Effects of thermal treatment on fracture characteristics of granite from Beishan, a possible high-level radioactive waste disposal site in China

In-situ experimental investigations on fracture behavior of Beishan granite under loading after 125-600 °C thermal treatments have been carried out through high temperature Scanning Electron Microscope (SEM) testing system. Thermal microcracks and non-uniform expansion of grains, long and deep thermal cracks along the grain boundary were observed on the surface of Beishan granite with thermal treatments at different temperatures. The initial direction of microcracks and the main crack propagation path are affected by the shape of mineral grains and the distribution of thermal cracks. The effect of the distribution of thermal cracks becomes more dominant with the increasing temperatures. There are multiple mechanisms of thermal effects on strength and fracture toughness of Beishan granite, namely, thermal cracking, water evaporation, hot melting and phase transition of quartz. The main controlling mechanism changes with treatment temperatures. While elastic modulus slightly increases at 275 °C because of hot melting, in general, it decreases with the increasing temperature due to increasing thermal cracks. Stress intensity factor for short-span (the ratio of span to height is 2) three-point bending specimen is determined using boundary collocation method. The relationship between fracture toughness and heat treatment temperature can be approximately described by linear decreasing function.