Mixed mode fracture tests and inversion of FPZ at crack tip of overlying strata in underground coal gasification combustion cavity under real-time high temperature condition

Abstract

Measuring the fracture parameters of rock materials under real-time high temperature is of great significance for accurately evaluating the stability and safety of high-temperature underground tunnels. At present, the related research is focused on the fracture properties of rocks after treatment with high temperature. To this end, we independently developed a set of testing apparatus with effect of THMC coupling of high temperature and high pressure. Using this equipment, a serious of mixed mode I ± II fracture experiments of overlying strata (red sandstone) in combustion cavity under real-time high temperature conditions (20–700 °C) are carried out. And, the variations of fracture toughness, fractal dimension of fracture trajectory and fracture initiation angle with respect to temperature and notch angle are clarified. In addition, in order to predict the fracture process and fracture behavior of red sandstone at real-time high temperature, the experimental fracture envelopes are compared with that predicted by traditional fracture criterion. It is concluded that the fracture process zone (FPZ) size at the crack tip plays a crucial role in the effectiveness of the fracture criterion, and it is temperature-dependent. Finally, based on the generalized tangential stress criterion (GMTS), the effective FPZ sizes of red sandstone under different high temperature conditions are determined by using particle swarm optimization (PSO) algorithm combined with least square method. The research results of this paper are of great significance for evaluating the stability and safety of overlying strata (red sandstone) in combustion cavity.