Abstract: This paper analyzes the whole brittle failure process of rock from energy point, which enumerates and classifies variety of energy forms in the process of rock failure. It presents that thermal energy and mechanical energy must be distinguished when energy method is used to analyze the failure process of rock, because they have different effects. The relationship among input mechanical energy increment , releasable strain energy increment and dissipated energy increment is discussed through a symbolic cyclic loading-unloading curve; in addition, their transformation laws in different stages throughout the whole process are presented. The coefficient of brittle stress-drop has some shortcomings; and a new coefficient of energy-drop is presented in detail. Finally, this paper calculates energy-drop coefficient through the stress-strain curves in a series of confining pressure conditions of marble and granite, and obtains the relationship between the energy-drop coefficient and confining pressure. In addition, the effects of damage variable and Poisson's ratio on energy-drop coefficient are discussed. The comparison between brittle stress-drop coefficient and energy-drop coefficient with the existing experimental data shows that our model is reasonable and can be applied to more wide confining pressure conditions.

Key words: rock mechanics, stress-drop, energy method, releasable strain energy, energy-drop