Brittleness evaluation based on the energy evolution throughout the failure process of rocks

Abstract Brittleness is an important mechanical property of rock masses and a key index used in various engineering practices particularly to evaluate hydraulic fracturing, rock fragmentation and rock burst. However, rock brittleness is a concept remaining to be determined since there are no unique standards available. In this study, two methods to determine the brittleness index are proposed by considering the pre-peak failure energy and post-peak fracture energy, which are regarded as key factors in determining whether brittle failure of rock occurs. By taking the Longmaxi Formation shale as an example, the energy evolution laws of shales under different confinements are first analyzed, and the evaluation methods are then examined and applied to these rocks. The results show that these brittleness indexes for shales are capable of describing the transition from absolute plasticity to absolute brittleness. The applicability and performance of the proposed indexes are further validated against large uniaxial and triaxial compression test datasets for different rock types. Application of these brittleness indexes to the tested rock types clearly demonstrate that the two indexes have opposite variation trends with rising confining pressure. Compared with the previous evaluation methods, the rationality of the methods proposed in this study is also verified. These results indicate the proposed brittleness indexes can reflect the brittle characteristics of the rock and the whole process of deformation and failure of the rock. Accordingly, these methods seem to offer reliable evaluations of the brittleness of rock in various engineering practices, although further validation is necessary.