Progressive microcrack development in tests in Lac du Bonnet granite—I. Acoustic emission source location and velocity measurements

Abstract Three large blocks (52 × 52 × 22 cm) of Lac du Bonnet granite from the Cold Springs Quarry were tested under various load conditions to investigate the effect of stress path on the formation of damage around holes. Two blocks had throughgoing central holes, and were subjected to uniaxial and biaxial loads, respectively. Acoustic emission (AE) count rates were monitored during loading of these two blocks. The 3rd block initially had a partially drilled central hole and was subjected to a biaxial load. Following loading the hole was lengthened and then the sample was reloaded, until on the 4th cycle the hole was throughgoing. AE studies were used to determine the location and mechanism of microcracking during the load cycles on the incrementally drilled sample, in addition to the AE count rate. Results indicate that activity was associated both with fracturing close to the hole and with the boundary conditions of the applied load. Most AE events had either shear of complex source mechanism, though many of the events close to the hole were apparently implosive events. Notch formation initiated in this sample when the tangential stress in the hole was approx. 160 MPa, significantly higher than the unconfined compressive strength of the sample (260 MPa). The damage associated with the advancing hole was apparently quite localized and did not appear to have significantly affected notch development under these test conditions. Velocities measured parallel to the hole axis decreased following each load cycle; this decrease correlates primarily with the formation of large crack planes oriented parallel to the unconfined face.