Fracture Behaviors and Damage Evolution Anisotropy of Granite Under Coupling of Multiaxial Confinement and Dynamic Loading.

Granite, as a typical anisotropic hard and brittle rock, is generally in a uniaxial, biaxial, or triaxial state of stress in deep underground engineering and is frequently influenced by extreme dynamic loads including earthquakes, occasional burst, and excavation disturbance. A series of dynamic compression tests were conducted on granite using a true triaxial split Hopkinson pressure bar (SHPB) system on initially anisotropic granite. Using X-ray three-dimensional (3-d) computed tomography (CT) scanning technology and ultrasonic testing, the mechanical properties and fracture behaviors of anisotropic granite under combined dynamic and static loading conditions were studied. Results show that the failure mode of granite changes from damage, to splitting failure, and finally to crushing failure with the rising strain rate under dynamic uniaxial compression. In addition, the axial prestress increases the proportion of transmitted energy in the rock and significantly influences failure strain of the rock. Under biaxial static prestresses, the granite is split along the direction of its free faces and the failure mode is affected by the value of the anisotropy index in the direction of free faces. Under the triaxial static prestresses, the rock is very unlikely to be damaged and the damage effect occurs only after multiple impacts. In the same prestress state, the strain rate of granite increases with the increasing impact velocity and the prestress constraint reduces the dynamic strain rate in the granite. The initial anisotropy of the longitudinal wave velocity of granite is gradually enhanced as the cyclic impact continues. Increasing the strain rate will induce more significant evolution of initial anisotropy of granite, while the prestress constraint restricts the development of initial anisotropy. The results improve understanding of the evolution of impact damage and the dynamic failure characteristics of anisotropic rocks under high geostress. Highlights: Conducted a series of dynamic fracture tests of granite under unprestressed, uniaxial, biaxial and triaxial prestressed conditions. Revealed the dynamic impact damage evolution mechanism of granite under multi-axial stress constraints from a microscopic perspective. Investigated the confining pressure effect and strain rate effect on dynamic damage anisotropy of granite under multi-axial stress constraint. [ABSTRACT FROM AUTHOR]