The evaluation criteria for rock brittleness based on double-body analysis under uniaxial compression

The brittleness of rocks plays an important role in the fracture network fracturing process of unconventional oil and gas reservoirs, the drilling efficiency of rock breaking and wall stability, and mining engineering. Rock brittleness has become one of the key parameters in the study of many rock mechanics and related engineering problems. However, there is still no widely accepted definition of brittleness. Although many criteria have been proposed to characterize rock brittleness, their applicability and reliability have yet to be verified. Therefore, brittleness evaluations require further study. In this paper, we divided the rocks under an external load into the unruptured area (body I system) and ruptured area (body II system). We considered that the body I system provided energy for the fracture of the II body. The brittleness of rock is understood as the ability of energy release when a post-peak fracture occurs in the II system. Therefore, the quasi-static energy-balanced equation of a double-body system was established and a new brittleness index was defined as the elastic energy release between an unsteady quasi-static state to the stable state of body II. The new index for brittleness evaluation proposed in this paper can not only quantify the brittleness of rock through the relation between the extremum of a softening modulus at the inflection point in the post-peak deformation curve of body II and the elastic modulus of body I but also ensure the position of the failure strain in the post-peak stage as well as reviewing the instant characteristics of the rock’s brittle crack. It is a new method to evaluate rock brittleness. Taking sandstone with different contents of brittle minerals as an example, the variation of the brittle index with the content of brittle minerals was studied.