Abstract: The mechanical features based on unloading mechanical parameters are not accordant with the mechanical characteristics of rock under unloading condition; in order to construct strength criterion which is widely used and accordant with characters of unloading rock, the work and results are carried out as fellows. Firstly, different initial damage levels between unloading experiment of rocks and underground engineering under excavation unloading conditions are discassed. Secondly, the internal relations between elastic strain energy, the second invariant of stress deviator, the second invariant of stress tensor and Poisson’s ratio are explored. Thirdly, the internal relations between elastic strain energy and strength criterion are also explored. Lastly, based on elastic strain energy, the mechanical features of rocks under unloading condition are studied by using loading mechanical parameters. The experiment results show that existed strength criterion is possible to violate the first thermodynamic law or the law of conservation of energy. It is suitable to increase or decrease for the limited bearing loads of rock under the range of unloading condition based on elastic strain energy, and the mechanism of above-mentioned result is analyzed.

Key words: unloading mechanical characteristics, elastic strain energy, Poisson’s ratio, second invariant of stress tensor