Abstract: In order to study the evolution law of acoustic emission and wave velocity of rock under true triaxial step-loading. The self-developed rock true triaxial testing machine,acoustic emission monitoring system and ultrasonic testing system are adopted. The acoustic emission location monitoring and ultrasonic testing experiments of rock under true triaxial graded loading were carried out. The study results show that the change trend of AE cumulative energy and cumulative count can be used as the precursor information of rock fracture. In the first stage,the increase is small as the rock is compacted. In the second,third and fourth stages,the internal fractures of sandstone began to develop and their growth rate increased slowly. The fifth stage is the main stage of sandstone damage,with an increase of more than 80%. It is inferred that the sandstone in the fifth stage is unstable due to the stress reaching the peak strength. Acoustic emission location technology can well characterize the crack morphology and propagation law. In the first,second and third stages,acoustic emission localization events rarely occur in the sandstone and the distribution is relatively scattered. In the fourth and fifth stages,a large number of acoustic emission localization events occurred and concentrated on the fracture surface and fracture zone on both sides of the minimum and intermediate principal stress directions. The variation of ultrasonic wave velocity has a good response to the internal deformation and fracture propagation of rock. The rock wave velocity is anisotropic during loading. The wave velocity in the direction of maximum principal stress increases as the rock is compacted. And the increase of wave velocity decreases with the decrease of rock compaction. The wave velocity in the direction of the intermediate principal stress increases first as the rock is compacted. And then the wave velocity decreases with the formation of rock fracture surface. The wave velocity in the direction of the minimum principal stress first increases with the compaction of the rock. Subsequently,as the rock forms a penetrating fracture surface and is accompanied by a fracture zone,the wave velocity rapidly attenuates and plummets to a lower level.

Key words: sandstone,true triaxial step-loading,acoustic emission,source location,ultrasonic wave velocity