Investigation of Micro- and Macro-Scale Fracturing Mechanisms in Sandstone Exposed to Freeze-Thaw Cycles Using DEM Simulations and Theoretical Analysis

Freeze-thaw (F-T) cycles play a pivotal role in rock degradation and expedite rock fracturing in cold regions, thereby leading to catastrophic failures in rock engineering. Therefore, it is of great significance to understand the impact of F-T cycles on rock fracturing and to formulate a theoretical framework for rock engineering projects in cold regions. This study employed experimental testing, discrete element method (DEM) modeling and micromechanical theoretical analysis on notched semi-circular bend (NSCB) specimens of red sandstone to assess the fracturing mechanism under Mode-I loading. A linear parallel bond model (LPBM) within the DEM framework was calibrated to replicate the experimental load-displacement response, acoustic emission (AE) counts, and macroscopic fracture trajectory. Results reveal that increasing F-T cycles significantly diminishes peak load and fracture toughness (KIC) and leads to earlier microcrack initiation, as evidenced by simulated AE counts. The analysis of internal stress distribution and pore structure indicates that frost cracking promotes the transition of micropores to macropores, thereby greatly altering water retention behavior and accelerating rock degradation. A micromechanical model is adapted to correlate pore-scale properties with fracture development. The results show that swelling pressure induced by ice formation within the pores increases the stress intensity factor (SIF) (Kcdf) at crack tips. When Kcdf exceeds the material's KIC, subcritical crack growth is initiated. The validated DEM model also demonstrates its reliability in capturing micromechanical damage mechanisms associated with F-T cycles. The obtained results may provide insightful theoretical and experimental framework for evaluating the structural stability and predicting the fracture hazard in rock engineering operations in cold climates.