Anisotropic Fracture Evolution and Size Effect of the Fracture Process Zone in Laminated Shales Derived from Digital Image Correlation

Real-time digital image correlation (DIC) experiments were performed during three-point bending tests of shale with 7 bedding angles and 5 sizes to investigate the anisotropic fracture behavior and size effect. The results revealed that the bedding plane orientation significantly influenced the fracture propagation. Microcracks at the crack tip and the subsequent fracture of the shale specimens with bedding angles of 0° and 90° were primarily attributed to tensile strain. In contrast, the shear strain and its effect on crack initiation and propagation in specimens were higher at bedding angles from 15° to 75°. The relationship curve between the dominant crack parameter (DCP) and the bedding angle had a V-shape. DCP values greater than 2.51 (0° ≤ β < 45°, β = 90°) indicated minimal shear effects, resulting in crack propagation in the original direction, whereas values less than 2.51 (45° ≤ β ≤ 75°) implied crack deviation toward the bedding plane due to shear effects. Furthermore, the fracture parameters, including the critical crack tip opening displacement, inelastic zone length (l_r), fracture process zone (FPZ) length (l_FPZ), and effective FPZ length (c_f), exhibited a W-shaped trend with bedding angle. The values decreased and increased in two stages with a bedding angle of 45° as the demarcation point. The size effect was pronounced; l_FPZ increased linearly with the specimen size; l_r increased with the specimen size, but the growth rate was lower. DIC analysis accurately reflected the anisotropic characteristics of crack evolution and strain concentration zone. However, the FPZ length derived from DIC was larger than the theoretical results obtained from nonlinear fracture mechanics and Bažant’s size effect law. This discrepancy was attributed to the fact that the FPZ derived from DIC included marginal regions with minimal microcracks and strain concentration.