Dynamic characteristics and damage model of freezing thawing red sandstone under impact load

To study the dynamic mechanical properties and damage evolution characteristics of freeze-thaw rocks under impact loads, 0, 15, 30, 45, and 60 freeze-thaw cycle tests and dynamic impact tests were conducted on red sandstone in cold regions. The effects of strain rate and freeze-thaw damage on the dynamic mechanical properties of red sandstone were analyzed. Based on the damage evolution equation considering the damage threshold, a damage viscoplastic dynamic constitutive model of freeze-thaw red sandstone was constructed based on the combination element model theory and statistical damage theory, and the rationality and applicability of the model were verified. The results show that red sandstone exhibits freeze-thaw cycle effects and strain rate effects, and its dynamic peak strength decreases with the increase of strain rate or the decrease of freeze-thaw cycle times. The higher the strain rate, the stronger the sensitivity to freeze-thaw damage; the introduction of dynamic peak strength loss γ can eliminate the strain rate effect under impact loads and more intuitively reflect the impact of freeze-thaw cycles on the dynamic peak strength of rocks under different impact loads. The damage threshold point is 45 freeze-thaw cycles, and after 45 cycles, the freeze-thaw damage tends to stabilize; compared with unfrozen and thawed environments, a larger scale crack network is formed internally, and the failure mode changes from splitting to shear failure. The mechanical properties deteriorate significantly in the freeze-thaw environment; the linear elastic stage of sandstone exhibits obvious behavior after freeze-thaw cycles. Based on the D-P failure criterion, a damage evolution equation considering the damage threshold is derived, which is more reasonable. Based on the Zhu−Wang−Tang constitutive model, a viscoelastic plastic dynamic constitutive equation for constructing damaged bodies is introduced, reflecting the comprehensiveness of the constitutive model; the dynamic constitutive model established in this article well reflects the complex characteristics of damage softening, viscoelasticity, and viscoplasticity exhibited by red sandstone during load deformation, and also reflects the influence of freeze-thaw damage on the dynamic mechanical properties of red sandstone; the conclusions obtained can provide reference for studying the dynamic impact failure of rocks in seasonal high-altitude and cold regions.