Time-dependent rock deformation is considered to precede dynamic failure in many rock-engineering projects and natural geohazards. In order to understand long-term performance of brittle rocks, we gather mechanical load, strain, acoustic emission (AE) and digital image correlation (DIC) data to describe the evolution of damage and cracking in response to tensile loading during multi-stage relaxation experiments on inverted single edge notch bending (iSENB) specimens. The source locations of the AEs correspond to a process zone ahead and around the notch indicating the evolution of the microcracks. The results showed that the cracks first start aseismicaly under subcritical growth until 10mm from the tip of the notch and then they grow seismically by showing seismic signals in the form of acoustic emissions. It was observed that the process zone obtained by DIC is smaller than the cloud of the AE locations. This can be partially because DIC only shows surface deformation and partially because of the errors associated with the AE source locations such as ignoring anisotropic velocity, sampling rate, sensor locations, etc. Moment tensor analyses of the AE signals showed that both tensile and shear cracks are involved in the micro-scale although in the macroscopic scale the damage process is mostly considered as tensile. The results showed that microcracks start as tensile, and then continue as shear, especially at the end of the crack where the specimen experiences compression loading.
Rocks subjected to constant load, or constant displacement boundary conditions can be observed to deform slowly and eventually fail at stress levels less than otherwise expected from short-term strength tests (Amitrano and Helmstetter, 2006). Similarly, rock mass failures commonly don’t occur immediately after a forcing event (e.g. excavation or intense precipitation), but instead may occur months or years after the disturbance, indicating failure is partly controlled by a gradual decrease in rock strength. Understanding progressive rock deformation, and in particular the transition from sub-critical to critical fracture velocities, is therefore important to predict the onset of dynamic and catastrophic failure associated with landslides, earthquakes, and volcanic eruptions (Brantut et al., 2013). Understanding this process can also provide important insight into the integrity of underground mines and excavations, the long-term storage of hazardous wastes, effective recovery of hydrocarbon and geothermal energy resources with controllable induced seismicity, and last but not least safe and economic CO2 sequestration [Brantut et al., 2013]. United Nations Educational, Scientific and Cultural Organization (UNESCO) has recently emphasized our current lack of knowledge in this area, and "Understanding Slow Deformation before Dynamic Failure" has been suggested as one of the two priority research areas within the Natural Hazards theme of its International Year of Planet Earth (Ventura et al., 2009, Brantut et al., 2013).