Quasi-static loading rate effects on fracture process zone development of mixed-mode (I-II) fractures in rock-like materials

The development of a fracture process zone (FPZ) (microcrack zone) ahead of a fracture tip is a prominent fracture characteristic of rock-like materials. At present, understanding of the quasi static loading-rate effect on the FPZ development of a mixed-mode (I-II) fracture remains challenging for rock-like materials. In this paper, the centrally cracked Brazilian disk (CCBD) specimens of artificial rock-like materials were tested to create mixed-mode (I-II) fractures at different quasi-static loading rates (0.02-2 mm/min), and the centrally crack in each specimen is prefabricated at 15 degrees (tensile-shearing mixed-mode), 45 degrees (compressive-shearing mixed-mode) and 0 degrees (pure mode-I) to the loading direction. The digital image correlation (DIC) was employed to identify FPZ, with the discontinuous characteristics of DIC-measured displacement and strain field ahead of the fracture tip. Based on test results, a couple of outcomes were obtained. (1) The geometries of mixed-mode (I-II) fractures changed barely at different quasi-static loading rates. (2) At mixed-mode (I-II) fracture tips, the DIC-measured tensile displacement fields presented remarkable discontinuity; in contrast, the DIC-measured sliding displacement fields were nearly continuous. Consequently, at different quasi-static loading rates, the tensile-shearing and compressive-shearing mixed-mode (I-II) just represents the fracture bearing tensile-shearing and compressive-shearing stresses for rock-like materials. Still, at the mixed-mode (I-II) fracture tip, the deformation in FPZ and the generation of the real fracture surface is tensile. (3) With the loading rate increasing, the FPZ length of mixed-mode (I-II) fracture increased roughly from 5 mm to 17 mm, which is similar to the pure mode-I fracturing (FPZ length: 4.5-17.3 mm). The rate-dependent FPZ length of mixed-mode (I-II) fracture follows a power-law, consistent with the mode-I fracture. (4) The peak load (an index indicating the fracture resistance) of mixed-mode (III) and pure mode-I fracturing was strengthened with the increasing loading rate, linearly correlated to the increasing FPZ length. It indicates that rate-dependent FPZ is supposed to strengthen the fracture resistance of mixed-mode (I-II) cracks with loading rates increasing. This research can provide a basis for controlling mixed-mode (I-II) fracturing of rock-like materials in underground excavation projects and geo-energy exploitation, such as the deflecting propagation of hydraulic fractures.