Effects of microstructures on dynamic compression of Barre granite

摘要

The distribution and characteristics of microstructures (microcrack and grain) of Barre granite (BG) were investigated, and three orthogonal weak planes associated with the preferred orientations of microcracks were identified. It has been demonstrated that both the fracture toughness and the longitudinal wave speed depend on the direction of these weak planes. In this study, disk samples cut from one BG block are prepared for split Hopkinson pressure bar (SHPB) test. The axial directions of the samples are chosen to be parallel to the preferred direction of microcracks and the samples are grouped and denoted by Y (lowest P-wave velocity), ZETA (highest P-wave velocity), and CHI (intermediate P-wave velocity). Pulse-shaper technique is adopted to achieve equilibrium of dynamic stresses on both ends of the sample and constant strain rate during the dynamic loading. For samples within the same orientation group, the maximum stress achieved shows clear strain-rate sensitivity. The effect of microcracks on the dynamic compressive response of BG depends on the strain rate for a fixed loading duration (approx 230mu s). For low strain-rate loading (approx 70s~(-1)) and high strain-rate loading (approx 130s~(-1)), the maximum dynamic stress achieved is not sensitive to the microcrack orientation; for intermediate strain rate (approx 100s~(-1)) loading, the maximum achieved stress for Y-samples is the largest. In addition, three dynamic compressive rock failure modes are identified: quasi-elastic, cracked, and fragmented. The correlation between the failure modes and the shape of the stress-strain curves is discussed.