# Study on Dynamic Mechanical Properties and Failure Pattern of Thin-Layered Schist
## 3 Dynamic Mechanical Properties
### Characteristics of Stress-Strain and Dynamic Strength  
The \"three-wave method\" theory can be used to process the test data and calculate the stress and strain values of specimens [26]. The typical stress-strain curves of each thin-layered schist specimen are obtained by eliminating the obvious abnormal results, as shown in Figure 5.  
Figure 4: Typical stress wave diagram of thin-layered schist.  
Figure 3: The schematic diagram of SHPB system.  
According to the stress-strain curve of the specimen, combined with the deformation characteristics of the test, it can be found that the strain process of specimens with different schistosity angles is different, but there are the following stages.  
In the first compaction stage, the stress-strain curve shows an obvious downward sag trend; the micro-cracks and pores in the specimen gradually close and shrink under the impact load, and the overall deformation resistance of the specimen is enhanced. Then, the stress-strain relationship of the specimen exhibits linear behavior, that is, linear elastic compression deformation. As deformation progresses, the stress-strain curve gradually becomes concave as the slope decreases and strain increases at a slower rate, and the specimen exhibits plastic deformation.  
Since then, variations in curve shapes are observed for thin-layered schist with different schistosity angles. Upon reaching peak stress, there is a rapid decrease in stress with increasing strain, leading to the appearance of a distinct post-peak curve that signifies irreversible deformation damage and failure. The post-peak stress-strain curve of specimens at 0\({}^{\circ}\), 60\({}^{\circ}\)and 90\({}^{\circ}\) angles has the same downward trend, and the curve is steep, showing a certain brittleness. The curve exhibits closed shapes due to limited fracture without reaching the yield limit, resulting in several large rock blocks. In contrast, the post-peak stress-strain curves of specimens at 30\({}^{\circ}\)and 45\({}^{\circ}\) angles have a moderate downward trend, which display opening shapes. The rock failure is caused by the shear slip of schistosity.  
In addition, the dynamic compressive strength of schist has obvious anisotropy, and different schistosity angles have a significant influence on the overall compressive performance of specimens. When the schistosity angle is 0\({}^{\circ}\) and 90\({}^{\circ}\), the specimens show high dynamic compressive strength, and the rock block size after fracture is relatively large. In contrast, specimens with schistosity angles of 30\({}^{\circ}\), 45\({}^{\circ}\) and 60\({}^{\circ}\) show lower compressive strength values and a higher degree of breakage. The SHPB test results and mean compressive dynamic strength of specimens are shown in Table 1 and Figure 6.  
It can be seen from Table 1 and Figure 6 that the dynamic critical failure strength of each thin-layered schist conforms to the typical anisotropy characteristics of \"U\"-type strength. The compression strength of specimens with schistosity angles of 45\({}^{\circ}\) is much lower than in other cases. Combined with the analysis of the failure morphology of the specimens, it can be seen that the specimens with a schistosity angle of 45\({}^{\circ}\) fail along the schistosity slip, which is the easiest way to destroy it, so the dynamic compressive strength is the lowest.  
Figure 5: Stress–strain curves of thin-layered schist with different schistosity angles.