Damage evolution and fracture propagation in sandstone during liquid oxygen cryogenic fracturing using DIC technology

To investigate how the length of liquid oxygen storage influences rock failure behavior and the evolution of the fracture process zone (FPZ) during phase-change expansion fracturing, four types of oxygen- storage charges were applied to cubic bluestone sandstone specimens. Ultra-high-speed imaging combined with Digital Image Correlation (DIC) technology was employed to capture full-field displacement and strain evolution throughout the loading process. Based on this data, crack propagation patterns, failure modes, strain-time responses, fractal dimensions, and energy dissipation characteristics were analyzed. The results indicate that under medium-to-low storage lengths (5–7 cm), crack growth is primarily governed by axial through-fractures, transitioning from a “multi-crack linkage” mode to a “single continuous penetration” mode. The strain-time response exhibits a characteristic three-stage behavior, comprising rapid increase, peak stabilization, and nonlinear attenuation. The crack initiation time decreases linearly with increasing storage length, while the fractal dimension rises from 1.700 to 1.749, accompanied by a shift in fragmentation from large blocks to more uniformly sized medium fragments. When the storage length exceeds approximately 7 cm, excess phase-change energy triggers competitive crack propagation, resulting in the formation of a composite network of axial and eccentric cracks. This leads to a sharp increase in the fractal dimension to 1.801 and a notable non-uniformity in fragmentation. Further analysis reveals that storage length plays a critical role in controlling displacement field patterns and FPZ evolution. Under low storage conditions, fragments rotate outward around the bottom contact point; as storage increases, symmetric horizontal ejection becomes dominant. Beyond a critical range, the competitive propagation of eccentric cracks inhibits the development of axial cracks, resulting in limited displacement of central fragments. Consequently, FPZ length shifts from a decreasing trend to an increasing one, indicating a fundamental change in the energy dissipation mechanism.