Experimental and numerical study on the dynamic behavior of a transversely isotropic rock

The present work investigates the dynamic mechanical behavior of transversely isotropic Jhiri shale at five anisotropy angles (0 degrees, 30 degrees, 45 degrees, 60 degrees, and 90 degrees) using a large diameter (76 mm) split Hopkinson pressure bar. Jhiri shale, a type of Vindhyan shale, is collected from the Rewa region in the Madhya Pradesh state of India. Samples are loaded at strain rates of -150/s, -230/s, and -320/s under compression. It is found that the strength is ratedependent and increases as the strain rate rises. At elevated strain rates, U-type anisotropy is noted. The anisotropy ratio lowers from 3.50 to 3.37 when the strain rate changes from quasi-static to -320/s, indicating that the influence of bedding planes diminished at higher strain rates. All the samples subjected to dynamic compressive loading are pervasively fragmented and undergo complete pulverization. As the strain increases, the degree of fragmentation intensifies, and the mean fragment size reduces. Samples with intermediate anisotropy angles (30 degrees, 45 degrees, 60 degrees) are subjected to a higher degree of fragmentation than those with 0 degrees and 90 degrees. The rock fragments exhibit fractal distribution, which can be described using a power-law relationship. The dynamic experiments are numerically simulated using the discrete element method. The grains of the rock are generated as randomly sized and distributed polygonal blocks using Voronoi tessellation. The initiation and propagation of micro-cracks and failure mechanisms are explored via numerical simulations. It is found that samples mainly undergo shear failure dynamically. As the strain rate increases, the number of cracks increases. For samples with intermediate anisotropy angles, shear cracks begin along the bedding planes and then spread internally. At any given strain rate, they have a higher number of cracks than 0 degrees and 90 degrees samples, indicating a higher degree of fragmentation, as noted experimentally.