Biaxial compression fracture behavior and energy damage evolution mechanism of deep fissured sandstone

• The coupling mechanism of the two key factors of fissure angle and strain rate on macroscopic mechanical parameters, characteristic stress, crack propagation and energy evolution is revealed. It is clearly pointed out that the increase of fissure angle and strain rate will enhance the strength of sandstone, but the regulation effect of the two on the number of cracks and AE activity is completely opposite. This comparative finding provides a more comprehensive and profound perspective for understanding the mechanical behavior of rock mass. • The relationship between crack propagation and energy distribution is revealed. The process of sandstone failure is clearly divided into four stages of energy evolution. It is found that although the total energy increases and decreases with the change of fissure angle and strain rate, the ratio of elastic energy to dissipated energy remains stable at each characteristic stress. The relationship directly relates the macroscopic mechanical response to the microscopic damage process, and provides a theoretical basis for predicting the critical state of rock failure through the energy criterion. • A new rock damage evolution model based on energy dissipation is constructed and verified, and the anisotropic effect of fissure is successfully reproduced. In this study, a new damage variable based on dissipated energy is defined, and a rock energy damage evolution model considering the influence of fissures is proposed, which can provide theoretical support for evaluating the stability of engineering rock mass and predicting its failure. Engineering rock mass is a discontinuous medium rich in macroscopic and microscopic fissures. To study the failure behavior and energy damage mechanism of fissured rock mass, this paper takes the sandstone of Gaojialiang Coal Mine in Ordos as the research object. Based on the particle flow code (PFC2D), numerical models of sandstone with different fissure angles were established. Biaxial compression experiments at different strain rates were carried out to analyze the influence of fissure angle and strain rate on the mechanical behavior and energy damage mechanism of sandstone. The results show that as the fissure angle increases, the initiation stress, expansion stress and characteristic stress ratio rise, the inflection point of volumetric strain moves forward, the AE ringing count increases, the "quasi-ellipse" direction deflects, with its long axis approximately perpendicular to the fissure, the contact force fluctuates, and energy accumulation and damage intensify. An increase in strain rate leads to an increase in the initiation stress and expansion stress, but the change in characteristic stress ratio is not obvious. The AE count decreases, the "quasi-ellipse" shape does not show a clear pattern, the contact force increases, and the energy accumulation decreases. A rock energy damage model considering the influence of fissures was established, and the rationality was verified through biaxial experiments. The research results can provide a theoretical basis for the failure mechanism and disaster prevention of deep fissured rock masses.