Observation of fracture process zone and produced fracture surface roughness in granite under Brazilian splitting tests

In many rock engineering structures, the fracture network's complexity and the generated fractures' roughness are immensely important for understanding the hydraulic conductivity of the rock mass. Despite significant progress, the mechanisms governing nucleation and growth of fracture process zone (FPZ) are still debated. The present work investigates the characteristics of the FPZ and the surface roughness of the produced fracture in granite under tensile loading at two different loading rates. Acoustic emission (AE) and digital image correlation (DIC) techniques were simultaneously employed to monitor the development of the FPZ in disc specimens of granite subjected to Brazilian splitting tests. In addition, several roughness parameters of the produced fractures were measured, and it was tried to link the locations and the magnitudes of the AE events with the height distribution of the fracture surface. Our results indicated that the loading rate significantly influences the extent of the FPZ. The tensile strength of specimens also increased with the loading rate. The FPZ estimated from the AE and DIC data were in good agreement; however, AE provided a slightly wider process zone for all specimens, regardless of the loading rate. It was observed that reducing the loading rate led to a larger FPZ and generated macroscopic fractures with higher roughness characteristics. Moment tensor inversion of the AE signals revealed that in all tested specimens, macroscopic fractures, which are conventionally considered tensile fractures, are composed of three main cracking mechanisms at the microscale, including tensile, shear, and compressive sources. Focal mechanisms distribution showed that the cracks might initiate under tensile, shear, or a combination of both mechanisms. This study’s findings can help in better understanding the rock fracturing processes under tensile loading in granitic rocks for field applications.