Abstract: The fracture of rock is the results of a comprehensive role in the process of energy conversion, and the release of the strain energy stored is the essence of rock abrupt failure. The energy characteristics of rock size effect under low / intermediate strain rates have been studied. It is found that the total energy U, the elastic energy Ue and the damage energy Ud at failure decrease with the increasing sample size. Rock strength is related to the stored energy, especially to the stored elastic energy, namely the more the elastic energy, the higher the strength. The stored elastic energy reduces gradually with the specimen length changing from 50 mm to 125 mm. Splitting is the main fracture mode for the short specimens while the shearing fracture mode dominates the longer specimens with evident strain localization. Energy is the inherent impetus which leads to differences of failure in micro, meso and macroscopic characteristics under different strain rates, which is also the intrinsic motivation of the size effect of rock strength.

Key words: rock mechanics, low / intermediate strain rates, size effect, energy characteristics