Anisotropic influence of fracture toughness on loading rate dependency for granitic rocks

Granite is a significantly anisotropic rock that is affected by the pre-existing microcrack distribution. In rock engineering applications in a variety of fields, it is crucial to understand the influence of the internal microcrack distribution on the mechanical properties of the rock. In this paper, static and dynamic fracture toughness tests were conducted with straight notched disc bend (SNDB) specimens of Youngju granite to investigate the relationship between the microcrack-induced fracture toughness anisotropy and the loading rate dependency. Three groups of specimens were manufactured with three different orthogonal directions, which were determined by the P-wave velocity measurement. In order to conduct the static and dynamic fracture toughness tests, a servo-controlled Material Testing System (MTS) machine and a split Hopkinson pressure bar (SHPB) apparatus were respectively used. In the dynamic test, to achieve the stress equilibrium state in the measurement, a careful pulse shaping technique with a copper and rubber pulse shaper was used. The results of the entire range of tests in this study revealed that the fracture toughness of Youngju granite has a clear loading rate dependency. Further, we found that the preferred orientation of pre-existing microcracks causing strong anisotropy of fracture toughness under static loading rates became significantly weaker in the dynamic loading state. Finally, a possible reason for this relationship between the fracture toughness anisotropy and the loading rate dependency is discussed with different fracturing processes of rock under the static and the dynamic loading states using image analysis for generated cracks.