Dynamic behaviour and nonlinear ultrasonic characteristic of sandstone under intermittent cyclic impact loading

Tunnel excavation constitutes a cyclic construction process characterized by defined time intervals between successive cyclic advancement phases. During drilling-blasting operations in rock tunneling, the surrounding rock is subjected to intermittent cyclic blast-induced impact loads. In this study, a Split Hopkinson Pressure Bar (SHPB) system was employed to conduct the first systematic experimental study on sandstone under intermittent cyclic impacts. The effect of intermittent time on dynamic characteristics, damage and energy evolution of sandstone were investigated. Additionally, the nonlinear ultrasonic characteristics of the sandstone during cyclic impact tests were examined using a nonlinear non-destructive technique. The research results indicate at lower impact pressures, the presence of intermittent time between two adjacent impact cycles can effectively enhance the sandstone's impact resistance. At 0.35 MPa, during the final impact, the peak stress and dynamic elastic modulus of the specimens under intermittent cyclic loading are 73.13 MPa and 9.58 GPa, respectively, while under continuous cyclic loading, they are 58.83 MPa and 7.83 GPa, respectively. Both intermittent and continuous cyclic impact loading result in axial splitting failure of the sandstone specimens. Under identical impact pressure conditions, the mesoscopic fractal dimension (Dc) of the sandstone fracture surfaces under intermittent cyclic impacts demonstrates significantly lower values compared to continuous cyclic impacts, particularly at 0.35 MPa. As the number of impacts increases, changes in dominant frequency amplitude and nonlinear coefficient remain relatively small, indicating a slower progression of cumulative damage in sandstone specimens during the intermittent cyclic impact process. These findings provide valuable insights into the behavior of sandstone under intermittent loading conditions, which more accurately represent real-world tunneling engineering conditions.