## 3 Test Results and Discussion
### Results of the Single Impact Test
#### 3.1.2 Dynamic Mechanical Properties  
According to Equations (6), (7) and (10), the collected, reflected and transmitted wave signals were processed. The stress-strain curves of sandstone specimens under different impact velocities are shown in Figure 5. With the increase in the impact velocity, the peak stress and the corresponding strain both tend to increase. Taking a single impact test with a strain rate of \(54.6\) s\({}^{-1}\) as an example, the stress-strain curve in Figure 5b is roughly divided into four stages. (1) The linear elastic stage--line AB. The stress-strain curve at this stage is linear, and the stress increases sharply, while the strain only increases slightly. The slope at this stage is large, which reflects the large dynamic elastic modulus of sandstone. The physical process at this stage is as follows. The sandstone reaches the elastic limit instantly under the loading of the stress wave, and the original fractures do not have the compaction section as in the static compression test. However, these fractures generate a large number of new fractures in the rock before they are compressed under the action of the stress wave and then enter the plastic stage. This is shown in the constitutive curve as follows. The elastic stage of the dynamic stress-strain curve is convex, while the static stress-strain curve is concave. (2) The new crack formation stage--line BC. The elastic deformation reaches the limit, meaning only the strain increases while the stress growth is very small, and the line is approximately horizontal with a very slow slope. The physical process at this stage is as follows. The original cracks expand, and a large number of new cracks are  
Figure 4: Strain rate–time curve.  
Figure 3: Waveforms of typical SHPB tests at different impact velocities.  
induced, but no macrodamage is formed. The role of the stress wave is mainly reflected in the expansion of the original cracks and the formation of new cracks, resulting in a large increase in strain and almost no increase in stress. (3) The plastic strengthening stage--line CD. At this stage, the plastic development and stress growth of the sandstone are carried out at the same time, but the slope of the curve is greatly reduced compared with the elastic stage. The strength at this stage grows increasingly slow and gradually reaches the peak strength. This peak is the dynamic strength of the sandstone under a certain stress wave parameter. The physical process at this stage is that cracks continue to develop, and the solid part without cracks in the sandstone provides the strength growth. At the same time, the peak strength also indicates the ability to resist impact damage. Under the same stress wave amplitude, the larger the stress peak is, the greater the ability to resist impact damage. When the plastic strain is small, the internal damage does not cause obvious macroscopic damage; when the strain reaches a critical value, the specimen fails. The stress of the constitutive curve increases before point D, which is called the loading section. The plastic development stage accounts for the majority of the stress-strain curve. (4) The unloading stage. When the strain exceeds the peak stress, the strain continues to increase slightly while the stress decreases rapidly, and the curve positive unloads. There is no elastic recovery in this process, which indicates that irreversible damage or even failure has occurred in the rocks.