Dynamic Mechanical and Fracture Behaviour of Sandstone Under Multiaxial Loads Using a Triaxial Hopkinson Bar

Variations in stress conditions of rocks have been observed during blasting for excavation or large-scale seismic events such as an earthquake. A triaxial Hopkinson bar is developed to apply initial pre-stresses achieving various in situ stress conditions, including uniaxial (principal stresses sigma(1)>sigma(2)=sigma(3)=0), biaxial (sigma(1)sigma(2)>sigma(3)=0) and triaxial (sigma(1)sigma(2)sigma(3)0) confinements, and then to determine properties of materials under multiaxial pre-stress states at high strain rate. A series of tests was conducted on sandstone specimens to investigate dynamic responses under multiaxial pre-stress states. A high-speed camera at the frame rate of 200,000 fps with a resolution of 256x256 pixels was used to capture the fracture characteristics rocks under biaxial compression tests. Experiments show that under the same impact velocity, dynamic properties (e.g. dynamic strength, elastic modulus, fracture modes) of sandstone exhibit confinement dependence. Dynamic strength decreases with increasing axial pre-stress sigma(1) along the impact direction, while it increases with the increase of lateral pre-stresses sigma(2) and sigma(3). The elastic modulus increases with the confinement varying from uniaxial, biaxial to triaxial compression. Rocks are pulverised into powder under uniaxial pre-stress impact, and fragments are ejected from the free face under biaxial compression, while they show slightly damaged or a macroscopic shear fracture under triaxial compression. The 3D imaging of fracture networks in the damaged/fractured specimens was acquired via the X-ray computed tomography system.