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

• Published in: Engineering fracture mechanics 2020-12, Vol.240, p.107365, Article 107365
• Main Authors: Xing, Yuekun, Huang, Bingxiang, Ning, Erqiang, Zhao, Long, Jin, Feng
• Format: Article
• Language: English
• Subjects: CCBD specimen; Compressive properties; Crack propagation; DIC method; Digital imaging; Discontinuity; Displacement; Fracture process zone; Fracture surfaces; Fracture toughness; Hydraulic fracturing; Loading rate; Microcracks; Mixed-mode (I-II) fracture; Peak load; Prefabrication; Quasi-static loading rates; Rock-like materials; Shearing
• ISSN: 0013-7944; 1873-7315
• DOI: 10.1016/j.engfracmech.2020.107365

•Mixed-mode fracture geometries change barely at different quasi-static loading rates.•Rate-dependent deformations in FPZ are tensile at mixed-mode fracture tip.•Rate-dependent FPZ length of mixed-mode and mode I fractures follow a power-law.•Rate-dependent FPZ length strengthens fracture resistances of mixed-mode cracks. 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° (tensile-shearing mixed-mode), 45° (compressive-shearing mixed-mode) and 0° (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 (I-II) and pure mode-I fracturing was strengthened with the increasing loading rate, linearly correlated to the increasing FPZ length. It indicates that rate-dependent