Brittleness is one of the most important mechanical properties of rock, which has a strong influence on the failure process of rock mass in mining or tunnelling activities. however, the definition of brittleness in rock mechanics is not yet to be precisely defined. Dozens of brittleness index have been proposed to characterise hard rock behavior under compression, but there is no unified criterion as to which criterion is the most reasonable and reliable. From the point of view of energy, the relationship between the crack initiation elastic strain energy ratio and brittleness index is discussed in this paper. The release of elastic strain energy is often accompanied by the compression process of brittle rock, so there must be some relationship between brittleness index and elastic strain energy. The crack initiation stress is determined by crack volumetric strain method or lateral strain response method, and the crack initiation elastic strain energy ratio is defined as n. A functional relationship was established between brittleness index and crack initiation elastic strain energy ratio from fracture mechanics and energy theory, which is verified by mechanical tests of different kinds of rocks. The results show that there is a linear function relationship between brittleness index and crack initiation elastic strain energy ratio. A large number of indoor uniaxial compression and Brazilian splitting test examples show that the brittleness index can be redefined by using crack initiation elastic strain energy ratio. This method has certain reliability, enriches the calculation method of brittleness index, and provides a new idea for the study of brittleness index.
Brittleness is the property of rock mass to break under very small plastic deformation, and the release of elastic strain energy is accompanied by brittle failure. As an important basic characteristic of rock mass, brittleness evaluation has important guiding significance for reservoir reconstruction in oil and gas field mining, stability of underground roadway and surrounding rock and prediction of rockburst disaster (Xia et al. 2016). At present, many scholars have done a lot of research on the evaluation of brittleness indicators (as shown in Table 1), but there is no uniform standard for the definition and test methods of rock brittleness indicators. From the point of view of stress-strain curve, Tarasov and Potvin (2013), Bishop (1967), Hajiabdolmajid et al. (2003), Hucka and Das (1974) proposed brittleness indexes B1-B6. From the point of view of the ratio of UCS to tensile strength of rock, Hucka and Das (1974), Altindag (2002, 2003, 2010) proposed brittleness indexes B7-B10. From the point of view of hardness test, Lawn and Marshall (1979), Quinn and Quinn (1997) proposed brittleness indexes B11 and B12. Some researchers (Blindheim and Bruland 1998, Yagiz 2006, Copur 2003) have defined rock brittleness indexes B13-B15, from the point of view of impact test and the punch penetration test.