Abstract
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.
Highlights
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)
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 specimens
The source locations of the AEs correspond to a process zone ahead and around the notch indicating the evolution of the microcracks
Summary
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). The traditional models which are mainly based on the evolution of strain with time, fail to capture the full range of behavior from elastic deformation at the beginning of loading, viscous deformation in primary and secondary creep and especially plastic deformation as cracking progresses toward dynamic failure during tertiary creep. This is, in part, due to the lack of reliable experimental data for validating the models. AE monitoring and DIC are coupled with traditional mechanical measurements in order to provide new insights into spatiotemporal micro-crack evolution, fracture mode, and aseismic/seismic transition during SCG in brittle rocks
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