Fracture Energy-Based Brittleness Index Development and Brittleness Quantification by Pre-peak Strength Parameters in Rock Uniaxial Compression

Brittleness is a fundamental mechanical rock property critical to many civil engineering works, mining development projects and mineral exploration operations. However, rock brittleness is a concept yet to be investigated as there is not any unique criterion available, widely accepted by rock engineering community able to describe rock brittleness quantitatively. In this study, new brittleness indices were developed based on fracture strain energy quantities obtained from the complete stress–strain characteristics of rocks. In doing so, different rocks having unconfined compressive strength values ranging from 7 to 215 MPa were examined in a series of quasi-static uniaxial compression tests after properly implementing lateral-strain control in a closed-loop system to apply axial load to rock specimen. This testing method was essential to capture post-peak regime of the rocks since a combination of class I–II or class II behaviour featured post-peak stress–strain behaviour. Further analysis on the post-peak strain localisation, stress–strain characteristics and the fracture pattern causing class I–II and class II behaviour were undertaken by analysing the development of field of strains in the rocks via three-dimensional digital image correlation. Analysis of the results demonstrated that pre-peak stress–strain brittleness indices proposed solely based on pre-peak stress–strain behaviour do not show any correlation with any of pre-peak rock mechanical parameters. On the other hand, the proposed brittleness indices based on pre-peak and post-peak stress–strain relations were found to competently describe an unambiguous brittleness scale against rock deformation and strength parameters such as the elastic modulus, the crack damage stress and the peak stress relevant to represent failure process.