Effect of intermediate principal stress on mechanics characteristic of brittle rock material under true triaxial unloading testing

A series true triaxial unloading experiment with acoustic emission (AE) testing for siliceous rock and limestone two brittle rock materials are finished by RT3 which is a rock high pressure true triaxial machine. The result shows that the peak strength and energy release before fracture or failure increases at first then decreases, for the same rock material, with increasing of medium principal stress (sigma(2)) under the unloading condition along the minimum principal stress (sigma(3)). The result of regression analysis by PSO algorithm indicates the law which the peak strength follows can be described using a parabola with negative quadratic coefficient.