Study on the mechanical properties and dynamic damage evolution mechanism of green sandstone under freeze-thaw loading conditions

From macroscopic to microscopic perspectives, the influence laws of the number of freeze-thaw cycles on the physical properties and macro-microscopic damage of the specimens were revealed. It was clarified that 30 freeze-thaw cycles were the inflection point for the specimens to shift from brittle to plastic failure. A damage model under freeze-thaw cyclic loading was developed, and three distinct evolutionary stages of damage were identified: damage quiescence, damage acceleration, and damage stabilization. It was found that the initial damage value is directly influenced by the number of freeze-thaw cycles. Furthermore, it was concluded that the damage acceleration stage represents the critical phase for damage deterioration and sample instability. Due to seasonal variations and diurnal temperature fluctuations, cold regions develop distinct frozen rock masses. With seasonal transitions, these frozen rock masses undergo phase changes into thawed states, generating substantial tensile and compressive stresses that induce irreversible damage to the rock structures—particularly in sandy rock formations, where the effects are more pronounced. Once initiated, such damage can trigger a “domino effect,” leading to severe economic losses and potential casualties. In light of this, the present study systematically employs experimental techniques, including nuclear magnetic resonance and acoustic emission monitoring, integrated with damage mechanics theory, to investigate sandy rock samples collected from a specific open-pit mine in a cold region. The research focuses on the behavior of these samples under freeze-thaw cycles ranging from − 30 °C to 30 °C, with experimental analyses conducted under combined freeze-thaw and mechanical loading conditions. Several innovative findings have been achieved: The evolution characteristics of acoustic emission (AE) signals and the fracture morphology of specimens under freeze-thaw and loading conditions exhibit distinct patterns. The variations in cumulative event count and frequency band distribution are generally consistent with the trend observed in the stress-strain curve. However, both strength and structural stability progressively deteriorate with an increasing number of freeze-thaw cycles, leading to a reduction in macroscopic fracture time. A critical threshold is observed at 30 freeze-thaw cycles, marking the transition point from brittle to plastic failure behavior. Following 40 freeze-thaw cycles, brittle failure is no longer present, and a well-defined macroscopic fracture surface emerges. The number of freeze-thaw cycles directly influences the initial damage level of specimens under load, with higher cycle counts corresponding to greater initial damage. After 40 cycles, the initial damage value reaches as high as 0.68. Furthermore, the damage evolution process under combined freeze-thaw and mechanical loading can be classified into three distinct stages: damage quiescence, damage acceleration, and damage stabilization. Notably, the damage acceleration stage represents the critical phase preceding unstable rupture.