Dynamic Failure Characteristics of Combination Rock under Different Strain Rates: Insights from SHPB Tests

In order to investigate the dynamic mechanical behavior of composite rock in deep-buried tunnels, the dynamic impact tests of the combinations with three lithologies (granite-H, cyan sandstone-S1, and red sandstone-S2) and six kinds of composite forms (H-S1, S1-H, H-S2, S2-H, S1-S2, and S2-S1), were carried out in this study. Additionally, the effects of the combination forms and strain rates on the stress-strain curve, peak stress, energy utilization rate (EUR), failure process, displacement field, and failure mode of the composite sample were analyzed. The results show that the peak stresses of all the combinations are linearly and positively correlated with the strain rate. When the strain rate is close, the peak stress and elastic modulus of the combination with front hard and rear soft (FHRS) rocks are larger. Furthermore, the EUR of different combinations is proportional to the incident energy. The EUR of the front soft and rear hard (FSRH) combination is generally higher than that of the FHRS combination. In general, the rock with low strength always fails first in the composite. When the strength disparity between the two rocks of the composite is small, the failure of the composite with FSRH is relatively small, and the main deformation occurs in the rock with low strength. Finally, the fractal dimension of the fragments after the impact of the FSRH combination is larger, i.e., the lumpiness of fragments is closer.