Granite is a strong anisotropic rock affected by distribution of pre-existing microcracks. In various rock engineering applications, it has a significant meaning to understand the influence of internal microcrack distribution on the mechanical properties of rock. In this paper, static and dynamic fracture toughness tests were performed with straight notched disc bending (SNDB) specimens of Yougnju granite to investigate the microcrack induced fracture toughness anisotropy. In order to conduct dynamic fracture toughness test, split Hopkinson pressure bar (SHPB) apparatus using pulse shaping technique was adopted. The crack propagating behavior was observed by high speed camera and the crack opening velocity (COV) was also measured with laser gap gauge (LGG) system. The fracture processes of the specimen with different loading rate conditions were analyzed by comparing the images of high speed camera and micro focus X-ray CT. As a result of this study, the fracture toughness of Youngju granite shows obvious loading rate dependency through entire loading rates. It was revealed that the preferred orientation of pre-existing microcracks caused strong anisotropy of fracture toughness under static loading rates which the anisotropy of that became weaker with increasing loading rates.
In the field of rock engineering such as blasting, hydraulic fracturing and drilling, the understanding of fracturing behaviors in rocks under various loading conditions is important. Fracture toughness, as an example, is an indication of resistance against crack growth at the tip of crack. Generally it is well known that granites show strong anisotropy of mechanical properties caused by pre-existing microcrack.
Many researchers have reported that fracture toughness of granites is affected by the preferred orientation of pre-existing microcracks and that increases with increasing the loading rate [Nara 2006, Dai 2010, Cho 2015].
