Study on Composite Fracture Characteristics and Hydraulic Fracturing Behavior of Hard Rock

To investigate the influence of non-singular terms (T stress) in the stress field on the composite fractures of hard rock Type I-II, such as rock splitting failure and hydraulic fracture propagation, this study focused on hard rocks in metallic mines. Through splitting tests and hydraulic fracturing experiments, the impact of T stress on the characteristics of Type I-II composite fractures in hard rocks was analyzed. Utilizing the generalized maximum tangential (GMTS) stress criterion considering T stress, the stress intensity factors of hard rock Type I-II composite fractures with different pre-existing crack angles were predicted. The critical fracture pressure expression for hard rocks was derived based on the maximum tangential stress (MTS) criterion. The results indicate that the GMTS criterion, considering T stress, is more suitable for describing the characteristics of Type I-II composite fractures under rock-splitting loads. However, under hydraulic fracturing, T stress has a minor influence on the fracture characteristics of hard rock hydraulic fractures. Therefore, when predicting the critical fracture pressure of hydraulic fractures, T stress can be disregarded. This study provides a scientific basis and guidance for hard rock hydraulic fracturing engineering.