Micro-scale Fracturing Mechanisms in Rocks During Tensile Failure

Understanding the fracturing mechanisms of rock on both macro- and micro-scale is important for properly designing rock engineering applications. However, there is still a lack of understanding of why macro and micro-scale fracturing mechanisms differ. In this study, acoustic emission (AE) and digital image correlation (DIC) techniques were employed to track the microcracking processes in granite specimens subjected to indirect (Brazilian) and direct tensile loadings. The moment tensor inversion of the AE waveforms and the DIC strain field data revealed that the ultimate so-called tensile macro-fracture was predominantly composed of shear microcracks in Brazilian tests and tensile microcracks in the direct tension tests. The different contributions of shear and tensile microcracks to the formation of the macro-fracture explain the difference between direct and indirect tensile strengths. Our results showed that the compressive stress in the Brazilian test due to its biaxial stress field and the grain size are the two critical factors affecting the microcracking mechanisms in the tested coarse-grained granite. Characterizing the surface of the generated macro-fractures and the results of a series of complementary tensile tests performed on fine-grained mortar specimens suggested that reducing the compressive stress and grain size decreases the contribution of shear microcracks. The results of this study can be used in rock fracture applications in granitic rocks such as hydraulic fracturing for geothermal energy extraction, where the knowledge of the cracking location and mechanisms is critical for enhancing the reservoir's productivity.