Ductile-brittle quantitative evaluation of rock based on post-peak properties under true triaxial stress

To quantitatively evaluate the influence of high true three-dimensional stresses on the ductile-brittle behaviours of rock in deep underground engineering, a series of true triaxial compression tests with different stress levels were carried out on marble and four kinds of granite. The influences of true triaxial stress states (sigma(2), sigma(3)) on the post-peak characteristics were analysed, and a new normalized ductile-brittle evaluation index was proposed based on post-peak energy conversion characteristics of rock under true triaxial stresses. The ductile-brittle behaviours of rock were divided into four qualitative levels, namely, ductile-brittleness, transitional, brittleness and super-brittleness, and the influences of true triaxial stress states on the ductile-brittle behaviours of rock were quantitatively investigated. The research shows that as sigma(2) increases or sigma(3) decreases (that is, the differential stress between sigma(2) and sigma(3) increases), the brittleness of rock increases, and its increase rate gradually decreases and tends to be stable, transforming from ductile-brittleness to transitional, brittleness and super-brittleness and resulting in super-brittleness being easily induced by low-sigma(3) and high-sigma(2) conditions. When the differential stress between sigma(2) and sigma(3) is small, the intrinsic characteristics of rock itself have an obvious influence on ductile-brittle behaviours. When the differential stress between sigma(2) and sigma(3) is large, all kinds of rocks can exhibit super-brittle behaviour. The change of stress controls the evolution of rock ductile-brittle behaviours, and high-stress controls rock brittleness. The rock brittleness under true triaxial stress is significantly higher than that under conventional triaxial stress at the same sigma(3). sigma(2) induces an increase in rock brittleness and causes the decay rate of brittleness to decrease with increasing sigma(3), and sigma(2) increases the upper limit of sigma(3) for brittle failure of rock. The enhancement effect of sigma(2) on rock brittleness must be considered when evaluating the brittle failure of deep surrounding rock under high-stress conditions; otherwise, the risk of brittle failure may be underestimated.