Fracture characteristics and process zone evolution in sandstone under freeze-thaw cycles

This study investigates the fracture characteristics and the fracture process zone (FPZ) of mode Ⅰ fracture in sandstone, aiming to analyze the propagation behaviors of mode Ⅰ crack under different freeze-thaw cycles. Semi-circular bending tests (SCB) were conducted using different freeze-thaw cycles to evaluate mode Ⅰ fracture toughness, FPZ dynamics, and macroscopic microscopic features. Digital image correlation (DIC) and scanning electron microscopy (SEM) techniques were employed for detailed analysis. Experimental results reveal that freeze-thaw cycling leads to the widening of both preexisting and newly formed microcracks between internal particles. Under external loading, crack propagation deviates from prefabricated paths, forming serrated crack patterns. The FPZ initiates at the prefabricated crack tip and extends toward the loading end, exhibiting an arc-shaped tip shape. The FPZ length increases with loading but decreases after reaching a peak value. With additional freeze-thaw cycles, the maximum FPZ length first increases and then diminishes.