Mechanical anisotropy of coal under coupled biaxial static and dynamic loads

Abstract

Understanding the mechanical properties and fracturing behaviours of coal is significant for the safety of underground mining engineering. Coal failure characteristics are highly influenced by the anisotropy and loading conditions. In this study, the coupled biaxial static and dynamic tests are conducted on coal specimens with five bedding orientations θ (i.e., 0°, 30°, 45°, 60°, and 90°) with respect to the normal direction to loading. A Triaxial Hopkinson bar (Tri-HB) system is adopted to apply the biaxial quasi-static stress first and then dynamic loading at four impact velocities (i.e., 10, 13, 17, and 21 m/s). Real-time processes of coal fracturing are recorded by two high-speed cameras, and accordingly, full-field deformation and ejection velocities are identified by the three-dimensional digital image correlation (3D-DIC) technique. Moreover, the internal fracture morphology of coal specimens is characterised using synchrotron-based X-ray computed tomography (CT). Experimental results show that at similar strain rates, the peak stress against θ shows a “U” shape with the lowest value at θ = 60°. The peak stress increases with increasing impact velocity, while its growth rate exhibits a downward trend revealing a decreasing sensitivity to strain rate. Coal ejection velocities are positively rate-dependent, and the highest ejection velocity is found shifting from θ = 45° to θ = 90° with increasing impact velocity. The average fragment size of coal specimens is negatively related to impact velocities and energy absorption, and the finest fragmentations are observed at around θ = 45°. Dynamic behaviours of coal under biaxial pre-stresses are dependent on bedding structures and strain rates, while the bedding effect becomes weak as strain rate increases.