Mechanical Response Characteristics and Progressive Instability Mechanical Model of Pre-cracked Sandstone Under Cyclic Impact Load

In engineering practice, fractured rock masses subjected to frequent disturbance loads have become increasingly common. Understanding the mechanical response characteristics and establishing a dynamic energy damage model of pre-cracked rock under cyclic impact loads are crucial to the long-term stability of deep rock mass engineering structures. In this study, the split Hopkinson pressure bar (SHPB) test system was used to conduct constant-amplitude cyclic impact tests on pre-cracked red sandstone. The dynamic response and fragment distribution characteristics of the pre-cracked red sandstone were investigated. The energy dissipation and damage evolution patterns of specimens were revealed, and a progressive instability mechanical model for pre-cracked rock was established. The experimental results show that under cyclic impact loading, the peak stress of the specimens decreased with increasing number of impact cycles. At the same number of cycles, the peak stress exhibited an irregular V-shaped distribution as the angle of prefabricated cracks increased, while the strain rate demonstrated an inverse variation trend compared to the peak stress. The dissipated energy density of the specimens exhibited a characteristic initial energy absorption-accelerated energy dissipation stage as the number of impact cycles increased. This indicated a slow growth rate during the early stage of cyclic impacts, followed by a sharp increase in the later stage. However, when the specimens’ integrity was significantly influenced by the prefabricated crack, the initial energy absorption stage was nearly absent. Meanwhile, the average size and distribution range of post-failure fragments first increased and subsequently decreased with increased prefabricated crack angle. The specimens exhibited nonlinear damage accumulation under cyclic impacts, with the damage variable growing logarithmically as impact cycles increased. At the same number of cycles, the damage variable reached a maximum at a prefabricated crack angle of 30°, with increasing angle beyond this critical value resulting in a gradual decrease. Based on the characteristics of energy dissipation and damage evolution, a progressive instability mechanical model of pre-cracked rock was established. Two control strategies for enhancing ultimate bearing capacity and reducing the energy absorption rate were suggested, with corresponding field implementation measures provided. The findings established a theoretical basis for stability control in deep underground engineering structures.