The simplified characterization of rough rock fracture morphology is fundamental to understanding shear mechanical behavior. This study presents a method for constructing fracture surfaces by integrating profiles with varied inclined dentate asperities. The resulting five surfaces (D_1 to D_5) were quantitatively validated using the roughness index θ * max /(C+1) , demonstrating distinct roughness gradients. Utilizing three-dimensional (3D) printing technology, these fracture types were cast into mortar specimens for direct shear testing under varying normal loads. Experimental results allowed for a detailed analysis of shear stress, normal displacement, and their correlation with initial roughness and normal stress. Post-test 3D scanning revealed significant spatial variations in macroscopic surface damage. Investigation into the reduction and residual values of surface roughness indicates that degradation is closely linked to initial topography. Specifically, the roughest regions sustain the most significant damage and contribute most substantially to shear resistance. This research offers a feasible and novel approach to evaluating the mechanical properties of rock fractures.
This study investigates the microstructural effects, energy evolution, and damage progression of sandstone under uniaxial cyclic freeze-thaw (F-T) conditions. Using the roof sandstone from Shuangma No. 1 Mine, uniaxial compression tests were performed on samples subjected to varying numbers of F-T cycles. PFC3D simulations were employed to reproduce the failure process and analyze microcrack evolution.Results show that both the compressive strength and elastic modulus of sandstone decrease with increasing F-T cycles. When loaded perpendicular to the freeze-thaw direction, the elastic modulus of sandstone is significantly higher than that under parallel loading; conversely, the peak strain and compressive strength show the opposite trend. Post-failure analysis reveals an increase in the total number of cracks, tensile cracks, and shear cracks with more F-T cycles for both loading directions. However, the total and tensile crack counts in the parallel direction remain consistently higher than in the perpendicular direction.Energy analysis indicates that the proportion of elastic energy at peak stress continuously decreases with more F-T cycles, though it remains higher in the parallel direction. A rebound occurs at 140 cycles. The cumulative elastic energy ratio also decreases with increasing cycles, with the parallel direction consistently exhibiting higher values. Damage evolution equations, derived from the principle of minimum energy dissipation, show that the damage threshold in the parallel direction continuously increases and stays higher than in the perpendicular direction, where it first decreases and then increases. Final damage values in both directions initially rise and then decline, with transition points at 140 and 110 cycles, respectively. These findings provide theoretical guidance for rock engineering in cold regions.
Predicting compressive–shear fracture in rock masses containing complex flaw distributions remains a major challenge in rock engineering. We propose an improved non-ordinary state-based peridynamics (NOSB-PD) model to simulate rock fracture behavior in this work. A stabilized NOSB-PD formulation is developed by incorporating a bond-level deformation gradient strategy to effectively suppress the zero-energy mode inherent in conventional NOSB-PD formulations, thereby ensuring deformation compatibility and numerical robustness. More importantly, the triple-shear energy criterion is introduced into the PD framework for the first time, enabling a more accurate characterization of shear fracture in rocks under complex stress states. The proposed NOSB-PD model is validated using two examples, demonstrating its excellent capability in suppressing the zero-energy mode and capturing fracture behavior in rock under compressive–shear conditions. Subsequently, the proposed model is used to systematically investigate the influence of flaw distribution on crack propagation and failure modes in rocks. The results indicate that variations in flaw distribution alter the local stress field, leading to a change in the rock fracture mode. Consequently, the rock bridge failure mode transitions from shear-dominated direct coalescence to mixed tensile-shear failure, and finally to tension-dominated indirect failure. The overall rock specimen is more prone to tensile–shear-mixed failure under conditions of shorter rock bridges with larger inclinations, or longer rock bridges with smaller inclinations. These findings provide new insights into the role of flaw distribution on rock fracture behavior.
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.
