Effects of microstructures on dynamic compression of Barre granite

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), Z (highest P-wave velocity), and X (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 (similar to 230 mu s). For low strain-rate loading (similar to 70 s(-1)) and high strain-rate loading (similar to 130 s(-1)), the maximum dynamic stress achieved is not sensitive to the microcrack orientation; for intermediate strain rate (similar to 100 s(-1)) loading, the maximum achieved stress for Y-samples is the largest. In addition, three dynamic compressive rock failure modes are identified: quasielastic, cracked, and fragmented. The correlation between the failure modes and the shape of the stress-strain curves is discussed. (C) 2007 Elsevier Ltd. All rights reserved.