Microscopic and Macroscopic Creep Damage Behavior and Constitutive Model of Sandstone Subjected to Static and Dynamic Coupled Loading

Understanding the instantaneous and creep mechanical responses of rocks subjected to the combined influence of static and dynamic loading is of great importance for designing underground works. This study developed an impact-disturbance creep loading apparatus and conducted a series of multistage impact-disturbance creep experiments on sandstones, varying static creep loads, and dynamic disturbances. The investigation analyzed specimen deformation, acoustic emission (AE) signals, and nuclear magnetic resonance (NMR) porosity changes to determine creep characteristics from microscopic damage to macroscopic failure. The experiment revealed the profound influence of dynamic disturbances on sandstone's instantaneous and creep behavior. Initially, these disturbances induce specimen densification under low creep stress but exacerbate damage propagation beyond a certain creep stress threshold. Moreover, increased dynamic disturbance energy correlates with higher creep failure strains but lower long-term specimen strengths. AE monitoring highlighted the role of dynamic disturbances in promoting tensile failure and the dominance of tensile cracks throughout the creep process. Notably, impact disturbances cause a significant increase in specimen porosity postcreep failure, especially in large pores. To capture sandstone creep behavior comprehensively under dynamic disturbance, a viscoplastic disturbance-damage creep model is proposed by integrating a disturbance-damage instantaneous element and dynamic-damage viscous element. The model can effectively describe sandstone creep behavior under dynamic disturbance conditions, offering valuable insights for underground work design.