Mixed-mode I + II tensile fracture analysis of thermally treated granite using straight-through notch Brazilian disc specimens

Abstract

The destruction and fracture failure of heat-treated rock structures have been major issues in the underground engineering, such as disposal of highly radioactive nuclear waste, deep mining and exploitation of geothermal resources. Although several literatures have focused on studying the mode I fracture toughness and mode I fracture characteristics of heat-treated rocks, studies of the thermal effect on the mixed-mode fracture toughness and mixed-mode fracture characteristics are rare. Additionally, there is a lack of systematic study to analyse the fracture characteristics of heated rocks on theoretic aspects. In this paper, lots of mixed-mode I + II fracture tests were conducted on a heat-treated granite using straight-through notch Brazilian disc specimens. A total of six specimen groups (25, 100, 200, 400, 600 and 800 °C) and four loading mixities (Mode I loading, mode II loading and two mixed-mode loadings) are selected to apply the fracture tests. The results show that temperature treatment can both significantly affect the brittleness and the mixed-mode fracture toughness of the granite, but the fracture initiation angle is not sensitive to high temperature. The mode I fracture toughness and mixed-mode fracture toughness develop similar trend with increasing treatment temperature. The theoretical fracture models can be effectively applied to predict variations of the mixed-mode fracture parameters. This research also indicates that fracture parameters at high temperature are likely to be predicted by the well-known fracture criterion by only taking the rock physical and mechanical parameters at natural temperature. This work could provide a reference to the research on the mechanical properties and fracture failure of rocks during restoring of post-fire rock structures or hydraulic fracturing of hot dry rocks.