Study on the Mechanical Properties and Dynamic Damage Evolution Mechanism of Green Sandstone under Freeze-thaw Loading Conditions

In cold regions, the pronounced seasonal and diurnal temperature variations lead to the formation of distinctive frozen rock masses. With the alternation of seasons and day-night cycles, these frozen rock masses transition into a thawed state, inducing substantial tensile and compressive stresses between mineral grains and primary pores. This process can result in irreversible damage to rock masses with low cementation strength, particularly sandy rock masses, which are inherently characterized by a loose structure and reduced strength upon water exposure. Once such damage occurs, it may trigger a "domino effect," leading to cascading failures that can cause severe economic losses and casualties in engineering contexts. In light of this, this paper employs a comprehensive approach utilizing nuclear magnetic resonance, acoustic emission, and other testing methods, integrated with damage mechanics theory. The study focuses on an open-pit mineral sandy rock within the freeze-thaw temperature range of 30 to -30℃ as the research subject, conducting experimental investigations under freeze-thaw cycles and loading conditions. The following innovative outcomes have been successfully achieved: Under varying freeze-thaw cycles (10, 20, 30, 40), the internal pore quantity and structure of the sample exhibited continuous alteration as the number of cycles increased. Following 40 freeze-thaw cycles, a macroscopic fracture surface developed on the sample's exterior;Under freeze-thaw loading, the evolution of acoustic emission (AE) signals in both the time and frequency domains, as well as the fracture patterns of the samples, exhibit variations. However, the change rules of AE cumulative event numbers and frequency band distribution characteristics are consistent with the development trend of the stress-strain curve. This indicates that the strength and stability of the samples progressively deteriorate with an increasing number of freeze-thaw cycles, leading to a shortened macroscopic rupture time. Notably, 30 freeze-thaw cycles mark the inflection point where brittle failure transitions to plastic failure, and brittle failure ceases to occur after 40 freeze-thaw cycles༛The number of freeze-thaw cycles is a critical factor in determining the initial damage value of the sample under load. Generally, an increase in the number of cycles correlates with a higher damage value. Specifically, after undergoing 40 freeze-thaw cycles, the initial damage value of the sample reaches up to 0.68༛The failure process of samples under freeze-thaw loading can be categorized into three evolutionary stages: damage initiation, damage acceleration, and damage stabilization. Among these, the damage acceleration stage serves as the critical phase associated with failure and instability.