Mechanical response and energy dissipation law of double-fractured sandstone under dynamic load

To study the dynamic mechanical response and energy dissipation law of fractured sandstone, an SHPB device combined with a high-speed camera system was used to carry out impact compression tests with seven rock bridge angle specimens. The dynamic strength characteristics of fractured sandstone and the crack initiation stress at the fracture tip was explored. The fractal characteristics of debris were quantitatively described, and the relationship between energy dissipation and fractal dimension and average debris size was analyzed. In addition, a significant rate effect between rock angle and dynamic load was observed with peak stress degradation value of the sample been significantly affected by the rock angle. The results show that the strain value of peak stress increases first and then decreases with rock angle increase; the peak stress values of the intact specimen show a sensitive change characteristic, and the prefabricated fractures significantly weaken the peak stress. The maximum and minimum rock angles are 0°and 90°, respectively. Under low dynamic load and small rock bridge angle, cracks are single and dispersed; under high dynamic load and large rock bridge angle, crack types are diverse, showing a composite failure of shear and tension. Dynamic load and rock bridge angle have a significant impact on the stress and deterioration performance of the samples, especially under higher dynamic load and larger rock bridge angle, where the stress deterioration of the samples is more pronounced. The crack initiation stress at the fracture tip decreases with rock angle increase and is lower than the peak stress. There are significant differences in the initiation time of cracks (damage velocity) at the fracture tip. The damage velocity is significantly affected by dynamic load; there is a significant rate effect between the incident energy and the reflected energy and the dynamic load. The energy dissipation rate and fractal dimension increase with dynamic load; the energy dissipation rate has an opposite relationship with the exponential curve of fractal dimension and average fragment size, and the 60° specimen shows more sensitive fractal variation characteristics.