Rock brittleness evaluation based on energy dissipation under triaxial compression

Evaluating the brittleness of rock is significant for strategy determination of hydraulic fracturing, such as candidate selection and parameter optimization. A series of definitions and indices of brittleness have been proposed to characterize the mechanical behavior rock failure. However, these existing indices failed to consider the residual state and the confinement effect. Based on the analysis of energy evolution during the whole process of rock failure, the brittleness was redefined in this study as the comprehensive capability of dissipating little energy during the pre-peak stage and self-sustaining complete failure during the post-peak stage. Accordingly, a new energy-based brittleness index B was proposed in terms of the complete stress-strain curve to quantify this capability from following three aspects: the ratio of accumulated elastic strain energy and total absorbed energy during the pre-peak stage (B-1); the proportion of elastic strain energy in all energy source consumed for sustaining rock failure (B-2); and the dissipation extent of accumulated elastic strain energy during the post-peak stage (B-3). To verify the reliability of the new method, uniaxial and triaxial compression tests were performed on different types of rock samples. The application and comparison of various indices showed that the new brittleness index precisely characterized the stress-strain curves and failure behavior of rock samples under different confinement levels. The variation trends of brittleness with confining pressure were obviously distinct among different rock types. Three independent brittleness indices B-1, B-2, and B-3 were helpful for analyzing sensitivity difference of brittleness to confining pressure among different rock types. Accordingly, this new energy-based method can provide reliable evaluation of rock brittleness.