Dynamic response and damage evolution of freeze–thaw-fractured sandstone under SHPB impact: a coupled FDM–DEM approach

In this study, a three-dimensional split Hopkinson pressure bar (SHPB) impact numerical model was established through the FDM–DEM coupling method to explore the mesoscopic damage accumulation and dynamic mechanical response of fractured sandstone under freeze–thaw cycles. Based on the volume-expansion theory, a discrete-element model of the phase-change expansion of pore-water–ice was constructed. Combined with the parameter calibration optimized by the genetic algorithm, the damage evolution of the rock during the freeze–thaw process was simulated. The research results show that: (1) The discrete-element simulation results show high consistency with experimental data. Taking the 40-mm rock bridge as an example, the maximum relative errors of peak strength and elastic modulus under different freeze–thaw (FT) cycles are 8.54% and 3.49%, respectively, meeting accuracy requirements. This validates the reliability of the particle expansion model and FT damage analysis method. (2) Under uniaxial compression, rock-bridge length significantly influences the mechanical properties of FT sandstone. Specimens with 50-mm rock bridges exhibit the highest elastic modulus and peak strength. However, FT cycles induce nonlinear degradation in compressive strength. (3) Dynamic impact tests reveal that FT cycles exacerbate rock fragmentation. With increasing impact velocity and FT cycles, strain rate rises, leading to nonlinear attenuation of dynamic strength and decelerated growth of the dynamic increase factor (DIF). The presence of rock bridges further causes multistage evolution characteristics in dynamic stress–strain responses.