilibrium factor. Normally, the longer the stress equilibrium time, the better the stress uniform can be achieved before the damage of the specimen occurs. This can provide more energy for rock crushing, and a higher energy utilization for rock crushing.  
Nevertheless, we found that the stress equilibrium time is not as long as possible. During a stress equilibrium time of 60-70 \(\upmu\)s, the energy dissipation rate of granite specimen is lower than other durations. The stress equilibrium time in the granite specimens will be unstable with higher strain rate, caused by the martensitic transformation under higher strain rates [21]. To make it simple the energy used to crush the rock is minimal. According to the length-diameter ratio of the granite specimens and impact loading, our results suggest a possibility of matching relationship between the length-diameter ratio and the impact loading. A large aspect ratio with low impact loading and a small aspect ratio with high impact loading are not better options.  
Before peak strength, the granite specimens with length-diameter ratios of 0.6, 1.0, 1.2, and 1.4 achieved stress equilibrium at impact velocity of 8 m/s and 10 m/s. The length-diameter ratio of 0.8 achieved a stress at impact velocity of 8, 10, 12, and 14 m/s. Compared with different length-diameter ratios and impact load, 0.6 and 0.8 length-diameter ratios can maintain stress equilibrium easily. [PERSON] et al. [8] drew a similar conclusion in their study about the stress uniform of coal samples. Moreover, the critical aspect ratio is related to the achievement of stress equilibrium. Further studies are needed on whether the stress equilibrium time after peak strength is effective. If the stress equilibrium time after peak is  
Figure 14: [PERSON]’s modulus curve.  
invalid, a future study can combine the stress equilibrium time judged by the equilibrium factor and the peak strength time in order to fit a new relationship between the energy dissipation rate and stress equilibrium time.  
Overall, we have noticed that smaller length-diameter ratios with an impact velocity of over 12 m/s gives better results in the SHPB test. Compared with the five length-diameter ratios, the granite specimens with a length-diameter ratio of 0.8 can maintain the longest stress equilibrium and achieve the most stable energy dissipation rate. The optimal matching relationship between an impact velocity of 12 m/s and a length-diameter ratio of 0.8 is presented in this study.