# Experimental Study on Dynamic Mechanical Properties of Sandstone Under Different Damage Degrees
## 6 Triaxial Dynamic Compression Experiment and Result Analysis
### Analysis of Energy Consumption During Triaxial Dynamic Loading  
As illustrated in Figure 19 (taking the impact pressure of 1.6 MPa as an example), as the confining pressure for HS rock samples rises from 4 MPa to 16 MPa, the energy absorption of the rock decreases from 8.25 J/cm\({}^{3}\) to 7.54 J/cm\({}^{3}\). The energy absorption of other types of rock specimens follows the same pattern. This occurs because the cracks within the rock become compacted under high confining stress, and as the confining stress increases, a larger proportion of the cracks are compacted. Once the microcracks are compacted, it facilitates the propagation of stress waves, but the energy absorbed from the stress waves decreases. When the confining stress decreases, the uncompacted cracks can absorb more energy from the stress waves for crack propagation, leading to a significant reduction in compressive strength as the confining pressure decreases. Figure 19 additionally indicates that, under identical confining stress, HS rock samples exhibit greater energy absorption compared to LS and NS rock samples. This is because, more microcracks are generated in the surrounding rock of HS rock specimens, suggesting that more cracks can participate in propagation and absorb energy from the impact stress waves.  
From the data in Table 3, when the confining stress decreases from 16 MPa to 4 MPa, the reduction in compressive strength is approximately 20%. The failure modes of HS and NS rock samples under varying confining stress, with a pneumatic cannon impact pressure of 1.4 MPa, are shown in Figure 20. The failure modes of the rock samples after loading indicate that as the confining stress increases, the size of the rock fragments after failure gradually increases, suggesting that higher confining pressure reduces the rock's energy absorption. The fragment sizes after failure at each confining pressure level in Figure 20a are smaller than those in Figure 20b. This is because the rock specimens in Figure 20a were subjected to high-impact power loads during drilling.  
The stress-strain curves for each group of rock samples reveal that strain tends to diminish in the later stages, suggesting that the confining pressure is limiting the rock's deformation. This is because the rock is not yet damaged in the early stages where the stress waves are relatively large, making the confining pressure's restriction on the strain not evident from a single curve. However, according to the strain rate data of each group in the experiment (Table 3), the strain rate diminishes as the confining stress increases, which means that the rate of deformation decreases as the confining stress increases. By comparing Figure 20 with Figure 15, it is obvious that although the rock specimens with confining stress have higher pneumatic cannon impact pressure and absorb more energy, the fragment size after failure is significantly larger than that of rock samples with no confining stress. This also well explains the restriction of confining stress on deformation and crack propagation.  
Figure 19: Variation relationship of energy absorption with confining pressure for different rock samples.  
Drilling is also a process of rock fragmentation, which inevitably leads to rock deformation. Therefore, the restriction of confining pressure on rock deformation inevitably brings great difficulties to rock-breaking in underground geotechnical engineering.