To study the strength characteristics and 3D fracture evolution law of granite under unloading confining pressure

3 different stress paths tests including conventional triaxial compression

unloading confining pressure-loading axial pressure

and graded unloading confining pressure-loading axial pressure cyclic loading and unloading were carried out on granite

and corresponding axial and radial stress-strain curves were obtained. Meanwhile

3D reconstruction technique of CT scanning was used to obtain 3D images of the distribution of internal fracture during and after rock unloading confining pressure process. Results show that: 1) Compared with the conventional triaxial compression test

the brittle failure characteristics of specimens under unloading confining pressure were more obvious

and graded unloading confining pressure-loading axial pressure cyclic loading and unloading test could enlarge the post peak ductility of the granite

as well as reduce the axial compression of instability and the fierce of failure. 2) Both unloading confining pressure schemes reduced the bearing capacity of the granite by about 30%. 3) The macro-fracture of the granite under loading was a combination of tension and shear

and the transition between tension and shear was not obvious. The apparent fracture was the result of the outward expansion of the internal fracture. 4) The amount of cracks of the granite generated before the peak under unloading was small

while a large number of cracks were generated after the peak

and the fracture was sudden and transient. When the confining pressure was low

the macroscopic cracks were firstly generated at the edge of the sample

and when the confining pressure was high

the macroscopic cracks were firstly generated in the middle of the sample.