Fracture Process Zone Evolution in Tight Sandstone Under Crack-Parallel Stress: A DIC Study

Crack-parallel stress modifies the near-tip stress state and may influence the development of the fracture process zone (FPZ) in rock. However, the spatial and temporal evolution of the FPZ during hydraulic fracturing cannot be reconstructed from the final fracture morphology alone. In this study, visual hydraulic-fracturing experiments were conducted on seven tight-sandstone specimens, with crack-parallel stress varied from 0 to 10 MPa while the other experimental conditions were kept consistent. Time-resolved full-field digital image correlation (DIC), combined with displacement–strain cross-calibration, was used to continuously track the initiation, expansion, localization, and coalescence of the FPZ, as well as the evolution of the traction-free crack tip and crack opening displacement (COD) on the specimen surface. The observations showed that the macroscopic traction-free crack did not form instantaneously but developed through progressive localization and coalescence of distributed damage within the FPZ. At the specimen level, the tests under nonzero crack-parallel stress exhibited shorter maximum FPZ lengths (14.7–30.6 mm) and lower critical COD values (10.5–27.5 μm) than the single 0 MPa reference specimen (80.9 mm and 38.2 μm, respectively). Given the limited replication, these differences are treated as descriptive specimen-level observations. The critical COD also varied non-monotonically across the tested stress levels. Three specimen-level FPZ–crack initiation patterns were identified: localized, matrix-nucleation, and diffuse-to-localized patterns. Their occurrence indicates that crack-parallel stress modifies near-tip confinement and crack-opening conditions, while specimen-scale heterogeneity and local defect distribution influence damage localization and the crack initiation site. These time-resolved observations reveal the spatiotemporal transition from distributed FPZ damage to traction-free crack formation, providing process-level information that cannot be obtained from the final fracture state alone.